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Draft: v1.5 (6 May. 17) A STEP: White paper ___________________________________________________________________________ Antarctica Search for Transiting Extrasolar Planets White paper for internal use of the team for studies report and prospects 1 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Executive summary We present hereafter A STEP, an Antarctica Search for Transiting Extrasolar Planets. Our goal is the detection of the extrasolar planets that transit in front of their parent star because the measurement of their radius, and (by radial velocimetry) of their mass, informs us directly on their composition and thus on the processes responsible for their formation. We show that Dome C is potentially the best site on Earth for transit surveys because of the 3months continuous Antarctic night, excellent weather conditions, and relatively slow variations of the environmental conditions. We show that present transit searches are limited by day / night cycles and systematic errors in the photometry, errors which are not well understood and appear to be different from one site to another. On the basis of this analysis, we propose A STEP, a precursor mission to assess the potential capability of a future massive transit search program from Dome C. A STEP consists in a cold-qualified CCD camera, to be placed at the focus of a Newton 40cm telescope at Dome C, for a continuous survey of ~10,000 stars of V magnitude 11 to 16.5 during the Antarctic winter. We aim at reducing systematic errors in the photometry to less than 2 mmag, the level estimated for the best transit search program so far. Depending on the properties of the site, and on the field of view that is necessary to achieve the highest photometric accuracy, A STEP could detect up to several transiting extrasolar planets per season. 2 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper 1. Scientific case The field of extrasolar planets has grown extremely rapidly in the past 10 years, and we now know of more than ~200 planets or planetary systems orbiting solar type stars in our neighbourhood. The discovery of more planets, smaller planets and the ability to characterize them directly impacts our ability to understand how planets form, how the Solar System formed, and to better prepare future, more ambitious missions aiming at detecting habitable planets and possibly biosignatures. Among the planets that are known today, most are just detected through the wobble of their parent star. To this date, eight of them have also been detected photometrically because they transit in front of their star at each orbital revolution. This is extremely important because the combined radial velocity and photometric transit measurements allow a measurement of both the mass and radius of the planet, and therefore a first constrain on their composition. In fact the transit method is the only one that is able to provide a constraint on the planetary radius in the short and medium-range future. It is on this basis that two space missions, COROT (launched in December 2006) and Kepler (launch in 2008), have been selected by CNES and NASA, respectively. They should detect tens, maybe hundreds of transiting extrasolar planets. Given the fact that these space missions come on top of several tens of ground based transit surveys (all of them unsuccessful so far, except two of them), why yet another ground based survey? Our analysis is that the limitations of present surveys, but also of the soon to come space surveys is due to unknown systematic effects of very different nature. We believe that at Dome C, we can potentially design a transit search program that can compete with a spacebased program for a fraction of the cost. However, this depends on two things: our ability to really understand the limitations of the transit surveys and design an ambitious proposal accordingly; the quality of the site for transit photometric surveys. This is why we propose a project which has three goals: qualify the site specifically for planet transit surveys (i.e. obtain the level of red noise, depending on various parameters-see next section); determine the main limitations of transit surveys at Dome C (e.g. seeing, differential refraction, temperature fluctuations...etc.); if compatible with (1) and (2), detect several transiting giant planets per observation season. We are considering that the number of targets that will be probed in space is not high enough to get enough discoveries to give statistical significance to distribution laws that will be observed. Indeed, The CoRoT and KEPLER space missions are both limited by the amount of data they could send back to the Earth, limiting their combined number of targets to 400000 stars, and thus a maximal combined possible yield of ~500 planets discovered, at different periods, mass, radius, and around different kinds of stars. Even that maximal yield wouldn’t be enough to statistically properly map the exoplanets distribution laws. The document is as follows: Next section describes our current understanding of the limitation of transit surveys. Then we present the advantages of Dome C, including results from ongoing 3 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper winter period, for a Dome C based transit search. We eventually describe the A STEP telescope configuration possibilities and our survey strategy. 4 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper A STEP: The team Observatoire de la Côte d'Azur (Laboratoires Cassiopée et Gemini): Tristan Guillot (PI) Francois Fressin (IS) Alain Blazit Nicolas Crouzet Vincent Morello Djamel Mekarnia Jean Gay Yves Rabbia Yves Bresson Jean-Pierre Rivet Alain Roussel Yves Hugues Dominique Albanese Scientific preparation, operation supervision, modelling tools, analysis of the results and scientific interpretation Scientific preparation, telescope:mechanics, telescope:optics, camera, modeling tools, analysis of the results and scientific interpretation Responsible of the camera team; telescope:optics, softwares Scientific preparation, telescope:optics, modelling tools, analysis of the results Scientific preparation, modelling tools Antarctization, Dome C logistics Telescope:optics, modelling tools Scientific preparation Telescope: optics Telescope environment, flat fielding system Telescope: mechanics Telescope: mechanics Camera, softwares Laboratoire Fizeau (Nice): François-Xavier Schmider Karim Agabi (PM) Scientific and technical preparation (telescope), Dome C logistics, analysis of the results and scientific interpretation Technical preparation(Antarctization, telescope, softwares), Dome C logistics Jean-Batiste Daban Eric Fossat Carole Gouvret François Jeanneaux Cécile Combier Guillaume Cuissot Lyu Abe Yan Fantei Responsible of the telescope team, technical preparation(teslescope, Control systems) Dome C logistics, analysis of the results and scientific interpretation Optical studies, technical preparation(telescope) Mechanical study of the camera environment Softwares Telescope: mechanics Quality control, tests and installation Softwares, data pipeline Observatoire Astrophysique de Marseille Provence (LAM & OHP): François Bouchy Michel Boer Alain Klotz Claire Moutou Magali Deleuil Marc Ferrari Antoine Llebaria Hervé Le Corroler Auguste Le van Suu Jérome Eysseric Claudine Carol Scientific preparation, follow-up of transit candidates Telescope control system, scientific interpretation Telescope and camera control software, scientific interpretation Scientific preparation, follow-up of transit candidates, photometric reduction Scientific preparation, follow-up of transit candidates Consulting on optical properties of the telescopes, tests and optical simulations Image processing, stellar photometry Scientific interpretation Computer interfaces, telescope control system System engineer Computer engineer Observatoire de Genève: Frédéric Pont Scientific preparation, analysis of the results, follow-up of transit candidates, scientific interpretation Deutsches zentrum für Luft und Raumfart (Berlin): Anders Erikson Data pipeline, camera, softwares, experience with BEST Heike Rauer Data pipeline, analysis of the results, scientific interpretation, experience with BEST University of Exeter: Suzanne Aigrain Nick Tothill Mark McCaughran 5 Data pipeline, experience with SWASP and MONITOR, analysis of the results, scientific interpretation Antarctization, telescope: mechanics, experience with South Pole Scientific preparation, scientific interpretation Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper A STEP team: workpackages Work Package 1: Mechanical studies Responsible: JB Daban - Design of the Newton 40 cm Telescope. - Thermal enclosure for focal instrumentation - Study of cold temperature qualification and differential stretching - Interface with mount and concrete pillar Work Package 2: Optical studies and camera Responsible: A Blazit - Study of the optimal PSF and pixel sample for transit photometry - Optical study of the telescope image with environmental fluctuations (mechanical, atmospheric) - Tests of the Proline FLI16801E camera on CoRoTcam testbench - Study of optimal photometry - Guiding device Work Package 3: Command and control softwares, hardware at Dome C Responsible: K.Agabi - Telescope command software - Automated procedure for field following, flatfields, back rotation of the telescope each day - Photometry quality check and daily report back to Europe. - Dome C logistics and installation on site Work Package 4: Data Pipeline, survey strategy, analysis and follow-up Responsible: T.Guillot - Re-use of BEST2 data pipeline, analysis of SWASP, Monitor, XO and OGLE pipelines - Field selection and use of other observational information - Follow up of transit candidates in radial velocimetry 2. The photometry of transits Since the photometric detection of the transit of HD209458b in 1999, more than two dozen photometric searches for surveys have been going on. On paper, the procedure seems trivial enough: monitoring a few thousand stars for 20-30 nights would lead to the detection of several transiting Hot Jupiters. When considering the score of projects devoted to the detection of planets by transit photometry, the present harvest appears meager. The 11 planets to date by ground-based transit projects (see table XXX), prove the detection of exoplanets possible for a large array of observational strategies (from deep field several-meter telescopes to wide field 10 cm reflectors), but the results of all these surveys is at least one order below their respective initial expectation. The a-posteriori analysis of these surveys show four main factors explaining the meager detection rates: 1. Due to technical problems or a limited number of good photometrical nights, most operating surveys have not been able to meet the duty cycles required to detect extrasolar planets. 6 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper 2. Dense stellar fields introduce severe crowding problems for transit photometry.Most existing surveys have been designed without taking this problem into account, e.g. by choosing a telescope/CCD combination that can not separate from each other a substantial number of stars and therefore with low detection probabilities as a result. 3. The window function (i.e. observations only a few hours per day during the night), which introduces a strong selection effect and prevents the detection of transiting planets except at certain favorable periods (Gaudi et al. 2005, Pont & Bouchy 2005). 4. Systematic effects in the photometry, which impose a much higher threshold for the detection of transit signals than initially estimated. The source of these systematics is many-fold. They result from an interaction between the atmospheric parameters, like airmass, extinction, temperature, seeing, sky background, and the instrumental parameters, like the precision of the flatfield, individual pixel response, telescope tracking, PSF shape. Table XXX – Transiting planets known in 2006 2.1 Window function Observations at Dome C are not affected by the day/night intermittence, and will be able to operate more or less continuously. This makes an enormous difference for transit searches. It not only quadruples the total time span of the observations (24h/day vs ~6hr day), it also removes the period selection effects completely, and allows the detection of transiting planets 7 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper regardless of their period (up to a certain cutoff period imposed by the total length of the survey). Importance of phase coverage Detection Probability Transit Period (days) Fig XXX - This diagram shows the detection probability of a transiting planet for a 60-day coverage. The transit depth is one percent of the stellar magnitude. The black curve shows the probability as a function of planet period for a telescope located in Chile. The red curve is the probability for the same telescope with uninterrupted phase coverage at Dome C. As most known hot Jupiter have periods around 3-4 days, continuous phase coverage is a crucial point. The difference between the two curves is even larger for fainter transits, which represent most of the cases 2.2 Systematic effects In order to detect hot Jupiter transits with some efficiency, a photometric search much be able to pick up transit signals of the order of 1 % with periodicity of a few days. The vast majority of transit surveys have fallen far short of this target. On paper, their capacity to detect shallow transit looked solid, but in actual fact, the shallowest detected eclipsing binary contaminators are all deeper than 3% (for instance the UNSW survey, Webb 2005, the MACHO candidates, Alcock et al 1997, the WASP candidates, Kane et al. 2004, the HATNet candidates, Hartman et al. 2004). The reason for this mismatch is the presence of “systematics” in the photometry. The OGLE survey, which did reach 1% transit depth and provided, additionally to the five detected transiting hot Jupiters, more than one hundred shallow eclipsing binaries, has permitted a much finer understanding of how systematics affect the detection threshold of transit surveys (Pont et al. 2005, Pont 2006 OHP meeting). In short, trends at the millimagnitude levels due to changing airmass, seeing, temperature, etc.., cause an increase of the detection threshold by a factor up to 3-5 compared to theoretical estimates! 8 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Fig XXX – Level of noise for target stars in a SUPERWASP field as a function of magnitude. Black points are the mean noise values during one exposure. Green points would be the noise value integrated on 2 hours timescales if all the noise sources were Gaussian (white). The red points are the real noise values integrated on 2 hours, showing a strong influence of red noise. The lower panel shows the same noise levels after applying the SYS-REM (systematic removal) procedure. The noise/magnitude curves for SUPERWASP are very similar to other successful surveys. 9 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Fig XXX - Comparison of detection threshold with “white noise” (blue line), which does not undergo temporal correlation and detection limit with “red noise” (linked to systematic effects - green line) – The pink to red part of the diagram shows the density of Hot Jupiter planets at given transit depth and target star magnitude. These effects, combined with the unfavorable window function, severely lower the detectability of hot Jupiter transits from the ground at normal latitudes. The detection rates are down to values of about 1 per 10'000 targets even in the best surveys, and down to negligible values for surveys with higher systematics. Given this new understanding, it can even be questioned if ground-based transit surveys at low latitude (i.e. non-polar) with high systematics are a reasonable use of resources. Source of the systematics The source of these systematics is many-fold. They result from an interaction between the atmospheric parameters, like airmass, extinction, temperature, seeing, sky background, and the instrumental parameters, like the precision of the flatfield, individual pixel response, telescope tracking, PSF shape. They can be different for different surveys. The experience of ground based transit searches, however, shows dominant tendencies: the systematics diminish with better sampling of target stars PSF. (ranging from about 10 mmag for one-pixel surveys to 1 mmag for 4-m class monitoring) the main component is related to airmass changes for the OGLE survey (Zucker 2005), and it has an important effect on photometry through differential colourextinction and refraction for other surveys (Irwin 2006) pixels answer and flatfielding may be main component for wide field surveys (XXXSuperwasp 2006) The understanding of the principal factors influencing the level of systematics is still incomplete at present. However, it is clear that they result from an interaction between atmospheric and instrumental parameters. Moving to more stable atmospheric conditions -especially for the airmass factor -- is certainly going to reduce the levels of the systematic trends. With its high latitude, Dome C allows the monitoring of southern fields during the whole winter at almost constant airmass, a condition inaccessible to more equatorial locations. 3. Dome C characteristics and impact on transit photometry 3.1 Duty cycle The evident advantage of Dome C for photometry is the fact that you can cover a field during several months. First of all because it is situated bellow the Antarctic polar circle, so the “full night” is three months long. 10 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Fig XXX - Left panel: observed window for Dome C, taking into account the meteorological conditions recorded in 2006 and the Sun altitude (at least 4± below horizon). The simulation also considers 20 minutes overhead per day, at 12:00 local time, for calibration and back rotation of the telescope. The right panel shows the variation with time of the duty cycle averaged over 10-day periods (full line) and the integrated duty cycle (dashed line). (from Mosser Aristidi 2006) Even on June 21st, there is a time during the day in which the Sun comes closer to the horizon. We can consider the night entirely dark when the sun is under -15° bellow the horizon. There is a contamination of the sky brightness when it is over this limit. We are considering the data affected by this increase of sky brightness differently and will try to apply an accurate removal of this effect for the data retrived during the “twilight” hours, as we can model it precisely. The height of the Sun below the horizon and its effect of sky brightness probably creates systematic noise effects, but at much smaller scale than the day/night intermittence for most of other ground based projects. We are also considering the solution of using a polarizer and observe at 90° of the ecliptic plane. As we could expect the sky brightness to be highly dominated by Rayleigh diffusion, and not by aerosols or particles effects, the sky should be highly polarized. That solution should be tested on site at Dome C before applying it. The moon will mainly influence the choice of the target stellar fields. We will have to avoid the proximity of a full moon to that field. It is still a contamination to add to the sky brightness. 