[daip] IF handling

[Orla Aaquist]

Sirs,

I am a little confused how the two IFs are handled by IMAGR (AIPS version
15APR98).

I am in the process of comparing, via UVSUB, two data sets form poorly
resolved planetary nebulae.  I am looking for expansion across the minor
axis (cf. Collin Masson, 1996).  Epoch 1 is from before 1987 and Epoch 2
data is from 1998.  When we originally handled the Epoch 1 data, we combined
the two IF's using the aips task DBCON (after initial calibration), and the
lower IF (14.9149 GHz) was usually shifted to the upper IF (14.9649 GHz)in
the concatenation.  Now, when imaging this old DBCONed data using the task
IMAGR, the map frequency shown in the image header (using IMHEAD) is 14.9649
GHz.  However, when I use IMAGR on our latest uv-data, the frequency in the
image header is the average of the two IFs (14.9399 GHz).  Strangely,
however, the corresponding (new) uv-data set reports only the upper IF in
the header.  Why is this?

For our new data, we split the source data from the calibration file using
SPLIT.  In the past, we loaded the two IFs as separate files (using UVLOD I
think, but it is a long time ago) and then used DBCON. With the new data,
the two IFs were extracted into the same file and DBCON did not have to be
used.  I am unclear how the two IFs are treated in these two cases.  When I
use IMAGR on the old data, the frequency reported in the image header
remains at the upper IF.  When I use IMAGR on the new data, the frequency
reported in the image header is the average IF. Why does this happen?

I have the impression that a change (decrease) in frequency will created a
slight, self-similar expansion of the image.  This worries me because a
small expansion due to a frequency shift may give rise to a false
interpretation of the data.  Is there some way to shift the frequency of the
lower IF to the value of the upper IF in the new data so that the data at
both epoch are being reported as the same frequency?  I could restrict
myself to only one IF, however I would like to know if there is another
solution, or if I am worrying needlessly.

I have tried to use UVFIX, but it shifts the coordinates to Epoch 2000, and
the image I obtain is smeared.

One final, probably related, question.  The number of visibilities reported
in the image header of the old data is the sum of the visibilities in the
two concatenated files, whereas the number of visibilities reported in the
header of the new data seems to show only the number collected by one IF.
Is this correct, or am I miscounting?

I hope someone can clarify and offer some suggestions.

Thank you.

Orla Aaquist