Re: long term variation and instrumental drift

[Chris Lloyd <cl@astro1.bnsc.rl.ac.uk>]

Long term changes can be caused by aging of the filters but this should
result in a very slow evolution of the transformations coefficients. I'm
not sure if I understood correctly about the transformations coefficients
being re-determined every night - this is not a good idea. Your
instrumental system should be stable enough that once the coefficients
have been well determined they can be applied for some time without
change. If you find that they change then either your instrumental system
is unstable or you are just chasing the errors around. Bad news either
way. Finally, there will inevitably be improvements to the pipeline
processing so the raw data, in some form, needs to be held in a way that
allows it to be reprocessed.

Chris