CA-forum #2
30 June 1994
1. Mosaicing follow-up:
a. Digital Sync. demods. K. Cambridge will now
take on this task. 6 units are already available; they
need modifying to provide the 3 bandpass inputs (G.Grave's
design).
first prototype available for the august shutdown;
the full set for the december shutdown.
** Several further changes are needed:
The ACC will need to implement the "start/stop integration"
events;
The event lines will need to be installed - between the
Beresford junction box and the conversion rack. Ron Beresford
undertakes to equip the prototype antenna.
The question of getting the HOLD time out to the ACC has to be
resolved. CAOBS will need to define an ACC message, to be sent
prior to each cycle. The question of computing the HOLD also
needs to be addressed.
b. Strobing of the conversion rack functions
(the "Digital Interface"). G. Graves will look to providing
a firm estimate of the time required.
c. ACC revised closed loop slewing and the drive-time
algorithm. Dave McC. has made some progress, but a further
round is planned for the august shutdown.
The sched drive time routine has been updated
to include both acceleration and decelleration, but
it needs dave's algorithm before it is satisfactory.
d. The planned tests of the samplers (how do poor
sampler statistics relate to fringe amplitudes) were not
successful - the ACC processing of the setting requests appears
to conflict with the servoing of the sampler levels. Dave McC.
will look into this question during the august shutdown; further
tests by NEBK/MJK will follow.
Testing this question in the lab. seems difficult as WEW
doesn't have a partially correlated signal source.
2. POINTING - Narrabri.
Report AT/31.6.7/019 ... "Pointing Calibration of Compact Array
Antennas - Progress Report" by D.McConnell & R.Wark provides some
background information.
current performance:
~5"/axis pre-fit rms immediately after a pointing check;
~10" rms on the EL axis 12 hours later, ~5" AZ axis.
The change is thought to be primarily due to the thermal distortions
(After each reconfiguration a 15-20 source pointing schedule
is run. The data are analysed for the 4 parameters which are
likely to change with the antenna move : tilt of the antenna AZ axis;
zero point of the az and elevation encoders.
A subsequent 5-10 source schedule is run after the update of the
pointing model parameters).
The pointing models were stable during the extended one-configuration
session last year (Oct - jan), except for the encoder episode in
antenna 1.
"Pointing errors are said to be worse for Northern sources".
-- The observatory staff would like to see the evidence of this
so they can plan a suitable response -- perhaps modifying the
selection of sources for the pointing check.
---> Tasso/John Reynolds will look to the data.
Antenna 6 is consistently the worst performer. Even though
it is fixed, its pointing model has had to be modified from time
to time.
Reference Pointing and associated issues.
A. It is certain that our pointing machinery will always
have some level of error.
B. Reference pointing is a procedure which corrects for local
defects in the pointing machinery - provided that errors
in measuring the "reference pointing" are less than the
errors of the model.
C. Reference pointing is not without cost - the poorer
the pointing model, the shorter the validity range (in time,
az,el) of the reference pointing correction.
D. Reference pointing would be beneficial right now.
E. The pointing model would be improved if the local weather
data were included. We currently assume 15C; 1003 hPa; 9 hPa water.
F. IRAM experience with tiltmeters is positive: they do improve
the pointing machinery. The AT antennas, although not having
had the benefit of a proper thermal analysis, nonetheless
have a clean, simple structure. Tiltmeters would probably help.
G. Future developments?
1. Referencing the reflector surface directly to an external
frame, as in the GBT, is very attractive. But likely very
expensive for the AT .. with 36 stations.
Use Parkes as a test?
2. Ring gyros .. thought to be expensive, but decreasingly
so - should be followed.
(Both these schemes probably pose some interesting questions
for the servo system -- at least the 2-encoder scheme maps
simply to the drive machinery).
--------------------------- Specifics -----------------
1. Reference pointing. Dave McConnell will implement this
in a 2-stage operation:
i. A "manual, test version" similar to the EARLY operation.
A second task would be activated to cooperate with CAOBS to process
a pointing observation, and offer to update the pointing parameters.
ii. An automatic version, triggered by a sched request.
To be resolved:
a. Who decides whether the "reference pointing" parameters
or the "official" parameters apply - ie, how is the validity
range defined? (Do the reference pointing parameters expire
with change of project? after 1 hour? etc).
b. Is the current pointing procedure optimum?
(all antennas follow a 5 point pattern).
2. Weather data: Dave McC. will modify ARRAY.
It will dispatch the latest weather data out to the antennas at
regular intervals (~ 1/2 hour). At this stage the main
problem is the data: the weather station has yet to recover
fully from last year's lightning strike ... the temperature
probes are too close to the building.
3. Tiltmeters - on hold till the reference pointing issue
resolved.
4. Antenna 6 - The pointing data from the past 18 months
should be reprocessed to a common setting to get some measure
of ther problem. (mjk/rw).
------------------------------------------------------------
3. Pointing MOPRA
Current performance : no better than 10"/axis, long term.
The pointing data are collected from a 10 pt pattern
(5 pt, az; 5 in elevation; on source, 1/2 power either side;
3 beamwidths either side for background).
A single dish, with modest collecting area, and no beam switching,
has its problems - most particularly with any background
gradients.
The measurement rms in the latest series is about 5"/axis,
(each source is observed twice), but the post-fit rms to
the model is 10"/axis. But at least the solution is stable:
the data in june has all been consistent with the same pointing
model.
The scatter, particularly in the azimuth range
180-330 is poor.
Robina/MJK will look to some further observations to
clarify this problem - we should be able to do better.
POINT will be modified to cooperate with the correlator for
spectral sources - Methanol (12 GHz) and SiO. This will
at least temper the background difficulties.