A1956 alfa precursor proposal: extra galactic ultra deep
28sep04: A1956 first use of driftchase
pattern. pixel positions off at the start.
21oct05: A1956 drift chase pattern. updated
Project a1956 is one of the alfa precursor proposals.
It will do multiple drift scans (78 seconds drift, then reacquire the start
of the strip) to increase the integration time of the map.
Project a1956 took some data on 28sep04 to test things
out. It used the driftchase pattern. The drift scan lasted for 78 seconds.
For one of the scans the ra/dec positions for the pixels were off at the
begining of the scan (all scans were not checked). These are the values
stored in crval2a,crval3a of the fits header. The plot shows the
ra,dec positions (.ps) (.pdf)
. These are for pixel 0 (the central beam).
first use of driftchase pattern. pixel positions off at the start.
Fig 1. These plots are the first 15 seconds of the drift. The top
plot is the azimuth, the bottom plot is the zenith angle. The telescope
continued to move for the first 5 seconds of the drift (this problem has
been looked into).
Fig 2 top: This is the ra during the drift. The black line
is taken from crval2a, the red line is computed from the encoder az,za,time
offline in idl (wasalfacmpradec). The units are hours.
Fig 2 2nd plot: This is the difference (crval2a-idlcomputed)
Fig 2 3rd plot: The declination during the drift in degrees. The
black line comes from crva3a. The red line is computed from the az,za,time
offline in idl.
Fig 2 bottom: This is the difference (crval3a-idlcomputed)
Project a1956 started their observing run on 21oct04.
The drift chase procedure had been worked on to correct the positioning
errors seen on 28sep04. On 21oct04 5 driftchase scans were done each
lasting 80 seconds. I took the ra/dec positions from the header (crval2a,crval3a)
for beam 0 and compared them with ra/dec positions computed using
the encoder az, za, and encoder time (interpolated to the data times in
the header). The plots show the
ra,dec positions (.ps) (.pdf)
21oct04: A1956 drift
chase positions after working on pattern. (top)
The ra,dec positions from the header values crval2a,crval3a has improved.
It jumps around by about +/- 7 asecs max for this data set. The az,za
encoder values changed by at most 1 arcsecond, so the jumping around
is a computational problem and not the telescope moving.
Fig 1 top. A linear fit was done to the ra values for each 80 seconds.
The black lines are the ras from the header (crval's). The red lines are
computed offline using the encoder values. The y axis is great circle arc
seconds (i included the cosine of the dec).
Fig 1 2nd : This has the declination minus the median declination
plotted for each of the 5 scans. Black comes from crvalxx while red
is computed offline from the encoder values. The y axis is arc seconds.
Fig 2 3rd,4th: The bottom two plots show the (az - median(az))
and (za -median(za)) for the 5 strips. The y axis is great circle arcseconds.
Since these where drift scans the az,za should have been fixed for each