JCMT Pointing Data Acquisition
JCMT Pointing Data Acquisition
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
achieves the same effect : the opening of a logger file ready to accept
parameters describing the position of the antenna.
The subsequent explicit command
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
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
/jac_logs/yyyymmdd/*.pnt in TPOINT
On the VAX icl> log causes a similar dump of antenna
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
The processing of pointing.log produces a file called
ptyymmdd_hh.dat which is also copied to the pointing archive,
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
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 :
Continuous Tracking Experiments
The heterodyne FIVEPOINT command
Five measures of the continuum flux, one on-source, 4 off-source in
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)
- 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
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
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
There are occasions when we wish to 'track' a source for
a considerable time, say > 1 hour. One such reason was to
monitor the effect of the central bearing
race defect upon azimuth
pointing residuals. Such an experiment might comprise a sequence of
regular pointings as decribed above. But, with automation of some of
these steps, there are better ways to do this :
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
- 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 :
Anything longer than 10s per point will produce poor resolution.
a 6s/pt FIVEPOINT takes ~100s to complete, while
a 10s/pt FIVEPOINT takes ~125s to complete.
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 :
- 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
HARP tracking - - - 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