Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Lovell Telescope wikipedia , lookup
Spitzer Space Telescope wikipedia , lookup
James Webb Space Telescope wikipedia , lookup
Allen Telescope Array wikipedia , lookup
Hubble Space Telescope wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Optical telescope wikipedia , lookup
Very Large Telescope wikipedia , lookup
Telescope Optics and related topics http://www.eso.org/~rhook/NEON/Asiago_2012_Hook.ppt Richard Hook ESO, Garching Sept 2012 NEON Observing School, Asiago 1 Introduction • I will focus on general principles, mostly optics but also some related topics like mountings • Later in the week Michel Dennefeld will talk about the history of the telescope and more about the E-ELT Sept 2012 NEON Observing School, Asiago 2 Some Caveats & Warnings! • I have selected a few topics, lots of things are omitted (eg, adaptive optics) • I have tried to not mention material covered in other talks (detectors, photometry, spectroscopy…) • I am a bit biased by my own background, mostly Hubble imaging. I am not an optical designer. • I have avoided getting deep into technicalities so apologise if some material seems rather trivial. Sept 2012 NEON Observing School, Asiago 3 Scope of Talk From the sky and through the atmosphere and telescope, but stopping just before the detector! • • • • Telescope designs Optical characteristics Telescope mountings The atmosphere Sept 2012 NEON Observing School, Asiago 4 403 years of the Telescope ESO VLT, 2000, d=8.2m (x4) Galileo, 1609, d=2.5cm E-ELT, 2022, d=39m Sept 2012 NEON Observing School, Asiago 5 Basic Telescope Optical Designs Most modern large telescopes are variants of the Cassegrain design. Sept 2012 NEON Observing School, Asiago 6 Basic Properties of Telescopes Optics Aperture = D, Focal Length=f, Focal ratio=F=f/D For telescopes of the same design the following holds. • • • • • • • • • Light collecting power - proportional to D2 Theoretical angular resolution - proportional to 1/D (1.22 D) Image scale (“/mm) - proportional to 1/f (206/f, “/mm, if f in m) Total flux of an object at focal plane - also proportional to D2 Surface intensity of an extended source at focal plane - proportional to 1/F2 Angular Field of view - normally bigger for smaller F, wide fields need special designs Tube length proportional to fprimary Dome volume (and cost?) proportional to f3primary Cost rises as a high power (~3) of D Sept 2012 NEON Observing School, Asiago 7 Mirrors and Lenses All optical telescopes contain mirrors and/or lenses: Lenses: • Have to be made of material with uniform optical properties for transmitted light (expensive) • Have refractive indices that are a function of wavelength - hence chromatic aberrations • Can only be supported at the edge Mirrors: • Fold the optical path so some designs are lead to vignetting • Have to have surfaces with the correct shape, smoothness and reflectivity • Can be made of anything that can be held rigidly, figured to the correct smooth shape and given the right coating In practice all large modern telescopes are mainly reflecting, with refractive elements reserved for correctors and small components within instruments. Sept 2012 NEON Observing School, Asiago 8 Telescope Aberrations Aberrations are deviations from a perfect optical system. They can be due to manufacturing errors, alignment problems, or be intrinsic to the optical design. • There are five basic monochromatic (3rd order) aberrations: – – – – – Spherical aberration (~y3) Coma (~y2) Astigmatism (~y2) Distortion (~3) Field curvature Where y is the linear distance away from the axis on the pupil and is the off-axis angle. The last two only affect the position, not the quality of the image of an object. • Systems with refractive elements also suffer from various forms of chromatic aberration Sept 2012 NEON Observing School, Asiago 9 Spherical aberration Sept 2012 NEON Observing School, Asiago 10 Optical Aberrations - continued Spherical Aberration in action Sept 2012 NEON Observing School, Asiago 11 Zernike Polynomials Aberrations may be represented as wavefront errors expressed as orthogonal polynomial expansions