[daip] [!5480]: aips - CVEL AIPS 2

[Eric Greisen]

Eric Greisen updated #5480
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              Status: Closed (was: Open)
                 Due: - Cleared - (was: 24 October 2014 11:44 PM)

CVEL AIPS 2
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           Ticket ID: 5480
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/5480
           Full Name: Carolina Rodriguez
               Email: ca.rodriguez at crya.unam.mx
             Creator: User
          Department: AIPS Data Processing
       Staff (Owner): Eric Greisen
                Type: Issue
              Status: Closed
            Priority: Default
      Template Group: Default
             Created: 11 September 2014 10:45 PM
             Updated: 23 October 2014 02:37 PM



You have not understood at all what CVEL is intended to do.

When you observe at a fixed frequency for, say, channel 64, then
the Earth's rotation and movement around the Sun and also velocity
wrt the Local Standard of Rest means that the velocity of channel 64
is a function of time.  CVEL shifts each observed spectrum so that
channel 64 has a fixed velocity while its frequency varies with time.
Thus each visibility point is shifted differently.

This process is needed so that IMAGR can simply take all the data
from channel 64 and make an image at the desired velocity.  Without
CVEL, IMAGR would have to take data from channel 64 at the first time
but then interpolate to take data from channel 64.02 at the next time,
64.04 at the time after that and so forth.

Thus - after CVEL, velocity -17.6 is where you wanted it in channel 64
although before CVEL it was at the 3.3 and 1.9 pixel offsets.  The frequencies
in channel 64 become those of the shifted spectrum and are in fact a
function of time (this function is recorded in the CL table or in an FO table).

CVEL changes your data to be what you would have liked to have had
you been able to observe exactly when you expected to observe AND
been able to vary the frequency as a function of time to keep the desired
velocity in channel 64.  Believe me, CVEL has done what you wanted.

Eric Greisen




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