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 CCD Imaging Basics CCD Chips.
Download ReportTranscript 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 CCD Imaging Basics CCD Chips.
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 CCD Imaging Basics CCD Chips and Cameras Considerations when choosing a CCD Camera Colour Imaging Comparison with Eyepiece View and Film CCD Images Components of a raw CCD Image Image Reduction and Processing (Light, Dark and Flat Frames) CCD Cameras Example CCD Targets Typical CCD Results compared to Eyepiece View Moon, Planets Asteroids, Comets Stars, Clusters & Nebula Galaxies, Supernova Science with CCD Camera Astrometry Photometry Example CCD Targets Planets and other Solar System Objects Nebulae Stars and Clusters Galaxies Typical CCD result compared with Eyepiece View CCD (processed) Notebook Drawing, 1997 Eyepiece View M51 (Ursa Major) 15 x 1 min exposures Simulated 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 Deep Sky - Abell 744 Galaxy Cluster CCD Image, 3 x 60 sec exposure (summed) The image records distant galaxies down to magnitude +17 CCD Imaging – The Basics CCD Camera (CCD Chip, Circuit Board, Electronics, Shutter, Cooling Equipment, Housing) Object Telescope CCD Chip Focuser Attachment Photon Shutter Light Sensitive Area photons recorded as electrons in ‘square light buckets’ 0 0 0 0 0 0 0 1 5 7 67 2 8 0 0 0 1 3 1 0 0 0 0 0 0 Electronics USB or Parallel Cable 0 0 0 0 0 Computer Ram Hard Drive Software 0 0 1 5 7 67 2 8 0 0 0 1 3 1 0 0 0 0 0 0 Computer Screen CCD Imaging involves some work Final Image Single Raw Image Raw CCD Image Light from Sky / Aberdeen Light from Galaxies and Stars Defective Pixel(s) Satellite Or Aircraft Trail Noise Cosmic Ray Light Gradient Noise Noise Dust Shadows Single Raw Image Vignetting Dark Current Read Out Noise Pixel to Pixel Variation in Sensitivity Let’s examine the components of this image Stacking increases S/N Single Raw Image (realtime contrast) (summed, nosummed) alignment) 15 stacked frames (aligned and Single Raw Image (adjusted contrast) 15 stacked frames (aligned & median combined) 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 300 250 200 250 200 150 100 50 Light from 150 3 Objects 100 0 50 -50 0 Cross-Section through a CCD (2) 250 300 200 250 Light from 3 Objects 150 200 100 150 (after dispersion through the atmosphere) 50 100 500 -50 0 Dust Vignetting Read Noise Thermal Noise Light Pollution Object Object Hot Pixels Cross-Section through a CCD 300 250 250 200 Raw Image as recorded 150 200 100 150 50 100 500 -50 0 1 Object Object Threshold Sky brightness Cross-Section through a CCD (3) 250 300 200 250 150 200 Addition of Sky Glow / Light Pollution 100 150 50 100 500 -50 0 Dust Vignetting Read Noise Thermal Noise Object Hot Pixels Light Pollution Object Effect of Vignetting and Dust and Pixel-to-Pixel Variation in Sensitivity Av. 40 x 0.5 sec flat frames (tee-shirt flats) Cross-Section through a CCD (4) 250 300 200 250 Vignetting at edge of frame 150 200 100 150 50 100 500 -50 0 Dust Vignetting Read Noise Thermal Noise Light Pollution Object Object Hot Pixels Cross-Section through a CCD (5) 250 300 200 250 Absorption of light from dust on lenses and CCD window / chip 150 200 100 150 50 100 and 500 -50 0 Dust Vignetting Read Noise Thermal Noise Light Pollution Object Object Hot Pixels Variation in Pixel to Pixel Sensitivity Dark Current (electrons counted due to ‘heat’, even in the absence of light) Cross-Section through a CCD (6) 250 300 200 250 Addition of thermal electrons during exposure (includes noise) 150 200 100 150 50 