in terms of angular position (and radial distance ( on the exit pupil he first few are: Sept 2012 NEON Observing School, Asiago 12 Simplest case - one reflecting surface • A concave spherical mirror suffers from severe spherical aberration and has limited use without additional optics (more about this later) • A concave paraboloid focuses light to a perfect image on its axis but suffers from coma (~1/F2) and astigmatism off-axis • For long focal ratios (f/4 and greater), in the Newtonian design, this leads to acceptable image quality and is widely used in smaller telescopes • For larger apertures a shorter focal ratio is essential and the field of tolerable aberration becomes very small • Large telescopes, if they have a prime focus, need correctors (more later). • Hyperbolic primaries can be easier to correct and occasionally appear as “hyperbolic astrographs”. Sept 2012 NEON Observing School, Asiago 13 Two-mirror Designs • The primary is concave. • The secondary is either convex or concave and may be inside or beyond the prime focus. • Most common designs have the secondary acting as magnifier so that the final effective focal length is greater than that of the primary • If the primary is paraboloidal the secondary will be hyperboloidal (Cassegrain) if convex and elliptical if concave (Gregorian). The aberrations of the final image will be the same as those of a single parabolic mirror of the same focal length - but the telescope will be much shorter. • The Cassegrain is more common as it is more compact but Gregorians may be easier to make and can be better baffled in some cases. Sept 2012 NEON Observing School, Asiago 14 Two-mirror classical systems www.telescope-optics.net Sept 2012 NEON Observing School, Asiago 15 Other two-mirror systems • The primary does not have to be paraboloidal • The conic constant (K= -e2) of the secondary can be adjusted to correct for spherical aberration of the final image • Of particular interest is the case where coma is eliminated the aplanatic Cassegrain is the Ritchey-Chretien (RC) Sept 2012 NEON Observing School, Asiago 16 Why the Ritchey-Chretien? There are many options for two-mirror telescopes: • Classical Cassegrain - parabolic primary, hyperboloidal secondary (coma) • Dall-Kirkham - elliptical primary, spherical secondary (easy to make, more coma) • Ritchey-Chretien - hyperbolic primary, hyperbolic secondary (free of coma) • All suffer from mild astigmatism and field curvature The RC gives the best off-axis performance of a two mirror system and is used for most (but not all) modern large telescopes: ESO-VLT, Hubble, 3 CAHA telescopes etc. Classical Cassegrain Sept 2012 Ritchey-Chretien NEON Observing School, Asiago 17 Three mirrors and more… • Many three mirror designs are possible and, with more degrees of freedom, wide fields and excellent image quality are possible. • The main problems are getting an accessible focal plane, avoiding excessive obscuration and construction difficulties. • No very large examples have yet been built - but become attractive for ELTs. Sept 2012 NEON Observing School, Asiago 18 A future large, widefield groundbased survey telescope with a three mirror design plus corrector - the LSST Sept 2012 NEON Observing School, Asiago 19 The E-ELT 5 mirror design: Segmented primary (39m) Convex monolithic secondary (4m) Concave tertiary (3m) Adaptive flat M4 (2.5m) Fast-moving flat M5 (2.7m) Field - 10 arcmins (diffraction limited) Final focal ration - f/16. Sept 2012 NEON Observing School, Asiago 20 Getting a wider field • Two mirror designs work well for large general purpose telescopes • Typically the usable field is less than one degree and the final focal ratio is f/8 or greater • Aberrations rise quickly with off-axis angle and as focal ratio decreases • Survey telescopes need a wider field and faster optics Sept 2012 NEON Observing School, Asiago 21 Schmidt Camera • Spherical primary • Stop at centre-of-curvature • No coma/astigmatism/distortion • Only spherical aberration and field curvature • SA is corrected by a thin correcting plate at the radius of curvature • Excellent image quality at f/2 and 6 degree field. • Tube