Starlight Xpress SXV-M5 - Manual (Page 12)

Handbook for SXV-M5 Issue 1 June 2004
12
If you are happy with the image, go to the ‘File’ menu and save it in a convenient
directory.
Now you need a ‘dark frame’, if the best results are to be extracted from your raw
image. To take this, just cover the telescope objective with the lens cap, or drop the
flip mirror to block the light path to the CCD (make sure that this is light tight), and
take another 60 second exposure. This image will 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. The SXV-M5 generates very little dark signal and so dark
frames are not essential for short exposures of less than a few minutes, but it is a good
idea to record at least one for each exposure time used during an imaging session. 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).
‘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 the deep-sky image:
1) Make sure the ‘Auto Contrast Stretch’ is switched off and load your image into
SXV_M5. Select ‘Merge’ and then ‘Subtract Dark Frame’. Pick the appropriate dark