Handbook for the SXVR-M26C Issue 1 March 2010
15
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 SXVR-M26C, 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, or methods such as ‘Sigma combine’ may be used.
In the unlikely event that 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 with 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.
‘Bias frames’ are somewhat more useful than dark frames when using the SXVR-
M26C. A bias frame is essentially a zero exposure dark frame and records any minor
readout defects that the CCD may possess, so a ‘bias frame subtraction’ can clean up
any ‘warm columns’ or shadings that are created during readout. To record a bias
frame, cover the camera aperture and take a 1000
th
of a second exposure. If you take
at least 10 such frames and average them together, the resulting ‘master bias’ can be
used to clean up readout defects for many months before CCD ageing changes require
another set to be recorded.
‘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:
Your raw image may look something like the one shown below – a 20 minute
exposure of the region of the Rosette, taken through a 100mm F5 refractor with an
IDAS light-pollution rejection filter.
(28 pages)
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