Documenting orientation (PA) relative to other sources (e.g., pysiaf)

[cgarling]

When running romanisim on single SCAs it is useful to be able to get the SCA center in sky coordinates given the WFI_CEN to generate appropriate catalogs for each SCA. I wrote some code with pysiaf that I thought should do this and had difficulty debugging it. It turned out I was misunderstanding the position angle arguments between romanisim and pysiaf (the argument to their attitude function). I think the most relevant docs here are

romanisim/romanisim/wcs.py

Lines 52 to 54 in 42b504c

	pa_aper : float
	position angle (North relative to the idealized Y axis) at the aperture
	V2Ref/V3Ref

and

romanisim/docs/romanisim/running.rst

Line 42 in 42b504c

--roll ROLL Position angle (North towards YIdl) measured at the V2Ref/V3Ref of the aperture used. (default: 0)

The former is more informative but the latter is more user-facing and I didn't appreciate what YIdl meant when I saw it at a quick glance. A search for YIdl in the RDox page on coordinate systems doesn't turn anything up either.

I think it would be nice to explain that you are using the idealized frame with a bit more clarity. Maybe an additional note on the "running" documentation page with some explanatory text / link to the RDox page would also be appropriate? I can put together a PR if you'd like.

[schlafly]

Thanks Chris. Yes, feel free to make a PR; I'll admit the different coordinate systems aren't completely clear to me either.

And FWIW, I think the "right" way to implement this in romanisim would be to add more of the apertures from the SIAF into romanisim. The v2/v3 of each SCA can be obtained via something like
https://github.com/spacetelescope/romanisim/blob/main/romanisim/util.py#L287-L291
(in the absence of pysiaf)
So probably one could replace the current --boresight argument with a more general --aperture argument that defaults to WFI_CEN, but could be any other aperture, and then rotates accordingly.

It will probably take a little care to make sure all of the SCAs share a common WCS in this approach, though.