[daip] Confused about APCAL

[Leonia Kogan]

Al,

You wrote:

>One gripe is that APCAL (at least the 15OCT99 version that
>we're using) only accepts an ASCII weather file, not the
>WX table produced by the automatic calibration.  I don't
>know if that's true for the Classic AIPS, since we haven't
>installed it owing to the unavailability of a binary-only
>installation.

The version 31DEC99 accepts the WX table produced by the automatic 
calibration.

Look the following correction made by Ketan Desai:

10420.  November 15, 1999        APCAL                      Ketan
        APCAL would refuse to do opacity corrections if source fluxes
        were not found in the SU table.  Now, APCAL will go ahead assuming
        a source flux of zero - a reasonable default - with a stern
        rejoinder to the user that they may have wanted to run SETJY.
        Also, APCAL would refuse to do opacity corrections if a text file
        containing weather information was not supplied.  Internally, the
        only use of this file was to find the maximum recorded ground
        temperature for each station.  The WX table is now supplied by
        VLBA calibration transfer and APCAL needs to be made to read that
        table.  For now, the behaviour of APCAL has been changed not to
        crash with opacity corrections are requested and no weather text
        file is supplied.  The new behaviour is to assume a default
        maximum ground temperature of 20 C at each station, _if_ the
        INFILE is left blank [with a warning of course...].
        Moved nowhere.

If you have a problem having 15DEC99 version I can send you the fortran codes 
of the last APCAL version. Having had the codes you can create your 
private version of APCAL that coincides with the 15DEC99 version.

Let me know please if you want it. 

The problem you describe at the beginning of your message may dissapear 
using the last version of APCAL.

Leonia
----- Begin Included Message -----

>From owner-daip at kochab.cv.nrao.edu Sun Apr  2 09:19 MDT 2000
Date: Fri, 31 Mar 2000 18:25:10 -0500 (EST)
From: Alan Marscher <marscher at buast0.bu.edu>
To: aipsmail at cv3.cv.nrao.edu
Subject: [daip] Confused about APCAL
Cc: marscher at buast0.bu.edu
X-Sun-Charset: US-ASCII
Sender: owner-daip at kochab.cv.nrao.edu
Precedence: bulk
Content-Type: text
Content-Length: 1575
X-Lines: 36
Status: RO

Hi,

Since I and my collaborators reduce a lot of 43 GHz data,
we need to correct for opacity using APCAL.  We investigated
the behavior of APCAL on a dataset in March 1998 using various
settings of the APCAL parameters.  We found that if we set
TREC equal to the nominal values from the gains.key file and
specified an approximate value for the zenith opacity, the
APCAL solutions looked OK but the flux scale ended up being
too low by about 30%.  (This was true for another 5 epochs as
well.) Setting TREC=0 produced a flux scale that was about right. 

However, for epochs starting in Apr 99 (after the automated
calibration was installed), things seem to have changed. Setting
TREC=0 leads to very high fluxes and odd looking time
variability of the opacity corrections. This occurs for data taken
during good weather as well as bad. APCAL seems to be too poorly
constrained. TREC=nominal values seems to work better than it did
before.  Has something changed?

The EXPLAIN file for APCAL isn't very clear about what is done
to the data, so the best parameter settings are not obvious.  The
Cookbook doesn't help much, either.  What are the recommended
settings for APCAL in cases in which the opacity corrections are
fairly high?

One gripe is that APCAL (at least the 15OCT99 version that
we're using) only accepts an ASCII weather file, not the
WX table produced by the automatic calibration.  I don't
know if that's true for the Classic AIPS, since we haven't
installed it owing to the unavailability of a binary-only
installation.

Thanks for looking into this,

Al


----- End Included Message -----