[daip] CVEL in AIPS

[Ross Burns]

Dear AIPS helpdeskers

My name is Ross, I'm at Kagoshima university and am a 1st yr PhD student.
Here at our lab we use AIPS to reduce VLBI data from the VERA
interferometer (it's like a small VLBA).

We usually get along fine with all of the AIPS tasks that are used in our
data reductions, however it seems that the latest version has some
modifications that has caused us some trouble. The problematic change seems
to be in either SETJY or CVEL or both.

During past reductions it became apparent, long ago, that CVEL seemed to
work better when the array name was set to 'VLBA' and then change back to
'VERA' after CVEL. Here is the code we would use as standard:


*******
*TABED
*******
tget tabed
default
indisk 2; getn 1; keyword 'arrnam'; keystrng 'vlba'; optype 'key';
aparm (4)=3; inext 'AN';inver 1; outver 1; bcount 1; ecount 0; clro;
go; wait
*Change array name to VLBA

*******
*SETJY
*******
tget setjy
default
indisk 2; getn 1;source 'X_Hya
restfreq 2.223e10, 5080000;
veltyp 'lsr'; veldef 'radio';
bif 0; eif 0;
aparm (1)=187;
sysvel 23.66;
go; wait

*******
*CVEL
******
tget cvel
default
indisk 2; getn 1; outdisk 2;outclass 'CVEL''
doband -1; gainuse 1; source 'X_Hya
aparm 187 0;
go; wait; pcat
* Creating a CVEL file.


*******
*TABED
*******
tget tabed
default
indisk 2; getn 2; keyword 'arrnam'; keystrng 'VERA'; optype 'key';
aparm (4)=3; inext 'AN';inver 1; outver 1; bcount 1; ecount 0; clro;
getn 1; go; wait
getn 2; go; wait
* Change array name back to 'VERA'


Where 187 is the frequency channel of the emission peak
and 23.66 is the velocity of the emission peak

However. In the latest version, 31DEC13 we found that this procedure
wouldnt work. The velocity offsets were far too big and often seemed to
fail altogether.
We realised that CVEL was always setting the system velocity to zero.

So we changed our procedure by first locating the velocity and frequency
channel of the emission peak, and then calculating the frequency
channel corresponding to the zero velocity. This worked quite well, though
it may someday become problematic if the bandwidth does not include the
zero velocity, then we would calculate negative values for the
corresponding frequency channel, and thats no good.

The other problem is that this method only allows us to allocate a system
velocity that is a multiple of the velocity resolution (0 km/s +- some
number of channels) this gives us less control on the values we use to
define the peak of the emission.

Generally, it is a pain for any new students who cant get hold of an 'old'
version of AIPS. For anyone in our lab running 31DEC12 or earlier this
problem does not occur, though I recently re-installed the current 31DEC12
using binary and an internet connection and found the CVEL problem, so
perhaps it was included as part of a recent patch?
My old (Downloaded around Jan2012) 31DEC12 did not have this issue.

I think this problem could be something to do with the name change to
'VLBA' though I am not sure. I cant imagine such a fundamental change would
have been made to CVEL itself.




If you could shed some light on what might be going on here it would be a
great help for me and all my fellow lab members! Sorry about the long
message but i though I should be thorough in the explanation of whats
troubling us.
Kind regards
Ross
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listmgr.nrao.edu/pipermail/daip/attachments/20131217/8769ecff/attachment.html>