Starlight Xpress SXVR-H16 - Manual (Page 11)

Handbook for the SXVR-H16 Issue 1 June 2009
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The faint image data will then appear in considerable detail and I think that you will
be impressed by the result!
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 SXVR-H16 generates relatively 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 can 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 Maxim DL).
‘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:
Below you will see typical examples of a dark frame and an uncalibrated raw image
of M16.
A typical 5 minute dark frame exposure. Note the random scatter of ‘warm pixels’