Alun and Robin,
Thanks for your advices.
I will take measurements as suggested.
A tear down sounds like a good idea which should sort any instability problems.
1. There is a problem with wavelength shift any idea what’s causing this see attached.
2. The calibration lines are in focus at start and can go out of focus at the end of the session.
Both problems occur on and off, before motorising the collimator lens adjustment, I don’t expect that to make any difference.
Has anyone else had these problems ? if so how did you overcome them ?