[daip] AIPS handling of weights and flags

[Eskil Varenius]

Dear Amy and Eric,
Thank you for two very good answers! Sorry for my late reply, I've been
home for a week having a cold. Now I'm almost well again and will once
again approach AIPS, this time a bit wiser thanks to you.

Have a nice day,
Eskil Varenius

2012/2/22 Eric Greisen <egreisen at nrao.edu>

> Eskil Varenius wrote:
>
>> Dear AIPS support,
>> I wonder about how AIPS handles weights and flags. Please direct me to
>> the appropriate place if this address is not the correct one. I've searched
>> the net, cookbook and explain-pages without a clearcut answer. I would be
>> very happy if someone could increase my knowledge around the following
>> questions.
>>
>> What is the meaning of negative weights for visibilities? Is there a
>> general way in which such visibilities are handled throught AIPS?
>> Suppose that I modify weights on some visibilities using WTMOD (or
>> ParselTongue) to -1. If I then use the task FIXWT to modify weights to
>> reflect the amplitude scatter of data, will this also modify the negative
>> weights? Or, will it only modify positive weights?
>>
>
> Negative or 0.0 weights are viewed as flagging the data and the visibility
> values for such data are seldom changed and never used.  I cannot promise
> what individual tasks might do, but the <= 0 status of a weight should
> never change.
>
>
>  Finally I wonder about SPLIT. SPLIT can apply an FG-table. If I make a
>> single-source file using SPLIT from a multi-sourcefile, and chose to flag
>> data using the multi-sourcefile-FG-table, will SPLIT remove every single
>> flagged visibility? Or - since I don't see any FG table in the produced
>> single-source file - are some visibilites still transferred for some reason
>> but with modified weights? What about visibilites with negative weigths,
>> will these count as flagged when using SPLIT?
>>
>>  When an FG table is applied, the calibration routines change the weights
> to - abs(weight) and if all weights in the visibility record (one baseline,
> one time but all IFs and all polarizations and all spectral channels) are
> <= 0, the record is discarded and will not appear in the output data set.
>  Thus some vis may appear with <= 0 weights (say you flagged LL but not RR
> for some time) in the output whilst others will disappear (you flagged both
> RR and LL at that time).  Visibilities with negative weights in the input
> set to SPLIT continue to have negative weights and will disappear (or not)
> depepnding on the other parts of the vis record.
>
> Eric Greisen
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listmgr.nrao.edu/pipermail/daip/attachments/20120223/b5201f94/attachment.html>