Starlight Xpress SXVR-H674C - Manual (Page 17)

Handbook for the SXVR-H674C Issue 1 March 2012
17
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-
H674C. 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:
1) Make sure the ‘Auto Contrast Stretch’ is switched off and load your image into
SXV_hmf_usb. If you intend to subtract a dark frame, select ‘Merge’ and then
‘Subtract Dark Frame’. Pick the appropriate dark frame and the software will then
remove the dark signal from your image, leaving it somewhat darker and slightly
smoother than before.