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Telescope Optics and related topics http://www.eso.org/~rhook/NEON/LaPalma_2013_Hook.ppt Richard Hook ([email protected]) ESO, Garching July 2013 NEON Observing School, La Palma 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 many topics will be covered in more detail by other lecturers July 2013 NEON Observing School, La Palma 2 Some Caveats & Warnings! • I have selected a few topics, lots of things are omitted • 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. July 2013 NEON Observing School, La Palma 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 July 2013 NEON Observing School, La Palma 4 404 years of the Telescope ESO VLT, 1999, d=8.2m (x4) Galileo, 1609, d=2.5cm E-ELT, 2024, d=39m July 2013 NEON Observing School, La Palma 5 Basic Telescope Optical Designs Most modern large telescopes are variants of the Cassegrain design. July 2013 NEON Observing School, La Palma 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 July 2013 NEON Observing School, La Palma 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. July 2013 NEON Observing School, La Palma 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 July 2013 NEON Observing School, La Palma 9 Spherical aberration July 2013 NEON Observing School, La Palma 10 Optical Aberrations - continued Spherical Aberration in action July 2013 NEON Observing School, La Palma 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: July 2013 NEON Observing School, La Palma 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”. July 2013 NEON Observing School, La Palma 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. July 2013 NEON Observing School, La Palma 14 Two-mirror classical systems www.telescope-optics.net July 2013 NEON Observing School, La Palma 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) July 2013 NEON Observing School, La Palma 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, etc Classical Cassegrain July 2013 Ritchey-Chretien NEON Observing School, La Palma 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. July 2013 NEON Observing School, La Palma 18 A future large, widefield groundbased survey telescope with a three mirror design plus corrector - the LSST July 2013 NEON Observing School, La Palma 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. July 2013 NEON Observing School, La Palma 20 E-ELT Optical Design July 2013 NEON Observing School, La Palma 21 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 July 2013 NEON Observing School, La Palma 22 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) July 2013 The ESO 1-metre Schmidt at La Silla NEON Observing School, La Palma 23 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. July 2013 NEON Observing School, La Palma 24 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 July 2013 NEON Observing School, La Palma 25 Prime focus correctors • Wynne corrector (eg, WHT on La Palma): – – – – July 2013 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, La Palma 26 More exotic Prime focus correctors: • Suprime-Cam, on the Subaru 8m Copyright: Canon July 2013 NEON Observing School, La Palma 27 An even more exotic corrector: • Hobby-Eberly 11.1x9.8m, fixed altitude spherical segmented mirror. Penn State July 2013 NEON Observing School, La Palma 28 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 July 2013 NEON Observing School, La Palma 29 VISTA - ESO’s IR Survey Telescope 4.1m, f/1 primary Large integrated corrector/IR camera Modified RC optics 1.65 degree field July 2013 NEON Observing School, La Palma 30 VISTA in action: the Flame Nebula (NGC 2024) and the Horsehead Nebula in Orion in the nearinfrared July 2013 NEON Observing School, La Palma 31 Atmospheric distortion correctors (ADCs) The problem: July 2013 NEON Observing School, La Palma 32 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. July 2013 NEON Observing School, La Palma 33 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: – – – – – July 2013 Rigid and free of resonances Accurate tracking Good sky coverage Compact (smaller dome) Space and good access for instruments NEON Observing School, La Palma 34 Mountings: examples • The German equatorial: Jacobus Kapteyn 1m, La Palma, 1983. (ING/IAC) Great Dorpat Refractor, 1824. (Graham/Berkeley) July 2013 NEON Observing School, La Palma 35 Mountings continued: • The English or “yoke” mount: The Mount Wilson 100in. From a book by Arthur Thomson, 1922 (Gutenberg project) July 2013 NEON Observing School, La Palma 36 More modern mounts: • Pioneer: Hale 5m - horseshoe, 1948 • Also used for many 4m class telescopes in the 1970s/1980s (CAHA 3.5m, CFHT, ESO 3.6m, AAT 3.9m, Kitt Peak 4m etc) Hale 200in, (Caltech) July 2013 NEON Observing School, La Palma 37 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) July 2013 NEON Observing School, La Palma 38 Mirror coatings July 2013 NEON Observing School, La Palma 39 The Atmosphere - transmission J H K A July 2013 NEON Observing School, La Palma 40 The Atmosphere - emission (at a good dark observatory site, La Palma) July 2013 NEON Observing School, La Palma 41 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) July 2013 NEON Observing School, La Palma 42 July 2013 NEON Observing School, La Palma 43 Part II: Astronomical Digital Imaging (a very brief introduction) July 2013 NEON Observing School, La Palma 44 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 Undersampling and drizzling FITS format and metadata • Colour • Software - the Scisoft collection July 2013 NEON Observing School, La Palma 45 Four Examples: The power of imaging HUBBLE: A supernova at z>1 VISTA Survey: Part of the VISTA VVV survey of the Milky Way bulge and disc in the infrared (JHKs). ALMA mm: Spiral structure about R Scl July 2013 NEON Observing School, La Palma ESO VLT NACO, nearIR, adaptive optics: the centre of the galaxy Image