ATM When to re-coat your mirror

[Andrew.Martyn@dao.defence.gov.au]

I have just started to restore an old 6" scope in order to make myself a
nice portable scope. The mirror is at least 15 years old. It appears in
pretty good nick having being used mainly in a country/small city
environment. There are a few noticeable coating problems near the edge of
the mirror but they only come in a few mils. I intend to just put a thin
mask around the edge of the mirror. What I really want to know is how you
tell what the reflectivity state of the mirror is. I know there is supposed
to be a gradual degradation, but how do you tell if its still at 90% or has
slipped down to 70%?

Andrew Martyn
Canberra
Australia