Handbook for the SXVR-H18
Issue 1 March 2010
16
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
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_H18
‘Slew & Sum’ imaging:
The SXVR-H18 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.
To take an S&S image, go to the camera interface window and select an exposure
time for one image of the sequence. Do not use a very short exposure time, as the
read-out noise will become dominant. About 30 seconds is a reasonable minimum.
Now go to the ‘Multiple Exposure Options’ and select a number of exposures to take.
You can also select to average the images, rather than adding them, and there is a
‘Alternative Slew Mode’ available, which uses the correlation of image areas, rather
than a single star. This mode can be better in dense star fields.
Another option is ‘Auto remove dark frame’. This is advisable with S&S images, as
the slewing will mis-register the images with a single dark frame that is applied to the