100 500 -50 0 Dust Vignetting Read Noise Thermal Noise Light Pollution Object Object Hot Pixels Dark Current vs Time All Frames -25 deg C and identical white-black range 10 sec (Black = 0 ADU / White = 1000 ADU) 60 sec 120 sec 300 sec Dark Current vs Temperature All Frames 60s exposure and identical white/black range -5 deg C (Black = 150 ADU, White = 300 ADU) -15 deg C -25 deg C Colder Astronomical Cameras typically cool CCD chips to 30 deg C below ambient (using Peltier cooling) Dark Current vs Camera Simulated 60s exposures shown with identical white/black ranges Low Spec Camera -15 deg C Mid Spec Camera -15 deg C High Spec Camera -15 deg C High Spec Cameras Cosmic Rays Dark Frame Light Frame Dark Frame Dark Frame Read Out Noise (Bias Frame – a 0 sec exposure) -15 deg C Cross-Section through a CCD (7) 250 300 200 250 Addition of Readout Noise (+/-) 150 200 100 150 50 100 500 -50 0 Dust Vignetting Read Noise Thermal Noise Object Hot Pixels Light Pollution Object Cross-Section through a CCD (9) 300 250 250 200 Raw Image as recorded 150 200 100 150 50 100 500 -50 0 1 Object Object Threshold Cross-Section through a CCD (10) 300 250 200 250 Raw Image with Black Threshold applied 150 200 150 100 100 50 Compare with light from 3 objects 500 250 200 -50 0 150 1 100 50 0 Object Object Threshold -50 Object 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 Longer Exposure – Higher S/N Reduction Steps (1) Raw Light Frame Dark Reduced Frame Dark Frame Chart Title 300 300 250 250 250 200 150 200 150 100 150 - 50 100 200 100 = 50 100 0 0 50 150 1 -50 0 50 0 1 Dark 1 Object Object Threshold Dark Reduced Signal Removal of Dark Frame (an image with same exposure length but taken with closed shutter) Done in order to reduce read-out & thermal noise Reduction & Processing Example Raw Light Frame (60s) Dark Frame (median of 9) Final Image (15 frames stacked) Reduced Light Frame Reduction Steps (2) Raw Flat Frame Even Light 300 250 250 200 250 200 150 200 150 100 150 100 50 100 50 0 50 0 -50 0 -50 1 Flat Light Raw Flat Object Flat Frame Dark Frame (same Raw Flat Frame (after dark subtraction) Chart Title exposure as flat frame) 300 300 250 250 250 250 200 200 200 150 200 150 150 150 - 100 100 = 100 50 50 500 0 100 50 1 0 -50 150 100 50 0 0 -50 Dust Vignetting Read Noise Thermal Noise Hot Pixels Light Pollution Flat Light Object Creation of Flat Frame 1 Dark Raw Flat Object Flat Frame Av. 40 x 0.5 sec flat frames (tee-shirt flats) Reduction Steps (3) Flat Normalised Flat 250 300 2.000 200 250 1.800 1.600 150 200 150 100 / 100 50 500 Average Flat Field Value 1.400 = 1.200 1.000 0.800 0.600 0.400 0.200 0 -50 1 0.000 1 Raw Flat Object Normalised Flat Normalised Flat Dark Reduced Frame 300 250 2.000 250 300 250 200 250 200 200 150 150 150 100 100 100 50 50 1.800 250 200 1.600 / 200 150 150 100 1.400 1.200 1.000 0.800 100 50 0.600 0.400 50 0 0.000 0 -50 1 1 0 500 0.200 0 -50 Final Image -50 1 Object Dark Reduced Signal Object Normalised Flat Light Object Final Processing Final Reduced Image Final Image (with Black Threshold Set) 300 250 300 250 200 250 250 200 150 200 200 150 100 150 150 100 50 100 100 50 0 50 50 0 -50 0 0 -50 1 1 Object Light Light Final Reduced Image 300 250 250 200 200 150 Wavelet Object Threshold Processed (Deconvolved) Image (assumed shape of atmospheric dispersion) 300 250 300 200 150 100 250 150 Deconvolved 150 with 100 50 50 0 0 -50 = 200 100 50 0 100 1 1 50 Deconvolved Light Light 0 Object Read Noise Dust Vignetting Thermal Noise Hot Pixels Threshold 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 Reduction/Stacking Example IC 434 (Horsehead Nebula) 60s Raw 11 aligned frames summed 60s Reduced (dark subtract) Final Image Reduction/Stacking Example NGC 2903 60s Raw 60s Reduced (dark subtract) Average 10 x 60s CCD Cameras SBIG (USA) e.g ST-7e, $1995 (US) Starlight Express (UK) e.g HX-916 (Mono) £1395 Apogee (USA) WebCam eg Philip ToUCam Pro II, £75 Low Light Video e.g. Watec 120N, £579 HX7-C (Colour) £995 e.g. Astrovid, $ 995 (US) Example range of CCD Cameras Cookbook CCD Cameras TC-211 (Mono) 13.8 x 16um, 192 x 164 px, 2.6 x 2.6mm £50-100 Electronic Eyepieces Meade Electronic Eyepiece TV/VCR/Camcorder connection £90 WebCam Based Cameras Philips ToUCam Pro , Video 5.6 x 5.6um, 640 x 480 px, 4.6 x 4.0mm £75 Digital Cameras Various £200 - £400 Long Exposure Video CCD Cameras Minitron Watec 120N 8.6 x 8.6 um, 752 x 582 px, 6.5 x 5.0 mm, 0.00002 lx , 0.15 kg £299 £579 Smaller CCD Cameras Starlight Express MX5 (Mono) 9.8 x 12.6um, 500 x 290 px, 4.9 x 3.6mm, Starlight Express MX5C (Colour) £495 £620 ‘Standard’ Size CCD Cameras Starlight Express MX716 (Mono) 8.6 x 8.3um, 752 x 580 px, 6.47 x 4.83mm, 0.2kg, SBIG ST-7XME, 9 x 9 um, 765 x 510 px, 6.9 x 4.9 mm, 0.9 kg, £895 $1995 (US) Large Format CCD Cameras Starlight Express HX916 (Mono) 6.7 x 6.7um, 1300 x 1030 px, 8.71 x 6.9mm, 0.25 kg, SBIG ST-9X 20 x 20um, 512 x 512 px , 10.2 x 10.2 mm SBIG ST-8XME, 9 x 9 um, 1530 x 1020 px, 13.8 x 9.2 mm, 0.9 kg, £1345 $3195 (US) $5995 (US) Very Large Format CCD Cameras Starlight Express SXV-M25 (Col) 7.8 x 7.8um, 3000 x 2000 px, 23.4 x 15.6mm, SBIG STL-11000CM 9 x 9 um, 4008 x 2745 px, 36 x 24.7mm (26 sec download) Spring 2004 $8995 (US) 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 Example Spectral Response Curves CCD Chip Sizes Compared with 35mm Film TC211 ST7 ST8 KAF0400 KAF1600 New Large Format Cameras SLR 35mm film Camera 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 Image Scale at 2x2 binning = 206 x 18/2032 = 0.9 arc sec/pixel = 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 seeing With 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 Matching CCD and Telescope (2) Calculating Field Of View Field (Horizontal) in arc mins Field (Vertical) in arc mins) = Image Scale x No. pixels (horizontal) / 60 = 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) Field (Vertical) = 0.9 x 765/60 = 11.4 arc min = 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 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 view Important to use Good Quality Lenses ST7e with 200mm lens 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) Simulated unguided image of M51 12 min exposure Two main solutions to the problem - Take short (60 sec) exposures, then align & stack - Guide the telescope during the exposure Long Exposures / Guiding (2) CCD manufactures have developed several alternative guiding solutions : Track and Accumulate (SBIG) Expose Separate CCD Camera (e.g Meade) Guide Expose Guide Finder Expose Off-Axis Guide Camera Main Camera Self Guided (SBIG) Telescope Guide CCD Main CCD Star2000 (Starlight Express) Guide Frames Camera Interline CCD Image Frame Colour Imaging (1)– Single-Shot Cameras Colour Imaging (2)– Using Filters Colour Filter Wheel SBIG CFW-8A Red, Blue, Green, Clear Filters Option to take and image in other filter bands e.g UBRVI for photometry Colour Imaging – with Filters Red (Av. 3x10s) Green (Av. 3x10s) Blue (Av. 3x20s) Colour Image (LRGB) Luminance (Av. 6x10s) M42 (Orion) CCD Imaging compared with Eyepiece Viewing +ve Can ‘see’ fainter objects (i.e. can ‘see’ objects impossible to see with the naked eye) Much easier to record and share what has