[daip] frequency channel information in IMHEADER

[Eric Greisen]

Joseph Lazio writes:

 > I'm attempting to make a cube out a set of (spectral-line) images to
 > match up with a previous analysis.  I have a fairly basic question
 > about frequency channels.  Is the frequency information for a channel
 > aligned with the lower edge of the channel, the center, or the upper
 > edge?

   All coordinates are for the center of the voxel (N-dimensional
pixel) since that is the only point in the voxel that is invariant to
rotation.

This is a FITS standard but one has to be careful about the source of
the data since some places screw this up.

Eric Greisen


 > 
 > To make this more specific, the velocity information in the header is
 > the following:
 > 
 > AIPS 2: VELO-LSR     1   4.2761687E+04     -98.00 -2.0610034E+02    0.0
 > 
 > The velocity for the first channel in the cube is then 22.1516533 km/s
 > [= 42.761687 km/s + (1 + 1 + 98)*(-0.20610034) km/s].
 > 
 > Now suppose that I combine two channels.  The new channel width will
 > be -0.4122006836 km/s.  Will the first channel still be 22.1516533
 > km/s, or do I have to adjust this frequency as well.
 > 
 > (I've checked various AIPS Cookbook chapters and my copy of Going
 > AIPS without finding this described explicitly.)
 > 
 > Thanks in advance,
 > 
 > -- Joe
 > 
 > -- 
 > T. Joseph W. Lazio, Ph.D.        voice: +1 202 404 6329
 > Remote Sensing Division          fax:   +1 202 404 8894
 > Naval Research Lab, Code 7213    Joseph.Lazio at nrl.navy.mil
 > Washington, DC 20375-5351 USA    http://rsd-www.nrl.navy.mil/7213/lazio/
 > 
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