3.2 Cloud coverage during 2006 winter (Aristidi 2006) 11 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper For the first time, estimations of the clear sky fraction were made this year. It was estimated visually several times a day, using a scale from 0 (no visibility) to 1 (cloud-free sky). Considering the period covered by the winterover (Jan 1st – Oct 31st), the clear sky fraction was greater than 0.9 78% of the time. Other numbers are presented in Fig. 1. This excellent clear sky fraction is of course of great importance for astronomical observations, in particular for asteroseismology, where extremely long integration times (several weeks) are needed. A simulation by B. Mosser (Observatoire de Paris) showed that Dome C asteroseismic observations should provide performance better or similar to a 6-site network at mi-latitude (a paper is was recently submitted to the Publ. of Ast. Soc. of Pacific). Time % for clear sky > 0.9 > 0.85 > 0.5 # of consecutive clear days (fraction >0.9) Average : Max : # of consecutive bad days (fraction <0.25) Average : Max : 78% 80% 91% 5.3 14.9 0.5 1.6 Figure XXX :Left: Statistics of the clear sky fraction during the period Jan 1st – Oct 31st . Right: Time percentage for two given clear sky fractions as a funtion of the month. Figure XXX :Clear sky fraction during the year 2006 3.3 Atmosperic fluctuations – seeing (Aristidi 2006) The DIMM, or “Differential Image Motion Monitor” is a telescope equipped with a mask with sub-apertures of diameter 6 cm distant 20 cm. This mask is placed at the top entrance of the telescope. One of the holes is equipped with a small angle prism (deviation 30 arcsec), the other one with a glass parallel plate. We use a Schmidt-Cassegrain Celestron C11 telescope (diameter 280 mm) with a 2xBarlow lens (equivalent focal length 5600 mm). It is placed on an equatorial mount (Astro-Physics 12 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper 900). The mounts is fixed to a massive wooden foot. The DIMM is operated from the top of a 5 m high platform to avoid the contribution of the ground layer turbulence. A digital CCD camera is placed in a thermostated box (temperature around –20°C), the box and the camera being located at the focus of the telescope. All this equipment has been customized to work in Antarctic cold conditions. Picture XXX : Left, the DIMM system. Note the 2 hole mask at the telescope top. The box at telescope back contains the camera. Right : typical short-exposure frame of the star Canopus at the focus. The two images move with turbulence, analysis of their differential motion provides the seeing. Seeing statistics for the winter 2006 (Feb 1st – Oct 31st ) The seeing conditions we found are similar to those of the previous winter. Statistics of the last two winterovers are presented in the table hereafter. The behaviour of the seeing with the time (see Fig. 2) is also similar to what was observed last year: very good values in summer turn into poor seeing at spring and remain around 1.5 -- 2 arcsec until November. Campaign Number of data Median seeing (“) Mean seeing (“) Std deviation (“) Max (“) Min (“) WO 2005 WO 2006 55385 1.22 1.30 0.77 6.49 0.08 67305 1.34 1.51 1.02 9.61 0.09 Figure XXX : Left: seeing statistics for the last two winterovers. Right: monthly-averaged seeing versus month for the two winterovers. Day-by-day values are shown in figure 3. As it can bee seen, the DIMM ran almost every day with short down time due to mechanical problems, bad weather (including wind speed > 8 13 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper m/s) or loss of the star. The histogram of the seeing values is plotted in figure 3, and shows the classical 2-bumps structure with a clear maximum near 2 arcsec that vanishes in summer where the shape of the histogram becomes nearly a gaussian centered on 0.6 arcsec. Figure XXX :Left: daily averages of the seeing during the winterover Right: seeing histogram. 3.4 Austral Auroras Another asset we had to check was the exact influence of austral aurorae. There has been several auroras observable from Dome C low on the horizon and not susceptible to affect observations and only one visible in a larger part of the sky with potential annoying effects. The cause of the low affectation of the site by these auroras is the fact that it is close to the south magnetic pole. As auroras only appear on a ring centered on the magnetic pole, they are mainly bellow the horizon from Dome C. They have been confirmed during 2005-2006 winter campaigns to be a negligible nuisance at Dome C. Picture XXX – The only significant Austral Aurora recorded in 2005. 14 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper 3.5 Temperature fluctuations at Dome C Temperature fluctuations have similar amplitudes than in temperate sites, however, there is no typical timescale for fluctuations as in temperate site linked to day/night cycles, where periodic fluctuations of several hours can mimic planetary transits and therefore represent an important source of noise. Fig XXX shows that there is a strong link between ground temperature, wind speed and global sky cloud coverage. Fig XXX – Clear fraction of sky, wind speed and temperature during the 2006 year. There is a strong correlation of sky coverage with higher wind speeds and average temperatures. We could anticipate dome enclosure from these two increasing indicators. (Aristidi 2006) 3.6 Dome C and environmental systematic effects Systematic effects are linked to both environmental and instrumental issues. They can not be removed properly as their timescale is close to transit length and smoothing lightcurves does not remove them. We do not have the exact knowledge of what these effects are and which ones are the most versatile, but we do know that they are strongly linked with environment of the observations, i.e. temperature fluctuations, air mass variations and differential extinction on target stars, seeing fluctuations. These effects are not properly removed through data treatment and they are not reduced by adding successive measure points in the same night. If we have not completely hierarchised these effects, we know they are closely linked to the day / night cycle for other ground based programs. The extremely good atmospheric conditions at Dome C and the full winter period should considerably reduce these environmental systematic effects. 15 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper 4 A STEP photometry specifications 4.1 Image sampling We believe that an optimal spatial sampling is crucial to a successful transit survey. Part of the objective of the initial phases of the ASTEP project is to determine which is the optimal sampling to get the best gain from the conditions at Dome C in terms of systematic trends in the photometry. For small telescopes and reflectors choosing a wide field in order to have a sufficient number of targets, inter-pixel differences and tracking drifts are likely to dominate over atmospheric constraints. For large telescopes (diameter ~1 m) with deep fields, we know from the OGLE survey that the sampling is sufficient (~10 pixel per seeing disc) so that the limiting factor is atmospheric stability. It is not clear yet where the limit between these two regimes is situated. The optimal instrument/observation strategy at Dome C will partly depend on this limit, which we plan to determine as part of the A STEP project. The following points are to be considered: First of all, a great number of target stars is mandatory, as even a 3 day period planet only has a 10 % chance of transiting. On the basis of radial velocity surveys, one can estimate the number of transiting planets in the Jupiter-Saturn mass range to be about one in 1500 solar-type stars in the field. Ideally, one should choose a field as large as possible in order to detect planets around bright stars. These are easier targets for the follow-up studies. Conversely, programs running on very deep fields (mv>18) yield transit events that cannot be confirmed by radial velocimetry and should be avoided. However, the wide field approach is limited due to the generally poor photometry. Among the more than 20 running surveys, OGLE is by far the most successful and uses one of the smallest field of view (i.e 35’, for an average 6° for other surveys). This shows that spatial sampling is important for the detection of extrasolar planets. We chose a compromise between field size and spatial sampling under the following conditions: o We are using a single camera instead of a difficult-to-operate matrix of CCDs, with driving software already developed. We are thus limited to the 16 million pixel sampling, the highest available number of pixels for commercial cameras. o We privilege optimal photometry to a really high number of targets. A number of ~4000 cool main-sequence stars per field still is a requirement in order to have enough targets to be in the conditions of differential photometry and provide us a sufficient average amount of hot Jupiter in the fields to guarantee a few detections. Sky background brightness, star photon noise and easiness of the RV following limit the maximum magnitude of target stars at ~16. The requirement for the field size thus is ~0.5 to 1°. 