Starlight Xpress SXV-M25C - Manual (Page 18)

Handbook for the SXV-M25C Issue 1 June 2004
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To use your flat fields, they must first have a dark frame subtracted. Although this
may appear to be unimportant with such brightly lit and short exposures, there is the
‘bias offset’ of the camera in each image and this can produce an error in the final
correction. As we are mainly interested in the bias, any very short exposure dark
frame will give a good result. The dark subtracted images should then be averaged
together before use.
After the above procedures have been executed, the flat field will be ready for use.
Load up your image for processing, subtract the dark frame and then select ‘Apply
flat field’ in the ‘Merge’ menu. The result should be an image with very few signs of
the original artefacts and you can then process it in the normal way
Processing a deep-sky image:
The following instructions include the subtraction of a dark frame, but this may be
regarded as optional.
1) Make sure the ‘Auto Contrast Stretch’ is switched off and load your image into the
SXV_M25C 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’ icon button 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, followed by normalising the background to black with a normal stretch, is
the usual procedure.