[daip] [!6834]: AIPS - VLBA continuum data reduction---how to remove instrument delay residuals for weak phase calibrator?

[Hui Zhang]

Hui Zhang updated #6834
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VLBA continuum data reduction---how to remove instrument delay residuals for weak phase calibrator?
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           Ticket ID: 6834
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/6834



Dear Mr/Mrs,
       I am Hui Zhang  from Shanghai Astronomical Observatory, Chinese Academy of Sciences. 
       I am learning to process the VLBA continuum data. But there are several problems I don't know how to solve.
I wil just hope you to help me for solving one problem first in this email. Maybe I can solve other problems 
after that.
    
       My problems is:   why the phase calibrator's phase looks still messy instead of flat after I removing the 
instrument delay residuals  with the task 'VLBAPCOR'?  And I am sure there is a 'PC' table in the data, and the 
phase is flat in all baselines for the fringe finder/bandpass calibrator after 'VLBAPCOR'

     According to my understanding from Appendix C in AIPS Cookbook,  the phase should be flat, and there is no phase jump after doing with task 'VLBAPCOR'
for fringe finder/bandpass calibrator. And there should also be no phase jumps or slope for phase calibrator, the phase should also be flat for
phase calibrator.  But what I got was not the case after 'VLBAPCOR'.    

      For example(spectral line observation):  1.png is the phase before I removing the instrument delay residuals (with VLBAMPCL),  
and 2.png is the phase after that for the phase calibrator.   This example is from VLBA tutorial : 
http://www.aoc.nrao.edu/~amiodusz/sumschool14/vlbaspectut.shtml   
I processed the data by following the steps in this tutorial, and got the result shown in 1.png and 2.png. The phase of the phase calibrator is
not perfect after removing the instrument delay residuals.  I guess this is because of the phase calibrator is weaker.

     Here is another example (continuum observation): 3.png and 4.png are the results before and after removing the instrument delay residuals for the phase calibrator, respectively. 
The phase is still messy, so how to make the phase looks almost flat for this phase calibrator?  

    In fact I also tried to process other data,  in that case,  there was obvious phase slopes in some baselines, and also messy phases in other baselines before removing the instrument
delay residuals. But after I did the instrument delay residuals with 'VLBAPCOR',  the phase  became flat for the baselines with phase slopes before 'VLBAPCOR', but the phases were till 
messy for the baselines with messy phase before 'VLBAPCOR'. 

     In all the examples above, the phases were flat for the bandpass calibrators after I removed the instrument delay residuals with 'VLBAPCOR' for continuum data or with 'VLBAMPCL' for 
spectral line data.  And I am sure I have selected the time range on bandpass calibrator with good fringe for all baselines, and I tried to selected this time range in the middle part of the whole observation.
  
      I don't how to make all the phase be flat for the phase calibrators after removing the instrument delay residuals (I think the difficulty is that the phase calibrator in our example is weak, about several tenth mJy)?  And I also become puzzled, should I really have to make all the  phases be flat on all baselines for the phase calibrator?  And why the phase can be flat in some baselines and be still messy in other baselines for the same phase calibrator after 'VLBAPCOR' or 'VLBAMPCL'?

       I hope I have described my questions clearly.  I will be very appreciate if you can help me to solve this problem. Thank you very much.

Best Regards,
Hui Zhang


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