Ron and I did some tests March 19th (2003) to check typical move and settling 
times for the GBT, at the request of Lincoln Greenhill.
The results are of general interest to anyone who wants to move frequently 
back and forth between a source and a calibrator or between an on-source 
and off-source position.

The calibrator 3C147 was observed at 9.8 GHz and we moved to the source 
from distances ranging from 10' to 4 degrees in azimuth and 0.5 to 2 deg 
in elevation. The "on-source" position was actually at the half-power point 
of the source, so that we could look at feed-arm vibrations (but that's
another story).

The total move time is how long it takes from when a scan is requested 
to start until the antenna is tracking the source.  The scan coordinator 
gives a countdown, which appears to start about 2 seconds after the scan 
start button is pushed.  After the countdown, the data taking starts. 
The data shows that most of the time, the antenna is not quite on source 
when data taking starts.  The first  amplitudes in a scan are about
60-75% of the on-source value. The amplitude reaches the on-source value
about 5 seconds after the scan starts. The total move times that we 
list are the sum of the countdown time plus 2 seconds, plus the time after 
the scan starts for the amplitude to reach the on-source value 
(usually 3-6 seconds).

Results are as follows:

Distance of move   Total Move time
-----------------  ------------------
 dEL +0.5 deg         18 sec
     +0.5             16
     +1.0             21
     +2.0             28
     -0.5             16

 dAZ +1.5 deg         29 sec
     +0.5             21
     -0.5             20
     -0.5             19
     -0.5             21
     -1.0             26
     -2.0             26
     -4.0             35
     -0.17            12