Monday, July 5, 2010

Inconsistencies between UVPLT and TVFLG?

Here's a question from Anonymous:

I am re-reducing 1667 MHz OH spectral line data from project AC319 (JUN-92, polarizations RR, LL, RL, LR). I chose one spectral channel from one day, and I plotted the visibility amplitudes with both TVFLG and UVPLT. The data are split, and they are for the program source W22.

I plotted the visibility amplitudes in UVPLT (stokes 'rr'), and I don't see any amplitudes over 20 Jy for baseline lengths 4 - 9 kilolambda. However, when I plot the same data in TVFLG (stokes 'rr', sorted by length), I see a number of amplitudes higher than 20 Jy in the same baseline range.

Why would the higher amplitudes be displayed by TVFLG but not by UVPLT? In both cases, I chose flagver =-1, so no flagging should have been applied when I ran either task.

I'm not sure of the exact answer to your question, but there are lots of ways for TVFLG and UVPLT to give slightly different pictures of your data:

--Is XINC set in UVPLT?
--Are you sure your looking at the same channel in each case, and not averaging channels in either case?
--TVFLG may be secretly time averaging your visibilities. To make sure that it's not time averaging, find out what your integration time is and input this as DPARM(6). Then, once the data are actually loaded, look at the AVG parameter stated in the bottom left of the tv screen. If it is set to a number greater than 1, than time averaging is occurring. To change this averaging time, click on 'enter smooth time'.


amanda said...

My first thought was to double-check your stokes parameters. Different flagging/display tasks have different defaults, which can cause confusion. However, since you've check that Laura's suggestions are good ones, especially the time averaging in TVFLG. TVFLG will spit out a line in the beginning of the output about what what time averaging it's using. Double check that this number is less than your integration time.

Anonymous said...

How do I set XINC? XINC is set to 1 currently.
I checked for averaging and dparm settings. I am looking at the same channel and it is not averaging. Also, Dparm(6) is set to the integration time. When I display data in TVFLG, AVG is 1.

Laura said...

I just meant to make sure that XINC was not greater than 1. XINC=1 is good.

Hmmm...Why don't you send us your UVPLT and TVFLG parameters?

amanda said...

TVFLG output (or just the first 30 or so lines would be good too).

TVFLG will ignore DPARM(6) in order to fit all the data on the TV, so you need to make sure that your TIMERANG is small enough that DPARM(6) will work. The TVFLG output tells you what TVFLG how is actually averaging.

Adrienne said...

You could also try looking at your data in WIPER instead of UVPLT. I've found it to be faster for looking at lots of data points. I'm not sure if that will fix your problem, though.

Anonymous said...

Thank you all for your suggestions.
The problem seems to be solved for now. Actually I was plotting entire UVRANGE in TVFLG and I assumed that the x-axis scale would be linear. But when I plotted only for UVRANGE=4, 9; I did not see any amplitudes higher than 20 Jy(same as seen in UVPLT).
This is because the TVFLG does not sort the data strictly in UV length order.