The first Step in processing a CCD image is to calibrate the image, i.e. subtracting a master dark frame and flat fielding with a master flat in that order. The result is called the calibrated image. For the ST6 images the master dark image was prepared by taking 40 one-minute dark images and averaging them to obtain the master dark. The individual dark frames are one minute in duration since the light image is the accumulation of one minute exposures. The master flat field was prepared by taking 40 dark images, averaging them and subtracting this image from the average of 40 light images. The duration of the flat field light and dark are equal and have a duration which yields about 50% saturation (a 16-bit pixel value of about 30,000). The flat light images were made by using the sky as a light source at dusk.
In the next step of processing the ST6 images a log scale was done between 0-65535 counts. This allows the faint areas and bright areas to be viewed at the same time by brightening the fainter parts of the image logarithmically relative to the brighter areas. To sharpen the image an unsharp mask was applied. The camera-supplied software (CCDOPS) has a very strong sharpening filter which is too powerful except for images with a very high signal-to-noise (S/N) ratio. For images which have a lower S/N ratio the sharpening routine may still be applied by first applying a weak smoothing filter to reduce noise.
The image is then displayed and the background and range are automatically set. I prefer to increase the range parameter by about 50% which allows the brighter areas of the image to be displayed in more detail. The background parameter can also be manually set to suit one's personal preference for the background brightness. If a hardcopy is desired, an inkjet printer using HP glossy paper will do a fine job.
Ed Grafton
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