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CCD Imaging

CCD Imaging. David Richards 2004-04-13 All astronomical images taken by David Richards, 2001-2004 (Meade 8” LX200 SCT / SBIG ST-7E ). CCD Imaging. Introduction Example CCD Targets Typical CCD Results compared to Eyepiece View CCD Imaging Basics Components of a raw CCD Image

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CCD Imaging

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  1. CCD Imaging David Richards2004-04-13 All astronomical images taken by David Richards, 2001-2004 (Meade 8” LX200 SCT / SBIG ST-7E )

  2. CCD Imaging • Introduction • Example CCD Targets • Typical CCD Results compared to Eyepiece View • CCD Imaging Basics • Components of a raw CCD Image • Image Reduction and Processing (Light, Dark and Flat Frames) • CCD Cameras • CCD Chips and Cameras • Considerations when choosing a CCD Camera • Colour Imaging • Comparison with Eyepiece View and Film • CCD Images • Moon, Planets • Asteroids, Comets • Stars, Clusters & Nebula • Galaxies, Supernova • Science with CCD Camera • Astrometry • Photometry

  3. Example CCD Targets Planets and other Solar System Objects Stars and Clusters Nebulae Galaxies

  4. Notebook Drawing, 1997 Typical CCD result compared with Eyepiece View CCD (processed) Eyepiece View M51 (Ursa Major)15 x 1 min exposures Simulated

  5. Longer Exposure – Greater Magnitude Reach Consecutive CCD images (star field in Milky Way in Cygnus)2003-08-05  5.2 x 7.6 arc mins (suburban site, Dorset, UK) The 10 sec exposure reaches to mag +12.0 whilst the 40 sec exposure reaches to +13.5

  6. Deep Sky - Abell 744 Galaxy Cluster CCD Image, 3 x 60 sec exposure (summed) The image records distant galaxies down to magnitude +17

  7. CCD Camera (CCD Chip, Circuit Board, Electronics, Shutter, Cooling Equipment, Housing) Object Telescope CCD Chip Focuser Photon Attachment Shutter Computer Screen Computer Light Sensitive Areaphotons recorded as electrons in ‘square light buckets’ RamHard DriveSoftware 0 0 0 0 0 0 1 5 1 0 0 7 67 3 0 0 2 8 1 0 0 0 0 0 0 0 0 0 0 0 0 1 5 1 0 0 7 67 3 0 0 2 8 1 0 0 0 0 0 0 Electronics USB or Parallel Cable CCD Imaging – The Basics

  8. Single Raw Image CCD Imaging involves some work Final Image

  9. Light from Sky / Aberdeen Light from Galaxies and Stars Defective Pixel(s) Satellite Or Aircraft Trail Cosmic Ray Light Gradient DustShadows Single Raw Image Dark Current Vignetting Read Out Noise Pixel to PixelVariation in Sensitivity Raw CCD Image Noise Noise Noise Let’s examine the components of this image

  10. 15 stacked frames (aligned & median combined) 15 stacked frames (summed, no alignment) 15 stacked frames (aligned and summed) Stacking increases S/N Single Raw Image (realtime contrast) Single Raw Image (adjusted contrast)

  11. Cross-Section through a CCD Image (1) Simulated image of light reaching camera in earth orbit Simulated image of light reaching camera at Sea Level Cross Section Light from 3 Objects

  12. Cross-Section through a CCD (2) Light from 3 Objects (after dispersion through the atmosphere)

  13. Cross-Section through a CCD Raw Image as recorded

  14. Sky brightness

  15. Cross-Section through a CCD (3) Addition of Sky Glow /Light Pollution

  16. Effect of Vignetting and Dust and Pixel-to-Pixel Variation in Sensitivity Av. 40 x 0.5 sec flat frames (tee-shirt flats)

  17. Cross-Section through a CCD (4) Vignetting at edge of frame

  18. Cross-Section through a CCD (5) Absorption of light from dust on lenses and CCD window/ chip and Variation in Pixel to PixelSensitivity

  19. Dark Current(electrons counted due to ‘heat’, even in the absence of light)

  20. Cross-Section through a CCD (6) Addition of thermal electrons during exposure(includes noise)

  21. Dark Current vs Time All Frames -25 deg C and identical white-black range(Black = 0 ADU / White = 1000 ADU) 10 sec 60 sec 120 sec 300 sec

  22. Dark Current vs Temperature All Frames 60s exposureand identical white/black range(Black = 150 ADU, White = 300 ADU) -5 deg C -15 deg C -25 deg C Colder Astronomical Cameras typically cool CCD chips to 30 deg C below ambient (using Peltier cooling)

  23. Dark Current vs Camera Simulated 60s exposuresshown with identical white/black ranges Low Spec Camera -15 deg C Mid Spec Camera -15 deg C High Spec Camera -15 deg C High SpecCameras

