Minutes, ca-forum #10
date: thursday 29 june, 1995; 11:00 -> 12:30
agenda:
1. DSD (WEW, ML)
-> changes to caobs to make use thereof.
2. System modifications and tests
(When/how does a task get promoted to at$run). (mjk)
3. Data Quality observations (NEBK)
4. Mosaicing and environments (mjk)
5. Ref. pointing (DMcK)
6. S-band poln. (NEBK, mjk)
7. Band definitions - eg, observations below 1.2 GHz
8. 12 GHz Holography (mjk)
-> Need for remote focus control - at least manual in control room, with
readout.
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1. Digital Synchronous Demodulators.
The unit has been modified to provide separate pre-scalers for the
ON and OFF; this seems to have cured the earlier noise problems.
Tests now show the digital units superior to the analogue units
by the expected factor of two or so.
New problem: the ACC prescaler factors need to be reduced by a factor of 4.
** Dave McC will attend to this next week. (3-7 july).
CAOBS modifications:
The routine to compute Tsys needs to be modified to read the DSDs.
CAOBS will support either mode ... user choice via SET DSD or SET ANALOGUE
in CAOBS, with DSD. The choice will be persistent,
so that CAOBS will start up with the setting it had on exit. The choice
will be visible in the SHOW TCAL page. An alternative might be to
reset to DSD whenever the project ID changes.
(should there be a "reset to factory defaults" switch in CAOBS ? )
** MJK will look to this by mid-week.
2. System modifications.
We should formalise the process of modifying and checking out on-line
software. The narrabri group will look into this. Discussed briefly
was the idea that a modification should be preceded by a
note describing the details along with a list of check-out tests.
This would be sent to Dave Mc.C.
3. Data Quality Observations.
NEBK has produced a set of Miriad scripts to process the observations.
These were checked out last week. The procedure is not entirely
automatic .. any data editing will need to be done by hand.
Three fields are processed in each RF band - 1934-638 (or 0823-500)
on boresight; offset by a few synthesized beams; and a near by empty field.
First impressions:
- dynamic ranges of 5000 - 10000 were easily achieved;
- but low level artefacts were present, particularly at L-band.
- 1934 has some weak companions at L-band (~10-20 mJy, at 900").
The plan is to have a folder which will accumulate the results.
At this stage it is not clear what form the archiving should take - ie,
which plots/figures/tables should be kept on hand for future reference;
** Narrabri will experiment over the next few data quality observations to
get a feel for the useful items.
Additional point: given that many potential calibrators, including
1934-638, give trouble at L-band, we should look to producing a database
of these calibrators. The analysis tasks (eg, Miriad) would be able to
access this so as to render these sources useful as calibrators.
** JER to look to this.
4. Mosaicing.
MJK made some small changes to the environment machinery to address
the "first field" problem with the CenA mosaic. The tests were inconclusive.
There are also some reports that the machinery now starts the pattern before
the antennas are on-source. Some checks are desirable.
The scheme to allocate a different environment to each field in the
mosaic is still limited to 20 fields (after which the environments
are re-cycled). The advertised limit (128) is on hold until the
narrabri group are able to do the major re-compilation of all the software
that uses the global common. This may not occur before august.
5. Reference pointing.
This is now available to friendly experts.
The scheme is invoked by setting the scantype to RPOINT.
The scheme is still evolving -
- it needs a check on the reasonableness of the solution, and
the ability to reject bad solutions.
- the validity range (in time/azimuth/elevation) needs to be
explored.
- user control over the implementation: can the reference solution
be accepted/rejected on a scan-by-scan basis? Should the solution
automatically be cleared with change in project ID ? (This is not
relevant if the user must explicitly request the reference solution
with each scan).
** DMcKay will look to these matters.
6. S-band polarisation problems.
The problem was outlined briefly last forum. The results are now a bit
clearer; the consequences for polarisation work have still to be resolved.
- At 2400 MHz we find that the beam patterns are elliptical with an axial
ratio (minor axis/major axis) of 0.95 (X) and 0.91 (Y); the major
axis (X) is orthogonal to the (Y) major axis; there is also
some suggestion of a small squint between the beams.
The S-band system is not focussed; indeed, all observations are made
with the subreflector in the optimum C/X position. Lowering the
subreflector to be as close to the S-band focus as possible improved
the beam shape (much better defined sidelobes), but did not reduce
the minor:major axial ratio.
- The Mopra SETI feed showed a similar effect, although the squint was
more pronounced.
Polarisation work on extended objects at S-band are therefore
problematic at present.
6A. Remote (automatic) control of the focus setting.
The hardware is ready; the software requires a modest amount of
work (1-2 days). What is lacking is an understanding of the options
and the consequences:
- do we let CAOBS choose the optimum position?
- do we track with elevation?
- do we simply provide a stand-alone task (a_test ?) for experts?
Some questions:
- does the focus setting change with elevation?
- does the beam shape vary significantly with focus setting?
7. RF. Band definitions.
In response to requests from J.Reynolds and Lister Staveley-Smith
the L-band lower limit was set to 1170 MHz.
Observations of 1934-638 showed no degradation in performance.
Some scope remains for further extension, and could be tested on
request.
8. 12 GHz holography.
Antenna 6 was surveyed using the Optus B-1 satellite (12.75 GHz).
The surface has an rms of ~0.35mm; the system was badly defocussed
(1.5cm movement of the subreflector is required). a lateral
subreflector shift of ~0.5 cm may be required. Further observations are
required.
The report is numbered AT/39.3/051
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Next meeting:
Friday, July 21, at 11:00.
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