@adonath I am wondering what the reason for removing this section is. I see it was moved to the __call__ function of `NonLinearLSQFitter` but it was meant to be applicable to all fitters. The `SLSQPFitter` can also be passed an analytical derivative and supposedly  other fitters in the future should be able to take one too. Isn't moving this section of the code to the `NonLinearLSQFitter` only going to make this impossible?