[daip] New Ticket - [!LTT-971236]: velocity conversion

[Lynn D Matthews]

New Ticket: velocity conversion

I have a question about conversion of EVLA data from frequency to velocity via SETJY. I seem to be getting a result that is off by the width of one channel.

I set up my observation so the band center was specified as:
"velocity 7.3km/s LSR_KINEMATIC RADIO"

As I understand it, if there are 8 IFs, this velocity should be assigned to the first spectral channel of IF5. The original SU table lists for the source LSR velocity 
7300.00000  m/s and Rest Freq. 1420405752.0 Hz for all IFs, and the header gives pixel 1.0 as the FREQ reference pixel.

If I then run SETJY on my multi-source data with OPTYPE ='VCAL',  VELTYP ='LSR', VELDEF='RADIO' it reports:

IF =  5 velocity =       6.96 (km/sec calcd)

This is off from the specified observing velocity by approximately the width of one channel (1.95 kHz=0.41 km/s). Am I making a mistake or some flawed assumption here?

Thanks,
Lynn Matthews

Ticket Details
===================
Ticket ID: LTT-971236
Department: AIPS Data Processing
Priority: Default
Status: Open
Link:  https://help.nrao.edu/staff/index.php?_m=tickets&_a=viewticket&ticketid=3452