may14:Alfa calibration scans, rotation angle
Alfa was reinstalled in the dome on 21may14
after being worked on in the lab. Spider scans (heiles
scans) were taken with alfa on 24,26may14 using beams0..6
This data was taken with model18A with the tiedown active.
- 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 were used .
- The program cycled 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 27may14 the alfa model was updated to remove the constant beam0
- 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 3: coma, 1st side lobe height, beam efficiency (main
beam) and (main beam + first side lobe).
- Page 4: Pointing error (all beams)
- Page 5: Pointing error (beam0).
- beam0 mean offsets will increase the rms of all beams.
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).
- 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.
- We probably need to redo with the beam0 offsets removed.
- The gain,sefd, cals look ok
- gain looks a bit high for beam0 on a few sources. (cals a
little off or source flux?)
- sefd is : beam0:2.5 jy , outer beams about 3Jy
- pointing errors:
- rotating by + 1.1 degrees reduces the non beam 0 rms from 14
to 12 asecs.
- Pointing Notes:
- i removed the beam0 mean offsets from the alfa model on
- the large rms in all beams is partly due to the beam0 mean