trigonometric calculations

Hi everyone:

I'm new to this group, and looking for assistance. I've been struggling to
get to grips with 3D spherical geometry, and I could use some good
references to useful books and/or online resources.

My problem is to transform azimuth/elevation coordinates to the relative
azimuth/elevation as measured from a tilted observation plane. I have been
trying to work through the compound angle calculations from first
principles, but it occurs to me that this must have been done before. This
is not student homework; I am a professional optical engineer working on
solar collection systems.

Googling has found me many resources for celestial sphere calculations, and
some stuff on spherical geometry, which is great, but I haven't yet turned
up anything that seems to directly help with my coordinate transformation
problem.

Thanks in advance for any pointers
Ron Gibbs
Gibbs Associates