Tag: mirror

Testing a Mirror

As I have observed lately, most of the small telescope mirrors available right now are out of China, most of those produced by one company, Guan Sheng Optical or GSO. If you want a small mirror, say a 6″ or 8″ mirror, there is not a lot of choice, the mirror makers in the US generally do not do anything smaller than 10″.

Foucault knife edge test
A textbook Foucault knife edge test image on a 6″ f/4 mirror

The GSO mirrors range from decent to pretty bad, with no way of knowing what you will get when you order, just luck of the draw.

But how do you tell?

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Getting a Mirror Re-Coated

The classic Cave Astrola telescope has become my roll-out, quick observing session ‘scope, often found in my driveway. I have also used it a few times at darker sites when I expect the weather to be damp or dewy as a Newtonian is more protected.

The restored 8" f/6 Cave Astrola under a dark sky at Kaʻohe
The restored 8″ f/6 Cave Astrola under a dark sky at Kaʻohe

While the restoration job was finished some time ago, I never got around to re-coating the optics. Meanwhile the telescope has seen good service on many occasions as I enjoyed this fun-to-use instrument.

The optics did need some attention… The primary mirror from the Astrola appeared to have not been re-coated since it was made in 1978. Thus the aluminum coating was over 40 years old. While the coating looked bad, it was still serviceable, producing reasonable images.

Still, the loss of light due to the old aluminum coating was probably reducing the effectiveness of this 8″ telescope to something more like a 6″ telescope. I had meant to get it re-coated some time ago, but we know how these things work.

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