Formation in One Equation I = SÄO ÄP + 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 Ä 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. July 2013 NEON Observing School, La Palma 47 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 July 2013 NEON Observing School, La Palma 48 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. July 2013 The radial profile of a typical groundbased star image. NEON Observing School, La Palma 49 Lucky Imaging Improving the PSF – 1) Lucky Imaging Wikipedia July 2013 NEON Observing School, La Palma 50 Lucky imaging (cont) C14: 35cm, sea level with lucky imaging (Damian Peach) Hubble: 2.4 metre, orbit July 2013 NEON Observing School, La Palma 51 Improving the PSF – 2) adaptive optics July 2013 NEON Observing School, La Palma 52 PSFs in Space Mostly determined by diffraction and optical aberrations. Scale with wavelength. ACS, F814W - well sampled (0.025” pixels) WFPC2, F300W - highly undersampled (0.1” pixels) July 2013 NEON Observing School, La Palma 53 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 July 2013 NEON Observing School, La Palma 54 Making Hubble PSFs July 2013 NEON Observing School, La Palma 55 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. July 2013 NEON Observing School, La Palma 56 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 July 2013 NEON Observing School, La Palma 57 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 July 2013 NEON Observing School, La Palma 58 Some real image defects (Hubble/WFPC2): Bleeding Ghost Cosmic ray July 2013 NEON Observing School, La Palma 59 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 July 2013 NEON Observing School, La Palma 60 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 July 2013 NEON Observing School, La Palma 61 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. July 2013 NEON Observing School, La Palma 62 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 calibration performed with the best reference files and processed data are available from the archive July 2013 NEON Observing School, La Palma 63 Image Combination • Multiple images are normally taken of the same target: – – – – – July 2013 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) To build contiguous images from multi-chip cameras NEON Observing School, La Palma 64 Mosaic Cameras • Detectors cannot be made larger indefinitely • But larger images are vital for many purposes • So, many recent imagers are mosaics of detectors with gaps between the chips • Examples: – – – – – July 2013 WFI @ 2.2-metre (La Silla) (8) VISTA (16 – IR) VST (32) MegaCam @ CFHT (36) …and many others. NEON Observing School, La Palma 65 Dither patterns example: VISTA July 2013 NEON Observing School, La Palma 66 Combining mosaic camera images • Basic reduction of each chip • Astrometric calibration of each image against reference catalogues (eg, 2MASS, GSC etc) • Mapping and resampling each input image onto the output grid • Using appropriate weighting (so, ignoring bad pixels) July 2013 NEON Observing School, La Palma 67 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. July 2013 NEON Observing School, La Palma 68 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 (so, typically 0.2 arcsec pixels at a good site) – normally no special treatment needed July 2013 NEON Observing School, La Palma 69 Combining Undersampled Images • Undersampled images present particular problems • Sub-pixel stepping (as done by Hubble) can help reduce the effects of undersampling • Special combination tools are needed for best results July 2013 NEON Observing School, La Palma 70 Undersampling and reconstruction Truth After pixel July 2013 After optics After linear reconstruction NEON Observing School, La Palma 71 Drizzling • A general-purpose image combination method – focussed on undersampled images • 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 – as well as Herschel/LuckyCam etc etc. • NOTE: for well-sampled images other, standard tools work as well. July 2013 NEON Observing School, La Palma 72 Drizzling July 2013 NEON Observing School, La Palma 73 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. July 2013 NEON Observing School, La Palma 74 The Effects of Resampling Kernels July 2013 NEON Observing School, La Palma 75 Implemented as MultiDrizzle for HST - www.stsci.edu/pydrizzle/multidrizzle July 2013 NEON Observing School, La Palma 76 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. July 2013 NEON Observing School, La Palma 77 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 multidimensional data array. • There are many other FITS features, for tables, extensions etc. • For further information start at: http://archive.stsci.edu/fits/fits_standard/ July 2013 NEON Observing School, La Palma 78 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. …. July 2013 NEON Observing School, La Palma 79 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) July 2013 NEON Observing School, La Palma 80 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) July 2013 NEON Observing School, La Palma 81 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. July 2013 NEON Observing School, La Palma 82 Making Colour Images Developed by Lars Christensen and collaborators: www.spacetelescope.org/projects/fits_liberator July 2013 NEON Observing School, La Palma 83 Original input images from FITS files Colourised in Photoshop Final combined colour version: July 2013 NEON Observing School, La Palma 84 Software Scisoft is a collection of many useful astronomical packages and tools for Linux 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 included: IRAF, STSDAS, TABLES etc ESO-MIDAS SExtractor/SWarp ds9,Skycat Tiny Tim, CASA, THELI Note – new 64bit version coming soon! July 2013 Python …etc, etc. + VO tools (new in Scisoft VII) NEON Observing School, La Palma 85 The End July 2013 NEON Observing School, La Palma 86