Full size solar filter -autocollimation?

[miki63au]

Hi people,
would this be a silly idea?
Full aperture glass solar filter is used for high resolution photograph.
So way not use the reflection for a Cassegrain or Newtonian testing or collimation?
A full size test glass with good quality cost thousands! Not this one!

And if it good enough, way not try to get rid of the coating (acid etching?)
and use it as a cheap optical window?

Is this a stupid idea?
Miklos.

[Geoff45 (Geoff)]

A novel idea. There are a couple of possible downsides.
1. A refracting surface only needs to be figured to a quarter of the accuracy of a relecting surface, so the solar filter, while it may be good enough as a refracting surface, may not be up to scratch as a reflecting test surface.
2. If the solar filter has slightly curved, but parallel surfaces, it may still work as a solar filter, but be no good as an autocollimator. I don't know how they make these things, so I'm not sure whether or not this is a likely scenario.

However, if you have a paraboloid of known accuracy, or better still a spherical mirror, you could check the accuracy of the SF. If it's OK, you've scored a cheap flat mirror.
Geoff