AAO Newsletter October1996 - Page 11
I am writing this on the last afternoon of the 6 night run. It is looking
very cloudy and if we are wiped out tonight (which is looking highly likely)
then we will have had a total of about 1.5 clear nights-par for the course
for 2dF, I'm afraid.
Despite this on-going bad luck with weather, I have been very encouraged
by the performance and reliability of the positioner. In contrast to the
previous run, which suffered extensively from the gripper crash and the
subsequent knock-on effects, this run has seen not one PMAC/VME induced
error. What positioner failures we have seen (very few) have been induced
by external factors (poor back-illumination, occasional retractor problems)
which are entirely understandable and tractable. The system has configured
a large number of complex (200-fibre) fields without incident at a variety
of telescope orientations. There is no doubt that congratulations are in
order for the whole team in achieving this level of reliability and performance,
especially those in software and electronics who worked so hard and effectively
between the runs to debug and improve the control system.
The spectrograph also continues rapidly to progress. We now have very
precise grating encoding and convenient back-illumination and slit drive
control. More fundamental, however, is the successful installation of the
back-illumination labyrinth which appears to give a satisfactory light
seal even in the extreme condition of having all LEDs on at once and continuously.
Although there are several adaptions that still need to be made, there
is no doubt that Alan has now cracked the problem which after all is fundamental
to the whole 2dF double-buffering concept.
In the clear conditions prevailing at the beginning of last night, we
were able to successfully acquire one faint QSO-galaxy red-shift field
and integrate for a period of ~ 1.5 hours. Spectra appeared to be coming
in very well. We set up for a second field but the clouds rolled in forcing
us back into FPI astrometry tests. Clearly fibre astrometry remains high
on the priority list for 2dF commissioning. Several telescope rasters indicated
that fibre positioning accuracies are better than 1" with formal errors
giving ~ 0.25" rms. However there appear to be zones on the field
plates which are substantially worse, with difference vectors approaching
1". The small number (~ 30) of good PPM stars available in each 2dF
field make pinning this effect down very difficult and time-consuming and,
given the poor weather, only a small amount of progress has been made on
this front.
Furthermore, in attempting to acquire a field at a declination of -71°,
we encountered acquisition difficulties which looked like a field rotation
problem. JAB had warned us that we might face problems in fields close
to the pole and it has become clear that we need a concerted, poor condition,
campaign strategy to do FPI position checks in these zones to diagnose
the problem.
Retractors are becoming another issue of concern. Over the last month
we have encountered several (~ 4) retractor failures where the bottom pulley
has become stuck, leaving a loose fibre which can become entangled and
cause damage to other fibres if it goes undetected. We need to address
this problem with some degree of urgency since it is potentially quite
dangerous.
Keith Taylor
Vale IPCS
On the 17th of September the IPCS control desk was removed from the
control room. This ended nearly 20 years of service to the astronomical
community. It seems an appropriate time to review
