JCMT dish temperatures vis-a-vis pointing

Date    : 01 May 1997
From    : Fred Baas

 . . . the pointing is primarily affected by
gradients in the supporting A-frame. Overall temperature differences
between this frame and the dish structure should have NO effect on the
pointing whatsoever, provided that the gradients in the telescope
structure remain reasonably small. Therefore, time lags are unimportant
too.

Analysis of ALL the temperature probes and results from FEA analysis on
the April 11 dataset (EXTREME observing conditions) gives maximum pointing
errors of 0.5 arcsec, caused by a large (not normally observed) thermal
gradient across the backing structure which peaked at around 16:00 in the
afternoon following comet observing.

Normally, pointing offsets in both AZ and El are of the order of 0.1 to
0.2 arcsec in our FEA simulations. We can take these corrections into
account and marginally improve the pointing this way. However, pointing
offsets due to thermal gradients in the dish and its supporting structure
are so small that it will be hard, if not impossible, to verify them.

Date    : 09 Jul 1997
From    : Fred Baas

It may interest you to know that as of now, a (yet untested)
version of the FEA controller is installed and running from MWTTEL.
What is does is sampling temperatures across the telescope for 1 minute,
fit a parabola to the temperature induced distortions in the dish surface,
print the resulting DAZ and DEL plus Z-focus offsets, and display a (black
and white for now) image of the surface. The offsets and image are updated
every minute.

This information may be very helpful for you to understand the pointing
behaviour of the telescope. Although the FEA results have not yet been
verified with holography measurements, the trends that it shows should at
least be correct.

A complication might be that the calculated offsets are already partly
(hopefully fully) compensated by the telescope software.




Iain Coulson


Updated: 23 July 1998