Starlight Xpress SXV-M7C - Manual (Page 19)

Handbook for SXV-M7C Issue 1 June 2004
19
check the ‘Autosave Image’ checkbox near the bottom of the window. If you now
click on ‘Take Picture’ the automatic sequence will begin and will not stop until you
press a computer key. The images will be saved in FITs format with sequential names
such as ‘Img23, Img24….’ and will be found in the ‘Autosave’ directory (or a sub-
directory of Autosave, set up in the program defaults menu).
The exposure time needed for good planetary images is such that the image histogram
has a peak value at around 127 and does not extend much above 200 (Ignore the
major peak near zero, due to the dark background). If you use too short an exposure
time, the image noise level will be increased, and if too long a time is used you will
saturate the highlights and cause white patches on the decoded image. With the
recommended focal length, Jupiter and Mars will both need an exposure time of
between 0.1 and 1 seconds and Saturn will need between 0.5 and 2 seconds.
Processing a planetary image:
Planetary images have one major advantage over deep sky images, when you come to
process them – they are MUCH brighter, with a correspondingly better signal to noise
ratio. This means that aggressive sharpening filters may be used without making the
result look very noisy and so some of the effects of poor seeing can be neutralised.
A raw image
First, synthesise the colour image in the usual way. Now try applying an ‘Unsharp
Mask’ filter with a radius of 5 and a power of 5. This will greatly increase the
visibility of any detail on the planet, but the optimum radius and power will have to
be determined by experiment. In general terms, the larger the image and the worse the
seeing, then the wider the radius for best results. My Jupiter shots are usually about
half the height of the CCD frame and I find that the ‘radius 5, power 5’ values are
good for most average seeing conditions. If you have exceptionally good conditions,
then a reduction to R=3, P=3 will probably give a more natural look to the image, as
too large a radius and power tends to outline edges with dark or bright borders.