Starlight Xpress SXV-H9C - Manual (Page 18)

Handbook for the SXV-H9 Issue 1 June 2002
18
1) Make sure the ‘Auto Contrast Stretch’ is switched off and load your image into the
SXV_H9C program. 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.
2) The next step is to process the image into colour, but you may find that the
application of a gentle contrast stretch to the image before synthesis will improve the
colour result. Don’t overdo the stretch, as the colour range can then exceed the
dynamic range available, leading to un-correctable colour errors. Keep a copy of the
original file – just in case! Set the colour synthesiser to apply the anti-alias filter and
then run it.
3) The resulting image will probably still look faint and dull, with a pale orange or
green background due to light pollution. Try using the ‘Start point’ adjustment in the
Colour balance menu to get something close to a neutral background colour. It is now
time to process the ‘luminance’ (brightness and contrast) of the image to get the best
visual appearance. First, use the ‘Normal’ contrast stretch to darken the background
by setting the ‘Black’ slider just below the main peak of the histogram. Alternatively,
you can use the ‘Remove Background’ option to let the software decide on the best
setting. This will greatly reduce the background brightness and the image will begin
to look rather more attractive, if dark. You can now try brightening the highlights with
another ‘Normal’ stretch, in which you bring down the ‘White’ slider to just above the
main image peak. The best setting for this is rather more difficult to guess and you
may need several attempts before the result is ideal. Just use the ‘Undo last filter’
function, if necessary, to correct a mistake. In many cases, a ‘Normal’ contrast stretch
will give a good result, but may ‘burn out’ the bright regions and leave the faint parts
of the image rather lacking in brightness. To combat this, many imagers will use a
combination of ‘Normal’ and ‘Non-linear’ contrast stretches. The best settings are
different for different objects, but performing a non-linear or power law stretch,