  24. Cosmic Rays Dark Frame Light Frame Dark Frame Dark Frame

  25. Read Out Noise(Bias Frame – a 0 sec exposure) -15 deg C

  26. Cross-Section through a CCD (7) Addition ofReadout Noise (+/-)

  27. Cross-Section through a CCD (9) Raw Image as recorded

  28. Cross-Section through a CCD (10) Raw Image with Black Thresholdapplied Compare with light from 3 objects

  29. Getting Good Images A principal aim during imaging (and subsequent reduction) is to maximise the Signal-To-Noise (S/N) in order to get the best image of the astronomical object. Techniques include : • Minimise noise from sky light by imaging from a dark site (if possible) • Cool the CCD Chip as far as possible (temperature control important) • Use longest exposure that telescope can track for without drifting, and without over-saturating the chip. • Using on camera pixel binning (may decrease resolution – but not if seeing limited) • Use camera with low read out noise / low dark current • Reduce images to remove dark current, allow for the varying response of each CCD pixel and remove the impacts of vignettting and dust on CCD chips or telescope optics • Minimise read-out and dark noise (using Median of multiple Dark Frames) • Use average (or median) of multiple Flat Frames • Use stacking to ‘add’ light from target, whilst cancelling noise – thereby increasing the S/N

  30. Longer Exposure – Higher S/N

  31. Reduction Steps (1) Dark Reduced Frame Raw Light Frame Dark Frame = - Removal of Dark Frame (an image with same exposure length but taken with closed shutter)Done in order to reduce read-out & thermal noise

  32. Reduction & Processing Example Raw Light Frame (60s) Dark Frame (median of 9) Reduced Light Frame Final Image (15 frames stacked)

  33. Reduction Steps (2) Raw Flat Frame Even Light Flat Frame (after dark subtraction) Dark Frame (same exposure as flat frame) Raw Flat Frame = - Creation of Flat Frame

  34. Flat Frame Av. 40 x 0.5 sec flat frames (tee-shirt flats)

  35. Reduction Steps (3) Flat Normalised Flat AverageFlat Field Value = / Normalised Flat Dark Reduced Frame Final Image /

  36. Final Processing Final Reduced Image Final Image (with Black Threshold Set) Wavelet (assumed shape of atmospheric dispersion) Processed (Deconvolved) Image Final Reduced Image Deconvolved with =

  37. The challenge of recording very faint objects Attempt at imaging 2004 DW (a mag +19 Kuiper Belt Object). Star field in Hydra with the predicted position of Kuiper object marked by green circle. 2 x 5 min exposure (summed)Faintest visible objects are mag +17.7

  38. Reduction/Stacking Example IC 434 (Horsehead Nebula) 11 aligned frames summed 60s Raw 60s Reduced (dark subtract) Final Image

  39. Reduction/Stacking Example NGC 2903 60s Raw 60s Reduced (dark subtract) Average 10 x 60s

  40. CCD Cameras SBIG (USA) e.g ST-7e, $1995 (US) Starlight Express (UK) e.g HX-916 (Mono) £1395 Apogee (USA) HX7-C (Colour)£995 WebCameg Philip ToUCam Pro II, £75 Low Light Videoe.g. Watec 120N, £579 e.g. Astrovid, $ 995 (US)

  41. Example range of CCD Cameras • Cookbook CCD CamerasTC-211 (Mono) 13.8 x 16um, 192 x 164 px, 2.6 x 2.6mm £50-100 • Electronic EyepiecesMeade Electronic Eyepiece TV/VCR/Camcorder connection £90 • WebCam Based CamerasPhilips ToUCam Pro , Video 5.6 x 5.6um, 640 x 480 px, 4.6 x 4.0mm £75 • Digital CamerasVarious £200 - £400 • Long Exposure Video CCD CamerasMinitron £299Watec 120N 8.6 x 8.6 um, 752 x 582 px, 6.5 x 5.0 mm, 0.00002 lx , 0.15 kg £579 • Smaller CCD CamerasStarlight Express MX5 (Mono) 9.8 x 12.6um, 500 x 290 px, 4.9 x 3.6mm, £495Starlight Express MX5C (Colour) £620 • ‘Standard’ Size CCD CamerasStarlight Express MX716 (Mono) 8.6 x 8.3um, 752 x 580 px, 6.47 x 4.83mm, 0.2kg, £895SBIG ST-7XME, 9 x 9 um, 765 x 510 px, 6.9 x 4.9 mm, 0.9 kg, $1995 (US) • Large Format CCD CamerasStarlight Express HX916 (Mono) 6.7 x 6.7um, 1300 x 1030 px, 8.71 x 6.9mm, 0.25 kg, £1345SBIG ST-9X 20 x 20um, 512 x 512 px , 10.2 x 10.2 mm $3195 (US)SBIG ST-8XME, 9 x 9 um, 1530 x 1020 px, 13.8 x 9.2 mm, 0.9 kg, $5995 (US) • Very Large Format CCD CamerasStarlight Express SXV-M25 (Col) 7.8 x 7.8um, 3000 x 2000 px, 23.4 x 15.6mm, Spring 2004SBIG STL-11000CM 9 x 9 um, 4008 x 2745 px, 36 x 24.7mm (26 sec download) $8995 (US)

