Tuesday, November 8, 2011

Remote login to AIPS

So, you're away from your computer (say it's in Wisconsin and you're in South Africa), but need to log in to AIPS remotely to get some (simple) work done. Since I do this about once every six months and have to relearn how to do it every time, here's the solution.
ssh into your machine, then instead of just typing at the terminal:

> aips
> aips da=<computer name=>
both of which give me errors or repeatedly ask for me to retype my password, try:
> aips notv da=<computer name>
Eh, voila! I can use AIPS without complaint.
True, you can't use the tv to view your data, but that would be a pain to do remotely anyway. When I make an image, I output an .PS file and rsync it to my local machine. rsync is nifty and worth learning, if you don't know it already.

Read more!

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.

Read more!

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'.

Read more!

Monday, June 7, 2010

Excluding single-channel flux peaks

Reader Linda asks

I'm trying to use the AIPS task BLANK to blank an HI data cube. I am using the method where you blank a convolved version of your cube to exclude flux < 2-3*noise and then use that convolved cube as a filter to blank your original image. With the help of other kind folks, I've got this part worked out. But I would also like to exclude single-channel flux peaks. Does anyone know of a way to ask AIPS to only keep pixels with signal that is above a set level in a specified number of consecutive channels? It seems like using OPCODE = 'FLTW' might be helpful, but my attempts at using this haven't produced anything useful.

I'd really appreciate any help!

Any suggestions?

Read more!

Friday, March 26, 2010

BPLOT is a really cool task

I recently learned about a task in AIPS which is very good for assessing bandpass stability---BPLOT. If you have multiple bandpass solutions, it will plot each one, stacking them one on top of each other as a time sequence. To the left is an example plot from BPLOT. Try it, you'll like it!

Read more!

Getting rid of distracting black lines in TVFLG/SPFLG

Have you ever loaded your data into TVFLG or SPFLG and seen what looks like lots of blank times (manifested as horizontal black lines cutting through your data)?

This is because TVFLG/SPFLG is trying to load your data with a time bin that is not a multiple of your integration time. So---the solution is to set DPARM(6)! Set DPARM(6) to your integration time in seconds; e.g., for GMRT data:

DPARM(6) = 16.7

(or, if you have time averaged your data, set it equal to your averaging time).

Read more!

Tuesday, March 23, 2010

Watch out for the STOKES parameter when plotting (even if you don't care about polarization)

When using the many plotting/flagging programs in AIPS, make sure to check the value of the STOKES parameter before plotting (try help stokes to see the options and select the one appropriate for that data set). Different tasks have different STOKES defaults, so sometimes displaying data in one program results in a totally different plot than in another program because one is including some polarizations the other is not. This particularly can be a problem if you have cross-polarization values you don't care about (yet).

Read more!