Custom equation error in fitting toolbox

Hi, I'm trying to fit my data with a specific function trough the fitting tool. The equation that my datas should fit is , it's the distribution of a laser light interference when it pass trought one fenditure.

By the way the fitting tool cannot fit my data, the error reported is: "NaN computed by model function, fitting cannot continue.

Try using or tightening upper and lower bounds on coefficient". I think the problem is cause by because when I dont add to the box there's no error.

[Alex Sha]

Hi, post out your data if possible, it will be convenient for others to try your problem

[Pietro Scapolo]

Done! Thank you!

Answer 1

[Chunru]

The following function will result in undefined value for sin(x)=0

sin(a*sin(x))/(a*sin(x))

You can change this into sinc function (which take care of 0/0 case):

sinc(a*sin(x)/pi)
% which is equivalent to the first expression and eliminate the problem at
% sin(x)=0. 
% doc sinc

[Pietro Scapolo]

Thank you! I tried but it doesnt work, the curve it generates it's not the one I wanted... In the screenshot i put the numeric value because it corrispond to the max value of y.

[Chunru]

% The whole function should be
b*(sinc(a*sin(x)/pi)).^2
% the above is part of the function that cause the problem

[Pietro Scapolo]

Thank you! It worked!