4.2 Optical simulations 4.2.1 Principle of the simulations 16 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper In order to get an optimized photometry on a sufficient number of targets, with an easy-toantarctize telescope, we fixed a range of specifications for the optical combination. A Newton telescope with a specific field corrector has been chosen for its simplicity and the weaker dependence of its properties from differential stretching. Here is the hierarchy of properties that have to be respected for the optical combination: We will use a simple Newton optical combination with an optimized 2 or 3 lenses field correction. The star PSF will be broadened with defocusing the telescope. The field available should be unvignetted on at least 95 % of the camera The diagonal of the camera should be the projection of a 0.5 to 1.5° line on sky, range in order to have a sufficient number of targets and keep a good angular resolution per pixel. The shape of the image and its stability are the main parameters to optimize as a function of: o seeing fluctuations o mechanical stretching of the optical components Secondarily, the homogeneity of the image across the field of view would make the difference between two equivalent configurations. The central obstruction size does not have a fixed limit, but increasing the projected diameter of the secondary mirror over 20 cm should be considered seriously for the loss of global flux. A “coudé” configuration of the correction and focal plane instrumentation could be chosen if it does not negatively alter the optical combination properties (i.e. through a modification of the secondary mirror size), and effectively increases the compactness of the telescope. In order to qualify the quality of each star image before testing photometry with the optimal ones (see section 3.3), a test could be done by integrating the difference in flux between the star image and a “top hat” function of equal energy. Several values for “top hat” size will be used for this test corresponding to the range of optimal star FWHM sizes estimated with the single star photometry simulator. (from 1.5 to 3 pixels) This “top hat” function is used because it corresponds to what is thought to be the optimal star image in a relatively poorly pixel-sampled photometry. Indeed, most ground-based surveys, often deeply under-sampled, use different techniques in order to broaden their star image (e.g. raster-scan technique UNSW 2005, telescope circular moves during the exposure HATNET 2005). In the case of A STEP, with a smaller field of view, this PSF broadening will be reachable just through defocusing. We are using this function as the optimal shape template because: It increases the number of pixels considered for photometry, and thus lowers the noises linked to pixel fluctuations and flat-fielding It lowers the saturation level of bright stars, allowing longer exposures. It has sharp edges, at a radius significantly smaller than the aperture of the photometry. It has all the flux inside the aperture. 4.2.2 Results of the optical simulation 17 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Fig XXX – Optimized optical combination using a 3-lense field corrector for the Newton 40 cm telescope for the simple and coudé configurations. Spectral range goes from 550 to 900 nm (R+I bands) in this simulation. Fig XXX – Defocused star image shape for the optimal configuration. 4.3 Single star photometry simulations The aim of the study we did was to determine the individual effect of different noise sources on a single star, in order to qualify: The best camera for transit photometry among the sample of high precision commercial cameras. The requirements for the optimal point spread function. 4.3.1 Principle of the simulations A grid of 100x100 pixels, representing a small part of the real CCD, is used to simulate the influence of noise sources on photometry for a single source. The optimal test should include realistic simulations of the different factors affecting photometry: The size of the CCD electrodes, reflecting a part of incoming flux on the camera (assumed to be a fraction of 30 % of surface in the case of front illuminated cameras) 18 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper The color-dependant quantum efficiency of the camera The pixel non-linearity (Gaussian distribution of 1 % mean value – peak 3 %) The pixel non-uniformity (Gaussian distribution of 1 % mean value – peak 2 %) The flat-fielding process and its effect reducing the pixel residual non-uniformity (see section XXX) The effective projected position of the star on the CCD and temporal motions (linked to breathing and guiding errors) on the CCD. The temporal spectrum of motions of stars is difficult to determine accurately without real tests. For our simulations, we assume these moves to be a random walk of amplitude with a Gaussian law of 0.2 arcsec FWHM each 1 second. We assume in those simulations two possible regimes for guiding: o The use of a guiding camera, leading to re-adjust the image each 1 second at a random position in the disk of 1/10 pixel size centered around the precedent star position. This fraction is the usual precision obtained with SBIG guiding devices. o The use of the main camera to update the same way the image position but each 20 seconds after readout of the precedent frame. The stellar image is considered o As the combination of 2 image outputs of the Zeemax telescope simulation (considering optical correction, central obstruction, defocusing and diffraction – see section3.2.1) for the spectral range cut into two parts of equal spectral size. The two images will be called “blue” and “red” in the following text, they are used to simulate colored effects at a first order. o These two images are convolved with a Gaussian of FWHM equal to the seeing value. The seeing is considered stable during one second time-frames. The seeing values are an interpolation in a random sequence of measures observed during the 2005 and 2006 winter campaigns. The star image is integrated on the pixel grid, from which we calculate a number of photo-electrons for each pixel on each 1 second frame. A sky background level is added in each pixel, with time dependant fluctuations. We assume a sky brightness equal to XXX It is difficult to properly simulate the effect of subtraction of global or local frames, frame to frame adjustment, and the different techniques of differential photometry. However, we can simulate easily: o Aperture photometry, that is conducted on each frame with different apertures equal to 1,2,3 and 4 times the FWHM of the star image. o The maximum signal to noise ratio (SNR) that could possibly be extracted for each target star. In order to do so, we are adding each pixel signal and noise in the neighborhood of the photocenter pixel of the star provided it increases the global signal to noise ratio. Solutions considering a gradient in “statistical weight” of each pixel as a function of its distance to the photocenter will be tested to provide the configuration that offers the maximal signal to noise ratio. 4.3.2 Results of the simulations XXX 4.4 Corotlux simulations for global survey strategy. 19 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper We have used the CoRoTlux simulator, a code initially developed to predict the yield of CoRoT space telescope (Baglin et al 2002) and quantify the need for follow-up observations. We here apply this code to simulate the yield of A STEP survey. The CoRoTlux code is described in details in Fressin et al. 2007. We summarize the assumptions made by the model in order to simulate results of the A STEP survey: The stellar population, with multiple stars and background stars is generated from Besançon model of the galaxy; We only consider massive planets (the mass of Saturn and more), as they are much more well characterized by observations than the first few hot Neptunes discovered recently; The giant planet distribution is set as a function of the metallicity of target stars (Santos 2004); The planet mass-period distribution is a carbon-copy of the sample known from radial velocimetry; The radius of the planets is calculated with the Guillot & Showman (2002) model of planetary evolution. It depends on considering the mass, amount of flux the planet receives from its host star, and the amount of heavy elements in the planet. With the assumption that the amount of heavy elements is a function of the metallicity of the host star, that model reproduces well the radii of known transiting exoplanets; The detection threshold considers the effect of time-correlated