Checking Out Lens Coatings
Jack Kramer
When an ad touts an eyepiece as having "fully multi-coated lenses", is that just advertising fluff or is it really meaningful? The purpose of coatings on lenses is to increase light transmission and reduce internal reflections. But the quality of coatings varies. Optics billed simply as "coated" normally have a single layer coating only on surfaces exposed to the outside air. "Multi-coated" means there's a more effective coating, with multiple layers applied. The best is "fully multi-coated," which indicates that all lens surfaces exposed to air, inside and out, have been coated with multiple layers of anti-reflective material. So it is a meaningful aspect of the lenses and is typical of premium-quality optics.
To some extent, you can gauge the quality of a coating, but it takes a practiced eye, since different lenses form images at different places. As you look at an eyepiece or objective lens straight on, your reflection should not be visible or should be very faint. When you look straight on at an especially well-coated lens, you should see no reflection at all - as if the lens simply isn't there.
Looking at a slight angle, the color of a reflection from a light bulb tells something about the lenses, but the different colors of lens coatings generally aren't significant. Fully multi-coated optics may reveal faint purple, green, or red-hued reflections. Eyepieces typically show several reflections, and a properly coated optic may show a variety of colors. This is because different types of glass are often used in the lens stack, and these glasses each need specific coatings in order for their performance to be optimized.
However, a mix of colored and white reflections could mean some lens surfaces are coated, while others are not. White indicates where the surface lacks coating. You may also see a white image of the light bulb formed by the lenses, but this will be brighter than the reflections.
On a large lens, such as a refractor objective or a corrector plate, if you see no prominently colored reflection of an overhead light this is a mark of a high-quality objective. Instead you'll probably see an image of the light seemingly suspended in front of and/or behind the objective. These are images formed by the lenses in the objective and a natural result of the curvature of the lenses. Moreover, each lens in the system may form a separate image. Thus an achromat or two-lens apochromat could have two images of the light, a three-lens APO three images, etc. For more info on eyepiece coatings, check this web site: http://voltaire.csun.edu/roland/coating.html
Published in the December 2006 issue of the NightTimes
