- Tue 31st August 7pm - 01:30am. Do preliminary tests of different scanning strategies, astigmatism correction using Active Surface.
- Thu 2nd/Mon 6th September 24 hours (flexibly scheduled). Do coma/astigmatism measurement at night time, X-band pointing with
new model during the day.
- Thu 23rd/Sun 26th September 24 hours (flexibly scheduled). Confirm coma/astigmatism using new focus tracking and pointing
models during night, X-band pointing with new focus-tracking/pointing model during the day.
The intent of this page is to summarize plans/results for measuring Coma and Astigmatism using simple cross-scans executed
at K-band (20GHz).
By displacing the subreflector in Xs (elevation) and Zs (cross-elevation) we introduce coma. We find the position corresponding
to minimum coma by performing a series of elevation (or cross-elevation) scans as a function of X (or Z) subreflector displacement,
and choosing the value of X (Z) which has mimimum FWHM. Coma can then be corrected by displacing the subreflector in X (Z)
appropriately.
By displacing the subreflector in Ys (axial focus) we can measure the FWHM in each of elevation and cross-elevation, and determine
the best focus position (minimum FWHM) for each co-ordinate. The amount of astigmatism present is related to the difference
betwween the two best focus positions. Astigmatism can then be corrected using the active surface.
Note, scanning in elevation and cross-elevation only measures astigmatism in the elevation/cross-elevation axes. Another
component of astigmatism may be present, rotated by 45 degrees. We should check for this at some point.
- Dynamic corrections should be turned off. Over the timescale of a series of measurements, corrections should be negligble,
and having them off removes one degree of complexity which we might get wrong.
- We should perform the coma measurements symmetrically about the expected Xs displacement, not around 0 Xs displacement,
so that we sample it well.
- We should do a careful standard peak and focus prior to each set of coma/astigmatism measurements to ensure we are starting
from the nominal position.
- Since we are not particularly concerned about absolute pointing or hysteresis, we should only do single "forward" scans in
each axis. This will allow us to double the number of independent coma/astigmatism measurements we will make.
- These is evidence that especially azimuth scans occasionally suffer from "feed-arm waggle". We should optimize our
Scanning Strategies (see below) before starting the main experiment.
Currently, we spend 2 secs going through the FWHM of the source, allow ~15 secs for the antenna to "settle" before getting on
source, and so perform 30 second scans total. At K and Q-band, "feed-arm waggle" is sometimes present. Whether this is seen
appears to depend critically on the trajectory the antenna needs to take to arrive on-source as requested. Hence small changes
in scan velocity (e.g. due to varying sidereal rate as a function of elevation) and/or start position (e.g. changing scan length)
can cause waggle to come and go. The antenna has a new paramteter "settling time", which we can now use to allow settling at
the start of the scan, without the corresponding overhead at the end of the scan. Nominal beam FWHM is ~0.6'. If we scan much
less than +/- 2', we won't get any baseline for large coma value.
We should try some alternative scan strategies, I propose standard, longer start delay, and slower scanning, as follows:
- Our standard strategy: +/- 4.5' scan, 0.3'/sec, 2 secs through FWHM, 30 seconds total scan time.
- Twenty second initial settling time, +/- 2' scan, 0.4'/sec, 1.5 secs through FWHM, 30 seconds total scan time.
- Ten second initial settling time, +/- 2' scan, 0.1'/sec, 6 secs through FWHM, 50 seconds total scan time.
Set Xs and Zs tilts to zero. This will change expected position of Xs and Zs best translation.
Coma
- Xs (elevation). Assume that we can estimate the best Xs offset (e.g. from earlier measurements), and then
offset from that value (in units of wavelengths): +5, -5, -3, +3, 0 (five measurements)
- Zs (cross-elevation). Displacement range is limited. Use +1.5, -1.5, -1, +1, 0.
Astigmatism
- Need a wider range of Ys (axial) focus since astigmatism will split best elevation and cross-elevation values.
Offset from the nominal (Ys focus curve derived value) in wavelengths: +3, -3, -1.5, +1.5, 0.
The following useful sources are >10Jy at 20GHz:
--------------------------------------------------------
CALFIND: GBT pointing calibrators stronger than 10.00 Jy
at 20.00 GHz (GBT FWHM beam = 37.0 arcsec)
and lying within 180.0 deg of
J2000 RA = 00 00 0.00, DEC = 00 00 0.0
--------------------------------------------------------
IAU NAME RA (J2000) DEC S(20.0 GHz) Offset
HHMM+DDMM HH MM SS.SSSS DDD MM SS.SSS Jy deg
--------------------------------------------------------
0319+4130 03:19:48.1601 +41:30:42.103 17.38 61.2
1256-0547 12:56:11.1665 -05:47:21.524 22.56 164.8
1642+3948 16:42:58.8099 +39:48:36.993 12.48 104.7
1733-1304 17:33:02.7057 -13:04:49.548 10.41 96.6
1924-2914 19:24:51.0559 -29:14:30.121 14.33 71.6
2253+1608 22:53:57.7479 +16:08:53.560 15.55 22.9
--------------------------------------------------------
Useful commissioning time runs from ~8pm EDT = 00 UT = 17 LST to ~8am EDT = 12 UT = 05 LST
For testing on 31st, we should just stick with 1733-1304, which will be ~transitting at 40 deg El.
For the overnight run, we should initially alternate between 1733-1304 and 1642+3948, with the occasional observation of 1924-2914,
then switch to 1642+3928 and 2253+1608, then finally to 2253+1608 and 0319+4130.
Assume the observing sequence is:
- Standard peak/focus (5mins)
- Xs (El) coma (4mins)
- Zs (Az) coma (4mins)
- Ys (az and el) astigmatism (7mins)
- slew / extra overhead (10 mins)
we can do 1 complete set in 30 mins, or 24 sets overnight. Based on the above set of sources, we could get the following elevation
range (see astig.obs in /home/groups/ptcs/obs/setup):
Start End
Az El Az El
175.546 38.373 185.048 38.342
281.305 77.084 281.935 70.689
281.935 70.689 283.451 64.903
204.016 35.200 212.882 32.138
285.552 58.686 287.931 52.578
187.851 21.873 195.548 20.543
290.604 46.393 293.471 40.350
125.456 57.337 139.113 62.510
296.764 34.035 300.270 27.965
156.835 66.197 177.957 67.700
63.839 39.693 66.540 45.718
198.915 66.716 217.166 63.521
68.978 51.968 71.051 58.407
231.506 58.718 242.562 52.901
72.531 64.987 72.845 71.660
251.338 46.474 258.635 39.679
70.070 78.347 53.321 84.622
264.925 32.760 270.521 25.918
324.317 86.169 292.881 80.679
275.668 19.264 280.703 12.654
287.690 74.389 287.121 68.006
285.759 6.151 290.962 -0.193
-- RichardPrestage - 27 Aug 2004
Revision r1.4 - 13 Dec 2006 - 00:48 GMT - RichardPrestage
Parents: WebHome > InformationForProjectTeam
|
Content copyright © 1999-2007 by the contributing authors.
All material on this collaboration platform is the property of the contributing authors.
|
| PTCS.AntennaCharacterizationComaAstigmatismSummer2004 moved from PTCS.ComaAstigmatismSummer2004 on 13 Dec 2006 - 00:48 by RichardPrestage - put it back |