[daip] NOT a TVFLG bug -- cal. pos'n changes between fast-switching scans
Michael Rupen
mrupen at aoc.nrao.edu
Sat May 3 14:55:54 EDT 2003
Dear Daip & Ken,
I sent in a gripe a while back, complaining of data which seemed to be
uncalibrated in TVFLG but not in UVPLT. Thanks to a suggestion by Leonia,
this was tracked down to a change in the name of one of the calibrators (see
below). The bottom line is that
* TVFLG is fine
* FILLM reports a change in position between different calibrator
scans, of order a few milliarcseconds; accordingly, it assigns a unique
name to this "new" (different position) calibrator, appending " 05"
to the original name.
* The offending scans are all fast-switching scans, but...
* ...the OBS. file shows *identical* positions for the calibrator in all
scans.
* Thus at the moment it's not clear whether this is a problem with FILLM
or with the positions actually used during the observations.
* The position differences are so small that this is not of any fundamental
importance, but it is an annoyance during calibration. At the higher
frequencies and in the more extended arrays these positional offsets might
even be noticeable in the data.
* I would think the next step would be to look at the actual data on
tape and see whether the coordinates are indeed different, as AIPS claims.
I would bet that they are, and that this is a problem in the on-line
system.
Details are given below.
Cheers,
Michael
p.s. I obtain the same results with both on-line and off-line FILLM.
=======
Details
=======
During a 26apr03 observation (AR508), I used fast switching for several
sources, and employed the same calibrator (given in the OF [offset] card)
for two sources at 6cm, and three sources at 4cm.
At 6cm, AIPS assigns the name
17009-26109
for the first two scans on the calibrator, corresponding to the first source;
but uses the name
17009-26109 05
for the remaining four scans, corresponding to the 2nd source.
Similarly at 4cm the calibrator is called
17444-31166
for the first, and
17444-31166 05
for the other two sources.
The most obvious explanation is that the calibrators were assigned different
coordinates. This is indeed the case, as shown by AIPS' scan listing
(LISTR/SCAN):
17009-26109 : 0000 A 17:00:53.1542 -26:10:51.729
17009-26109 05: 0000 A 17:00:53.1543 -26:10:51.725
^ ^
at one wavelength (6cm), and
17444-31166 : 0000 A 17:44:23.5822 -31:16:35.986
17444-31166 05: 0000 A 17:44:23.5826 -31:16:35.986
^
at the other (4cm).
Surprisingly, this coordinate difference is NOT reflected in the OBSERVE
file used for this run (/home/banshee/observe/obsArchive/471r508.OBS). The
relevant sections are as follows:
//* Maximum shadowing of 6.2 meters.
I1709B 18 48 28 17 09 02.2700 -36 23 33.000C CC 0000
//DS 3
//LO 0.0 0.0 3860 3810 0000
//FISF 100.000000 200.000000
//OF NOD SKY 17 00 53.1541 -26 10 51.725 A 170 70
//* Maximum shadowing of 5.1 meters.
I1746-32B 19 04 30 17 46 15.6100 -32 14 00.700C XX 0000
//DS 3
//LO 13.4 13.4 3940 3890
//FISF 100.000000 200.000000
//OF NOD SKY 17 44 23.5826 -31 16 35.986 A 120 60
X1720-31B 19 31 19 17 19 57.6000 -31 45 21.000C XX 0000
//DS 3
//LO 13.4 13.4 3940 3890
//FISF 100.000000 200.000000
//OF NOD SKY 17 44 23.5826 -31 16 35.986 A 180 60
//* Maximum shadowing of 5.9 meters.
X1720-31B 19 45 40 17 19 57.6000 -31 45 21.000C CC 0000
//DS 3
//LO 0.0 0.0 3860 3810 0000
//FISF 100.000000 200.000000
//OF NOD SKY 17 00 53.1541 -26 10 51.725 A 240 60
//* Maximum shadowing of 8.5 meters.
X1720-31B 20 19 11 17 19 57.6000 -31 45 21.000C XX 0000
//DS 3
//LO 13.4 13.4 3940 3890
//FISF 100.000000 200.000000
//OF NOD SKY 17 44 23.5826 -31 16 35.986 A 180 60
//* Maximum shadowing of 11.3 meters.
X1720-31B 20 37 03 17 19 57.6000 -31 45 21.000C CC 0000
//DS 3
//LO 0.0 0.0 3860 3810 0000
//FISF 100.000000 200.000000
//OF NOD SKY 17 00 53.1541 -26 10 51.725 A 240 60
This is a little hard to see; abstracting the interesting bits (the OF or
offset pointing cards) we have
6cm (CC):
//OF NOD SKY 17 00 53.1541 -26 10 51.725 A 170 70
//OF NOD SKY 17 00 53.1541 -26 10 51.725 A 240 60
//OF NOD SKY 17 00 53.1541 -26 10 51.725 A 240 60
vs. AIPS:
17009-26109 17:00:53.1542 -26:10:51.729
17009-26109 05 17:00:53.1543 -26:10:51.725
^ ^
4cm (XX):
//OF NOD SKY 17 44 23.5826 -31 16 35.986 A 120 60
//OF NOD SKY 17 44 23.5826 -31 16 35.986 A 180 60
//OF NOD SKY 17 44 23.5826 -31 16 35.986 A 180 60
vs. AIPS:
17444-31166 17:44:23.5822 -31:16:35.986
17444-31166 05 17:44:23.5826 -31:16:35.986
^
So the cards in the OBS. file are identical, but AIPS believes the resulting
coordinates are different.
I transferred the gains from '17009-26109' to '17009-26109 05' at 6cm,
and the resulting amplitudes, phases, and images look fine, and consistent with
those of '17009-26109' itself. Of course the resolution in D configuration is
not sufficient to measure the sorts of differences mentioned above.
I have done MANY of these sorts of observations, and I have used the same
calibrator for several different sources while fast switching several times
in the past. This is the first time this sort of error has occurred. [I
*have* on occasion inadvertantly used different coordinates for the same
calibrator, with AIPS behaving as above, but this is expected.] I do
occasionally see differences between the coordinates reported by AIPS in
LISTR/SCAN, and those given in the OBS. file, but those have been differences
of one in the last decimal, which I've attributed to minor round-off errors.
I do not however routinely check the coordinates to the last decimal place,
so I would only occasionally have caught slight offsets like the few
milliarcseconds found here.
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