[daip] [!5478]: aips - corrected ticket about CVEL AIPS

[Eric Greisen]

Eric Greisen updated #5478
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       Staff (Owner): Eric Greisen (was: -- Unassigned --)
                 Due: - Cleared - (was: 15 September 2014 06:47 PM)

corrected ticket about CVEL AIPS
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           Ticket ID: 5478
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/5478
           Full Name: Carolina Rodriguez
               Email: ca.rodriguez at crya.unam.mx
             Creator: User
          Department: AIPS Data Processing
       Staff (Owner): Eric Greisen
                Type: Issue
              Status: Open
            Priority: Default
                 SLA: NRAO E2E
      Template Group: Default
             Created: 11 September 2014 06:47 PM
             Updated: 11 September 2014 07:35 PM
      Resolution Due: 19 September 2014 06:47 PM (7d 23h 12m)



perhaps if you gave me the project code I could look at the data myself
and get a better idea what the situation might be.  With VLA data, the
velocity was usually specified by the user and the observations made
so that velocity at the reference channel remained fixed during the 
observation (the frequency then changed slowly)

CVEL was written initially for VLBI data that does not track velocity and
has now come in handy since the EVLA does not Doppler track either.

Note you describe the velocity as changing with day - if the data were
at a fixed frequency then the velocity at the reference channel you got
3 days late was different from what you wanted.  But it should have been
close and CVEL can shift data by small amounts okay - so long as all of
the lines are several channels wide.  Otherwise it does bad things.

You mention the velocity in the header - I do not know what you mean exactly.
When FILLM loads VLA data it writes a CL table and it velocity was tracked
then a column in that table records the shifts and CVEL reads it.  In that case
CVEL may not do much at all unless you request a new velocity at the reference
pixel.  This frequency shift information used to be lost when the multi-source
file was split but in recent versions of aips that information is retained in a FO 
extension file.  Normally multi-source files have no velocity info in the header
but CVEL updates the output header for single-source files.

I might be of more help if you send the project code, the IMHEADER for the CVEL
input and output files and your CVEL INPUTS.

Eric Greisen



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