Starlight Xpress SXVR-M25C - Manual (Page 24)

Handbook for the SXVR-M25C Issue 1 June 2009
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The two serial connections are in the form of standard RS232 PC style plugs and
provide TX, RX and Ground connections at RS232 levels. Access is via commands
sent through the USB connection and, at the time of writing, is limited to any serial
controls that are provided by the SXV software. It is expected that many more
functions will be added as the software is upgraded.
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Other features of the SXVR-M25C hardware and software
Using the ‘Binned’ modes:
Up to this point, I have assumed that the full resolution imaging mode is being used,
as binning will destroy the colour data. ‘Binned 2x2’ mode sums groups of 4 pixels
into one output pixel, thus creating a 1516 x 1008 pixel image with 4 times the
effective sensitivity. Using 2x2 binning, you can considerably improve the sensitivity
of the SXVR-M25C without losing a great deal of resolving power, so you may like
to use this mode for finding faint deep-sky objects. Other binning modes (3x3 and
4x4) are available and will further increase the image brightness and reduce its
resolution.
Taking and using a flat field:
Flat fields are images, which display only the variations of illumination and
sensitivity of the CCD and are used to mathematically modify a wanted image in such
a way that the errors are removed. Common flat field errors are due to dust motes on
the camera window and vignetting effects in the optical system of the telescope. Dust
motes act as ‘inverse pinholes’ and cast out-of-focus images of the telescope aperture
onto the CCD chip, where they appear as shadow ‘do-nuts’. Most optical systems
show some vignetting at the edges of the field, especially when focal reducers are
used. This causes a brighter centre to show in images, especially when there is a lot of
sky light to illuminate the field.
If dust motes are your main problem, it is best to clean the camera window, rather
than to rely on a flat field to remove the do-nuts. Flat fields always increase the noise
in an image and so physical dust removal is the best option. If you have serious
vignetting, first check whether the optical system can be improved. The most likely
cause of this problem is trying to use too powerful a degree of optical compression
with a focal reducer and you might want to try moving the camera closer to the
reducer lens.
If you really do need to use a flat field for image correction, then it must be taken with
care. It is most important that the optical system MUST NOT be disturbed between
taking your original images and taking the flat field. Any relative changes of focus
and rotation etc. will upset the match between flat field and image and the result will
be poor correction of the errors. The other necessity for recording a good flat field is a
source of very even illumination of the telescope field. This is surprisingly difficult to
achieve and many designs of light source have appeared in the literature and on the
Web. These usually consist of a large wooden box, containing several lamps and an