[daip] Rescaling at end of IMAGR

[Andrew Biggs]

Dear daip. I have a question regarding the rescaling that goes on at the
end of IMAGR when multiple channels have been imaged separately. After
each channel is imaged (I do not clean as the continuum has been
subtracted) I'm informed that the field is being rescaled to fix the units
and I think that this is something to do with the different size of the
dirty beams compared to what is written in the header. This has the
effect, I think, of making the measurement of a flux in the map correct. I
can check this by running imagr on all channels and then on just one. If I
run an IMSTAT on the same channel in each, the reported flux is the same
in each case.

However, and this is where I worry, the rms and max/min are not. Obviously
this must be the case given that the multi-channel images have been scaled
by some factor. However, I am going to stack these images (weighted by the
noise) and worry that I'm artificially inflating the noise unnecessarily.
For example, I can run IMAGR specifying a restoring beam (using BMAJ and
BMIN) and making a map of the same channel in the two different ways
produces identical images as the pixels weren't scaled during the
multi-channel imaging, but no actual restoring was done in either case (as
nothing is being cleaned). Should I in fact be preventing any rescaling
going on? Or would convolving each source to the same resolution,
post-IMAGR, be tantamount to the same thing?

The stacking, by the way, is of the 3-d variety, the map of a channel from
one source being combined with the map of a different channel from another
source. This is all taken from a wide-field dataset. I'm making small maps
of the individual sources after rotating them to the phase centre and then
stacking them in IDL. This, among other things, presumably allows UVLSF to
work better with a low-order fit as all the sources will be at the phase
centre.

Cheers,

Andy