Handbook for the TRIUS PRO-825C Issue 1 September 2020
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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 TRIUS PRO-825C 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 TRIUS PRO-825C, 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 TRIUS
PRO-825C. 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 1000th 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.
