7. Acquiring color information

To create color CCD image you need to accumulate sets of raw images taken through red, green and blue (RGB) or cyan, magenta and yellow filters (CMY). Because of the infrared sensitivity of a CCD detector, an IR-blocking filter has to be used with each of the above. CMY has an advatage of covering forbidden oxygen and hydrogen beta emission lines around 500nm (major components of the light from emission and planetary nebulas) which are frequently excluded by RGB filters. Also exposures needed to obtain same signal-to-noise ration are shorter with CMY then when using RGB (each of the CMY filters lets through two colors). In addition, using white overlay image (one obtained with IR-blocking filter alone) greatly improves contrast - see Richard Berry's Cookbook site (http://wvi.com/~rberry/cookbook.htm) for detailed discussion of this so called WCMY process. Following examples were taken with my favourite WCMY.

Set of raw images taken through each filter is first corrected for dark current and flat field as described above. Each of them is also linear scaled and averaged to assemble three separate master filtered images.

(linear scaled images are shown)

8. Assembling color CCD image

Above CMY images are then converted into virtual red, green and blue frames based on particular filter characteristics (see Cookbook site for discussion of filter calibration and conversion).

Then, in a final step of WCMY process, color balance of each pixel is calculated by comparing corresponding signals in red, green and blue channels, and resulting 24-bit color information is applied to the white image from step 6. In contrast, when using traditional RGB process, final image is formed by combining color channels.

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True power of CCD imaging comes from the ability to extract important information through image processing. Nothing was added in any of the steps above - all the information in the final color image was already present (in more or less subtle form) in raw exposures. To succeed, it is important to remeber that CCD imaging (as any astroimaging) is not point-and-shoot activity. It frequently takes more telescope time then traditional film astrophotography - exposures used to assemble the above image of Crab Nebula totaled 112 minutes (not including acquiring dark and flat frames). So if you want to venture into electronic imaging, be warned! But look at the bright side of it - if something goes wrong you will notice it on the computer screen and be able to correct the problem right away. Sure beats realizing a mistake after developing a roll of film a month later!

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