length is twice focal length • Legacy - the sky surveys (DSS) Sept 2012 The ESO 1-metre Schmidt at La Silla NEON Observing School, Asiago 22 Catadioptric Systems • There are many other possible systems with full aperture correcting plates. • Correctors can either be the thin/flatish Schmidt type, or a thick meniscus in Maksutov designs. • The corrector plate can be closer to the primary to make a more compact design, with a narrower field. • Very common as small telescopes as they can be compact and can use spherical mirrors for ease of manufacture. • Because of the difficulties in making and supporting large lenses these systems, like Schmidts, are rarely much larger than 1m aperture. Sept 2012 NEON Observing School, Asiago 23 Sub-aperture Correctors • Introducing refractive correctors close to the focus can suppress residual aberrations and improve image quality and field size. • Different types: – – – – Field flatteners Prime focus correctors Cassegrain focus correctors (Focal reducers) • Often part of instruments Sept 2012 NEON Observing School, Asiago 24 Prime focus correctors • Wynne corrector (eg, WHT on La Palma): – – – – Sept 2012 Expands useful field to around 1 degree at f/3 Spherical surfaces relatively easy to make Works for paraboloidal and hyperboloidal primaries Normally only slightly changes effective focal length NEON Observing School, Asiago 25 More exotic Prime focus correctors: • Suprime-Cam, on the Subaru 8m Copyright: Canon Sept 2012 NEON Observing School, Asiago 26 An even more exotic corrector: • Hobby-Eberly 11.1x9.8m, fixed altitude spherical segmented mirror. Penn State Sept 2012 NEON Observing School, Asiago 27 Cassegrain correctors • A relatively weak corrector in front of the focus of a two mirror telescope can flatten the field and improve image quality • If the original design of the whole telescope includes the corrector, moderately wide (two degree) fields are possible with excellent imaging • Older examples are the f/8 focus of the JKT (with HarmerWynne corrector and the 2.5m f/7.5 Dupont telescope at Las Campanas. • A new example, highly optimised is VISTA Sept 2012 NEON Observing School, Asiago 28 VISTA - ESO’s new IR Survey Telescope 4.1m, f/1 primary Large integrated corrector/IR camera Modified RC optics 1.65 degree field Sept 2012 NEON Observing School, Asiago 29 VISTA in action: the Flame Nebula (NGC 2024) and the Horsehead Nebula in Orion in the nearinfrared Sept 2012 NEON Observing School, Asiago 30 Atmospheric distortion correctors (ADCs) The problem: Sept 2012 NEON Observing School, Asiago 31 Atmospheric Dispersion Correctors • Need to be able to introduce dispersion opposite to that created by the atmosphere - which varies with zenith distance (two counter-rotating prisms). • Want to reduce the image shift introduced, to zero at a given wavelength - so need to make each component to be a zero deviation pair of prisms of different glasses itself. • Prisms can be thin and the wedge angle is small (typically 1.5 degrees) and they are often oiled together in pairs to increase throughput. • The design becomes more difficult in converging beams. • An example - the ADC on the WHT. Sept 2012 NEON Observing School, Asiago 32 Telescope mountings • Support the telescope at any desired angle • Track to compensate for the Earth’s rotation • Two main types: – Altazimuth, axes vertical and horizontal – Equatorial, one axis parallel to the Earth’s axis • Desirable characteristics: – – – – – Sept 2012 Rigid and free of resonances Accurate tracking Good sky coverage Compact (smaller dome) Space and good access for instruments NEON Observing School, Asiago 33 Mountings: examples • The German equatorial: Jacobus Kapteyn 1m, La Palma, 1983. (ING/IAC) Great Dorpat Refractor, 1824. (Graham/Berkeley) Sept 2012 NEON Observing School, Asiago 34 Mountings continued: • The English or “yoke” mount: The Mount Wilson 100in. From a book by Arthur Thomson, 1922 (Gutenberg project) Sept 2012 NEON Observing School, Asiago 35 More modern mounts: • Pioneer: Hale 5m - horseshoe, 1948 • Also used for many 4m class telescopes in the 1970s/1980s (CAHA 3.5m, ESO 3.6m, AAT 3.9m, Kitt Peak 