noise, or ‘red noise’, as described by Pont et al. (2006). We tested our model with two different red noise mean values of 2.2 and 1.5 mmag, a reasonable expectation in comparison with the values observed a posteriori on most ground based projects (from 2 to 4 mmag depending on telescope, environment and data treatment (from the analysis of Superwasp, Monitor, Hatnet and OGLE systematics on photometry from Pont and the ISSI team 2006)). Table XXX shows a quantitative simulation established on 100 monte carlo draws for the A STEP survey. The fiducial survey consists in a single field observation during the 2008 Antarctic winter and 3 alternate fields respectively during the 2009 and 2010 campaigns. We assume the duty cycle to be 100 % of the three full winter months. These results show that a permanent coverage on a field gives a higher average number of detections than an 8-out-of24-hours survey with the same total length of observation. Alternating different fields of view with successive exposures during 15 minutes on each target field seems to be the best strategy for giant planet catch optimization. Figure 1 shows the mass-period distribution of exoplanets simulated for a 3-year A STEP campaign. As well as giving more statistical significance for populations discovered by current transiting projects (no very close-in small radius planet), it could provide detections at longer periods and lower size. 20 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Table XXX - Expected giant planets yield from CoRoTlux simulations We chose here to present only reasonable expectations on close-in giant planets yield. Estimating properly the possible yield of Neptune-size planets would require a better knowlegde of the distribution and characteristics of these planets as well as a precise idea of the photometric accuracy reachable at Dome C. Recent observations with microlensing and with radial velocimetry as well as results from theoretical modelling of planet formation indicate that Neptune-size planets may be more common than giant planets. Depending on their frequency of occurence, A STEP may be able to detect several. Figure XXX - Mass versus period of expected transiting giant planets for A STEP. Model results are shown as black crosses for detectable events, and yellow crosses for those that are considered undetectable based on the photometric signal (see text). The known transiting exoplanets are shown as circles (red for deep field survey detections, orange for wide field survey detections and blue for planets found by radial velocimetry and confirmed in transit). The model results correspond to a 3-year campaign with 7 target fields observed. 21 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper This analysis is done using red noise value as a “big box” containing all time-correlated noise sources and systematics. As our knowledge of the different individual noises will increase, we will add them individually to CoRoTlux simulations. The further step of our global survey analysis will be the coupling of CoRoTlux planetarytransit-like events file with the single-star-photometry simulator. We will thus be able to add proper noise sources for each monte-carlo simulated event (planet-star couple and neigborhood). The ultimate step will be the end-to-end simulator, with the proper addition to CoRoTlux of individual noise sources (linked to environmental changes, motions of the field on the camera, coloured effects …). We will then generate successive stellar field images on CoRoTlux, and apply to them our data pipeline to extract light-curves and search for transit-like events. That study will provide us accurate estimations for the yield of giant planets, for the number of transit-shape like events from blend that will have to be discriminated through treatment and follow-up. It will also be useful to optimize the survey strategy (linked to fields of view, crowding, specific stars targeting) and qualify what would be the optimal 2nd generation transit search mission. 5. A STEP telescope technical design The science study has shown the need to design a precursor capable of achieving the highest photometric precision, but at a moderate cost. We have identified a 40 cm Newton telescope to be the best compromise for its simplicity, its suitability for intermediary wide fields (~1°), and its lower dependence to differential stretching for stabilizing the image (in comparison with Ritchey Chretien combination). The experience of our partner O&V on realization of these telescopes for temperate sites is a plus too. 5.1 - 40 cm Newton telescope XXX – Waiting input from Franck The tube of the telescope is built in carbon fiber for weight, robustness at cold temperature and extremely low dilatation coefficient between 0 and -100°. The mount of the telescope will be a German equatorial ASTROPHYSCIS 1500. This is one of the most reliable mount industrially produced and several tests have already been done by the LUAN team on this mount at dome C. 5.2 Telescope mount and support The Astro-Physics 900 and 1200 German-equatorial mounts have already been tested in Antarctica at Dome C. Our team (Fizeau, O&V) has developed an antarctization procedure in order to have them properly working at temperatures as low as -80°C. Still, there has been a few breakdowns with those mounts, especially when heavily loaded, which is the case for the INVAR tube on the CORONA experiment. The main failure causes were linked to motor heating and encoders. No real test has been done about the precise vibration frequencies of this mount on ice in Antarctica. 22 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper The Astro-Physics company is developing a new prototype, that should be available in spring 2007. The main differences with the Astro-Ph 1200 mount are: Its fiducial payload is 100 kg instead of 60 kg for the Astro1200 Its ceramic gears do not use grease and should not be affected by low temperature Its stability will be better than the Astro1200. From tests done with the Astro1200 by F.Valbousquet in a temperate area in poor environmental conditions, the average moves of the field of view on a camera have an average of 0.48 arcsec from one 4 s exposure to the following one in both directions. Fig XXX shows the histogram of the amplitude of the moves interpolated on 1 second scale. In our simulations for the prototype mount fixed at Dome C, we assume these moves to be a Gaussian random walk of 0.15 arcsec FWHM each second. Fig XXX – Histogram of the amplitude of telescope moves interpolated on 1 second timescales from F.Valbousquet’s tests on Astro-Physics 1200 Mount. There has not been up today any vibration studies of telescopes on ice at Dome C, but we know from the DIMM experiments that the amplitude of the vibrations on the platform are not negligible. In order to minimize the possible vibration spectrum due to building on ice, the summer 2006-2007 Dome C team (Agabi, Schmider, Valbousquet, IPEV) has installed a 4 meter concrete pillar (2 meter deep in the ice) on the compressed ice extension of the Concordiastro dome. 23 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Picture XXX – Installation of the concrete pilar by the Dome C summer 2006-2007 team 5.3 Thermal enclosure for the optical plane instrumentation In order both to keep all electronic devices in the specified temperature range and to stabilize the temperature of the camera box and correction lenses, we will build a thermal enclosure including all focal instrumentation: The scientific camera with its inside cooling device The 2-3 lenses used for optical correction The filter / dichroic + guiding camera The interface between USB2 output of the camera and fiber link to the command and control computer A possible chemical desiccant deposit in order to ensure that no frost will appear on the optical surfaces Temperature controls A glass shot of the aperture The enclosure has to be both thermally isolated from the telescope tube and correctly mechanically fixed on it, especially fixing the angle and distance from the first correction lens to the secondary mirror. 5.4 24 A STEP Camera Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Choice of the camera Our preliminary analysis opposed two kind of commercial cameras: 2K x 2K pixels back illuminated cameras 4K x 4K pixels front illuminated ones The results of this analysis showed that a better photometric precision was achievable with a better pixel sampling provided the simulated Gaussian-shape PSF was large enough (~2 pixel FWHM) not to be limited by the differential fraction of flux reflected by the readout electrodes as a function of the shape and moves of the PSF. In the field of commercial high-number-of-pixels cameras, the Fingerlake PL 16801 E camera shows better characteristics than other cameras for all considerations. Size 4096 x 4096 px, 36.88 x 36.88 mm. Pixel size 9 x 9 µm. 1 output. Peak of quantum efficiency 67 %. Saturation limit 100 000 e- / px. Readout noise 2 Mpx/s 15 e- rms (with a 10 s readout). Dark current 0,014 e- / px.s at -40°C. Photoresponse non-uniformity 1 % rms. CCD class and defaults We have ordered a class C2 camera in order to test it on the CoRoTcam testbench before ordering a class C1 camera for scientific use at Dome C. Class C2 one will be used for redundancy and support. The only difference between these cameras is the number of cosmetic defects of their pixels. Fig XXX shows the number of defects as a function of the Quality of the CCD, with the following definition for defects: Point Defect Dark: A pixel which deviates by more than 6% from neighboring pixels when illuminated to 70% of saturation, OR Bright: A Pixel with dark current > 7,000 e/pixel/sec at 25C. Cluster Defect: A grouping of not more than 5 adjacent point defects Column Defect A grouping of >5 contiguous point defects along a single column, OR A column containing a pixel with dark current > 20,000e/pixel/sec, OR A column that does not meet the CTE specification for all exposures less than the specified Max sat. signal level and greater than 2 Ke, OR A pixel which loses more than 250 e under 2Ke illumination. Defect Separation: Column and cluster defects are separated by no less than two2 pixels in any direction (excluding single pixel defects). Defect Region Exclusion: Defect region excludes the outer two (2) rows and columns at each side/end of the sensor. 