Handbook for the TRIUS PRO-694C Issue 1 September 2020
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A 1200 second exposure of the Rosette through an IDAS 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 TRIUS PRO-694C 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-694C, 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