4m etc) Hale 200in, (Caltech) Sept 2012 NEON Observing School, Asiago 36 The modern choice: altazimuth fork • Very rigid and compact • Access to Cassegrain and Nasmyth focii • Needs variable rate tracking on both axes • Field rotation compensation needed • Dead spot close to zenith 1.2 Euler telescope, La Silla (ESO) Sept 2012 NEON Observing School, Asiago 37 Mirror coatings Sept 2012 NEON Observing School, Asiago 38 The Atmosphere - transmission J H K A Sept 2012 NEON Observing School, Asiago 39 The Atmosphere - emission (at a good dark observatory site, La Palma) Sept 2012 NEON Observing School, Asiago 40 A Few References • Astronomical Optics, D. Schroeder (good overview) • Reflecting Telescope Optics (2 volumes), R. Wilson (comprehensive) • Telescope Optics, Rutten & van Venrooij (mostly for amateurs) • The History of the Telescope, King (somewhat dated) Sept 2012 NEON Observing School, Asiago 41 Sept 2012 NEON Observing School, Asiago 42 Part II: Astronomical Digital Imaging Sept 2012 NEON Observing School, Asiago 43 Topics • • • • • • • The imaging process, with detector included The point-spread function The pixel response function Artifacts, defects and noise characteristics Basic image reduction Image combination, dithering and drizzling FITS format and metadata • Colour • Software - the Scisoft collection Sept 2012 NEON Observing School, Asiago 44 Two Examples: The power of imaging A supernova at z>1 detected in the Great Observatories Origins Deep Survey (GOODS). z-band imaging with Hubble ACS/WFC at multiple epochs. Part of the data collection that led to Dark Energy. Part of the VISTA VVV survey of the Milky Way bulge and disc in the infrared (JHKs). This led to an 84 million star CMD and the discovery of huge numbers of variable objects. Sept 2012 NEON Observing School, Asiago 45 Image Formation in One Equation I = SO P + N Where: S is the intensity distribution on the sky O is the optical point-spread function (PSF, including atmosphere) P is the pixel response function (PRF) of the detector N is noise is the convolution operator I is the result of sampling the continuous distribution resulting from the convolutions at the centre of a pixel and digitising the result into DN. Sept 2012 NEON Observing School, Asiago 46 The Point-Spread Function • The PSF is the shape of the image of a point source (such as a star) at the detector • It determines the resolution and structure of an image • The two main influences on the PSF are the optics and the atmosphere • PSFs vary with time, position on the image, colour etc Sept 2012 NEON Observing School, Asiago 47 Groundbased Point-Spread Functions (PSF) For all large groundbased telescope imaging with long exposures - without adaptive optics - the PSF is a function of the atmosphere rather than the telescope optics, The image sharpness is normally given as the “seeing”, the FWHM of the PSF in arcsecs. 0.3” is very good, 2” is bad. Seeing gets better at longer wavelengths. The radial profile is well modelled by the Moffat function: s(r) = C / (1+r2/R2)b+ B Where there are two free parameters (apart from intensity, background and position) R, the width of the PSF and b, the Moffat parameter. Software is available to fit PSFs of this form. Sept 2012 The radial profile of a typical groundbased star image. NEON Observing School, Asiago 48 PSFs in Space Mostly determined by diffraction and optical aberrations. Scale with wavelength. PSFs for Hubble may be simulated using the Tiny Tim software (included in Scisoft). It uses a model of the telescope and Fourier optics theory to generate high fidelity PSF images for all of Hubble’s cameras. There is also a version for Spitzer. See: www.stsci.edu/software/tinytim (V6.3) ACS, F814W - well sampled (0.025” pixels) WFPC2, F300W - highly undersampled (0.1” pixels) Sept 2012 NEON Observing School, Asiago 49 From Optics to the Point Spread Function OPD = optical path difference = wavefront errors (often as Zernikes) A = aperture function = map of obscurations in pupil (spiders etc) Then, Fourier optics gives: P = A e (2 I OPD / ) = complex pupil function PSF = | FFT(P) |2 = point spread function Sept 2012 