25 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Fig XXX – Number of defects with tests performed at T=25°C. Insertion of the camera in the thermal enclosure As the thermal enclosure including all instrumentation will be cantilevered with the telescope tube, its size and weight matter in order to equilibrate the telescope. Weighting 2.6 kg, its dimensions are 158 x 158 x 102 mm as described in figure XXX FIG XXX – Dimensions of FLI PL16801 E camera The camera’s most critical piece according to mechanical issues is its shutter. It will have to undergo ~1.000.000 shuts in a completed winter season. FingerLake is to provide us a new shutter prototype specified for this number of shuts in spring 2007. 26 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Fig XXX - The PL 16801 E camera with CCD and full packaging. Its use in a thermalized enclosure does not require specific conditioning, but these cameras have been successfully tested down to -40°C, which could prove useful in case of breakdown or disfunction of enclosure warming. 5.5 Spectral Range The importance of atmospheric colored effects (differential refraction and extinction), the chromatic effects linked to optical pieces in the different parts of the field of view and the differential shape of defocused PSF as a function of stellar type are the three main factors that point towards the use of a filter. The sky brightness is thought to be an important source of red noise and limits the magnitude of stars to be valuable targets. In their generic study of transit search, Pepper et al 2005 show that the I filter is optimal for transit surveys as cool main sequence stars (G,K) are the main targets. They are using a simple sky brightness site-generic estimation and do not have a proper analysis of red noise, but I-band is considered optimal by several successful surveys (OGLE, SWASP). Fig XXX shows classical sky irradiance for a night at a temperate site. No real measurement has been published for sky brightness at Dome C. 27 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Fig XXX - Relative irradiance spectrum of the night sky in a temperate area. The integrated flux in R band is 1.85 times higher than the flux in I band. The assumption that I band is theoretically the best compromise is to be balanced with the fact that commercial cameras all have lower quantum efficiencies in I band. The FLI16801E is optimized in R-band (fig XXX). According to the Kodak specifications, there is a loss of 45 % of photons for a homogeneous source from R to I band. Fig XXX – Quantum efficiency of Kodak-16801E CCD. We have considered for our optical simulations a wide range from 550 to 900 nm, corresponding to R+I filter. Defocusing the image has a differential chromatic effect. The shape of stellar image will thus be different as a function of stellar types of targets, especially the brighter ones (see fig XXX). Even if the integrated flux of cooler stars in R and I band is similar, defocusing will increase the importance of chromatic atmospheric effects. 28 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Fig XXX – simulated monocolor defocused PSF images at the center of the field at 550 nm and 900 nm. Fig XXX – Normalized flux for stars with temperature of 3000, 4000, 5000, 6000, 7000, and 8000 K of equal R+I visual magnitude. The Kodak 16801-E CCD should not be affected by fringing in I band, but it is not specified for wavelength higher than 850 nm. We will test it up to 900 nm, but the fact to use it in a higher spectral range than specified could result in increasing its pixel fluctuation. A good compromise if optical simulations prove the necessity to decrease the spectral range, could be the use of an intermediary filter, ranging from 600 to 800 nm, for which: Sky brightness is reduced (of 20 % in comparison with R-band) Main target stars (4000 to 6500 K) have a flat intensity profile as a function of wavelength, thus less dependant to chromatic effects. CCD quantum efficiency is higher than 50 % 6 Survey strategy and data pipeline 6.1 Field of view 29 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper As the airmass fluctuations may be one of the most important source of “red noise”, the fact to choose a target field in the direction of the south pole would considerably reduce the influence of that parameter. Stellar density is mainly linked to the angular separation between galactic plane and target field. In order to select the best compromise for sampling, an angular separation around 5 degrees seems to be a good solution, as the field offers an important number of target stars for without making photometry dominated by crowding. A field similar to Eddington1, located between the galactic plane and the South Pole, in the Carina, is our nominal choice of field. Our simulations may make us choose a field closer to South Pole for better sampling and lower airmass fluctuations. Fig XXX - Main target stellar field used for simulations 1° x 1° in Carina – Our target fields will be located between the galactic plane and the south pole for low airmass fluctuations. The chosen field offers a compromise between number of target stars and crowding inside the CCD. 30 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper The first results of our simulations showed how crucial sampling is. Conducting a survey with several successive target fields is in a lot of cases a good alternative to a larger aperture. A STEP optimal survey could consist on observing successive target fields, then going back to the first one. As we need a sufficient number of measure points inside each transit and the telescope is not designed for fast moves, the optimal number of target fields would probably be around 3-4. In order to limit the possible failures during the failure causes during the first year of operation, we will only observe one field continuously. 5.2 Calibration Camera tests Using the CoRoTcam testbench, we will test different parameters of the camera with different colour sources and the integrating sphere. The different tests we want to operate are: Pixels different full well capacity Pixels non uniformity and non linearity Bad pixels, clusters, columns and check of the official specification list for class C2 – class CCDs Readout noise test as a function of readout time Temperature control of the CCD and check of different behaviours Check of quantum efficiency and uniformity at different wavelengths from 550 to 900 nm. The results of these tests will confirm the nominal use of the camera and provide the knowledge of optimal on site calibration. Flatfields (S.Aigrain) The knowledge of the CCD pixels and their answer is determinant for good photometric precision too. But getting valuable flats for wide fields experiments is tricky. Twilight flats may be obtained by pointing north just above the horizon at noon each day, but it is not clear exactly how bright the sky will be. Dark sky flats may be an alternative, provided 1) there aren't too many stars to stack out, and 2) the flats are not dominated by fringing rather than illumination response. It would also be possible to get dome flats every ~48h by setting up a screen and a lamp, but it is extremely hard to get decent flat fields for wide-field purposes using dome flats because it's practically impossible to get even illumination across the field. According to J. Irwin 5cambridge, multiple data pipelines designer), provided the pointing is highly repeatable, the illumination doesn't have to be perfectly uniform. The supernova project at KPNO, which also depends on precise relative photometry across wide fields, use a combination of dark sky and dome flats, and this may be our best bet if the pseudo-twilight flats fail. Pixel non linearity Nonlinearity is supposed to be stable and do not need to be calibrated often, and a lot of the time one never needs to bother since there are worse systematic effects to worry about. However, if we do need to do it, that would require a very stable light source, or some method of measurement of the illumination - that would be possible using a bootstrap method if strong vignetting is present. 