[evlatests] Results of ea21 Pointing tests (part 2)
Rick Perley
rperley at nrao.edu
Sat Jan 4 12:36:13 EST 2014
In yesterday's circular, I summarized the behavior of ea21, and an
unmodified VLA antenna, in response to the referenced pointing cycle. A
possibly key point to remember is that this cycle runs on a 20 second
duration, and the offsets are between ~3 and 6 arcminutes.
In this report, I summarize the results of utilizing the holography
modes. The results are in some cases rather different than those from
the referenced pointing. A complete explanation is wanting.
There were three SBs made up for the holography tests. For all
three, a pair of cuts were made, one in the vertical and one in the
horizontal. We used an oversampling of 5 (5 points per beam, as defined
by the higher tuning frequency), and made 21 measurements (i.e., to the
2nd null of the higher tuned frequency).
VLA holography does not use a continuous slew -- rather, it uses
fixed stopping points, each of a set duration. The details are given below.
Test 1) Two cuts at L-band. Each stopping point of duration 10
seconds. The stepsize was 4.7 arcminutes.
Test 2 Two cuts at X-band. Duration 10 seconds. Stepsize = 0.8
arcminutes.
Test 3) Two cuts at X-band. Duration 20 seconds per stopping
point. Stepsize = 0.8 arcminutes.
The results are graphically shown in the 24 attached .eps plots.
These have a simple naming convention:
Antenna-BandDur-Dir-Expand.eps
where: Antenna is either 21 (the modified antenna) or 12 (an
unmodified antenna)
Band is either X or L
Dur is either 10 (10 seconds per stopping point) or 20
(20 seconds)
Expand is either 'null' (the full cut) or Z -- a zoom
in of a single step
The data were calibrated to unity flux density. All plots are for
single baselines, with the target (moving) antenna vs. a fixed antenna.
For ea21, the reference antenna was ea28. For ea12, the reference was
ea10. (These are adjacent antennas). There were a total of 9 reference
antennas, so the SNR can be improved to a factor of three, should this
be needed. (To save time, I simply used single baselines).
All plots are in *voltage* units, and (unlike the pointing results
distributed yesterday) high power is up.
Results:
Test 1. L-band, 4.7 arcminute steps, 10 seconds per stopping point.
1-a) 21-L10-Az.eps
This shows the full horizontal (azimuth) cross cut through the beam
at 1465 MHz, from about the peak of the first sidelobe to the other
side. The high points at either end are calibrator scans. The steps
are clean with little sign of overshoot or resonances.
1-b) 21-L10-Az-Z.eps.
A zoom-in of a single step, for the modified antenna. There is no
sign of significant overshoot or 2 Hz resonance.
1-c) 12-L10-Az.eps
The full cut of an unmodified antenna. A significant overshoot
(upwards when moving toward beam center, and downwards when moving away)
is seen.
1-d) 12-L10-Az-Z.eps
A zoom-in of a single step for the azimuth cut. The overshoot is
about 1/3 of a step -- 1.5 arcminutes. The 2 Hz ringing is visible in
the early part of the move, but is lost in the noise shortly after.
1-e) 21-L10-El.eps
The full cut in elevation. There is essentially no overshoot.
1-f) 21-L10-El-Z.eps
A zoom-in of a single step, near half power. There is no sign of
overshoot or 2 Hz ringing.
1-g) 12-L10-El.eps
The full cut in elevation of the unmodified antenna. A significant
overshoot is seen.
1-h) 12-L10-El-Z.eps
A zoom-in of a single step near half-power. An overshoot of about
1.2 arcminute is seen. The 2 Hz oscillation is present (but mostly
masked by the noise). The overshoot settles down within ~ 5 seconds.
Test 2. X-band, 0.8 arcminute steps, 10 seconds per stopping point.
*** The results are are strikingly different than those from
L-band. The only change was the stepsize, so I'm hoping there is an
explanation based on this.***
2-a) 21-X10-Az.eps
This shows the full horizontal (azimuth) cross cut through the beam
at 8236 MHz, from about the peak of the first sidelobe to the other
side. The high points at either end are calibrator scans. In stark
contrast to the L-band cut, here we see a significant overshoot of about
one stepsize -- 0.8 arcminutes
2-b) 21-X10-Az-Z.eps.
