Starlight Xpress SXV-M25C - Manual (Page 16)

Handbook for the SXV-M25C Issue 1 June 2004
16
M42 from a 5 minute exposure at F5 with the FSQ106 + IDAS filter
Most competitive brands of CCD camera require a ‘dark frame’ to be subtracted from
your images to achieve the best results. A dark frame is simply a picture which was
taken with the same exposure as your ‘light frame’, but with the telescope objective
covered, so that no light can enter. It records only the ‘hot pixels’ and thermal
gradients of your CCD, so that these defects are largely removed when the dark frame
is subtracted from the light frame. The SXV-M25C CCD is quite different from those
used in other brands of camera and generates an extremely low level of dark noise.
Indeed, it is so low that subtracting a dark frame can actually INCREASE the noise in
your images! This is because the statistical noise of the dark frame can exceed the
‘pattern noise’ from warm pixels and hence add to that of the subtracted result. If your
test pictures have an exposure time of less than about 30 minutes (as above), then
don’t bother with a dark frame, just ‘kill’ any hot pixels with your processing
software. In SXV-M25C, the ‘Median filter’ can do this, but other software (e.g.
Maxim DL) will provide a ‘hot pixel killer’ that can be mapped to specific locations
in the image.
If you feel that dark frame really is necessary, please proceed as follows:
To take a dark frame, just cover the telescope objective with the lens cap and take
another exposure of the same length as that of the light frame. 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. 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 Maxim DL) and this can be useful as a time saver.