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Can Big Coatings
Increase Telescope Size?
by Humanoido
The idea is to have your telescope lenses and mirrors coated to increase light transmission and reflectivity to gain the equivalent performance to that of a much larger scope without the coatings.
They say you need to eyeball the situation rather than just trust the numbers, because there are many variables.
So I set out to countless star parties to compare as many telescopes as possible. Apples to apples and oranges to oranges, for consistent testing. First up was my 12.25" Newtonian with a coated mirror giving performance on light gathering with deep sky objects and it was the same as a 16" without the coatings. Similar tests showed the 14" comparable to the 18", an 8-inch compared to a 12-inch, and the 9.25" compared to approximately a 13-inch. There are still many 13-inch scopes around made from Coulter Optics from some years ago, for comparison. The 4.25" was like an 8-inch. Other scopes with coatings were consistently of larger diameter in performance, in terms of light gathering power, compared to plain scopes. Of course this test is entirely subjective, qualitative and may vary from user to user.
Optics
Enhanced VS Plain
4.25" - 8"
6" - 10"
8" - 12"
9.25" - 13
12.25 - 16"
14" - 18"
Remember that all optical surfaces need coatings. SCT correctors have a front and back, and often have extra lenses located internally near the visual back in EdgeHD versions. Eyepieces may have 7 optical sets that all need coating. These lenses also have two sides. The coating on a mirror should be a dialectric coating of proper thickness to generate constructive interference of light waves. For a glass lens, the maximum transmission is important while minimizing reflections. Glass lenses also have multiple surfaces to be considered. Therefore telescope testing may vary depending on a wide variety of other corrector lens and oculars. Some optical companies will offer objective mirrors with coated silver for the highest reflectivity but at a significantly higher cost. The extra cost of coatings can be well worth the extra performance indicative of larger diameter aperture size. In the case of Newtonian reflectors, it's important to coat both the primary mirror and the secondary diagonal as well, and inclusive of the ocular elements. Some FRs are uncoated and eyepieces too, so choose accessories wisely. In one example, an ocular was only half coated so double check your optics when testing.