NEON Observing School, Asiago 50 Making Hubble PSFs Sept 2012 NEON Observing School, Asiago 51 Simple Measures of Optical Image Quality • Full Width at Half Maximum (FWHM) of point-spread function (PSF) - measured by simple profile fitting (eg, imexam in IRAF) • Strehl ratio (ratio of PSF peak to theoretical perfect value). • Encircled energy - fraction of total flux in PSF which falls within a given radius. All of these need to be used with care - for example the spherically aberrated Hubble images had excellent FWHM of the PSF core but very low Strehl and poor encircled energy. Scattering may dilute contrast but not be obvious. Sept 2012 NEON Observing School, Asiago 52 The Pixel-Response Function (P) • • • • • • The sensitivity varies across a pixel Once produced, electrons in a CCD may diffuse into neighbouring pixels (charge diffusion) The pixel cannot be regarded as a simple, square box which fills with electrons The example shown is for a star imaged by HST/NICMOS as part of the Hubble Deep Field South campaign. The centre of the NICMOS pixels are about 20% more sensitive than the edges CCDs also have variations, typically smaller than the NICMOS example, but very significant charge diffusion, particularly at shorter wavelengths Can affect photometry - especially in the undersampled case Sept 2012 NEON Observing School, Asiago 53 Image Defects and Artifacts • Cosmic-ray hits - unpredictable, numerous, bright, worse from space • Bad pixels - predictable (but change with time), fixed to given pixels, may be “hot”, may affect whole columns • Saturation (digital and full-well) and resulting bleeding from bright objects • Ghost images - reflections from optical elements • Cross-talk - electronic ghosts • Charge transfer efficiency artifacts • Glints, grot and many other nasty things Sept 2012 NEON Observing School, Asiago 54 Some real image defects (Hubble/WFPC2): Bleeding Ghost Cosmic ray Sept 2012 NEON Observing School, Asiago 55 Charge Transfer (In)efficiency CCDs are read out by clocking charge along registers. These transfers are impeded by radiation damage to the chips. This effect gets worse with time and is worse in space, This image is from the STIS CCD on Hubble. Note the vertical tails on stars. Can degrade photometry and astrometry Sept 2012 NEON Observing School, Asiago 56 Noise • For CCD images there are two main sources of noise: – Poisson “shot” noise from photon statistics, applies to objects, the sky and dark noise, SNR increases as the square root of exposure time – Gaussian noise from the CCD readout, independent of exposure time • For long exposures of faint objects through broad filters the sky is normally the dominant noise source • For short exposures or through narrow-band filters readout noise can become important but is small for modern CCDs Sept 2012 NEON Observing School, Asiago 57 Geometric Distortion Cameras normally have some distortion, typically a few pixels towards the edges, It is important to understand and characterise it to allow it to be removed if necessary, particular when combining multiple images. Distortion may be a function of time, filter and colour. HST/ACS/WFC - a severe case of distortion more than 200 pixels at the corners. Large skew. Sept 2012 NEON Observing School, Asiago 58 Basic Frame Calibration • Raw CCD images are normally processed by a standard pipeline to remove the instrumental signature. The main three steps are: – Subtraction of bias (zero-point offset) – Subtraction of dark (proportional to exposure) – Division by flat-field (correction for sensitivity variation) • Once good calibration files are available basic processing can be automated and reliable • After this processing images are not combined and still contain cosmic rays and other defects • Standard archive products for some telescopes (eg, Hubble) have had On-The-Fly Recalibration (OTFR) performed with the best reference files Sept 2012 NEON Observing School, Asiago 59 Sampling and Frame Size • Ideally pixels should be small enough to well sample the PSF (ie, PRF negligible). Pixel < PSF_FWHM/2. • But, small pixels have disadvantages: – Smaller fields of view (detectors