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Graham secondaries aren’t worth recoating - buying another will be cheaper.
Dielectric coatings are rare on primary mirrors - the thickness of the stack of layers in the coating is several wavelengths thick, which is problematic vs surface accuracy better than 1/8 wavelength. It’s easy enough to achieve for small secondary mirrors and diagonals, but much harder for a primary mirror. OTOH a dielectric coating is very durable and should last a lifetime - even with washing - unless scratched. They’re also impossible to strip - the coating has to be removed by fine grinding and the mirror repolished and figured.
Last edited by Wavytone; 31-12-2019 at 08:24 PM.
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