Ultrastar C handbook Issue 1, August 2015
The focal length of your guide telescope is not especially critical for good guiding, as the guiding
software searches for the ‘centroid’ of the guide star image and can resolve shifts of far less than 1
pixel. Using a guide scope with a relatively short focal length (between 300 and 500mm) makes guide
star finding very easy and this is a recommended setup for general purpose guiding.
The alternative (and more mechanically stable) option, is to use an off-axis guide prism on the main
telescope drawtube. SX Ltd offer various assemblies for this purpose, such as our ‘Mini wheel with
OAG’, or our ‘Slim OAG + Filter Wheel’. The OAG can also be supplied on its own, with an extension
tube to compensate for the thickness of the missing filter wheel. The OAG utilises the C mount thread
in the Ultrastar C to give a secure attachment.
The control of your telescope mount will depend on what inputs are provided. The Ultrastar C output
connector supplies 4 pull-down lines and a common return line to simulate an ‘ST4’ style autoguider
output. The lines are isolated from the Ultrastar C electronics by opto-coupler chips and so there is no
risk of damage when connecting the outputs to mounts with unprotected electronics. A standard ‘RJ12’
style output lead is supplied with the Ultrastar C and this may be connected to any ST4 compatible
autoguider input socket on the mount. It is also possible to guide by using serial data from the
computer to an RS232 input on the mount, but this will be covered later and can be less effective than
the direct hardware connection. Note that the Ultrastar C guide cable is a ‘non reversing’ cable and so
is not compatible with some other guider cables, such as the SBIG versions.
The preferred guiding mode is generally 2x2 binned, as this gives both fast downloads and high
sensitivity. However, if operating with a very short focus ‘scope, or lens, high resolution mode may be
best.
Guiding with the Starlight Xpress software
With the Ultrastar C connected to the PC and mount, open ‘UltrastarC.exe’ and find the ‘Set program
defaults’ menu under the ‘File’ heading. If this is the first run of the software, you will get a warning
about the ‘ini file not found’, but ignore this and click on OK. You should now see a screen similar to
the one shown below:
