STC V1.21 posted as PR

[Alberto Micol]

Dear Arnold,

I'm sorry, I had so much in mind the observation data model,
actually the characterisation DM, that I forgot to specify the context
and got myself confused.

What I had in mind was a CCD image taken with a given roll angle, and
I was looking into the STC to see which definition for the roll angle
you came up with. So, I confused the roll angle with the position
angle. The roll angle is the position angle of the Y axis of the 
detector.
(and so I answered myself).

Sorry for bothering you all!

Alberto