Starlight Xpress SXVF-H9 - Manual (Page 13)

Handbook for the SXVF-H9 Issue 1 May 2007
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view. You can get to see a lot of this, without affecting the image data, if you go to
the ‘View’ menu and select ‘Auto Contrast Stretch Image’. 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.
A 60 second exposure of M42 through an H-alpha filter (non-linear stretched)
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 SXVF-H9 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 10 minutes (as above), then
don’t bother with a dark frame, just ‘kill’ any hot pixels with your processing
software. In SXVF-H9, 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, or drop the
flip mirror to block the light path to the CCD (make sure that this is light tight), and
take another exposure with the same length as that of the light frame. This image will