Astrometry HOWTO

Version 0.6 Last modified 2002/10/21

Shami Chatterjee & Walter Brisken


A quick overview of the data reduction and astrometry procedure.  For
more detailed usage notes, see the autoaips.howto file.

a) Data at each epoch needs to be calibrated: final outputs are
calibrated pulsar, in-beam [if available] and calibrator data, along
with flag commands used.

b) Preliminary astrometry: apply in-beam self-cal or ionospheric
calibration solution to the pulsar data. Final output is a JMFIT
position output file for the pulsar, referenced to the standard
calibrator position (in-beam if avilable, else nodding cal).

c) To Archive: 
	- Flag commands (in .x files)
	- Calibrated UV data for calibrator and in-beam (on local disk)
	- Reduction notes (online via SC)

d) Final calibration: After all epochs of data are avilable, a final
calibrator model is to be constructed with calibrated UV data from
*all* epochs, for both the inbeam and the nodding cals. Using these
final models, *each* epoch will be recalibrated and final astrometric
positions will be extracted.   This ensures that the same model is used
for each epoch.

Final output: astrometric positions for targets at each epoch. 

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Steps along the way, and things to watch out for:

1) Install autoaips. The autoaips.txt help file provides a detailed
recipe to follow during data reduction.  The most updated version of
autoaips is available at http://www.astro.cornell.edu/~shami/psrvlb/ .
Changes between autoaips versions are summarized in autoaips.c .

2) All key and log files are available online, at 
 http://www.astro.cornell.edu/~shami/psrvlb/ - links to epoch N.

3) It is vital to write out flags so that they can be reapplied later.
   Save these flags in the PSR.x.N file, as in the example below: 

flag1 = bif=2;eif=2;bchan=12;echan=14;ante=10
flag2 = ante=5;basel=6
flag3 = bchan=1;echan=1

Note: NO SPACES!  Everything right of the first '=' is directly interpreted
by AIPS in the task 'UVFLG'.
Also, inputs are not cumulative - each flag is independent of previous flags.