The process of acquiring a guide star at UES is more difficult than at
Cassegrain, due to the limited size of the fibres, which are the only way of
imaging the detector. Within the UES A&G box, there are two probe fibres,
one fine and one coarse, and a fibre to view the detector slit. Two modes of
operation are possible, using the reflections off the slit edges as the guiding
light source, or using an Off Axis guide star near to the star of interest. In
this case move the telescope such that the star of interest is over the slit
and then move the autoguider probe to place the guide star on the Guide fibre,
be it Fine or Coarse. As there is no other way to view the sky other than the
autoguider detector via its fibres, it is not simple to find a guide star. The
Guide Star Server is a useful ally.
Although this is harder than guiding with the slit, the results are better
since the autoguider has the centre of the star to use as its reference point.
First select the Coarse fibre with SELECT COARSE, then point the Telescope
such that the star of interest is roughly over the coarse fibre. Turn on
continuous acquisition with ACQUIRE ON. The coarse fibre (only) will be
displayed repeatedly. Adjust the acquisition time with ACQINT n until the star
is suitably imaged, and adjust the position of the telescope to centre the star
on the Coarse fibre.
Next, as the position of the autoguider probe is accurately known, tweak the
telescope to put the star of interest onto the slit. Enter ADD SLIT, and both
the slit and the coarse fibre will be displayed. Note that the loop time will
be slower since the two windows have to be reconstructed each time. With the
telescope tracking the star of interest open-loop, adjust the position of the
autoguider probe until a guide star is centred on the coarse fibre.
Next, turn off Continuous acquisition with ACQUIRE OFF, select the coarse fibre
for guiding with SELECT COARSE display the slit too with ADD SLIT and start
guiding with GUIDE ON . Both the slit and the coarse fibre should be
displayed.
Finally, look at the pointing error displays: if the errors are reasonable and
close to zero with the star near to the centre of the slit, close the guide
loop by entering autoguider on at the Telescope Control System terminal.
Since the two sides of a star image are reflected off the diffraction slit in
the spectrograph, this light source can be used to guide the telescope
directly. Here, the star of interest is acquired as for Off Axis guiding by
turning on continuous acquisition with the coarse fibre: SELECT COARSE then
ACQUIRE ON . Once the star is found, the telescope can be tweaked to move the
star onto the slit. The slit is then viewed with ACQUIRE OFF, SELECT SLIT and
ACQUIRE ON, and the star centred before guiding is enabled.
Since most of the light disappears down the slit, there are actually two points
of light, one on each side of the slit. This causes the autoguider to guide in
a bi-modal fashion, which is a result of the guide centroiding algorithm. A
better slit centroiding algorithm is available in this release.