RIX Roy: True North

[hugobuddel]

"Note that the angle we care about is the direction of ""North"" relative to the METIS instrument. This is related to the parallactic
angle (see e.g. Appendix A of https://www.overleaf.com/read/jjkcmjzvcgtv
We will get the direction of ""North"" relative to ""up"" on the Nasmyth platform from the TCS and the current value thereof should
be stored with each DIT. Apparently due to the optical design of the ELT, this direction can deviate substantially from what one
would calculate from the plain trigonomtry (as in the aforementiond appendix A) ignoring the details of the ELT design and operation."

Yet another header keyword that none of us knew about?

Sounds easy enough to just use that number.

https://www.overleaf.com/project/5f92c5d0f5223f0001d4362f is E-ANR-MPIA-MET-1095 "True North accuracy in METIS" 0-2 xx-06-2023

I recall there are several places in the document about the parallactic angle, so need to ensure we do them all correctly.

[hugobuddel]

Postponing this to after FDR