Starlight Xpress SXVF-H9 - Manual (Page 14)

Handbook for the SXVF-H9 Issue 1 May 2007
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be a picture of the dark signal generated during your exposure and it should be saved
with your image for use in processing the picture. If many such darks are recorded
and averaged together, the statistical noise will be reduced, but the gains to be had are
rather small compared with the effort involved.
As variations in ambient temperature will affect the dark signal, it is best to take the
dark frames within a few minutes of capturing your images. For the same reason, it is
not wise to use ‘old’ dark frames if you want the best possible results, however, some
software allows you to scale library dark frames to match the image (e.g. AstroArt)
and this can be useful as a time saver.
‘Flat fields’ are often recommended for optimising the results from your CCD
camera, but these are generally less important than dark frames, especially if you
make sure that the optical window of the camera is kept dust-free. The purpose of a
flat field is to compensate for uneven illumination and sensitivity of the CCD and it is
better to avoid the need for one by keeping the optics clean and unvignetted. I will
ignore flat fielding for current purposes and describe the process in detail at a later
stage.
Processing a deep-sky image:
The following instructions include the subtraction of a dark frame, but this may be
regarded as optional.
1) Make sure the ‘Auto Contrast Stretch’ is switched off and load your image into
SXV_H9. Select ‘Merge’ and then ‘Subtract Dark Frame’. Pick the appropriate dark
frame and the software will then remove the dark signal from your image, leaving it
somewhat darker and slightly smoother than before.