  42. Considerations when choosing a CCD Camera • Chip Size / Pixel Size / Number of Pixels / Pixel Shape • Match with Telescope Focal Length • Sensitivity of CCD • Dark Current / Read Noise • Cooling / Temperature Regulation / Shutter • Digitisation (12 bit/ 16 bit) • Linearity of CCD / Capacity of a pixel • Anti-Blooming (ABG vs NABG) • CCD Quality / Defective Pixels • Camera Weight / Size • Binning / Windowing Capabilities • Download Speed, USB / Parallel • Self Guiding Capabilities • Single Shot Colour / Filter Wheel attachment • Software • Cost • Reliability / Support

  43. Example Spectral Response Curves

  44. CCD Chip Sizes Compared with 35mm Film TC211 KAF0400 ST7 KAF1600 ST8 New Large Format Cameras SLR Camera 35mm film

  45. Matching CCD and Telescope (1) • Calculating Image Scale (arc secs per pixel)Image Scale = 206 x pixel size (in um) --------------------- focal_length (in mm) e.g for SBIG ST-7 and 8” f/10 SCT Pixel Size = 9 um Focal length = 25.4 x 8 x 10 = 2032 mm Image Scale at 1x1 binning = 206 x 9 / 2032 = 0.9 arc sec/pixel Image Scale at 2x2 binning = 206 x 18/2032 = 1.8 arc sec /pixel Typical seeing is 2-4 arc sec, so 2x2 binning (1.8 arc sec/pixel) is about right (At 2x2, sensitivity is better and downloads are much faster, but images are only 382 x 255)1x1 binning only really of benefit when imaging planets when there is benefit in sampling at <1 arc sec, and there is opportunity to benefit from brief moments of exceptional seeingWith Focal Reducer (63%) 1x1 binning = 1.3 arc sec/pixel, 2x2 binning = 2.5 arc sec/pixel • General rule : chose CCD (or choose Telescope) that gives around 2 arc sec /pixel

  46. Matching CCD and Telescope (2) • Calculating Field Of ViewField (Horizontal) in arc mins = Image Scale x No. pixels (horizontal) / 60Field (Vertical) in arc mins) = Image Scale x No. pixels (vertical) / 60 e.g for SBIG ST-7 and 8” f/10 SCT Pixel Size = 9 um, Focal length = 25.4 x 8 x 10 = 2032 mm Image Scale at 1x1 binning = 206 x 9 / 2032 = 0.9 arc sec/pixel (765 x 510) Field (Horizontal) = 0.9 x 765/60 = 11.4 arc min Field (Vertical) = 0.9 x 510/60 = 7.7 arc min With focal reducer (63%) Image Scale at 2x2 = 2.5 arc sec/pixel (382 x 255) Field (Horizontal) = 2.5 x 382/60 = 15.9 arc min Field (Vertical) = 2.5 x 255/60 = 10.6 arc min • General rule : Dependant of proposed Targets chose a Camera with a larger dimension CCD to gives a larger FOV (price will be a limitation).Alternatively select a low focal ratio telescope (eg f/4) or use a focal reducer

  47. CCD Cameras – with ordinary Camera Lens • CCD Cameras can also be used piggy-backed to a Telescope and fitted with ordinary camera lenses. This can provide wider fields of viewImportant to use Good Quality Lenses ST7e with 200mm lens

  48. Long Exposures / Guiding (1) • Unless a scope is perfectly polar aligned and has perfect tracking, stars will trail on long exposures (at focal length of 2000mm this might be observed after only 2 mins exposure) • Two main solutions to the problem- Take short (60 sec) exposures, then align & stack- Guide the telescope during the exposure Simulated unguided imageof M5112 min exposure

  49. Expose Guide Camera Guide Guide Expose Expose Main Camera Telescope Guide CCD Main CCD Camera Interline CCD Image Frame Guide Frames Long Exposures / Guiding (2) • CCD manufactures have developed several alternative guiding solutions : • Track and Accumulate (SBIG) • Separate CCD Camera (e.g Meade) • Self Guided (SBIG) • Star2000 (Starlight Express) Finder Off-Axis

  50. Colour Imaging (1)– Single-Shot Cameras

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