Thursday, May 17, 2007

WBR Step 3: Choosing a Reference Antenna

At several steps in your data calibration, AIPS is gonna ask you for a REFANT, a reference antenna. So we might as well choose one now!

First of all, run PRTAN to view your antenna setup. Really the only thing you need to do here is tell PRTAN to write to file (DOCRT = -1) and give it a file name (in OUTPRINT)

>tget prtan
AIPS 1: PRTAN: Task to print the Antenna (AN) extension of a uv file.
AIPS 1: Adverbs Values Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: USERID 0 Image owner ID number
AIPS 1: INNAME 'MULTI4.8' Image name (name)
AIPS 1: INCLASS 'CH0' Image name (class)
AIPS 1: INSEQ 1 Image name (seq. #)
AIPS 1: INDISK 1 Disk drive #
AIPS 1: INVERS 0 AN file ver. #
AIPS 1: NPRINT 0 No. records to print 0 => all
AIPS 1: DOCRT -1 > 0 => use terminal instead
AIPS 1: > 72 => terminal width
AIPS 1: Printer disk file to save

In the outputted file, you'll get a cute little map of your antenna locations that looks like this:

Location Of VLA Antennas

N18 ( 1)
N16 (26)
N14 (24)
N12 ( 5)
N8 ( 9)
N4 ( 7)
N2 (27)
N1 (25)
(20) W2 E2 ( 3)
( 8) W4 E4 (15)
(23) W6 E6 (12)
( 6) W8 E8 (10)
( ) E10 ( 2)
(21) W12 E12 (17)
( 4) W14 ( )
(11) W16 E16 (19)
(22) W18 E18 (28)
VLA:OUT (13)
VLA:OUT (14)
VLA:OUT (16)
VLA:OUT (18)
VPT:OUT (29)

(Yeah, this is data during the EVLA upgrade so there are quite a few dishes missing). For your reference antenna, you are going to want to pick a dish near the center of the array, but not exactly at the center (the centermost ones suffer most from RFI). You are also going to want to choose a dish that never crashed during the course of your observations. A good way to see what dishes went down during your observations is to download the observing logs. See if there are any notes from the telescope operator about the dish you are considering using as your REFANT.

Finally, you can take a quick look at the amplitudes of the data using the 'MATX' mode of LISTR. This will spit out amplitudes of your data (on each baseline) averaged over a given time increment. Sometimes you will see really anomalous amplitudes for a given antenna over some timerange (like, an order of magnitude off). If you see something really fishy for the dish you are considering for your REFANT, don't use it. Find a different REFANT that does not have crazy amplitudes.

So, to create such a file of amplitudes, run LISTR setting these parameters:
DOCAL = 2 (apply weights)
DPARM(1) = 0 (list amplitudes)
DPARM(4) = 0 (will list amplitudes averaged over the entire scan. You can change this if you want.)
DOCRT = -1 (write to file)
you could also specify a source, say your Phase Calibrator, if you want.

If an antenna passes all of these tests, than it's probably a decent REFANT!


amanda said...

I didn't know the matx refant trick. Very cool!

hans said...

thanks a lot