Background fit

lavn2

Hi,

I have some data with a background which cannot be fit well to a Chebyshev polynomial without using such a large number of coefficients that I get overfitting. In FullProf, I know it is possible to use an alternative method for the background, where we set background points manually and just refine their positions individually. Is such a technique possible in Topas? Please could anyone advise me on how to do this?

Thanks

alancoelho

Using a straight line instead of bkg may be of use, see http://topas.dur.ac.uk/topaswiki/doku.php?id=background_straight_line_segments

In Version 7 you can use the functions of Bkg_at and Yobs_at in the following manner:

	xdd ...
		penalty = 10000 (Bkg_at(#) - Yobs_at(#))^2;
		penalty = 10000 (Bkg_at(#) - Yobs_at(#))^2;
			...

The # numbers corresponds to x-axis values.

lavn2

Thanks Alan, it's very useful to know that this is possible.