A zoom-in of a single step, for the modified antenna. The 2 Hz
oscillation is plainly evident, along with the slower overshoot (which
shows a period of about 8 seconds).
2-c) 12-X10-Az.eps
The full cut of an unmodified antenna. A significant overshoot
(upwards when moving toward beam center, and downwards when moving away)
is also seen here, but is about half that of the modified antenna (i.e.,
about 0.4 arcminutes).
2-d) 12-X10-Az-Z.eps
A zoom-in of a single step for the azimuth cut. The overshoot is
about 1/2 of a step -- 0.4 arcminutes. The 2 Hz ringing is absent here
(!), despite being present for ea21, and for the corresponding L-band cut.
*** Comment: These perplexing results would be much easier to
understand if the antenna identifications were reversed! Note the much
slower rise in the step in this last plot -- looking like an accelerated
and deaccelerated profile -- while the rise for ea21 (21-X10-Az-Z.eps)
is much sharper, as if there was just a step-function impulse. But I
know of no way the antenna labelling could be reversed... ***
2-e) 21-X10-El.eps
The full cut in elevation. A small overshoot is seen.
2-f) 21-X10-El-Z.eps
A zoom-in of a single step, near half power for the modified
antenna. There is a small overshoot of about 0.1 arcminutes. The 2 Hz
ringing is strong and long-lasting.
2-g) 12-X10-El.eps
The full cut in elevation of the unmodified antenna. A significant
overshoot is seen.
2-h) 12-X10-El-Z.eps
A zoom-in of a single step near half-power. An overshoot of about
0.3 arcminute is seen. The 2 Hz oscillation is present (but mostly
masked by the noise). The overshoot settles down within ~ 5 seconds.
Test 3. X-band, 0.8 arcminute steps, 20 seconds per stopping point.
This test was to see whether the 20-second commanded stop has any effect
on the overshoot or ringing. The answer (surprising to me) is 'yes'.
3-a) 21-X20-Az.eps
This shows the full horizontal (azimuth) cross cut through the beam
at 8236 MHz, from about the peak of the first sidelobe to the other
side. The high points at either end are calibrator scans. In stark
contrast to the 10-second X-band cut, the overshoot is very much reduced!
3-b) 21-X20-Az-Z.eps.
A zoom-in of a single step, for the modified antenna. The 2 Hz
oscillation is plainly evident and of the same magnitude as the
10-second step. The overshoot, however, is much reduced, to about 0.15
arcminutes (from 0.8).
3-c) 12-X20-Az.eps
The full horiztonal (azimuty) cut of an unmodified antenna. The
overshoot is evident.
3-d) 12-X20-Az-Z.eps
A zoom-in of a single step for the azimuth cut. The overshoot, and
the absence of ringing, is the same as in the 10-second step experiment.
*** Comment: Same as for the 10-second experiment. ***
3-e) 21-X20-El.eps
The full cut in elevation for the modified antenna. . A small
overshoot is seen.
*** Note: The dropout seen just after 16:51:53 is unrelated to beam
characteristics. A single 10-second dwell point has gone missing --
there is noise, so the correlator is on, but there is no signal. This
dropout has nothing to do with antenna motion. These 'dropouts' were a
common feature in the past -- which I had thought had been diagnosed and
repaired. Apparently not ... ***
3-f) 21-X20-El-Z.eps
A zoom-in of a single horizontal step, near half power for the
modified antenna. There is a small overshoot of about 0.1 arcminutes.
The overshoot is the same as in the 10 second step. But this 20-second
step shows a much smaller 2Hz ringing.
3-g) 12-X20-El.eps
The full cut in elevation of the unmodified antenna. A significant
overshoot is seen, similar to that visible for the 10 second experiment.
3-h) 12-X20-El-Z.eps
A zoom-in of a single step near half-power. An overshoot of about
0.3 arcminute is seen. The 2 Hz oscillation is present (but mostly
masked by the noise). The overshoot settles down within ~ 5 seconds.
These characteristics are the same as in the 10 second experiment.
--------------------------------------------------------------------------------
Clearly, there are some phenomena here which need explanation. If
higher SNR is needed, please let me know, and I'll use the specialized
holography software to combine the baselines to all the reference
antennas. If other experiments, with alternate dwell times and
stepsizes are needed, please inform me. These are easy to set up and
execute.
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