Re: using rplot_counts with get_combined

On May 1, 2007, at 10:35 AM, juan luna wrote:

> -using plot_bsub_counts from .isisrc, if I choose
> popt.res=5, I got a Y axis in the residuals panel that
> has a \chi^2 label. Is this defined exactly as the
> delchi command in XSPEC, whose label is \Delta\sigma
> \chi^2?, what are the units of the ISIS's delchi
> definition?. I'm asking because I saw that Sherpa also
> add a different label with the delchi command.

Yes, it is the same as XSPEC's definition of \Delta\sigma\chi^2, I just 
went for a simpler label.  But basically, it's:

	sign(Data-Model) * (Data-Model)^2/\sigma^2

where \sigma will be automatically calculated properly whether you 
choose \sigma = square root of model counts, = square root of data 
counts, = Gehrels estimate from data counts, or = Cash statistic.

But lots of people, rather than using the above definition as the 
y-label use shorthand, like \chi^2.

Cheers,

Mike


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