[daip] Re: AIPS questions

[Amy Mioduszewski]

Hi Magdalena,

You should send your queries to daip at nrao.edu, I might not be around and 
daip goes to a several people, including me.

 > after the first run of
 > FRING (for phase calibrator) there is nothing on the map

On which map, the map of the phase calibrator or the map of the target? 
  Did you have many failed solutions from FRING?  FRING (and CALIB) will 
attempt to make the data look like the the model so can create a source 
if the source is sufficiently weak.  How strong was the resulting 
source?  You could try FRING fitting on the target with a point source 
or a different model and see what you get.  You could also try using 
CALIB, I find that CALIB works better for weak sources, if you have all 
VLBA antennas and are pretty confident that the delays and rates are 
small, since CALIB will not solve for them.  Also, by using a model and 
fringe fitting on the target you have pretty much lost all positional 
accuracy.

I am interested in why the phase referencing didn't work.  What did the 
CL table look like?  How far away was the phase calibrator and how 
strong was it?  I find that editing the last CL table and getting rid of 
spurious phase winds as one of the best way to improve your phase 
referenced image.

 > 2. Should I always run FRING on my target source with a
 > phase-referenced map as a model?

Well it depends on how strong your sources are and your scientific 
goals.  I would avoid using models from other frequencies unless you 
absolutely had to.  In the case where you had a pretty weak source as 
the model, I would not use it at all, but use a point source model if 
the phase referencing did not work, and then fix things up with some 
self-calibration.  If a source is strong enough to run FRING on it is 
strong enough for self-calibration.

 > 3. I have noticed that for some of my sources there is a difference in
 > coordinates between 18 and 6 cm in order of 0.004 arcseconds.

This can easily be caused by the ionosphere which is important at 18 cm 
but not at 6 and 4 cm.  This can also be a result of slightly different 
structures in the calibrator between 18 and 6 cm.  It is REALLY hard to 
do astrometry at 18 cm without a in-beam phase calibrator because of the 
ionosphere.

Cheers,

Amy