[daip] frequency channel information in IMHEADER

[Joseph Lazio]

Hi, 

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?

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/