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Handbook for SXV-M5C Issue 1 August 2004
22
The advantages of S.T.A.R. are very great, but there are two negative aspects, which
you should be aware of. The main loss is in the sensitivity of the CCD, as only half of
the pixel area is integrating an image at any one time. This is slightly offset by the
halving of the dark current, which also results, and can be completely compensated
for by increasing the exposure time – not a big problem when the camera can guide
itself! The other problem is that the CCD amplifier must be switched on momentarily
to read out each guider frame. Despite the short times involved, this results in some
contamination of the upper left hand corner of the image, by light emitted from the
amplifier transistors (electro luminescence), and this needs the subtraction of a special
dark frame to remove it. However, the glow is quite faint and may be ignored on
many images, or it can be removed with the ‘Gradient filter’ in the ‘Filters’ menu.
What you need:
To use S.T.A.R. 2000, you must have a telescope with ‘Auto guider’ capability. Most
modern SCTs have this facility in the form of a 6 pin ‘RJ11’ telephone style
connector on the mounting base or handset. If your ‘scope is ‘ST4 compatible’ it
should work with S.T.A.R. 2000. Adaptors for other mounts, such as the Vixen
Skysensor, are available from Starlight Xpress. In the case of a ‘difficult’ mount, an
add-on relay box is available to switch higher currents and opposite polarity circuits.
The system has been tested with the LX200, LX50 and Celestron CI700 German
mounts with good results, but almost any modern mount will work well. Please note
that the RJ11 cable supplied with the camera has the connectors fitted on opposite
sides of the 6 way cord – other cables may not be compatible!
Setting up the system:
For best results, it is important to set up your telescope mounting as accurately as
possible. Alt-Az mounts can be used with S.T.A.R. 2000, but will cause field rotation
during exposures and are often rather poor at smooth tracking. We recommend that
you always use an equatorially mounted ‘scope, with its polar axis accurately aligned
on the pole. If your mount is prone to sudden shifts of aim point during tracking,
usually due to poorly made gears or bearings, then S.T.A.R. will be unable to fully
compensate and some trailing will result. Slow and smooth errors in position are well
corrected and so this is what you should aim for. Check that all your drive gears are
firmly fixed to their shafts (loose gears are surprisingly common!) and that they
engage closely, without binding. The tracking of many mounts can be improved by
adding a deliberate bias to the East. A small weight on the Eastern fork tine will
usually be effective.
Connect up the computer and camera in the usual way and install the camera onto the
telescope. Next, take the 6 way RJ11 telephone lead and plug one end into the guider
output of the SXV camera. This output is a ‘switched’ signal, compatible with inputs
designed for use with an ST4 guider. It is also possible to use a serial connection to an
LX200 compatible RS232 input, but to do this you need to have a serial port to RJ11
cable from the back of the computer to the RS232 input of the mount. This connection
allows other ‘goto’ control signals to be sent to the mount, but may be less convenient
if you do not require this facility. Which output you use will also depend on your
telescope, but the Meade LX200 and its clones are compatible with serial control,
although the LX200GPS still has a software issue at the time of writing. All other