31 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Dark correction Kodak cameras could have roving hot pixels to the NIR arrays, and if the amps are not off during integration there will be a glow at the readout corner. Our data analysis could require dark correction in this case. The required frequency of dark correction is difficult to estimate, this will need to be tested. 5.3 Exposure time The main limitation to exposure time is the saturation of brighter stars in the field, and the possible annoying effects linked to overwhelming the pixels affected by saturation. Here is a calculation of the order of time for exposures to have stars of magnitude 11 in the considered spectral band saturating, with the following assumptions: The global loss of optical efficiency of atmosphere extinction mirrors and filters is 40 % The spectral range is 250 nm wide The central obstruction is 40 % in radius of the telescope aperture The CCD quantum efficiency in the considered band is 50 % The flux in the brightest pixel is 20 % of the global flux The star has an homogeneous flux as a function of wavelength The real full wheel capacity is 50000 photo-electrons per pixel > In those conditions, a star of magnitude 11 in the considered band would saturate in 10 s. Real data will soon be fixed to get the exact saturation limit, but this result is a linear function of most of this parameters and the real saturation limit should be of the same order. As saturating a small number of stars in the field could also be an acceptable issue, the exact exposure time will be completely defined after the study of: The exact number of stars of low magnitude (below 12) in the target field of view from the study of the 2Mass catalog The effect of saturating pixels on the local cluster and columns, that will be simulated with the CoRoTcam test bench There are a few other considerations to take into account for exposure time: If the scientific camera is used for tracking, it might be good to consider shorter exposures. Asteroseismology as a secondary scientific topic requires exposures as short as possible. From the specification file for the Proline camera 16801E, it is readable from 1 to 10 MHz for its 16Mpixel. A readout time of 4s (4Mhz) seems reasonable as readout noise is still a negligible noise source at that readout speed. 5.4 PSF broadening Undersampling the PSF of target stars is one of the main causes of poor photometry for ground-based surveys. We believe that we could broaden the PSF to the optimal size just 32 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper through defocusing, as A STEP fiducial sampling is larger than most ground-based projects. Still, defocusing increases coloured effects and is only valuable up to a certain size without losing a too high proportion of global flux and keeping the image homogeneous. For these reasons, we are also considering to test the broadening of the PSF using mechanical moves of the telescope. It is not clear whether intrapixel variations will mater given ASTEP's relatively well-sampled PSF. However, it could be a good thing to test a rastering pattern, as routinely used by the UNSW transit search on the 0.5m APT at Siding Spring Observatory. How this is to be implemented depends on the system. If guiding, use the guide star to move the field around by 1 pixel in a square pattern. A quantitative comparison of the pink noise level with and without rastering is a desirable thing. Rastering or mechanically broadening the image during the exposure could be valuable solutions to test and depends of the speed at which we want to move the telescope during exposure, as multiplying the moves of the telescope in Antarctic conditions could be a source of technical nuisance. Fig XXX – Example of PSF broadening of HATNET telescope (Bakos 2003). The telescope executes a moving pattern during the exposure. The pixel size is 14”. The amplitude of tiny movements is 10”. Consecutive numbers represent the successive moves of the telescope. The size of dots is proportional to the time spent at the respective grid points. The inner concentric circle shows the FWHM of a typical intrinsic PSF (1.7 pixels) and the outer circle shows the FWHM of the broadened PSF of 2.3 pixels, which is considered as much better for HATNET photometry. 5.5 Guiding The aim of guiding is to place each star on the same pixel, or as close to it as we can, in each exposure, to minimise drift across the CCD, because any such drifts are likely to induce red noise if the flat-field correction is imperfect, which is always at the ~1% level. The original thought is to use the target field for guiding, using a dichroic beam splitter inside the thermal enclosure to send some of the flux to a guide camera. It has the advantage of using an additional camera in the same conditions than the scientific camera, but there may be about chromatic effects if guiding in a different bandpass to the one used for observing. This use requires the results of the proper study of chromatic effects, as they are of the order of one arcsec for 300 nm differences in wavelength. 33 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper There are two additional things to take into account with that solution: Fluxes in blu-er wavelength band maybe significantly uncorrelated to the one of target stars, with its strong link to spectral type and sky brightness Chromatic differential effects as a function of time and position on the camera may both change the position of guiding stars of ~1 arcsec. It would be possible to quantify these effects in consider them in the guiding pipeline, but there would be a residual guiding error. A good dichroic could be delicate to realize for a large aperture (~f/3) as dichroic do not working perfectly with converging beams 6 Data processing 6.1 Generating Light curves (S. Aigrain) Co-located aperture photometry - Always pacing the apertures on the same position on the sky, as opposed to centroiding on each frame. Variable aperture size - We compute light curves using a range of aperture size, and for each star we select the aperture giving the "best" light curve. We use rms as a measure of the quality of the light curve, but it may make sense to use a diagnostic more appropriate for transit surveys, such as pink noise on the timescale of transits. Note that it may also be a good idea to keep the light curves in all the apertures: Dave Wilson (from Keele, working on the SuperWASP project) has come up with a neat way of identifying blended eclipsing binaries mimicking transits by comparing the light curves in the different apertures. Non-standard background estimation: we don't use the standard annulus technique, instead we interpolate across a grid of 64x64 pixel regions, where the background level is estimated using an iterative k-sigma clipping procedure. In our experience, this works better than the annulus technique, especially in crowded fields, though there will be a limit to how crowded one can go before the procedure starts to have problems too. Frame offset subtraction. As described in the paper, in each frame we generate a map of residuals from the medians of individual star's light curves, fit an nth-order 2-D polynomial to this map as a function of x-y position (weighted as appropriate to minimise influence from real variables). Actually, the biggest impact is from the zero-th order, but we typically use a 2nd order polynomial, and WASP uses a 3rd order. Storage format: We use a single fits file per field, each row of which contains the data for one object. The time, magnitude and error arrays are contained in columns of the table, each cell of which contains an array. This format is convenient for performing on-the-fly selection on any of the columns using the FITSIO extended filename syntax, but the file size may get unwieldy if you have only one field and huge numbers of data points, so you may want to adopt a format with a single object per file, or a tile, containing a certain fraction of the field. 6.2 Data flow & on-site pipeline 34 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper A small amount of data can be transferred North over a satellite link on a daily basis, the only possible shipment are diagnostics that photometry is done well. We therefore need to have an automated pipeline running at the telescope, using baseline calibration frames as well as a first order reduction pipeline in order to get out daily diagnostics, as well as a final reduction pipeline which can be more sophisticated and less automated if necessary. The questions for the automated version of the pipeline are then which steps the automated pipeline includes? All steps of the pipeline are already fully automated in most ground surveys data pipelines, and they are probably needed to compute the various quality check parameters to send back to France and for alarmmode on site. how and how often the calibration frames it uses are generated ? The master frame only needs to be made once per field, so this could be done manually either on-site by the person operating the telescope or remotely, if a selection of good images is sent North. what are the diagnostics to send back to Europe ? On a daily basis, we would want to send north the standard set of data quality check parameters, as well as frame offsets and rms. We will also send North the light curves of a selection of stars, to monitor that everything is behaving as expected, and enable us to carry out tests such as the red noise analysis. This selection should include stars spanning a range of magnitudes and present in 2Mass catalogue - of colours, not particularly variable, across the entire CCD. Finally, we should also send north the master and calibration frames, on whatever basis they are generated, to check that they are fine. In case of the occurrence of an unpredicted event that difficult to identify, we will have a simple procedure which would reboot for a new survey on the same field. Then, we will get back all the data at the end of the season. 