are finite and expensive) – More detector noise per unit sky area (eg, PC/WF comparison) • Instrument designers have to balance these factors and often opt for pixel scales which undersample the PSF. – Eg, HST/WFPC2/WF - PSF about 50mas at V, PRF 100mas. – HST/ACS/WFC - PSF about 30mas at U, PRF 50mas. • In the undersampled regime the PRF > PSF • From the ground sampling depends on the seeing, instrument designers need to anticipate the likely quality of the site. Sept 2012 NEON Observing School, Asiago 60 Image Combination • Multiple images are normally taken of the same target: – To avoid too many cosmic-rays – To allow longer exposures – To allow dithering (small shifts between exposures) – To allow mosaicing (large shifts to cover bigger areas) • If the multiple images are well aligned then they may be combined easily using tools such as imcombine in IRAF which can also flag and ignore certain image defects such as cosmic-rays • Combining multiple dithered images, particularly if they are undersampled is less easy… Sept 2012 NEON Observing School, Asiago 61 Undersampling and reconstruction Truth After pixel Sept 2012 After optics After linear reconstruction NEON Observing School, Asiago 62 Simple ways of combining dithered data • Shift-and-add - introduces extra blurring and can’t handle distortion, easy, fast. Useful when there are many images and little distortion. Fast. • Interlacing - putting input image pixel values onto a finer output grid and using precise fractional offsets. • In all cases you need a way to measure the shifts (and possibly rotations) • Need something more general… Sept 2012 NEON Observing School, Asiago 63 Interlacing, nice but hard to do… Four input images with exactly halfpixel dithers in X and Y are combined onto an output grid with pixels half the size by “interlacing” the input pixels. No noise correlation, very fast and easy. But - doesn’t work with geometric distortion and requires perfect sub-pixel dithers. Sept 2012 NEON Observing School, Asiago 64 Drizzling • A general-purpose image combination method • Each input pixel is mapped onto the output, including geometric distortion correction and any linear transformations • On the output pixels are combined according to their individual weights - for example bad pixels can have zero weight • The “kernel” on the output can be varied from a square like the original pixel (shift-and-add) to a point (interlacing) or, as usual, something in between • Preserves astrometric and photometric fidelity • Developed for the Hubble Deep Field, used for most Hubble imaging now • Other good alternatives exist (eg, Bertin’s SWarp) Sept 2012 NEON Observing School, Asiago 65 Drizzling Sept 2012 NEON Observing School, Asiago 66 Noise in drizzled images Drizzling, in common with other resampling methods can introduce correlated noise - the flux from a single input pixel gets spread between several output pixels according to the shape and size of the kernel. As a result the noise in an output pixel is no longer statistically independent from its neighbours. Noise correlations can vary around the image and must be understood as they can affect the statistical significance of measurements (eg, photometry) of the output. Sept 2012 NEON Observing School, Asiago 67 The Effects of Resampling Kernels Sept 2012 NEON Observing School, Asiago 68 Implemented as MultiDrizzle for HST - www.stsci.edu/pydrizzle/multidrizzle Sept 2012 NEON Observing School, Asiago 69 Cleaning Cosmics continued… The LA-Cosmic method (van Dokkum) Uses Laplace filter and needs good noise model - but works very well. IDL/Python/IRAF versions exist. Sept 2012 NEON Observing School, Asiago 70 THELI - a general tools for mosaic reduction A graphical interface to many tools (mostly from the Astromatic collection SExtractor/SWarp/WW/SCAMP etc) Can automate image reduction very effectively. Supports many instruments - from Canon Digital SLRs to VLT instruments. Sept 2012 NEON Observing School, Asiago 71 FITS format and Metadata • FITS is an almost universal data exchange format in astronomy. • Although designed for exchange it is also widely used for data storage, on