CCD Astrophotography and Image Processing

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CCD Astrophotography and Image Processing

• John Stone
• 2/11/2002

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What is a CCD?

• CCD, Charge-Coupled Device
• An image sensor technology
• Tiny compared to a 35mm film frame
• High quantum efficiency
• Good response in visual spectrum
• Used in video cameras, digital cameras,
• CMOS sensors are becoming common in everyday
  consumer devices due to their low cost, but much
  less efficient than CCDs

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Overview of CCD Sensors

• Square or rectangular pixel array
• Chips range from 3 to 10 millimeters in size
• Pixel sizes range from 5um to 20um
• Pixels are usually square
• Most CCDs have a wide variety of programmable
  exposure, and pixel sampling modes (explained
  later)

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CCD Camera Design
SBIG ST-237A CCD Camera

• Peltier cooling system, heat sink, and fan
• Cooling reduces thermal noise in images
• Compact aluminum case, good heat dissipation, low
  weight

CCD Sensor only 4mm or so per side, enclosed in
sealed chamber with optical window
Motor turns color filter wheel, which also acts
as a shutter
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Telescope Mount and Tracking

• Tracking a clock drive counteracts the Earths
  rotation about its axis
• Polar alignment the better the alignment of the
  equatorial mount with the polar axis, the better
  the telescopes tracking is
• Guiding no mount is perfectly aligned, so
  guiding is necessary for long exposures

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CCD/Film Image Comparison
ST237A CCD

• 8 SCT at f/10
• 8 sec CCD exposure, from back yard
• 45 sec film exposure, from dark site
• CCD has tiny field of view, so apparent
  magnification is higher
• Film fogs with long exposures due to light
  pollution and its exposure characteristics

35mm Slide Film
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More CCD Images
M15, 8 SCT f/10, 15 sec
M42 wide field 100mm Lens 20 sec, piggyback
M51, 8 SCT f/10, 90 sec, wind vibration
M42 wide field 100mm Lens 2 sec, tripod
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CCD Challenges

• Greater magnification than film
• Less tolerance for polar alignment errors
• Less tolerance for wind, vibrations

M51 Single 90 second exposure
Wandering star, indicates vibrations
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Grayscale vs Color CCDs

• Most CCDs appropriate for astronomical imaging
  have no built-in color filters
• CCDs with built-in color filters take color
  images, but are much less light efficient
• CCD chips without built-in filters can make color
  images with an extra filter wheel accessory
• Some cameras can have a filter wheel mounted
  internally, saves on weight and required
  telescope focus travel

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Color Filter Wheels

• Color interference filters
• Let narrow wavelengths into CCD sensor
• One exposure made for each color, by rotating the
  filter wheel.

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Creating Color CCD Images
Red Filter
Green Filter
Clear Glass, Luminance
Blue Filter
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Combined RGB Color Image
R B G Images
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Image Processing Examples
50 aligned and stacked images. Image noise is
cancelled out.
Medium strength unsharp mask processing. Brings
out details well.
Strong unsharp mask processing. Brings out
details, but increases graininess.
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CCD Imaging Web Sites

• Kodak CCD image sensor web site
• http//www.kodak.com/US/en/digital/ccd/products.sh
  tml
• http//www.kodak.com/US/en/digital/ccd/appNotes.sh
  tml
• Texas Instruments CCD image sensor web site
• http//www.ti.com/sc/docs/msp/ccd.htm
• Ron Wodaskis CCD imaging site, for his book
• http//www.wodaski.com/newastro/default.asp