[daip] [!6921]: AIPS - VLBA continuum data reduction----three questions

[Hui Zhang]

Hui Zhang updated #6921
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VLBA continuum data reduction----three questions
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           Ticket ID: 6921
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/6921



Dear Amy,

        I am now reducing another VLBA data, and there are several problems I don't know how to solve. 

        The information of the data is as follows: 
         It is a continuum observation, the fringe finder is 0133+476, the primary phase reference is 0219+428, the target is 3C66B.

         After I loaded the data, I used 'VLBAFIX' to fix the data,  I applied the ionospheric corrections, the corrections to the EOPs,
and flag the data below 10 degrees, then I did the amplitude calibration, and I corrected the antenna parallactic angles with
'VLBAPANG'.   Because it is a continuum observation and there is a PC table, so I want to remove the instrumental delay residuals
by 'VLBAPCOR', but here comes the problems:   when I run 'VLBAPCOR', it stopped and printed the message like in figure 'message.png'. And the information of the data and table is shown in figure 'imh.png'.   

So my first questions is: why the task can not be run correctly? Is it because there are only two IFs? or Only one stokes ('RR') ? 

        Then I used 'VLBAMPCL' to correct the instrumental delay residuals and it seems to succeed. This step created 'SN' table 3
and  'CL' table 7. It found 20 good solutions and no failed solutions during this step.

        Then I do the globe fringe corrections with the task 'fring' and 'clcal'. When I set the default values for most of the parameters
and set the parameters(such as calsour,gainu, refant,search,solint,sources,interpol) shown in Appendix C of AIPS COOKBOOK, it shows
that half of the solutions are failed. So I try to reset the parameter, such as dparm(2) and dparm(3) and solint,  I find that changing these 
parameters can not make me find more good solutions, there are still half failed solutions, so I think the results are not sensitive to parameters 'dparm' and 'solint' for the current data. Then I set the SNR threshold to be 3.5, and find that 1/3 solution are failed, 
2/3 solutions are good, this step created the 'CL' table 8 and 'SN' table 4.
         So my second question is what  the percentage of the good solutions is when we can say we have succeeded correcting the globe fringe? 

         After I finished doing the globe fringe, I plotted the results of 'CL' table 8 with 'POSSM'.  Figure 1a.png and 1b.png shows part of the results of the fringe finder 0133+476. Figure 2a.png and 2b.png shows the result of the target 3C66B, Figure 3a.png and 3b.png shows the results of phase calibrator 0219+428.   I also plot the 'CL' table 7 for phase calibrator 0219+428, which is shown in figure 4a.png and 4b.png.   Figure 1a.png and 1b.png shows that the fring has been corrected.    But figure 2a.png,2b.pnd, 3a.png and 3b.pnd shows that there was no solutions for one of the IF at some baselines, but figure 4a.png and 4b.png shows that the results of phase calibrator is good after correcting the instrumental delay residuals (CL#7).  So the strange result is caused during I do the globe fringe corrections.  

        So my third question is how to do the globe fringe? What caused the strange results shown in figure 2a.png,2b.png 3a.png and 3b.png? It seems that only when I reduce the SNR threshold, I can find more good solutions. But the flux of the target is about 400-500mJy/beam at 8.4 GHz, the phase calibrator is also not a weak source (see from figure 3a.png and 3b.png), so I think, SNR is 5 (default value) should be ok, but here I have to the reduce SNR to find more good solutions.

        Could you give me some suggestions to solve my problems? I hope I have supplied enough information to you. 
        Thank you very much!

Best Regards,
Hui Zhang


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Staff CP:  https://help.nrao.edu/staff
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