Alfa calibration scans, rotation angle after
Alfa was reinstalled in the dome on 20apr12
after being worked on in the lab (new amplifiers were installed).
Spider scans (heiles
scans) were taken with alfa on 27,28apr12 using beams0..6
This data was taken with model17 with the tiedown positions for
model17. Pointing model 18 was taken in mar12 but had not yet been
- The alfa rotation angle was set to 0 degrees.
- The wapps took data with 100 MHz bw centered at 1440 MHz. The
100 Mhz rf filter was not used (trouble switching it in).
- The program cycle through the various beams. Beam 0 was
sampled more often than the others to get a better measure of
the pointing offsets.
Calibration run results:
The plots show the
calibration results (.ps) (.pdf):
On 03may12 the alfa model was updated to remove the constant offsets
(3.09,7.34).This will get updated again when model 18 is installed.
- Page 1: the az/za coverage. Colors specify the beam that was
- Page 2: gain,Tsys,SEFD, avgBeam width. This used the standard
cal values. Colors show the different beams
- Page 4: coma, 1st side lobe height, beam efficiency (main
beam) and (main beam + first side lobe).
- Page 5: Pointing error.
beam 2 had large za errors. This has been seen in
previous years data. It is probably the offset we are
using for this beam.
Fitting for rotation angle error:
Part of the pointing error could come from the
alfa rotation angle being off. To test for this :
Plots showing the
error vs rotation angle offset (.ps) (.pdf):
- Compute the change in the pointing error if the alfa rotation
error was off by -5 to +5 degrees in .1 degree steps. Do this
for each beam separately.
- For each of the .1 degree steps recompute what the pointing
error would have been for each measured position.
- Plot the rms error and mean error for these newly computed
series. Do this separately for beam 0 (which is not affected by
rotation) and the other beams.
- Top: rms pointing error vs rotation angle offset.
- For each offset (-5 to .5 in .1 degree steps) the rms
pointing error was recomputed
- + beam 0, * beams 1,2,3,4,5,6
- Black is the
total rms (rmsAz, rmsZa added in quadrature)
- Red: Az rms pointing
- Green: Za rms pointing
- The total rms minimizes at a rotation offset of 1.1 degrees.
It is about 2 asecs less than the value with not rotation
offset (what is currently being used).
- beam 0 totRms is 6 asecs, nonBm0 0 rotation=13 asecs, nonBm0
- The az and za minimums occur and different rotation offsets.
This may be from the limited number of measurements we took.
- Bottom: mean pointing error vs rotation angle offset
- Black tot Mean error. az and za added in quadrature.
- green: mean za error,
- red : mean az error.
- minimum mean error occurred at .3 degrees..
- At .3 degree offset, the beam 0 and non beam 0 offsets are
- The gain,sefd, cals look ok
- Looks like all beams are now working ok.
- Back in nov11 after the last time alfa was reinstalled,
there were a few beams with Tsys problems.
- pointing errors:
- rotating by + 1.1 degrees reduces the non beam 0 rms from 13
to 12 asecs.
- Pointing Notes:
- the beam0 offsets should be close to zero after these offset
values are subtracted from the model (03may12).
- The offsets may be biased because we didn't cover all of the
dish (and we know there is a 1az tilt).
- The pointing model will get updated around 15may12 when
the new pointing model is installed. This should fix most of
the offset errors..
Still to do:
- look at the new amplifiers response to the radars.