JCMT Pointing Data Acquisition

CONTENTS

• The LOGGER task
  • The equivalent process on the VAX
• Observational considerations
• Discrete pointing observations
  • Pointing on Extended sources (NEW)
  • The heterodyne FIVEPOINT command
  • Heterodyne spectral line pointing
  • The SCUBA POINTING command
• Continuous Tracking Experiments
  • Heterodyne tracking using fives
  • SCUBA tracking using map16.t

The LOGGER task
The normal loading of the JCMT telescope control software on the VAX computer includes an implicit loading of the LOGGER task. Explicitly issuing the command

icl> logger_configure

achieves the same effect : the opening of a logger file ready to accept parameters describing the position of the antenna. The subsequent explicit command

icl> log

records values of those parameters at that time. Acceptance of the results of a POINTING measurement using SCUBA issues an implicit log command. The logger file is closed with an explicit

icl> logger_close

or whenever the project is changed (by icl> project) or whenever the software is run down.

The pointing data files are stored on JCMT computers as ASCII files called /jac_logs/yyyymmdd/*.pnt in TPOINT input format variant 1.

• On the VAX icl> log causes a similar dump of antenna parameters into the binary file [jcmtuser.observe.yyyymmdd]pointing.log. This is automatically processed by the VAX batch job AUTO_POINT which resubmits itself to the mwtsrv_batch queue each hour. Such files may also be reduced at any other time by OPERATOR using the PAU command procedure. The processing of pointing.log produces a file called ptyymmdd_hh.dat which is also copied to the pointing archive, jcmt_seed:[pointing.runs].

Observational considerations
The icl> log command is normally issued once the telescope is determined to be pointing at a target of known coordinates. Analysis of the variations of (daz,del) with az & el will then reveal deficiencies in the 7-parameter model of the antenna pointing, the current values of which are stored in /jac_sw/itsroot/src/tcs/thi/THI.tide.

The pointing takes a finite amount of time to complete, and the delay between the mid-point of the experiment and the logging of the antenna parameters may need to be accounted for.

Discrete Pointing Observations
There are different methods for pointing with the heterodyne receivers and with SCUBA :

• The heterodyne FIVEPOINT command
  Five measures of the continuum flux, one on-source, 4 off-source in the cardinal directions by a distance equal to the half-beam-width, should enable the determination of the centroid of the flux - the location of the source :
  • configure the FE (front-end) as one of the heterodyne receivers,
  • configure the BE as the CBE (continuum back-end)
  • set the lockin phase according to the FE and the sensistivity so as to avoid saturation.
  • beam-switch with a chop of 60" in azimuth at the nominal (7.8125Hz) frequency.
  • submit the icl> fivepoint command. 10s per point, once, is adequate for bright sources like the planets, 3c273, but as many as 4 such cycles may be necessary for fainter (1Jy) sources.
  • When completed and the result presented, you are asked to examine the display and accept or reject the result as appropriate.
  • If the result is acceptable, an icl> log command should then be issued to complete the experiment.

  If the initial pointing of the telescope is poor, the centroid determination may be rather crude and some iteration may be necessary. Note that S/N of ~10 on the central point and ~5 on each of the 4 off-center points is needed to reduce centroid fitting errors below 1arcsec.

• Heterodyne spectral line pointing
  Compact sources with weak continuum fluxes but strong submillimetre emission lines may also be used as pointing sources. These are listed in the pointing catalog . Spectra taken on-source then off-source by a half-beam-width should show a diminution in observed line strength, much as continuum line strength diminishes when going off-source of continuum sources. Analysis of 5 spectra taken in the same pattern as for the continuum case above then allows a determination of the centroid as before. Configure with the DAS as BE, and execute a FIVEPOINT command.
  For full details see Henry Matthews' page on this topic.

• The SCUBA POINTING command
  The relevant command to add to the SCUBA queue is, e.g.

  addback pointing.t source_name=3c273 n_integrations=3

  Really bright sources, like the planets and bright blazars may only need n_integrations=2, while it may be well worthwhile pointing on faint (0.3Jy) blazars for n_integrations=12 even : this will still only take 12*9*2 seconds = 3.6minutes. After the display you should again accept or reject the result. Acceptance automatically generates the icl> log command in this case.

  The analysis of POINTING data will benefit from on-line REMSKY , and the errors due to centroid fitting will be reduced below 1arcsec if the S/N of the data is more than ~10 (see a similar plot of pointing arruracy -vs- S/N from Nobeyama).

• Heterodyne tracking using fives - OLD - disregard
  A special icl procedure called fives.icl is available to do repeated continuum FIVEPOINTs. It does a FIVEPOINT, automatically accepts the result, and loops to another FIVEPOINT, so the source must be bright enough, and the initial pointing of the telescope good enough, to ensure meaningful results from each cycle in the loop.

  The duration of each FIVEPOINT, i.e. the integration time per point, and the number of FIVEPOINTs in the loop are controllable by the OPERATOR. To perform such an experiment do the following :

  • Point and focus in advance on something bright - remember that if the experiment is lasting an hour or so that you want the setup to remain stable throughout.
  • Issue the icl source command to go to your target - hopefully something bright.
  • icl> load [jcmtuser.observe.inc]fives
  • icl> fives
  • You are asked for the 2 parameters. Mars may be done with an integration time of as little as 6 seconds, but further efficiency are limited by the overheads :
    • a 6s/pt FIVEPOINT takes ~100s to complete, while
    • a 10s/pt FIVEPOINT takes ~125s to complete.
    Anything longer than 10s per point will produce poor resolution.
    You can set the number of cycles in the loop to match the expected elapsed time, or, if you're unsure of the end conditions, you can set a smaller number and repeat immediately upon completion, or a larger number and ^C out when necessary.
  • The sequence executes, and you can monitor the progress from the output line that looks like
            3 of 40,
    where, in this example, the number of cycles requested is 40.

• SCUBA tracking using map16.t - OLD - disregard
  Each integration of a SCUBA map may be analysed for the centroid of any image it contains; a map16.t being fully sampled at 850microns wavelength. The method of tracking with SCUBA is then to do a map16.t for as long as necessary to cover the time or azimuth demands of the particular experiment. The time resolution is then 32s. A command such as :

  addback map16.t source_name=3c273 n_integrations=100

  will run for approximately 50 minutes. Reduction of the map is achieved by issuing the following commands from a unix session in Hilo :

  • surf
  • kappa
  • source /home/imc/scuba/pointing/track.com

  The output is a file structured like the output of the LOGGER task on the summit Vaxes, so is ready for handling by pointing software.

• HARP tracking - - NEW - 20070711

  Although the characterization of these maps is yet to be determined, we are able to acquire and analyze fully sampled HARP spectral-line maps of line-sources like CRL2688 for centroids as we did for SCUBA continuum maps above; following the centroid yields the pointing residuals. We shall schedule checks of the race defect shortly.

  Analysis of these maps is described here.