Starlight Xpress SXVF-H9 - Manual (Page 16)

Handbook for the SXVF-H9 Issue 1 May 2007
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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.
Other things to try, include summing several images for a better signal to noise ratio.
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 (summing 4
images will double the signal-to-noise), but different exposures must be used.
Summing an image with itself will not change the S/N ratio!
Although I have concentrated on the use of a telescope for deep-sky imaging, do not
forget that you have the option of using an ordinary camera lens for impressive wide-
field shots! A good quality 200mm F3.5 lens with an infrared blocking filter will yield
very nice images of large objects, such as M31, M42, M45 etc. If you cannot obtain a
large IR blocker for the front of the lens, it is quite acceptable to place a small one
behind the lens, inside the adaptor tube. You can even try using a hydrogen-alpha
filter to bring out nebulae, reduce light pollution and sharpen the star images to pin-
points.
Taking pictures of the planets:
Planetary imaging is in many ways quite different from deep sky imaging. Most deep
sky objects are faint and relatively large, so a short focal length and a long exposure
are needed, while planets are bright and very small, needing long focal lengths and
short exposures. High resolution is critical to achieving good results and I have
already shown how a suitable focal length can be calculated and produced, using a
Barlow lens.
Many camera users comment on the difficulty of finding the correct focus when
taking pictures of Jupiter etc. This is usually due to poor seeing conditions, which are
only too common, but may also be due in part to poor collimation of your telescope.