Starlight Xpress SXVF-H36 - Manual (Page 16)

Handbook for the SXVF-H36 Issue 1 August 2007
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below the image from light leakage in the CCD structure. To minimise this, keep the
image brightness down with an ND filter or aperture stop, so that the exposure time is
not less than about 0.5 seconds. A H-alpha nebula filter often gives excellent results.
The exposure time needed for good lunar 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.
If you want to record an image sequence – perhaps during a lunar eclipse – you can
set up the SX software to ‘Autosave’ an image sequence. To start the Autosave
process, call up the SXV Camera Interface and select the ‘Continuous Mode’ check
box at the top (make sure the rest are unchecked). Now 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).
Processing a lunar image:
Lunar 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.
Try applying an ‘Unsharp Mask’ filter with a radius of 5 and a power of 5. This will
often greatly increase the visibility of any detail on the moon, 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. 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. As a finishing touch, the application of a Median filter or
a Weighted Mean Low Pass filter can be useful to smooth out the high frequency
noise after a strong Unsharp Mask.
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Other features of SXV_H36
‘Slew & Sum’ imaging:
The SXVF-H36 can be used in an automatic image-stacking mode, called ‘Slew &
Sum’. The camera is set to take several sequential exposures, which are automatically
‘slewed’ into alignment and then summed together by the software. This mode can
help to overcome a poor RA drive by summing images that have exposure times
shorter than the drive error period. The resulting image has more noise than a single
exposure of the same total length, but this method of imaging is still an effective way
of making long exposures.