Starlight Xpress SXVR-H674 - Manual (Page 14)

Handbook for the SXVR-H674 Issue 1 March 2012
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3) The resulting image will probably look faint and dull, with a pale background due
to light pollution. 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, although 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.
4) The image will now look quite impressive and I hope that you are pleased with
your first efforts!
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, followed by normalising the background to black
with a normal stretch, is the usual procedure.
Further small refinements are usually possible and you will become expert at judging
the best way to achieve these as your experience increases. As a rough guide, the
‘Filters’ menu can be used to sharpen, soften or noise reduce the image. Strong ‘High
Pass’ filters are usually not a good idea with deep sky images, as the noise will be
strongly increased and dark rings will appear around the stars, but a ‘Median’ filter
can remove odd speckles and a mild ‘Unsharp Mask’ (Radius 3, Power 1) will
sharpen without too much increase in noise.