Starlight Xpress SXV-M8C - Manual (Page 19)

Handbook for the SXV-M8C Issue 1 Jan 2005
19
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.
5)
The image will now look quite impressive and I hope that you are pleased with
your first efforts!
M27 from a 15 minute exposure at F5 with a C11
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 (hot pixels) and a mild ‘Unsharp Mask’ (Radius 3, Power 1)
will sharpen without too much increase in noise.
Other things to try include summing several images for a better signal to noise ratio.
This MUST be done AFTER colour synthesis, as summing raw images is highly
likely to destroy the filter grid pattern and so prevent the creation of a proper colour
image. Summing can be done in the ‘Merge’ menu and involves loading the first
processed image, selecting a reference point (a star) then loading the second image
and finding the same star with the mouse. Once the reference is selected, you can
either add directly, or average the images together. Averaging is generally better, as
you are less likely to saturate the highlights of the picture. The signal-to-noise ratio
will improve at a rate proportional to the square root of the number of summations