Wednesday, June 20, 2007

Reference Frequencies (Center v. Edge?)

I've come to the conclusion that for 1-channel continuum data coming from the VLA, the frequency that you see in the header and in your LISTR 'scans' ouput is the center of your bandwidth (not the beginning).

I come to this conclusion mostly because the L-band observing guide on the VLA web page says that "The two recommended center frequencies are 1365 and 1435 MHz." I used the defaults when I made my observe files, and now my LISTR output lists these two frequencies.

EDIT: June 21 2007
And these words from Eric Greisen

In FITS coordinates, the stated coordinate is at the center of each
voxel. It may be moved by having a non-integer reference pixel. In
normal VLA data we do not do that, so these frequencies are nominally
at the center of your band. I say nominally, because if the bandshape
is not symmetric about the center, then the effective frequency will
diverge from the one stated. True astrometry should never be done
with wide, asymmetric bandshapes (and the VLA at 50 MHz is

1 comment:

Kelley said...

Yikes...ok, I probably swapped the two in my post re: AIPS and MIRIAD. I'll check it out and edit the post as a appropriate.