been ‘seen’ Can generally ‘see’ more detail in objects (particularly nebula) Can find and locate objects more quickly (with appropriate software) Can even view from the leisure of indoors (with remote connection) Can playback /animate motion of slowly moving objects (eg Pluto) Can acquire the colour of faint objects (ones which look grey to naked eye) Can undertake more accurate (certainly easier) astrometry and photometry -ve Some objects more impressive with naked eye (eg red/blue double star , Jupiter + moons) Loose some of that ‘3D’ effect & feelings of awe Difficulty of claiming one actually saw / observed the object Realtime CCD images are often very noisy Typical realtime CCD image compared with Eyepiece View CCD (raw image on screen) Eyepiece View M51 (Ursa Major) 1 min exposure CCD – Comparisons with Film +ve CCD Images immediately available (no waiting on film lab) Digital (no need to scan in order to process further), Easier manipulation - ability to stack Light record is linear (no recripicty) With suitable software the image can be used to automatically locate telescope position or to guide the telescope. -ve Smaller image area FOV (typically only ~ 20% that of 35mm film) Comparisons of CCD Images with Film and Eyepiece Observations Recording of naked eye observation Film CCD Use and Sharing of CCD Images Astronomical Records World Wide Web Presentations Own records CCD Images (2001-2004) Moon Moon – Apollo 17 Landing Site Planets Venus 2004 Uranus 2002 Mars 2003 Jupiter 2003 Neptune 2002 Saturn 2001 Pluto 2003 Jupiter / Saturn / Uranus Moons Six of Saturn's moons appear in this CCD Image (2 sec exposure) Asteroids (Minor Planets) Animated Sequence of 10 CCD Images of Minor Planet Kleopatra (216) The animation records 58 arc sec motion of the minor planet over a period of 1 hr 56 min (= 30 arc sec/hour). Near Earth Asteroid Comets C/2002 T7 (Linear) 2004-Feb Comet C/2000 WM1 (LINEAR) 2001-Nov (passing through star field in Pegasus) (passing through star field in Aries) Clusters in Gemini (CCD Mosaic) M35 NGC 2158 M45 Pleiades (CCD Mosaic) Double Cluster In Perseus (7 x 6 CCD Mosaic, 20s exposures) Globular Cluster M15 (Pegasus), 6 x 10s Extra-Solar Planets ? HD 209458 (Pegasus) has a transiting Jupiter mass short period extrasolar planet.(HD 209458b). Every 3.5 days, the planet produces a dimming of the star of 1.7 % that lasts for about 3 hours. The dimming has been detected by Castellano and Laughlin using almost identical equipment to me (ie 8" telescope and ST-7E CCD camera), which presents me the opportunity to also have a go at trying to detect a extra-solar planet lying at a distance of 1.45 x 1015 km (153 light years) from Earth.. Nebula M57 Ring Nebula (Lyra) M16 Eagle Nebula (Serpens Caput) M27 Dumbbell Nebula (Vulpecula) NGC 2261 - Hubble's Variable Nebula (Monoceros) NGC 4567 / 4568 (Virgo) Galaxies NGC 7331 (Pegasus) M100 (Coma Berenices) M105 (Leo) NGC 2903 (Leo) M64 Black-eye Galaxy (Coma Berenices) NGC 2903 Spiral Galaxy Galaxy Cluster NGC 7320 Galaxy Cluster (Stephan's Quintet, Andromeda) The 5 main galaxies range from magnitude +13.6 to + 14.8 Faintest galaxy in image is +16.6 2002-10-02 21:44 to 21:51h UT CCD Image, 2 x 2 min exposure (2x2 binning) 11.4 x 7.6 arc min (#28003 & 28005) Supernova / Supernova Remnants M1, Crab Nebula SN 2001ib, 2001-Dec Colour Imaging - 2004 M42 Orion NGC 2392 Planetary Nebula (Eskimo or Clown Face Cluster) NGC 2903 Spiral Galaxy, Leo NGC 1857, Auriga Saturn Jupiter More Recent Images NGC 3628 Spiral Galaxy, Leo M63 Spiral Galaxy (Sunflower Galaxy) M65 Area Out-takes (1) Out-takes (2)