[daip] CALIB with model and GETJY

[Eric Greisen]

Olaf Wucknitz writes:

 > I have a question/suggestion for the use of CALIB with a model and 
 > subsequent GETJY.
 > 
 > If a model is used in CALIB for a source without an SU entry, CALIB 
 > determines the gains to scale the data to the flux of the model (sum of 
 > all clean components). That makes sense, of course. If we now call GETJY 
 > with this source in SOURCES (not in CALSOUR!, in other words we use our 
 > source as secondary calibrator), GETJY will determine the factor by which 
 > the flux of this source has to be multiplied to be correct. Since there is 
 > no flux in the SU table for our source, GETJY will assume 1 and scale this 
 > 1 to something which may or may not be close to the correct flux of the 
 > source. In the end, the flux of our source will be closer to 1 than to the 
 > real flux.
 > 
 > Do I understand this behaviour correctly? I know it by now, that I have to 
 > be careful when using models, but I notice that colleagues sometimes run 
 > into problems which are related to this. We just had a case where fluxes 
 > where incorrect by a factor of 500 or so. In such a case, even less 
 > cautious users will notice that something is wrong, but this will not 
 > always be the case.
 > 
 > One possible solution would be to modify the SU table and insert the flux 
 > of the model when CALIB is called for a source without a flux, but that 
 > may cause problems in other contexts.
 > Or CALIB could scale the model flux to 1 if the SU table does not have an 
 > entry.
 > In any case the user should be made aware of possible problems (assuming 
 > that my understanding is correct), maybe by warning messages or a line in 
 > the help file.
 > 
I know of no way to prevent people from "shooting themselves in the
foot" in this way.  I have added a message in FACSET which will be
stronger in CALIB than elsewhere and comments in the help files for
SETJY and GETJY.  Note that if the user uses SETJY to put some flux -
any non-zero, before CALIB will do - then GETJY will work as desired.
Also if one puts in the total flux reported from the model by CALIB
(routine FACSET) after CALIB but before GETJY, then GETJY will work as
desired too.

Eric Greisen