Starlight Xpress SXV-AO unit - Manual (Page 9)

Handbook for the Starlight Xpress AO unit
Issue 1 21/8/2005
Find an interesting object or starfield to image and adjust the ‘scope until a convenient
guide star can be seen in the guider field. Ideally, the star should be easily seen in an
exposure of only 0.1 seconds and, if necessary, binning may be used to gain sensitivity.
Once the guide star is identified, press the ‘Centralise AO’ button and watch the star
image – it should cycle through a cross shaped figure and then settle after about 2
seconds. Now press ‘Select guide star’ and click on the star which you think best suited
to act as your guide reference (bright and isolated). You can now press the ‘Train’ button
and the AO will cycle automatically while measuring the system sensitivity. If the
training cycle is successful, new rate values will be generated, although they will
probably be much the same as the default values. If the calibration fails, it will probably
be due to one or both of the AO direction settings being wrong. The defaults in the
software are for a guide camera mounted as shown in the frontispiece picture of the AO
assembly, with its indicator LED towards the camera end of the AO barrel. You can
either rotate the guide camera to this position, or try swapping the AO up/down and
left/right check box settings. Once you have the directions set and the motion rates
determined, calibration of the AO will not be necessary in future sessions as it is a
constant factor for all ‘scopes. Only one other setting is still necessary. This is the
calibration of the mount for ‘mount bumps’ during your guiding session. Although the
AO can take care of the small and rapid guiding errors, there will be steady drifts of the
telescope drive which will slowly shift the AO to the limits of its motion range. To
combat this effect, we need to send occasional corrections to the mount (‘bumps’) in
order that the AO remains near to the centre of its operating range. A relatively easy way
in which to determine the settings required is to set the AO guiding on your selected
guide star (press ‘Start guiding’) and then use the hand controller to move the star slightly
East or West until the data boxes on the guider image start to show offset values of more
than 50 steps. At this point you will see the text ‘Move mount’ appear, along with a
direction indication. If the result is that the AO error falls rapidly below 50 steps, then the
mount direction and rate is about right, but any sign that the error is rapidly increasing
will show that the ‘Swap e/w’ or n/s direction is incorrect. Stop the guiding, reverse the
‘Swap’ box for that direction and try guiding again. Once you have both directions set
correctly, you can experiment with the mount speed (pixels per second) settings to
optimise the amount of correction applied – ideally you want the mount correction to
move the AO back to the ‘zero offset’ position, but there is a large tolerance on this.
You are now ready to take an AO guided image!
Re-centralise the AO unit, select your guide star and then press ‘Start guiding’. Once the
guide star image is stable, press the main camera icon and set your image exposure time
in that dialog box. Now press ‘Take photo’ and wait for the result to appear!
Updating the AO firmware:
From time to time, improved versions of the AO firmware will become available and will
be posted on the Starlight Xpress web site. The AO unit is provided with a means of
updating its internal firmware via the PC serial port. You will need the serial cable as
provided with the AO unit and this should be connected between the AO unit input