6.3 Data storage and treatment Computer facilities As it is not possible to transfer any big amount of data from Dome C for the moment, we are planning to store all the data of one season at Dome C and completely treat them getting back the computer hard disks. We will define a survey procedure before the winter season for the telescope and we will only have safety precautions to monitor the survey with alarm signals, and reboot procedures. We are planning to use several computers dedicated to different tasks: A computer for operation control, telescope pointing, guiding, tracking. A telescope dedicated to camera readout. A computer dedicated to data storage. Another two computers for redundancy with separate storage facilities. Data Storage Our data storage is directly linked to the frequency of data we want to get. We estimate that 510 measure points inside the transit are enough to discover it with classical algorithms, but a larger number of measurements may be useful for event characterization, and blend 35 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper discrimination. From our experience in transit search algorithms, 5-10 points seems to be a minimal value in order to characterize the shape of the event found. That limit would imply a higher limit in sampling rate of 10 to 20 minutes for a one field survey. We would have to divide it by the number of target fields for a multiple-fields survey. Let us consider the amount of data for a simultaneous survey of 4 different fields with 2 minutes sampling in each field every ten minutes for a 4096 x 4096 pixel CCD. That would make an amount of 24.4 Go per day, which is to take into account. We do not think we can gain more than a factor 2 with data compression. We will need redundancy in our data storage which would at least double that amount. That maximal value we obtain still is in range to classical data storage facilities. The way we are planning for is multiple hard-disk storage and everyday couple of days/week DVD engraving. Stellar seismology requires shorter exposure times than transits, and the number of possible targets increases as a function of exposures shortness – having A STEP efficient in that secondary field may imply switching to larger storage facilities. Our exposure is limited by the saturation level of bright stars in the target field. A strategy to get both events for bright stars and fainter ones would be to alternate short and a long poses, as it is done for BEST. Still, we can sum different poses if precise temporal sampling is proven not to be crucial. 7. Future prospects and link with future programs 7.1 GIORDANO BRUNO - KEOPS GIORDANO BRUNO is the name given to the group of projects which lead to the implementation of large scale interferometry at Dome C. It implies several teams in different observatories. The LUAN is highly implicated in that project (KEOPS – the final part of it). A STEP and Giordano Bruno main links are the scientific goal, i.e. the discovery and characterization of exoplanets, and the fact these two projects are both conducted by laboratories from Nice. As the LUAN has most of the experience of “antarctisation” procedures and astronomical work on site, we will work in close link with LUAN team and share our results and experiences. 7.2 ICE-T The first draft of the German project ICE-T, conducted by Strassmeier, from Postdam Observatory, has just been released to potential collaborators. ICE-T is a project that would, by 2009, install two fully-automatic 60 to 80 cm telescopes at Dome C. These telescopes would be equipped with two 105 millions pixels CCD camera, with a principal scientific objective of detecting planetary transits. 36 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Appendix B – Instrumental noise sources Intra pixel variations Recent reports for currently running surveys proved the fact that intra-pixel fluctuations may be a majoring noise source for transit surveys. (UNSW – xx) In our simulations, we adopted intrapixel values as described by C. Karoff master thesis – Improving the Accuracy of space based photometry – Intra-pixel structure) Example of a longitudinal scan of EEV42-80 CCD pixel Ten scans of the same pixel with red light. The upper panel shows the scan in the vertical direction and the lower in the horizontal direction. The dots are the measurements, the red line is the mean at each scan position and the error bars show the variance of the mean. The y-axis shows the normalized number of counts. B.2 Inter pixel fluctuations An accurate map of the CCCD response is essential. The success of the project will probably be linked with the knowledge we have of the CCD we use. That map would drastically reduce the remaining inter-pixel variations. We are thinking about working together with a team having to test similar CCDs and generically test all CCDs. COROTCAM team and PICARD - SPACE team are respectively conducting or planning to make tests on CCDs EEV 42-80. 37 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper B.3 Filter and Spectral range First limit we have is linked with the capacities of the CCD we are thinking to use. Here is the average quantum efficiency of the CCD EEV42-80: We have no other limit for high wavelengths. As the sky is brighter at short wavelengths, we are considering cutting low frequencies and specializing our survey to G, K stars. B.4 Temperature control of the CCD Even if temperature during winter at Dome C is comparable with typical temperatures for cooled down CCDs, we have to control and regulate this temperature. We are going to test a classical one step Pelletier device on the twin telescope CCD, in order to have it work at a temperature around -50°. Testing it with thermometers will give us an idea of temperature fluctuations of this device. 38 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Appendix C: “White” noises C.1 Photon noise For unweighted aperture photometry, the variance in flux due to Poisson noise 2 ( f s) / g is P, f where f and s are the counts in the effective photometry aperture due to the star and sky respectively ( P, f , f, and s all in ADU), and g is the CCD gain. Expressed as an RMS variation in magnitude, provided P, f << f, this becomes: P a f f s g where a = 2.5/ ln 10 ≈ 1.086, and the magnitude of the star is m = z − 2.5 log f. The magnitude zero point, z, is the magnitude of a star which results in one ADU of detected flux at zero airmass. C.2 Scintillation Scintillation sets the theoretical minimum noise level for the brightest (unsaturated) stars. The magnitude scatter due to scintillation is given by (Kjeldsen & Frandsen 1992) sc int (0.09mag ) D 2 / 3 3 / 2 t 1 / 2 e h / 8 where D is the telescope diameter in centimetres, is the airmass, t is the exposure time in seconds, h is the altitude of the observatory in km. Using typical values for our observing program with A STEP (D = 40, = 1, t = 900, h = 3.2), the estimated scintillation limit is 0,15 mmag RMS for a 15 minutes exposure. 39 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Appendix D. Astrophysical noises D.1 Stellar variability Blind tests done for Corot satellite transit search algorithms indicated stellar variability as it was modeled is not a major noise for transit detection. On the other hand, recent results from MOST satellite indicated that stellar variability was really under-evaluated. D.2 Background stars and eclipsing binaries Background stars are a critical noise as we can not identify them. They undergo different fluctuations than the main target stars as they do not have the same color. They create a background noise that is a strong limitation for high magnitude stars. The expected most critical point for “second generation” transit search is the eclipsing binaries, and the transit blends create. In order to discriminate them, we can use radial velocity follow-up with HARPS instrument (which can point dome C targets). That follow-up is unavoidable because it provides the confirmation and the characterization of the planet (mass, radius, density). But for future transit search programs, two main problems will emerge: It will be technically impossible to confirm the fainter candidates. It won’t be possible to confirm a too long list of candidates. Once again, a good spatial sampling can considerably reduce the number of blends mimic-ing transits. A good point for dome C is the fact that we can keep all the data, which is impossible for space missions that have to sum their pixel before sending back their data. 40 Antarctica Search for Transiting Extrasolar Planets Draft: v1.5 (6 May. 17) A STEP: White paper Appendix E. Schedule of main actions for A STEP 41 Antarctica Search for Transiting Extrasolar Planets