1. July 2013NEON Observing School, La Palma1 Telescope Optics and related topics Richard Hook (rhook@eso.org) ESO, Garching http://www.eso.org/~rhook/NEON/LaPalma_2013_Hook.ppt

2. July 2013NEON Observing School, La Palma2 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

3. July 2013NEON Observing School, La Palma3 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.

4. July 2013NEON Observing School, La Palma4 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

5. July 2013NEON Observing School, La Palma5 404 years of the Telescope Galileo, 1609, d=2.5cm ESO VLT, 1999, d=8.2m (x4) E-ELT, 2024, d=39m

6. July 2013NEON Observing School, La Palma6 Basic Telescope Optical Designs Most modern large telescopes are variants of the Cassegrain design.

7. July 2013NEON Observing School, La Palma7 Basic Properties of Telescopes Optics Light collecting power - proportional to D 2 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 D 2 Surface intensity of an extended source at focal plane - proportional to 1/F 2 Angular Field of view - normally bigger for smaller F, wide fields need special designs Tube length proportional to f primary Dome volume (and cost?) proportional to f 3 primary Cost rises as a high power (~3) of D Aperture = D, Focal Length=f, Focal ratio=F=f/D For telescopes of the same design the following holds.

8. July 2013NEON Observing School, La Palma8 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.

9. July 2013NEON Observing School, La Palma9 Telescope Aberrations There are five basic monochromatic (3rd order) aberrations: –Spherical aberration (~y 3 ) –Coma (~y 2  ) –Astigmatism (~y  2 ) –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 Aberrations are deviations from a perfect optical system. They can be due to manufacturing errors, alignment problems, or be intrinsic to the optical design.

10. July 2013NEON Observing School, La Palma10 Spherical aberration

11. July 2013NEON Observing School, La Palma11 Optical Aberrations - continued Spherical Aberration in action

12. July 2013NEON Observing School, La Palma12 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:

13. July 2013NEON Observing School, La Palma13 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/F 2) 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”.

14. July 2013NEON Observing School, La Palma14 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.

15. July 2013NEON Observing School, La Palma15 Two-mirror classical systems www.telescope-optics.net

16. July 2013NEON Observing School, La Palma16 Other two-mirror systems The primary does not have to be paraboloidal The conic constant (K= -e 2 ) 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)

17. July 2013NEON Observing School, La Palma17 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 Ritchey-Chretien

18. July 2013NEON Observing School, La Palma18 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.

19. July 2013NEON Observing School, La Palma19 A future large, widefield groundbased survey telescope with a three mirror design plus corrector - the LSST

20. July 2013NEON Observing School, La Palma20 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.

21. E-ELT Optical Design July 2013NEON Observing School, La Palma21

22. July 2013NEON Observing School, La Palma22 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

23. July 2013NEON Observing School, La Palma23 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) The ESO 1-metre Schmidt at La Silla

24. July 2013NEON Observing School, La Palma24 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.

25. July 2013NEON Observing School, La Palma25 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

26. July 2013NEON Observing School, La Palma26 Prime focus correctors Wynne corrector (eg, WHT on La Palma): –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

27. July 2013NEON Observing School, La Palma27 More exotic Prime focus correctors: Suprime-Cam, on the Subaru 8m Copyright: Canon

28. July 2013NEON Observing School, La Palma28 An even more exotic corrector: Hobby-Eberly 11.1x9.8m, fixed altitude spherical segmented mirror. Penn State

29. July 2013NEON Observing School, La Palma29 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 Harmer- Wynne corrector) and the 2.5m f/7.5 Dupont telescope at Las Campanas. A new example, highly optimised is VISTA

30. July 2013NEON Observing School, La Palma30 VISTA - ESO’s IR Survey Telescope 4.1m, f/1 primary Large integrated corrector/IR camera Modified RC optics 1.65 degree field

31. July 2013NEON Observing School, La Palma31 VISTA in action: the Flame Nebula (NGC 2024) and the Horsehead Nebula in Orion in the near- infrared

32. July 2013NEON Observing School, La Palma32 Atmospheric distortion correctors (ADCs) The problem:

33. July 2013NEON Observing School, La Palma33 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.

34. July 2013NEON Observing School, La Palma34 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: –Rigid and free of resonances –Accurate tracking –Good sky coverage –Compact (smaller dome) –Space and good access for instruments

35. July 2013NEON Observing School, La Palma35 Mountings: examples The German equatorial: Great Dorpat Refractor, 1824. (Graham/Berkeley) Jacobus Kapteyn 1m, La Palma, 1983. (ING/IAC)

36. July 2013NEON Observing School, La Palma36 Mountings continued: The English or “yoke” mount: The Mount Wilson 100in. From a book by Arthur Thomson, 1922 (Gutenberg project)

