ca-forum #11 -- Minutes
date : Friday , 11 August, 1995. 11:00 am.
agenda:
1. DSD - status (ACC monitoring of HOLD) .. (DJM)
2. Tied-Array requirements (CACAL mods) ..
3. Narrabri System updates (procedures) (GB)
4. Data quality observations .. experience so far (MW)
---------------------------------------------------------
5. Reference pointing - update (DJM)
6. subreflector control (DJM)
7. Narrabri recommended frequencies .. S-band in particular
L-band (1344, 2368)
--- held over --------------------------
8. S-band beam pattern (update) .. (mjk)
9. 128 MHz birdies - low pass filter at the samplers (WEW)
-----------------------------------------------------------------
1. Digital Synchronous Demodulators (DSD) status.
The ACC code has been modified to provide a new monitor point: the
exact integration time used by the DSD. (It can vary from cycle
to cycle in mosaic according to the HOLD requirements).
This information is needed if a normalised value (counts/unit time) is
required - it is used to determine if the attenuators need resetting.
The DSD code will be installed at mopra on friday 11/aug, and at
narrabri on wed, 16/aug. Caobs will need to be modified by then.
(The code to use the DSDs to determine Tsys is in place and tested).
1.a Porting the DSD code to Mopra (needed for the mm beam switching)
showed that the two sites had drifted apart - in pntdef and camon,
as a result of SETI and other site-specific activities. The two
sites will be reconciled next week (14/aug ->).
2. Tied-array.
A. Current status ... The prototype is complete and has passed
its tests. The september 4 target date (Narrabri tests) is still realistic.
Boards are being manufactured and should be returned next week.
B. Requirements.
The on-line system calibration will need to set the gains and phases
so that the IFs are correctly phased and of equal gain at the adder.
The option of a phase shift of 90 degrees between X and Y is needed
when circular polarisation is requested.
C. Proposal:
- The observations are controlled by CAOBS. A new scan type -- "Tied-array"
will be required.
- CACAL will have a tied array option to set the phases with either
90 degrees (circular), or zero (linear) offset.
>> M. Wieringa will look into this.
- At a later stage the gain factors will be used to adjust the decision
level at the samplers - the ACC will be given revised target %.
(This is needed to extract the optimum signal/noise from the observations;
it requires some extensive ACC work, so is postponed for the moment.
The priority for this upgrade is set by the variations in Tsys - it
isn't needed if all systems are the same. The current variations
(Tsys) are around 10%).
Unequal gains will also have some impact on the polarisation purity,
>> M. Kesteven will look into the cost of not implementing this.
It is clear that we need to ensure that tied-array specific adjustments
(such as the non-17.3% sampler targets) do not carry over to normal
CA operations. The intention is to use the scantype as
the control mechanism. CAOBS will respond to this. CACAL may provide
several calibration settings (tied-array; normal operation), with
CAOBS choosing the appropriate value according to scantype.
[Update (15/8/95) : "scantype" is currently somewhat overloaded
as a messenger, so for the september tests the onus will fall
entirely on CACAL, with the correlator not requiring a message
from CAOBS. (The modification to CACAL is small - 3 lines of code).
SCHED will be revised shortly to provide a variety of keywords
for needs such as this. When this is done, the burden will be
transferred to CAOBS.]
D. Control of the tied-array machinery.
The correlator will expect a message from CAOBS ("Tied-array").
Thereafter fine tuning (user interaction) will be through a window
on the correlator workstation (Atria).
The correlator uses a dataset in the screened room to control the
hardware. In particular, there is a decision-level control operation
based on the correlator data - this is a cycle-by-cycle interaction.
E. Tied-array monitoring.
The output could be fed back to the correlator, concatenating up
to (approximately) 5 blocks for a grand spectrum.
There is a problem with simultaneous CA operation - at present there
are no free resources. (This simply means that the CA operation will
proceed as before; the S-2 (or pulsar machine) will use the tied-array
data; in two-frequency mode the correlator will likely have no free
resources to provide tied-array monitoring.
This matter is under investigation.
F. MX-mode (not discussed at the meeting, but touched on previously).
It is not out of the question to provide separate phase tracking for
two simultaneous targets. CAOBS would require some reworking; the
tied-array machinery none. Does the need warrant the effort?
G. Impact on the simultaneous CA operation.
The 90 degree phase offset
between X and Y means that the simple addition to get stokes I is not
sensible. Should ATLOD provide a warning? Beyond this there appear
to be no difficulties.
The utility VLBI_WATCH which logs "on-source" times for antenna 6
could profitably be extended for the tied-array operation to cover
all antennas.
>> J. Reynolds has volonteered to look after this.
3. Configuration Control.
Narrabri are reorganising the way system updates/revisions are coordinated.
G.Baines is looking into this and will report on this later - possibly
at a different venue.
4. Data quality observations.
A. experiencve so far. unhappy. N. Killeen's miriad scripts do
provide an automated procedure for the reduction of the observations,
but the observations processed so far have required editing - requiring
a large amount of narrabri time.
B. what is the goal of these observations?
It appears that the purpose of these observations is not clear -
the following is a loose reading of the discussion that followed:
a. The processing should not take more than an hour or so - ambitions
need to be scaled back, or skills enhanced if the time is excessive.
b. The immediate goals should be to provide a check on the system -
reasonable RMS in the blank fields; No sources at the centre of the
blank fields; reasonable dynamic range in the "on-source" fields;
no ghosts or other artefacts in the "off-source" fields. Automatic
editing (courtesy of M, Wieringa) should be applied; its operation
should be logged to provide a measure of the data quality -
N bad points/hour encountered.
c. The edited/cleaned images should be kept on-line for easy retrieval.
d. If possible all the data should be kept in a single archive so that
serious detailed examination remains an option at some later date.
5. (very brief reports ... these issues will be raised at the next
meeting).
A. Reference pointing .. This is not yet stable enough for general
release.
B. Subreflector control - similarly.
C. Recommended frequencies. L --- 1344 MHz. (best from interference;
second best for Tsys).
1472 MHz. better Tsys; worse interference.
S --- 2368 MHz.
C and X as before.
D. Extended frequency coverage.
The currently advertised bands should be marked "guaranteed"; the
extended ranges should be marked as "possible - subject to detailed
examination", or equivalent words.
The attached list is based on the available conversion chains - ie,
it does define an extreme view. RF filters, waveguide cut-offs and
receiver gains will likely make some of these frequencies unusable.
L : 1127 - 1807
S : 2183 - 2863
C : 3118 - 8143
X : 6877 - 11922
E. for the record:
The S-band beam pattern problems are detailed in AT/39.3/052
There is a discussion by B. Thomas of the current views on the
problem, and OMTs in general in AT/39.3/053