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ATNF ATUC MEMORANDUM

To: ATUC
From: Bob Sault
Date: 29 May 2004
Subject Millimetre systems on the ATCA


Performance of the 12mm system
The 12mm system is now well integrated into the ATCA system, with
performance being very good. System temperatures on typical winter days are
50-60K, and phase stability to 6km baselines is often not an issue.
Operation of the 12mm system is now routine.

The system temperature measures of the 12mm receivers are now calibrated,
and will need to be recalibrated after the 3mm installation period. 1934-
638 has been shown to be "constant" over more than a year. The spectrum of
1934-638 has been determined and installed into the offline calibration
software, as has an opacity correction. Flux measurements have been
completed on most 12mm calibrators, which are now reflected in the
calibrator database.

The weather does impact the 12mm system more than at centimetre
wavelengths. The 12mm observations bracketing summer were on 4 December and
15 February. Both of these observations were effectively abandoned because
of poor weather. In general 12mm observations should be avoided from mid
November to late March. In addition, observing at 12mm in poor weather is a
challenge for the ATCA observing system. In poor weather, the potentially
large atmospheric contribution to system temperature at 12mm makes
achieving a good power level balance more difficult.

There are some lingering issues:
. Water can condense on the 12mm dewar window. To prevent this, the 12mm
horns are now sealed and filled with dry nitrogen, but there have been
instances where this has failed. A longer term solution awaits
resources after the completion of the 3mm systems.
. There are some poorly understood issues in the 24.0-24.5 GHz region,
where most (all?) antennas can sometimes show inexplicably high system
temperatures. This is not always repeatable.
. An improvement in the attenuator system will be installed during the
upcoming shutdown. This will resolve a compression problem in some
systems.
. The frequency range from 25.1 to 26 GHz is not yet available. We hope
to resolve this in the 3mm installation shutdown.

Installation of the 3mm systems
The next major step in the MNRF high frequency upgrade started on 15 May,
with two millimetre receivers warmed, removed from the antennas, and
transported back to Marsfield to be retrofitted.

The main shutdown for the 3mm installation is 15 June-2 July. By the end of
the shutdown, all 12mm systems should be available, and the 3mm cryogenic
components should be installed and cold. However the 3mm LO/downconversion
system will be missing on some antennas. The current estimate is that all
five 3mm systems will be available by the time the bulk of the 3mm
observing starts on 23 July (although this slipped timetable is looking
tight). Three observing slots are affected by the predicted late delivery:
two of these will be rescheduled, and one will proceed with the available
3mm system.

More information on the progress of the installation will be available by
the time of the ATUC meeting.

Advice sought
ATUC's advice is sought on the following issues/questions:
. Are there areas where ATUC has suggestions for further refinement of
the 12mm systems.
.