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ATNF ATUC Memorandum

To: ATUC
From: Bob Sault
Date: 22 November 2004
Subject 3mm systems on the ATCA



Summary of 2004
The highlight of the year at the ATCA has been the installation of the 3mm
systems on 5 antennas. The first 3 new systems were first used for
observing on 10 September, and five systems being available on 20
September. This was appreciably later than the originally scheduled 6 July
start to the 3mm season, and was caused by some substantial engineering
challenges in the local oscillator system near the end of the project. The
new systems were brought into operation as quickly as possible, with
limited commissioning time. In some instances, the arrival of the hardware
at Narrabri and first use in scheduled observations was on the same day.
Scheduled 3-mm observing continued on the ATCA up until 15 October when the
"3-mm season" was declared closed. This is dictated by weather: the summer
weather pattern was already apparent by that stage. Almost all projects
from the time of the installation up until the close of the season were 3mm
ones. Scheduled 12-mm observing continued until 28 October.
Performance of the systems was generally good: system temperatures of 200-
300K were readily attainable, and the system sensitivity was 22-25 Jy/K.
With the limited time between delivery and the end of the season,
commissioning tests were being performed in tandem with scheduled
observing. Not surprisingly, there were a number of teething problems with
the new systems, which were addressed as promptly as possible. A number of
the lower priority or more long-term issues remain with us.
The late delivery of the 3mm systems required substantial rescheduling at
short notice, which was a disruptive process for all concerned. We thank
all those observers who graciously took the need to reschedule in their
stride, and assisted us despite inconvenience to themselves.
Although most projects were rescheduled, this was less than optimal for
some projects. Additionally only three or four antennas were available to
the earliest projects to use the 3-mm system. Expressed as a fraction of
total time in the term, 4.6% of projects originally scheduled needed to be
cancelled and replaced with other projects, 1.2% observed with a three-
antenna system, and 2.6% observed with a four-antenna system. The majority
of projects that were cancelled were for 12-mm observations: the
installation of the 3-mm system disabled the 12-mm system for a period.


Prospects for 2005
The are many issues that will be addressed over the summer (where possible)
with the aim of providing a polished system at the start of the 2005
millimetre season. An appendix to this paper lists some of these polishing
tasks.
Some significant issues include:
. The antenna efficiency of CA01 is substantially less than other
antennas, and the beam shape is poor. Although good progress has been
made to understanding this problem, there is some way to go. We have
contracted a photogrammetric survey of the subreflector and main dish
of CA01 to be performed on 15-16 December. This should give a
conclusive answer. Until this problem is better understood, the
subreflector actuation project is on hold.
. The system temperature of CA04 Y polarisation in the band 86-90 GHz is
significantly poorer than expected. This continues to be investigated.
. Above 102.3 GHz, system sensitivity abruptly degrades by about a
factor of two. The cause is well understood, and options to resolve it
are being assessed. It is unlikely that this will be resolved by the
2005 season.
. Although the polishing tasks are expected to give a better system, the
"headline" capabilities of the system in 2005 will be the same as in
2004. In particular, the frequency range offered remains essentially
unchanged (83.5-106 GHz), and noise diodes will not be installed on
the antennas (note that polarimetric observations require the noise
diodes). High frequency extension (106-115 GHz) and noise diode
calibration will wait at least until their successful demonstration at
Mopra.
One expected addition capability in 2005 is the provision of a second pair
of 16 MHz filters. This will make dual spectral line observation possible
for some set of projects.
At an operational level, we have removed the requirement that 3mm proposals
must include a team member with 3mm ATCA experience. At the same time,
proposals to use arrays more extended than H214 have been asked to provide
a specific justification on technical feasibility and scientific value of
using an "extended" array.
Advice sought
. ATUC is invited to consider the list of polishing tasks, and to give
feedback on issues that will improve system performance and scientific
usefulness of the 3mm systems. Prioritisation of items would be
useful.
. More broadly, ATUC is invited to make suggestions on how to best
exploit the 3mm systems and to maximise the quality of science
produced by them.

Appendix: 3mm polishing tasks

|Task |
| |
|Antenna optics |
|CA01 subreflector |
|CA02-CA05 subreflector |
|Focus |
| |
|Subreflector actuation |
|Project review |
| |
|Receivers |
|Investigate possible compression problems |
| |
|Correlator and IF Filters |
|2nd pair of 16 MHz filters |
|New correlator configurations |
| |
|System temperature and sensitivity |
|Upper sideband characterisation |
|Hot/cold system temperature measurements, and assess discrepancy |
|between Epping and on-sky system temperature values. |
|Evaluate accuracy/consistency of paddle Tsys |
|Characterise CA04-B,CA04-D performance near 88GHz |
|Investigate "drop-outs" in "Gain" plot of 27-Sep |
|Investigate oscillatory gain of CA05 |
|Gain/elevation curves |
|Bandpass stability |
| |
|System phase and baselines |
|Phase noise in LO system |
|Investigate sharp amplitude drop-outs in poor weather |
|Baseline calibration software |
|Baseline calibration and frequency dependence |
| |
|Pointing |
|Improved pointing model at 3mm |
|Assess the accuracy of reference pointing as a function of |
|distance to the pointing calibrator |
|Comparison between 3mm, 12mm and 3cm pointing |
|Evaluate pointing in windy conditions |
| |
|On-line software |
|Pointing tracks planets |
|"CACAL" table differentiation of settings includes L86 frequency |
|Improved CATAG |
| |
|Off-line software |
|Calibrator duty cycle tool (web) |
|Velocity to frequency calculator (web) |
|Miscellaneous improvements to atlod, uvsplit, gpboot, uvplt |
|(Miriad) |
|Improved algorithm for applying Tsys in atfix (Miriad) |
|Baseline-based calibration solutions to account for atmospheric |
|decorrelation (Miriad) |
| |
|Atmospheric monitoring |
|Evaluate seeing monitor data - compare models with measurements |
|Evaluate the WVR as an opacity meter |
| |
|Calibrators |
|Improve calibrator grid |
|Improved characterisation of calibrators at 3mm |
|Flux monitoring |
| |
|Bigger questions |
|Turret roller precision |
|Phase switching for sideband rejection |
|Subreflector actuation |
|Finer frequency setting |
|Front-end power equalisation |