[AstroPy] Info LevMarLSQFitter

Thu May 14 10:09:54 EDT 2020

Dear Diana,

I should use some particular constraints for the fitting I guess...and
> probably it will be the best solution.
>

Absolutely! I would definitely take advantage of imposing the constraints
on the parameters in whatever fitting algorithm I would use.
That's possibly the only way to make sure that you don't get physically
unreasonable results.

Best,
Zé

On Thu, May 14, 2020 at 10:00 PM Diana Scognamiglio <
dianasco at astro.uni-bonn.de> wrote:

> Dear Zé, Peter, Peter and Paul,
> thanks for your reply.
> I didn't find yet the solution to my problem.
> The problems could be due to the fact that the fit just converges in a
> local
> minimum and not in the global one or to the fact the the fit is not well
> constrained...in any case it sometimes completely fails giving for example
> flux=0!
> I should use some particular constraints for the fitting I guess...and
> probably it will be the best solution.
>
> About the Peter (Dwing)’s questions: I chose these ranges for the initial
> parameters: n_Sersic=[1.0,2.0,3.0,4.0,5.0,6.0] and  R_eff=[1.0,5.0, 10.0,
> 30.0, 50.0,100.0], and ini_flux=[100, 1000].
> If I repeat the run (using the same initial parameters), yes I get the
> same
> results.
>
> I also used the SimplexLSQFitter, but the things doesn’t change.
>
> Thanks again.
> Cheers,
> Diana
>
> On Tue, 12 May 2020 14:36:11 +0200
>          Peter Erwin <peter.erwin at gmail.com> wrote:
> > Hi Zé,
> >
> > As an aside, the fact that the Sersic2D model is nonlinear in its
> parameters is formally
> > irrelevant, since Levernberg-Marquardt is by design a *non-linear
> least-squares* algorithm.
> > (Levenberg’s original 1944 paper describing the algorithm was titled "A
> Method for the Solution of Certain Non-Linear Problems in Least Squares”.)
> >
> > In practice, I find that Levenberg-Marquardt *usually* finds the global
> minimum
> > in galaxy image-fitting problems (e.g., I get the same solution when
> running Imfit
> > with Levenberg-Marquardt or with the Differential Evolution algorithm,
> which
> > doesn’t use initial values), though there *are* cases where L-M does get
> trapped in
> > local minima. (The fact that Diana says she “always” gets a different
> result for different starting parameters maybe suggests something else is
> the problem.)
> >
> >
> > Diana — have you tried using an alternate fitting algorithm, such as the
> SimplexLSQFitter
> > class? (Simplex fitters tend to be less likely to be trapped by local
> minima than
> > Levenberg-Marquardt, though they’re usually significantly slower.)
> >
> >
> > cheers,
> >
> > Peter [Erwin]
> >
> >> On May 12, 2020, at 11:17 AM, Zé Vinícius <jvmirca at gmail.com> wrote:
> >>
> >> Hi Diana,
> >>
> >> From my understanding, LevMarLSQFitter uses the least-squares function
> as the objective to be minimized using the Levenberg-Marquardt algorithm.
> >>
> >> Because Sersic2D is a non-linear model on the parameters to be fitted,
> the problem is most likely non-convex, which means that Levenberg-Marquardt
> only guarantees to find a local solution, which will depend on the initial
> value for those parameters.
> >>
> >> Best,
> >> Zé
> >>
> >> On Tue, May 12, 2020 at 4:53 PM Diana Scognamiglio <
> dianasco at astro.uni-bonn.de> wrote:
> >> Dear Astropy Staff,
> >>
> >> I am Diana Scognamiglio and I have some problems to deep understand how
> LevMarLSQFitter works.
> >>
> >> I am trying to fit a mock Sersic galaxy using the LevMarLSQFitter
> classe, coding these lines:
> >>
> >>
> >> ini_mod =
> >> EllipSersic2D(flux=ini_flux, r_eff=ini_r_eff, n=ini_n,
> x_0=stampsize/2.0, y_0=stampsize/2.0, g1=0.0, g2=0.0)
> >>
> >> fitter = fitting.LevMarLSQFitter()
> >>
> >> fit_mod = fitter(ini_mod, x_array, y_array, stamp, weights=weights,
> maxiter = 1000, acc=1.0e-7, epsilon=1.0e-6,
> >> estimate_jacobian=False)   I have figured out that if I change the
> initial parameters in ini_mod (i.e., ini_flux, ini_r_eff, etc), I
> counterintuitively obtain always a different result although the galaxy I
> am fitting is the same. How does the fit depend on those initial parameters?
> >>
> >> Please, can you help me with this issue?
> >>
> >> Thank you in advance.
> >>
> >> I wait for hearing from you.
> >>
> >> Best regards,
> >>
> >> Diana Scognamiglio
> >>
> >>
> >> _______________________________________________
> >> AstroPy mailing list
> >> AstroPy at python.org
> >> https://mail.python.org/mailman/listinfo/astropy
> >>
> >>
> >> --
> >> Zé Vinícius
> >> https://mirca.github.io
> >> _______________________________________________
> >> AstroPy mailing list
> >> AstroPy at python.org
> >> https://mail.python.org/mailman/listinfo/astropy
> >
> > =============================================================
> > Peter Erwin                   Max-Planck-Insitute for Extraterrestrial
> erwin at mpe.mpg.de              Physics, Giessenbachstrasse
> > tel. +49 (0)176 2481 7713     85748 Garching, Germany
> > fax  +49 (0)89 30000 3495    https://www.mpe.mpg.de/~erwin
> >
> >
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> > https://mail.python.org/mailman/listinfo/astropy
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-- 
Zé Vinícius
https://mirca.github.io
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