I came across this situation that I need to build a transformation to describe the projection of a detector array specified in u.um to some fiducial plane which is used in lab testing the array.

We construct maps from such tests and would like to build a GWCS object as the WCS of these maps.

In this case, the WCS is an AffineTransform2D that takes inputs unit 'u.um' to 'u.pix', which requires pixel_scales specified as u.m / u.pix as the input_units_equivalencies.

I was wondering if this makes sense and I am glad to contribute a initial pull request. Mostly it gonna be a duplication of u.pixel_scale (say, u.pixel_scale_phys), but just replace u.deg there with u.m. Or it makes more sense to extend the current pixel_scale equivalency to cover this situation.