Starlight Xpress SXVR-H814 - Manual (Page 12)

Handbook for the SXVR-H814 Issue 1 February 2013
12
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 SXVR-H814 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 SXVR-H814, 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-
H814. 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.