search

Gabriel-p / photpy

Python CCD photometry
http://photpy.rtfd.io/
GNU General Public License v3.0
7 stars 0 forks source link

How is 'sigma' obtained by fitparams?? #11

Closed Gabriel-p closed 6 years ago

Gabriel-p commented 7 years ago

The photcal/fitparams IRAF task shows a sigma column when :vshow is used, with values for each star used in the fit. These values are used to obtain the reduced chi, and others statistical parameters.

How are these sigma values obtained? The code seems to write to that column a 1/sqrt(X) and the description of :results (in inlfit, see below) states that this is a weight.

fitparams uses INLFIT ("The interactive non-linear least squares fitting package") which uses the standard Levenberg-Marquardt non-linear least squares algorithm to fit the data.

Detailed descriptions of the algorithm can be found in the following two references.

  1. Bevington, P.R., 1969, Data Reduction and Error Analysis for the Physical Sciences, Chapter 11, page 235.
  2. Press, W.H. et al., 1986, Numerical Recipes: The Art of Scientific Computing, Chapter 14, page 523.

Related data:

Gabriel-p commented 7 years ago

I've almost figured out how sigma is obtained. I still need to understand how the average scatter is obtained, which is used to calculate sigma.

fitparams.py.zip

Gabriel-p commented 6 years ago

I give up. I'll use a different fitting method anyway.