Re: fits in channel space

Hi Maurice,

I'd like to add one thing to the thread about fitting data different 
from counts/bin.
Like Mike, I also prefer to use ISIS fitting engine instead of array_fit 
due to the better control of parameters (e.g. thaw/freeze, tie, 
set_par_fun, ...) and powerful related functions like conf_map_counts 
(and -- with the brand new ISIS 1.5 -- also because of parallel 
fit-algorithms).

Unless you follow David's suggestion
 > If you have real-valued data,
 > you can use define_flux(), fit_flux() instead.
, but use define_counts and fit_counts, you should know about the 
Minimum_Stat_Err variable, which might change your errors:

isis> help Minimum_Stat_Err
Minimum_Stat_Err

  SYNOPSIS
     Defines the minimum valid uncertainty for counts data

  USAGE
     (intrinsic global variable)

  DESCRIPTION
     When input data values are to be treated as counts, isis
     attempts to validate the uncertainty values, requiring them all
     to be greater than or equal to Minimum_Stat_Err, where
     Minimum_Stat_Err has a default value of 1.

     For example, defining a single-bin counts dataset with has bin
     value 1.e-5 and uncertainty 1.e-8:

     isis> define_counts (1.0, 2.0, 1.e-5, 1.e-8);
     1
     isis> print(get_data_counts(1).err);
     1

     Note that isis replaced the 1.e-8 uncertainty value because it
     was inconsistent with default expectations for this kind of
     data.

     To force isis to retain positive counts-uncertainty values
     smaller than unity, define a minimum (positive) allowable
     uncertainty value using Minimum_Stat_Err:

     isis> Minimum_Stat_Err=1.e-20;
     isis> define_counts (1.0, 2.0, 1.e-5, 1.e-8);
     2
     isis> print(get_data_counts(2).err);
     1e-08

     Here, isis kept the 1.e-8 uncertainty value.

     When input data is to be treated as flux, this kind of
     filtering does not take place:

     isis> define_flux (1, 2, 1.e-5, 1.e-8);
     1
     isis> print(get_data_flux(1).err);
     1e-08

     Uncertainties for flux values must be positive.

     To shut off warnings about invalid uncertainties being
     replaced, set Warn_Invalid_Uncertainties=0.


  SEE ALSO
     load_data, define_counts, define_flux, fit_counts, fit_flux


In my installation of ISIS, Warn_Invalid_Uncertainties is 0 by default, 
so one might not notice immediately that the errors aren't what one 
wants them to be. [Which is a common problem, right? ;-)]

Cheers,

Manfred
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