[daip] subtracting 'calibrated' source models from UV data

[Huib Intema]

Dear Eric,

I'm using AIPS tasks to subtract bright sources in large fields obtained 
from low frequency observations (<300 MHz). Each bright source generally 
has a unique calibration (SN table) that is valid in that particular 
viewing direction due to ionospheric phase disturbances. This peeling 
process involves many repetitions of the following call sequence:

1. SPLIT to apply a given SN table to the UV data
2. UVSUB/OOSUB to subtract a source model from the UV data
3. CLINV to invert the SN table used in step 1
4. SPLIT to apply the inverted SN table to the UV data resulting from step 2

I'm worried about applying and un-applying SN tables multiple times, as 
this introduces noise due to numerical round-off errors. A numerically 
more stable way is to generate a UV data set from the source model 
(through UVSUB/OOSUB opcode 'MODL'), apply the inverted SN table to this 
data set and subtract it (visibility by visibility) from the original UV 
data.

I'm aware of the AIPS task UVMTH, but this task does time-averaging on 
one of the UV data sets before combining two UV data sets, so this is 
not useable for my proposed scheme. As far as I know, what I propose is 
currently not possible in AIPS.

For a possible implementation of this functionality in AIPS I can think 
of two possibilities:
* Change in UVSUB/OOSUB so that one can apply an SN table to the UV data 
that is internally generated from the specified source model before it 
is subtracted from the specified UV data.
* Change in UVMTH so that one can subtract two UV data sets visibility 
by visibility without time-averaging, possibly allowing application of a 
SN table to the second UV data set (the one that is subtracted from the 
first).

Would you consider adding this functionality to AIPS. I would be most 
grateful.

Best regards

Huib Intema

PS: From the "help uvmth" text it was not clear to me which UV data set 
is time averaged, as at the start of this text says "The first input 
data set is averaged over all times ...", while under the description of 
OPCODE it says "'SUB' subtracts the time averaged values of the second 
input data set from the first ...". I guess the last one is right?

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               Huib Intema, PhD student
         Leiden Observatory (Leiden University)
      +31 (0)71 527 5852, intema at strw.leidenuniv.nl
         http://www.strw.leidenuniv.nl/~intema
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