How to use Rigid registration

Briefly, I want to gain the manual of Rigid registration in Fiji Plugins.

I took images in two-photon microscopy, in 2 different wavelengths.
The images are out of alignment slightly.
The images need registration for making ratio image.

I tried Fiji, Plugins> Registration> Rigid registration.
By using this at normal value, the gap is improved, but not perfect.
So, I want to find optimal parameters.

I searched this manual, but I can’t find it.
Could you tell me about the meaning of parameters? Or the manual’s URL?

About parameters, we can see “n initial positions to try”, “tolerance”, “level”, “stopLevel”.

Thanks.