Concat of visibilities

simonbihr
Posts: 1
Joined: Thu Mar 29, 2012 3:58 am

Concat of visibilities

Postby simonbihr » Thu Jun 27, 2013 9:55 am

Dear all,

I was asked to post a problem that we encountered:

We got old VLA data as lot of small uv fit files. These files were observed at different dates. To import these uv fits, we used importuvfits() and after that we wanted to create one big ms file out of these small ms files. We used concat() for that. Everything worked fine and we thought, we do everything correct.

But after the cleaning of the big ms file (vis='big.ms') we realized, that the spectral axis was shifted by about 5km/s. It took us while to locate the problem, but apparently it is the concat() command. If we use clean with all the small ms files separately (vis=['ms1','ms2','ms3'...]) it works fine.

I guess, that clean needs the header information of each ms file separate to work properly. I think otherwise the doppler tracking does not work... is that correct? Or does concat correct for that? Using concat we got no warning or error message, that we did something wrong...

In my experience I can not recommend to use concat for visibilities, but maybe we were a special case...

Even though the problem is solved for me, I would like to understand it and maybe this can help to improve casa.

Cheers,
Simon Bihr

PS: The next problem we had was a linux problem, because we were not allowed to open so many ms files at the same time, and clean stopped with an error like 'too many open files'. We solved this by adjusting the 'ulimit -n' parameter.

albert
Posts: 1
Joined: Thu Mar 06, 2014 11:26 am

Re: Concat of visibilities

Postby albert » Thu Mar 06, 2014 11:29 am

Hi,

I'm also having problems with concat. I simply have two ms files, GMRT data, which I calibrate seperately and try to concatenate together with concat. Upon trying to clean there is a segmentation fault, although separately they clean fine. I think it is something to do with the pointing tables. concat says no valid pointing tables present. result won't have either one.

Is there a better way to concatenate two ms files?

best regards,
Josh


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