Time to Re-coat your Mirror?
Jack Kramer (with help from others)
Every owner of a reflecting telescope at some point faces a decision as to whether it's time to re-coat the mirrors. And if so, is the standard aluminum coating good enough, or is an enhanced reflectivity coating better? Having faced that issue in the past, I did some research to see what the experts recommend.
First of all, mirrors should not need to be re-coated very often. A good standard coating may last for ten years - perhaps longer, provided the telescope is well protected during storage. A Schmidt-Cassegrain re-coating interval is even longer, inasmuch as the mirror is protected within an enclosed tube. Just because a mirror has minute spots doesn't mean the coating is breaking down. It may simply be a matter of cleaning the mirror. (And even cleaning isn't often necessary. Dust on the mirror surface has little effect.) If there are marks on the mirror surface that won't come off or transparent spots, then the mirror may need re-coating. So here are excerpts from some of the opinions about coatings.
Issue #46 of Telescope Making magazine:
"Enhanced coatings consist of a base layer of aluminum or silver overcoated with several layers of materials with varying refractive indices and thicknesses whose combination causes constructive interference of the reflected light, raising the reflectivity of the original metallic film. Some coatings enhance only a specific portion of the spectrum. Silver-based enhanced coatings tend to tarnish even if they're overcoated because overcoatings tend to be porous. Furthermore, enhanced coatings tend to be expensive and must be protected from acid dew that will degrade reflectivity and cause much scattering. Is the extra 10% light worth the added expense? It's hard to say, particularly considering that smog, acid dew and adverse atmospheric conditions may force you to re-coat every several years."
Mirror making guru, Carl Zambuto:
"We're presently using and recommend Nova Optical Systems to coat our mirrors, for a safe stripping and cleaning procedure and a uniform coating and overcoat. Nova theoretically reaches 92 percent with a single layer AL and a SIO2 overcoat of the thickness he targets. A very uniform job that we are quite happy with. More (theoretical) reflectivity will not be noticed."
Experienced Australian observer, John Bambury:
"Opinions on this are clearly divided amongst the expert opticians. IMO, some of the expert opticians talking down enhanced coatings do so because their preferred coating provider can't provide enhanced coatings. It requires different equipment and process to standard coatings. The naysayers in regard to enhanced coatings claim three main negatives in regard to them:
1) They affect the wavefront accuracy of the mirror if not properly applied.
2) They lose reflectivity very quickly and soon offer a similar reflectivity level to a standard coating.
3) They deteriorate faster than a standard coating and don't last as long.
In my opinion, a lot of the negative publicity regarding enhanced coatings stems from the early days when the application process was being developed and improved. I think things have progressed enormously from those early days, but many are still gun shy.
I have enhanced coatings on the 18" Obsession and they are superb. I couldn't be happier with them. In addition I regularly use nine different Obsession telescopes (with enhanced coatings) including my own. They are all faultless and perform superbly. I have compared the 18's with enhanced coatings and they equal a 20" with standard coatings. An optical system with a 98% reflective secondary and a 96% reflective primary offers a 17% brightness increase over 89% reflective coatings on both mirrors."
So considering the lack of unanimity, I've reached a few conclusions. Taking the opinions cited here, plus a number of others, it seems that:
- Enhanced coatings do improve reflectivity.
- For most people the difference is subtle at best.
- Enhanced coatings are not quite as durable as standard coatings.
- Enhanced coatings are tricky to apply and the extra layers may adversely affect the mirror's final figure (per Carl Zambuto).
- It's wise to also have the secondary mirror re-coated along with the primary.
I'm several years away from having to re-coat my present Newtonian, but at such time I will probably have a standard Beral finish applied by H. L. Clausing in Skokie. I've used them previously and am satisfied with the results. According to Clausing: "Beral is a hard metallic coating for front surface mirrors. Its reflective properties are quite similar to those of Aluminum. The average reflectance is 91% in the visible spectrum."
Because of Beral's hardness, it doesn't require a protective overcoating, and it holds up well to repeated cleanings. They also offer enhanced coatings, and their web site includes graphs of the response to different wavelengths of light for each type of coating.Published in the December 2008 issue of the NightTimes