Starlight Xpress SXV-M8C - Manual (Page 17)

Handbook for the SXV-M8C Issue 1 Jan 2005
17
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 as a FIT file in a
convenient directory.
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-M8C 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 SXV-M8C, 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 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 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.