Antenna Parking a possible influence on central bearing damage

Date: Thu, 9 Sep 1999 21:27:31 -1000 (HST)
From: Ian Pain 

Hi All

Firmin showed me the code for the commonly used 'park' routine. 

This selects a random azimuth between 82 and 102 degrees, using a
truncation of the Julian date as a seed for a random number generator.
Whilst it is likely that the telescope is often parked at a similar time
(ie. date fraction : say 10 am +/- an hour on average) we assume for the
moment that the numerical seed generator is highly non-linear and will
still produce a true random spread.

The number is however later truncated as an integer, hence provides only
21 discrete park positions which will be highly repeatable (arc.sec
precision) rather than a continuous spread of positions.

Looking at the emprical correction for the central bearing, only one
glitch fell in the 82-102 regime, and is infact precisely centered at 93.0
degrees. Indeed, no other glitch position (other than 273.0, which is 93.0
+ 180.0) occurs precisely at an integer azimuth as far as we can
determine from the limit of the tracking data.

Before getting too excited I need to check the transport strap position in
case that is also 93.0 degrees, but at the moment I would conclude that
the damage site (earthquake, vibration or whatever) _did_ occur when
'parked' at 93.0 degrees. It is not clear however, whether this was a
coincidental (ie earthquake or ESD hit whilst parked) or a causal link (eg
parking with drives on for extended period plus servo chatter created
wear). 

SKF are currently arranging for a full circle high sensitivity tracing of
the damage profiles, and if this reveal other features at a precise 1
degree spacing it would help point at 'park' damage. In parrallel, I will
check transport strap position and will probably also dig a bit deeper
into the random generator process and see if it is truly seed (time)
independent.

Best Regards

  Ian Pain

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