The impossible has been done again

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  • Member since
    March, 2008
The impossible has been done again
Posted by Antitax on Wednesday, September 25, 2013 1:14 PM

   Maybe it happened long ago and I notice only now but GSO is offering - against money, not as gifts Pirate - Ritchey-Chrétien reflectors with 99% reflectivity on both mirrors. I was pretty much convinced 97% was the maximum attainable with frontal reflection, and I would have found it great if this had the been the limit for ever. 99% is what bounces off my apo's diagonal mirror but the 45° angled reflection helps (bullets bounce off concrete at grazing angles but they crash against it at frontal angles).

   Thus I was quietly waiting for the moment when all but toy telescopes feature that "Hilux" 97% reflectance. My own GSO scope has 94% reflectivity, really good spec for an affordable no-frills dob, and really not far behind what I imagined was the 97% climax. Well, no. Not only is GSO selling - offering is not the right word, really - their whole range of Ritchey-Chrétien 6", 8", 10" and 12" scopes with that über-coating, but they're not expensive (look at the link to the "limited offer", 465€ for a 152mm).

   Thus you can expect other reflector makers to follow that lead and issue 99% mirrors as a matter of routine, or at least 96-97% coatings. If GSO can do it affordably, competitors should, well, compete. In truth those enhanced reflectivity (or reflectance) coatings slap 99% or 97% only for part of the spectrum, mainly the middle of it, yellow-green to blue-green, to which the dark-adapted eye reacts the most. Light loss is greater at the edges of the spectrum but mirrors never had a flat reflectivity curve with simpler coatings anyway.

   Lens coatings have reached more than 99% transmission many, many years ago, and over the full spectrum. Light loss at a modern lens' multicoatings is between 0.1% to 0.5%, the problem here is not light dropout but where it bounces off and how spread out or focused it is, because a tight reflection in an eyepiece can cause visible ghosting. Anyway, with 99+ percent lenses and 99 percent mirrors, optical yield is maxed out, only have to wait till this performance is generalized and lowered in cost with all makers.

TS 8x40 Wildlife, 10x50 Marine/Fujinon 16x70/TS 80mm triplet, 6x30 finder, EQ-3 mount, TS 2" 99% diagonal/Celestron C5+ and 6x30 finder, DIY tripod/5" Bahtinov/12" GSO dob, 8x50 finder/Meade 2" 24mm 82°/Hyperion 24,13,10mm 68°/TS Expanse 17mm 70°/SW 7mm Panorama 82°/Ultima 2x barlow/Astronomik UHC-E filter/Baader O-III/Astro Solar 5" & 80mm filters/Sky Atlas 2000/Rükl's Moon Atlas/Canon 400D/5mW green laser

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