These are chat archives for astropy/astropy
Cadairi think you will have to define a custom model for that one. It will be hard to add a constraint to the fitter I think. (I am not an expert on modelling though).
CadairI guess one way to do it would be to define a custom model where you reduce the number of fitting parameters by one by making one a function of another one
CadairI think what you want to do is have an
evaluatefunction that ties two of the parameters together and then calls the old evaluate method
Cadairon your new model subclass
Cadairthen the fitter should just work
Cadairbut for the sake of the fitter I think you will want to define your model to have one less parameter or else it will be trying to optimise a parameter that has no impact on the result