disk. • The basic FITS file has an ASCII header for metadata in the form of keyword/value pairs followed by a binary multi-dimensional data array. • There are many other FITS features, for tables, extensions etc. • For further information start at: http://archive.stsci.edu/fits/fits_standard/ Sept 2012 NEON Observing School, Asiago 72 FITS Header elements (Hubble/ACS): SIMPLE = T / Fits standard BITPIX = 16 / Bits per pixel NAXIS = 2 / Number of axes NAXIS1 = 4096 / Number of axes NAXIS2 = 2048 / Number of axes EXTEND = T / File may contain extensions ORIGIN = 'NOAO-IRAF FITS Image Kernel December 2001' / FITS file originator IRAF-TLM= '09:10:54 (13/01/2005)' NEXTEND = 3 / Number of standard extensions DATE = '2005-01-13T09:10:54' FILENAME= 'j90m04xuq_flt.fits' / name of file FILETYPE= 'SCI ' / type of data found in data file Fundamental properties: image size, data type, filename etc. TELESCOP= 'HST' / telescope used to acquire data INSTRUME= 'ACS ' / identifier for instrument used to acquire data EQUINOX = 2000.0 / equinox of celestial coord. System …… CRPIX1 = 512.0 / x-coordinate of reference pixel CRPIX2 = 512.0 / y-coordinate of reference pixel CRVAL1 = 9.354166666667 / first axis value at reference pixel CRVAL2 = -20.895 / second axis value at reference pixel CTYPE1 = 'RA---TAN' / the coordinate type for the first axis CTYPE2 = 'DEC--TAN' / the coordinate type for the second axis CD1_1 = -8.924767533197766E-07 / partial of first axis coordinate w.r.t. x CD1_2 = 6.743481370546063E-06 / partial of first axis coordinate w.r.t. y CD2_1 = 7.849581942774597E-06 / partial of second axis coordinate w.r.t. x CD2_2 = 1.466547509604328E-06 / partial of second axis coordinate w.r.t. y World Coordinate System (WCS): linear mapping from pixel to position on the sky. …. Sept 2012 NEON Observing School, Asiago 73 Image Quality Assessment: try this! (IRAF commands in ()) • Look at the metadata - WCS, exposure time etc? (imhead) • What is the scale, orientation etc? (imhead) • Look at images of point sources - how big are they,what shape? Sampling? (imexam) • Look at the background level and shape - flat? (imexam) • Look for artifacts of all kinds - bad pixels? Cosmic rays? Saturation? Bleeding? • Look at the noise properties, correlations? (imstat) Sept 2012 NEON Observing School, Asiago 74 A Perfect Image? What makes a fully processed astronomical image? • Astrometric calibration – Distortion removed (0.1pix?) – WCS in header calibrated to absolute frame (0.1”?) • Photometric calibration – Good flatfielding (1%?) – Accurate zeropoint (0.05mags?) – Noise correlations understood • Cosmetics – Defects corrected where possible – Remaining defects flagged in DQ image – Weight map/variance map to quantify statistical errors per pixel • Description – Full descriptive metadata (FITS header) – Derived metadata (limiting mags?) – Provenance (processing history) Sept 2012 NEON Observing School, Asiago 75 Colour Images • For outreach use • For visual scientific interpretation The Lynx Arc A region of intense star formation at z>3 gravitationally lensed and amplified by a low-z massive cluster. This image is an Hubble/WFPC2 one colourised with ground-based images. Hanny’s Voorwerp Spotted because of its colour. Probably a reflection of a now-switched off quasar. Sept 2012 NEON Observing School, Asiago 76 Making Colour Images Developed by Lars Christensen and collaborators: www.spacetelescope.org/projects/fits_liberator Sept 2012 NEON Observing School, Asiago 77 Original input images from FITS files Colourised in Photoshop Final combined colour version: Sept 2012 NEON Observing School, Asiago 78 Software Scisoft is a collection of many useful astronomical packages and tools for Linux (Fedora Core 6) computers. It can be downloaded from: www.eso.org/scisoft Most of the software mentioned in this talk is included and “ready to run”. There is also a Mac version. Packages on the include: IRAF, STSDAS, TABLES etc ESO-MIDAS SExtractor/SWarp ds9,Skycat Tiny Tim Python …etc, etc. + VO tools (new in Scisoft VII) Sept 2012 NEON Observing School, Asiago 79 That’s all - any questions? Sept 2012 NEON Observing School, Asiago 80