[AstroPy] Linearity correction by polynomial fitting
Chris Beaumont
beaumont at hawaii.edu
Thu Apr 18 15:33:28 EDT 2013
Hi Joe,
The astropy PR is good to know about -- keep me posted about what you
discover!
Fitting all ~3-5 million polynomial coefficients at once sounds like a bad
idea to me. The complexity of OLS increases roughly with ~coefficients^1-3
(depending on the method), so a single fit to 5-million unknowns is *much*
more computationally intensive than a million fits to 5-unknowns.
chris
On Thu, Apr 18, 2013 at 3:03 PM, James Turner <jturner at gemini.edu> wrote:
> Good.
>
> Tim Pickering also said that "something like scipy.optimize.fmin_powell"
> will handle vectors of coefficients so you can vectorize the whole thing
> but I think he forgot to copy you...
>
> James.
>
>
> On 18/04/13 15:00, Joe Philip Ninan wrote:
> > Hi James,
> > I think i shall try to keep the code as much pure python as possible.
> Installing
> > iraf on the controller machine just for one routine will probably be an
> over kill.
> > Nadia sent me link to pull request of astropy which can do fitting on 2d
> array.
> > So i only have to flatten by 3d to a 2d, with time along the column. (
> > https://github.com/astropy/astropy/pull/493 ).
> > This isn't merged to astropy yet, but i was told it will be merged soon.
> > Thanks for the suggestions,
> > -cheers
> > joe
> >
> >
> > On 18 April 2013 22:49, James Turner <jturner at gemini.edu
> > <mailto:jturner at gemini.edu>> wrote:
> >
> > > But I couldn't find anything for fitting a polynomial for every
> pixel
> > along an
> > > axis of an ndarray.
> > > Is there a tool to do this? Without implementing a python loop to
> loop
> > through
> > > each pixel and calculate coefficients?
> >
> > I'm not sure if there is anything yet (could tell you how to do
> > it in IRAF), as I think fitting has been a bit of a missing area
> > in Python until some recent work-in-progress. But wouldn't it
> > help to use Cython? I was also wondering whether a Python loop
> > will really be that slow if you're doing something relatively
> > non-trivial like a fit, but it probably will be since the axis
> > you're fitting will be much shorter than the image size...
> >
> > Cheers,
> >
> > James.
> >
> >
> > > Worst case scenario, i think i will have to write a C routine to
> do
> > polynomial
> > > fit and call it from python. But i would really love if a pure
> python
> > > alternative exists.
> > >
> > > Thanks
> > > -cheers
> > > joe
> > >
> > >
> > >
> > >
> > >
> > > --
> > > /---------------------------------------------------------------
> > > "GNU/Linux: because a PC is a terrible thing to waste" - GNU
> Generation
> > >
> > > ************************************************
> > > Joe Philip Ninan http://sites.google.com/site/jpninan/
> > > Research Scholar /________________\
> > > DAA, | Vadakeparambil |
> > > TIFR, | Pullad P.O. |
> > > Mumbai-05, India. | Kerala, India |
> > > Ph: +917738438212 | PIN:689548 |
> > > ------------------------------\_______________/--------------
> > >
> > >
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> >
> >
> >
> >
> > --
> > /---------------------------------------------------------------
> > "GNU/Linux: because a PC is a terrible thing to waste" - GNU Generation
> >
> > ************************************************
> > Joe Philip Ninan http://sites.google.com/site/jpninan/
> > Research Scholar /________________\
> > DAA, | Vadakeparambil |
> > TIFR, | Pullad P.O. |
> > Mumbai-05, India. | Kerala, India |
> > Ph: +917738438212 | PIN:689548 |
> > ------------------------------\_______________/--------------
>
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--
************************************
Chris Beaumont
Graduate Student
Institute for Astronomy
University of Hawaii at Manoa
2680 Woodlawn Drive
Honolulu, HI 96822
www.ifa.hawaii.edu/~beaumont
************************************
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