ca-forum #17

date: wednesday, july 24, 1996. 11: - 12:30

A. LO status - R. Beresford

A number of changes have recently been completed - these have raised the reliability of the LO system very significantly.

  1. The directional couplers which used to be installed in the station post have now been moved to the ELVEE rack within the air-conditioned pedestal room. There is a minor penalty - the LO power now has an additional length of coaxial cable added to its path. The benefit however is great - the directional coupler is kept in a thermally stable environment.
  2. The phase-lock circuit in antennas CA01 to CA05 is now essentially the same as in antenna CA06; that is, the 50 KHz signal is derived from the local crystal oscillator, rather than from the central site. The experience with CA06 has shown that the system is sufficiently stable as to make the remote 50 KHz unnecessary; the experience with the other antennas has shown that the 50 KHz poses many problems.
  3. The L42 module in the CA05 has been carefully reworked and realigned. This has resulted in a module which is distinctly less senstitive to the airconditioning cycling

    The performance since the completion of these changes is impressive, with no reported dropouts.

    A full description of the LO system and the proposed upgrade is now available on the web See : Outline [URL : http://ningadhun/~rberesfo/LOSYSTEM.htm]

    B. Tied array - update; W. Wilson

    The recent VBLI run on some OH sources will provide the information needed to label definitively the Left and Right hand senses of circular polarisation. John Reynolds will provide this information to the Narrabri group. The purity of the polarisation is thought to be in the "few %" range; this question will be pursued by JER and AKT. JER recommends that linear polarisation be used for high precision polarisation work. AKT raised the question of the circular polarisation forming machinery that is planned for Mopra: this is to be the single-antenna version of the tied-array, and Tasso's concern was the quality of the gain balance. (At narrabri we use the interferometer products). The noise diode could possibly be used, but it would need to be calibrated (as to balance, as well as to degrees K). The concensus seemed to be that it is unlikely to be any worse than the current analog (hybrid) scheme.

    C. DQO - J. Higdon/ R. Wark

    1. The DQO summaries are up to date.

    2. The blank field near 1934-638 has been further analysed by JLH. Near-thermal noise has been reached at 11, 6 and 4 cm, but not at 21 cm. (This is for the 8 field average). Jim has processed the data with great care, and concludes that the problem is not interference; The next line to pursue is to look at the circular polarisation for clues to the problem. There was some concern that ionospheric faraday rotation might be an issue. The blank field near 0823-500 will also be processed. The radio images are available on the web : see 20cm , 13cm and 3.6cm . (Warning: these are big images).

    D. Attenuator problems in mosaicing/SAC - M. Wieringa

    M.W. has recently completed a lengthy mosaicing run without encountering the attenuator instabilities seen by Bob Sault, which is difficult. However, MW did find a problem in CAOBS with the attenuator control, and this has been fixed (as of monday, 22/july).

    E. Antenna 4 polarisation leakage

    This problem (described in ca-forum#16) disappeared when the receiver connectors were cleaned and tightened. The receiver group (Mal Sinclair) was sceptical that the connectors could actually have been responsible, so further observations were requested. The most recent (monday, 22/july) showed no sigh of the problem.

    F. Subreflector Control - D. McKay

    The following summarises the hardware and software status of the remote control of the subreflectors:

    1. Hardware completed and tested.
    2. Full manual control available from the pedestal room.
    3. The subreflectors are now set to the optimum position for X-band (1990 determination).
    4. Monitor points have been incorporated into PNTDEF.
    5. A page exists for CAMON (called /focus).
    6. Monitor points hooked up, but not calibrated, nor have limits been set. There is still some wiring to be done in the LV rack. 1/2 day/antenna.
    7. Need to make a permanent LVDT=0 calibration mark on the antenna/focus structure.
    8. Need to complete CASUB (subreflector control software).
    9. Need to calibrate the gain-focus relationship as a function of frequency.

    Ron estimates 10 weeks before his work is complete. No estimate on the s/w at this stage.

    Problems pending

    These are matters which have been signalled as needing attention, and mostly are being attended to. Some will be discussed at a later forum.

    1. DUTC ... (wew/mw)
    2. Planetary data -> rpfits (wew/mw)
    3. Pulsar ephemeris data -> RPFITS (wew)
    4. C/X beam shapes and the pointing error algorithm -- (RS)
    5. SAC - need for revised machinery? (MJK)
    6. Software phase-switching (RS)
    7. Pointing discontinuities requiring ACC reset
    8. Tied array calibration. It would be possible to perform on-line calibration of the tied-array system when the target is a strong source - a "continuous CACAL", in effect.

    Next meeting : Tuesday, September 10 at 11:00 am.

    ATHELP is a new web page designed to assist observers with on- and off-line problems. {URL : http://www.atnf.csiro.au/observers/athelp.html]

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