Thursday, July 15, 2010

Gridding with SDIMG

I am trying to use SDIMG to grid some GBT data to 25 arcmin. The gridding parameters are set like in IMAGR, with XTYPE, YTYPE, XPARM, YPARM. Does anyone know how to make sense of the gridding parameters? Right now I use mysterious parameters given to me by Jay Lockman, which are idealized to the 9 arcmin L-band beam. I can't figure out what they mean to change them. The AIPS help files are just as mysterious to me.

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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'.

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