[daip] help with AIPS

[Melanie Gendre]

Hi,

I have been in communication with Eric Greisen about some issues I have
with AIPS, but I am still nowhere near understanding what's going on, and
I can't seem to get anything from the Cookbook.

Here's the situation:
I have VLA data at 90cm taken in BnA and CnB configurations, covering a 6
deg^2 field in 2 pointings (I have therefore 4 uv data files when I do
SPLIT).
I did the calibration of the data using the standard procedure FILLM,
LISTR, QUACK, SETJY, CALIB, EDITA, GETJY, EXTDEST, CLCAL, SNPLT, SPLIT,
UVSRT.

The difference between these data and what I usually do is that I have
several sources for each uv file (each pointings) and I'd like to get flux
and contour plots for each sources individually.

Eric suggested that I:
- use SETFC to get the recommended CELLSIZE
- change the CELLSIZE to a rounder close number and re-run SETFC to write
the boxfile
- use FLATN to make one large image of all the facets
- use SAD or JMFIT to get the flux

My (numerous) issues are:
(1) What are the differences between "pointings", "sources", "fields" and
"facets" in the AIPS jargon? I am confused between all those terms. If I
have several sources in one pointing, does that mean I have one field or
several fields?
(2) Why do I have to run SETFC twice?
(3) What exactly does the BOXFILE do? Does it isolate my sources?
(4) When I follow the procedure with SETFC, the output boxfile is empty.
Why? (My guess would be that it's because nowhere do I tell AIPS there
should be several boxes)
(5) I actually know where the sources should be (I have the list of
coordinates). Could that help in the reduction? If so, how?
I tried to input this list of sources in INFILE in SETFC, and I get a
BOXFILE output, but I am not sure of how reliable it is.
(6) Which leads to the question: how do I isolate my sources?
(7) In JMFIT, how do you point to each source individually? I can't see
any input coordinate/file


Maybe getting some insight of what the procedure is when you don't know
what sources you have in the image would help me understand how this all
works. I am getting really confused and a bit frustrated, and I'm sure
it's just because I'm missing (or misunderstanding) something small...

I include below the correspondence with Eric.

Thank you for your help
Melanie Gendre

--------------------------
Previous e-mails:

Melanie Gendre wrote:
> Hi,
>
> My name is Melanie and I am having some trouble with AIPS.
> I have data at 90cm, with 150 sources detected in 2 pointings of the VLA
(BnA and CnB configurations). I need to get contour plots of the sources
and estimate their 90cm flux.
> I am familiar with data reduction for one source per pointing, making
low
> resolution images at higher frequencies. It is my first time with
multiple
> sources per pointing though.
>
> As far as I understand, the calibration process for multiple fields is
similar to single field calibration. I use the standard procedure:
FILLM, LISTR, QUACK, SETJY, CALIB, EDITA, GETJY, EXTDEST, CLCAL, SNPLT,
SPLIT, UVSRT
>
> After that, I am a bit lost. I see that we can get recommendation for
each
> field size with SETFC and then use IMAGR to clean and make the image,
but
> I am a bit fuzzy on the details, especially the use of BOXFILE. So here
are my questions:
> - What is the format for BOXFILE if I want to use it in SETFC and IMAGR?
It is a different format that the BOXFILE use in the CLEAN section of
the
> cookbook as I have to specify FLDSIZ, RASHIFT and DECSHIFT...
> - What tasks should I then use to get images and flux of my separate
sources?

There is only one BOXFILE format used by all aips tasks.  SETFC will, if 
you set CELLSIZE and IMSIZE to 0.0, recommend a cell size and image  size.
 The latter will be a correct power of 2 but the former will be  some odd
number.  I recommend deleting the boxfile written by that  SETFC, and then
changing the CELLSIZE returns by SETFC to a nearby  rounder number.  Then
run SETFC again.  This will write a BOXFILE with  all of the fields
(facets) specified with center RA and Dec and with  default Clean boxes -
large circles that avoid the edges and esp corners  of the image.  You can
use the TV during IMAGR to change these boxes.  With SETFC and a BOXFILE
(and use OBOXFILE too in IMAGR to capture the  boxes you set with the TV)
there is no need to specify the adverbs  RASHIFT, DECSHIFT, FLDSIZE,
NBOXES, CLBOX.  If you already know that  there are 150 sources, I would
think that you must already have run IMAGR??

After the imaging and self-cal are done, use FLATN to make one large 
image of all the facets.  Then SAD or JMFIT can be used to fit the flux 
and size of the sources - the former will find them and try to fit them 
but will omit those it has trouble with and so is incomplete.  The  latter
has to be pointed by hand at each source individually.  If a  source is an
extended mess rather than a clean Gaussian, use TVSTAT or  IMSTAT
carefully to estimate its flux.

Eric Greisen
-------------------------------------------------------------------------
Melanie Gendre wrote:
> Hi Eric,
>
> Ok, I am completely confused. I think I might have missed a step somewhere
> but I can't see where...
> After I calibrate the data and split the file, I get 4 .SPLIT. files for
> each of my 4 fields (2 different fields observed with 2 different VLA
> configuration).
>
> As you suggested, I try to run SETFC to get the boxfile to be written.
> However, at no point do I specify the number of sources I have in each
> field or their position. Hence, I'm pretty sure I'm missing steps between
> splitting the file with SPLIT and cleaning with IMAGR... Help!

Mostly observers do not know what sources they have or where.

SETFC will set up facets that cover the requested area fully - they will
include all of your sources.  If you wish to restrict the Clean to those
sources then you can add Clean boxes in IMAGR (use OBOXFILE too so they
are remembered).  There are programs that will read a source list and
construct the boxes for you - task BOXES for one.

You are not missing steps - you know too much about your field so you
want to enter extra - which BOXES will let you do (after SETFC).

Please use daip at nrao.edu - I am about to leave on vacation.

Eric Greisen


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