[daip] AIPS question: IMFIT vs JMFIT

[Eric Greisen]

On 09/16/2015 03:18 AM, lisakov wrote:
> Dear Sir,
>
> I have faced a need to perform an image plane fitting of a structure of
> an AGN with several Gaussian components. Particularly it is a
> multi-epoch stacked image of the 3C273 at 7 mm.
>
> I decided to use AIPS and found 2 tasks which satisfy my needs: IMFIT
> and JMFIT.
> I found then that these tasks perform differently. With the same
> starting model (which is quite a good starting point, I believe) JMFIT
> works well but IMFIT complains on strange value of some parameters (I
> think, not the initial one, but the one which occurs during the evaluation)
> IMFIT1: STRANGE VALUE FOR COMP= 1 PARAM= 4 VALUE= 5.111E+02
> IMFIT1: RIDICULOUS VALUE FOR SOME PARAMETER AT NITER= 40
> IMFIT1: CHECK INPUT MODEL. MODEL VALUE IS CRAZY
>
> and then "Purports to die of UNNATURAL causes".
>
>
> Regarding this different behaviour I wonder, are there any caveats of
> using JMFIT instead of the IMFIT?
>
> And yet another question. I am not totally satisfied with JMFIT because
> I did not manage to make JMFIT use only pure circular Gaussians. I've
> tried to let only major axes to vary, but this performs as it should -
> it results in elliptical Gaussians. So my question: is there a
> possibility to tell JMFIT to use pure circular Gaussian components?
>
>
> AIPS version is 31DEC14.
> Attached to the e-mail are
> * image to fit
> * commands to set initial parameters and run both IMFIT and JMFIT

I may play with your data later - but I know that the mathematician that 
wrote both tasks did JMFIT second because he preferred that method.  So 
there is not problem with taking the JMFIT results.  The two give very 
similar results on relatively simple sources but with a more complex 
situation I am not surprised that one fails to converge.
I once followed the fitting in SAD (same math as JMFIT) and was amazed 
at how strange some of the models were during the fit even when it 
finally converged on something sensible.

Circular Gaussians would require significant changes in the code of the 
tasks (doable but not with very much demand).

Eric Greisen