37. July 2013NEON Observing School, La Palma37 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)

38. July 2013NEON Observing School, La Palma38 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)

39. July 2013NEON Observing School, La Palma39 Mirror coatings

40. July 2013NEON Observing School, La Palma40 The Atmosphere - transmission A J H K

41. July 2013NEON Observing School, La Palma41 The Atmosphere - emission (at a good dark observatory site, La Palma)

42. July 2013NEON Observing School, La Palma42 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)

43. July 2013NEON Observing School, La Palma43

44. July 2013NEON Observing School, La Palma44 Part II: Astronomical Digital Imaging (a very brief introduction)

45. July 2013NEON Observing School, La Palma45 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

46. July 2013NEON Observing School, La Palma ESO VLT NACO, near- IR, adaptive optics: the centre of the galaxy Four Examples: The power of imaging VISTA Survey: Part of the VISTA VVV survey of the Milky Way bulge and disc in the infrared (JHKs). HUBBLE: A supernova at z>1 ALMA mm: Spiral structure about R Scl

47. July 2013NEON Observing School, La Palma47 Image Formation in One Equation 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.

48. July 2013NEON Observing School, La Palma48 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

49. July 2013NEON Observing School, La Palma49 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+r 2 /R 2 )  + B Where there are two free parameters (apart from intensity, background and position) - R, the width of the PSF and , the Moffat parameter. Software is available to fit PSFs of this form. The radial profile of a typical groundbased star image.

50. Lucky Imaging July 2013NEON Observing School, La Palma50 Improving the PSF – 1) Lucky Imaging Wikipedia

51. Lucky imaging (cont) July 2013NEON Observing School, La Palma51 Hubble: 2.4 metre, orbit C14: 35cm, sea level with lucky imaging (Damian Peach)

52. Improving the PSF – 2) adaptive optics July 2013NEON Observing School, La Palma52

53. July 2013NEON Observing School, La Palma53 PSFs in Space WFPC2, F300W - highly undersampled (0.1” pixels) ACS, F814W - well sampled (0. 025” pixels) Mostly determined by diffraction and optical aberrations. Scale with wavelength.

54. July 2013NEON Observing School, La Palma54 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

55. July 2013NEON Observing School, La Palma55 Making Hubble PSFs

56. July 2013NEON Observing School, La Palma56 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.

57. July 2013NEON Observing School, La Palma57 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

58. July 2013NEON Observing School, La Palma58 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

59. July 2013NEON Observing School, La Palma59 Some real image defects (Hubble/WFPC2): Ghost Bleeding Cosmic ray

60. July 2013NEON Observing School, La Palma60 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

61. July 2013NEON Observing School, La Palma61 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

62. July 2013NEON Observing School, La Palma62 Geometric Distortion HST/ACS/WFC - a severe case of distortion - more than 200 pixels at the corners. Large skew. 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.

63. July 2013NEON Observing School, La Palma63 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

64. July 2013NEON Observing School, La Palma64 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) –To build contiguous images from multi-chip cameras

65. 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: –WFI @ 2.2-metre (La Silla) (8) –VISTA (16 – IR) –VST (32) –MegaCam @ CFHT (36) –…and many others. July 2013NEON Observing School, La Palma65

66. Dither patterns example: VISTA July 2013NEON Observing School, La Palma66

67. 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 2013NEON Observing School, La Palma67

68. July 2013NEON Observing School, La Palma68 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.

69. July 2013NEON Observing School, La Palma69 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

70. 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 2013NEON Observing School, La Palma70

71. July 2013NEON Observing School, La Palma71 Truth After optics After pixel After linear reconstruction Undersampling and reconstruction

72. July 2013NEON Observing School, La Palma72 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.

73. July 2013NEON Observing School, La Palma73 Drizzling

74. July 2013NEON Observing School, La Palma74 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.

75. July 2013NEON Observing School, La Palma75 The Effects of Resampling Kernels

76. July 2013NEON Observing School, La Palma76 Implemented as MultiDrizzle for HST - www.stsci.edu/pydrizzle/multidrizzle

77. July 2013NEON Observing School, La Palma77 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.

78. July 2013NEON Observing School, La Palma78 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/

79. July 2013NEON Observing School, La Palma79 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 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 …. Fundamental properties: image size, data type, filename etc. World Coordinate System (WCS): linear mapping from pixel to position on the sky.

80. July 2013NEON Observing School, La Palma80 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)

81. July 2013NEON Observing School, La Palma81 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)

82. July 2013NEON Observing School, La Palma82 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.

83. July 2013NEON Observing School, La Palma83 Making Colour Images Developed by Lars Christensen and collaborators: www.spacetelescope.org/projects/fits_liberator

84. July 2013NEON Observing School, La Palma84 Original input images from FITS files Colourised in Photoshop Final combined colour version:

85. July 2013NEON Observing School, La Palma85 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 Python …etc, etc. + VO tools (new in Scisoft VII) Note – new 64bit version coming soon!

86. July 2013NEON Observing School, La Palma86 The End