[daip] Problems with CVEL on WSRT and GMRT data.

[Nissim Kanekar]

Dear Eric,

I've recently come across some strange problems when using CVEL in 31DEC16 
andd 31DEC17 on GMRT and WSRT data. The data were taken with simple in-band 
frequency-switching, where a single IF band (of bandwidth=5 MHz or 6.25 MHz) is 
used and the line is kept within the band by shifting the band centre frequency 
by 2 MHz and then back to the original value every 5 minutes. We then run BPASS 
separately on the original data set and the frequency-shifted data set, and 
copy the BP tables from the original data set to the frequency-shifted data 
set, and vice versa. And finally run CVEL on the two single-source SPLIT files 
to shift each to the LSR frame before obtaining the absorption spectrum by 
averaging the visibilities in POSSM (the target sources are phase calibrators). 
The procedure has worked fine in the past (we used this for both GMRT and WSRT 
data in 2006-2011) but we find a velocity offset between the spectra obtained 
from the original and frequency-switched data sets after CVEL. The velocity 
offset between the spectra is different for different sources and is seen in 
both WSRT and GMRT data. I've also checked the header of the GMRT data and 
compared it to the old GMRT data (where CVEL used to work fine) and find that 
the headers look the same. Further, I ran dopset to work out what the velocity 
shift to the LSR frame should be (ignoring the 0.5 km/s due to earth rotation) 
and find that the shift computed by CVEL appears close to correct for the 
original data set but is wrong (by a few km/s in some cases) for the 
frequency-switched data set. I'm actually not sure whether the problem here is 
due to the frequency-switching or just something in CVEL on single-source files; 
the only place where frequency-switching has been done is applying the 
frequency-switched bandpass table and I wouldn't have thought that this should 
have an effect on the frequency axis. I was wondering whether you might have 
some idea as to what might be going on.

Many thanks indeed!

Best wishes,
Nissim