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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

CCD Imaging

David Richards2004-04-13

All astronomical images taken by David Richards, 2001-2004

(Meade 8” LX200 SCT / SBIG ST-7E )

ccd imaging2
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
example ccd targets
Example CCD Targets

Planets and other Solar System Objects

Stars and Clusters



typical ccd result compared with eyepiece view

Notebook Drawing, 1997

Typical CCD result compared with Eyepiece View

CCD (processed)

Eyepiece View

M51 (Ursa Major)15 x 1 min exposures


longer exposure greater magnitude reach
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
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)



CCD Chip





Computer Screen


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


USB or Parallel Cable

CCD Imaging – The Basics
raw ccd image

Light from Sky / Aberdeen

Light from Galaxies and Stars

Defective Pixel(s)


Or Aircraft


Cosmic Ray

Light Gradient


Single Raw Image

Dark Current


Read Out Noise

Pixel to PixelVariation in Sensitivity

Raw CCD Image




Let’s examine the components of this image

stacking increases s n

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)

cross section through a ccd image 1
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



Light from

3 Objects

cross section through a ccd 2
Cross-Section through a CCD (2)

Light from

3 Objects

(after dispersion

through the


cross section through a ccd
Cross-Section through a CCD

Raw Image as


cross section through a ccd 3
Cross-Section through a CCD (3)

Addition of

Sky Glow /Light Pollution

effect of vignetting and dust and pixel to pixel variation in sensitivity
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
Cross-Section through a CCD (4)

Vignetting at

edge of frame

cross section through a ccd 5
Cross-Section through a CCD (5)

Absorption of

light from dust

on lenses and

CCD window/ chip and

Variation in

Pixel to PixelSensitivity

cross section through a ccd 6
Cross-Section through a CCD (6)

Addition of

thermal electrons

during exposure(includes noise)

dark current vs time
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

dark current vs temperature
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


Astronomical Cameras typically cool CCD chips to 30 deg C below ambient (using Peltier cooling)

dark current vs camera
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

cosmic rays
Cosmic Rays

Dark Frame

Light Frame

Dark Frame

Dark Frame

cross section through a ccd 7
Cross-Section through a CCD (7)

Addition ofReadout

Noise (+/-)

cross section through a ccd 10
Cross-Section through a CCD (10)

Raw Image with

Black Thresholdapplied

Compare with light from 3 objects

getting good images
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
reduction steps 1
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

reduction processing example
Reduction & Processing Example

Raw Light Frame (60s)

Dark Frame (median of 9)

Reduced Light Frame

Final Image (15 frames stacked)

reduction steps 2
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

flat frame
Flat Frame

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

reduction steps 3
Reduction Steps (3)


Normalised Flat

AverageFlat Field Value



Normalised Flat

Dark Reduced Frame

Final Image


final processing
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


the challenge of recording very faint objects
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
Reduction/Stacking Example IC 434 (Horsehead Nebula)

11 aligned frames summed

60s Raw

60s Reduced (dark subtract)

Final Image

reduction stacking example ngc 2903
Reduction/Stacking Example NGC 2903

60s Raw

60s Reduced (dark subtract)

Average 10 x 60s

ccd cameras
CCD Cameras


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)

example range of ccd cameras
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)
considerations when choosing a ccd camera
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
ccd chip sizes compared with 35mm film
CCD Chip Sizes Compared with 35mm Film






New Large Format Cameras



35mm film

matching ccd and telescope 1
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
matching ccd and telescope 2
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
ccd cameras with ordinary camera lens
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

long exposures guiding 1
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

long exposures guiding 2


Guide Camera





Main Camera


Guide CCD

Main CCD


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)



colour imaging 2 using filters
Colour Imaging (2)– Using Filters

Colour Filter Wheel


Red, Blue, Green, Clear Filters

Option to take and image

in other filter bandse.g UBRVI for photometry

colour imaging with filters
Colour Imaging – with Filters

Blue (Av. 3x20s)

Red (Av. 3x10s)

Green (Av. 3x10s)

Colour Image (LRGB)

Luminance (Av. 6x10s)


ccd imaging compared with eyepiece viewing
CCD Imaging compared with Eyepiece Viewing


  • 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


  • 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
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
CCD – Comparisons with Film


  • 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.


  • Smaller image area FOV (typically only ~ 20% that of 35mm film)
comparisons of ccd images with film and eyepiece observations
Comparisons of CCD Images with Film and Eyepiece Observations



Recording of naked eye observation

use and sharing of ccd images
Use and Sharing of CCD Images

Astronomical Records

World Wide Web


Own records


Mars 2003

Venus 2004

Jupiter 2003

Saturn 2001

Uranus 2002

Pluto 2003

Neptune 2002

jupiter saturn uranus moons
Jupiter / Saturn / Uranus Moons

Six of Saturn's moons appear in this CCD Image (2 sec exposure)

asteroids minor planets
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).


Comet C/2000 WM1 (LINEAR)

2001-Nov(passing through star field in Aries)

C/2002 T7 (Linear)2004-Feb(passing through star field in Pegasus)

globular cluster
Globular Cluster

M15 (Pegasus), 6 x 10s

extra solar planets
Extra-Solar Planets ?

HD 209458 (Pegasus) has a transiting Jupiter mass short period extrasolar planet.(HD 209458-b). 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 Laughlinusing 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..


M57 Ring Nebula (Lyra)

M16 Eagle Nebula (Serpens Caput)

M27 Dumbbell Nebula (Vulpecula)

NGC 2261 - Hubble's Variable Nebula



NGC 4567 / 4568(Virgo)


NGC 7331


M100 (Coma Berenices)

M105 (Leo)

M64 Black-eye Galaxy(Coma Berenices)

NGC 2903 (Leo)

NGC 2903 Spiral Galaxy

galaxy cluster
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 UTCCD Image, 2 x 2 min exposure (2x2 binning)11.4 x 7.6 arc min  (#28003 & 28005)

supernova supernova remnants
Supernova / Supernova Remnants

M1, Crab Nebula

SN 2001ib, 2001-Dec

colour imaging 2004
Colour Imaging - 2004

M42 Orion

NGC 2392 Planetary Nebula (Eskimo or Clown Face Cluster)

NGC 2903 Spiral Galaxy, Leo


NGC 1857, Auriga


more recent images
More Recent Images

NGC 3628 Spiral Galaxy, Leo

M63 Spiral Galaxy (Sunflower Galaxy)