Re: How to use an optical flat

Charles Manoras wrote:
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What kind of coating and overcoating are now being used typically for
astro mirrors nowadays?

What are the enhanced coatings advertised in Sky a Telescope for example
made of (those for which you have to pay extra for)?

Those are usually multi-layer coatings which improve the throughput for visual or color photography at the expense of throughput in the UV and IR. For my research work I want smooth bandpass including the near UV to 300 nm and into the IR a bit. I usually specify aluminum with a slightly thicker than normal single layer SiO2 coating which has it's first peak in the red and a secondary (but broad and weak) peak at 325 nm. I'm more interested in long term stability than throughput. I've had best luck with mirrors coated by Navitar in Newport Beach CA. Not cheap but best quality I've found. I have 17 year old mirrors exposed nightly which are in good shape coated by them. Most others have needed recoating in under ten years.
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