Decoding Optical Coatings On Eyepieces and Binoculars

When choosing telescope eyepieces or binoculars, you’ll come across different types of coatings. What these mean in regard to the image you get to see through your instrument is a mystery when you’re a novice backyard astronomer.

Here I look at the different types and what each have to offer to decode that mystery.

What are the Different Types of optical coatings Used in eyepieces

You have options of multiple thin layers or a single layer of antireflection material, such as magnesium fluoride, on one or more of the glass elements in the eyepiece.

Optical coatings are transparent films placed on the glass to reduce reflection. This optimizes the light passing through to your eyes. You’ll get sharper and higher contrast images than you would with uncoated glass lens.

Today’s typical eyepiece (or ocular lens) has multiple coatings and multiple glass elements.

When light transfers from air to glass, a certain amount (~4%) is reflected or scattered (lost). The anti-reflective (AR) coatings reduce this scattering to give sharper and higher contrast images — important when viewing faint magnitude stars and other celestial objects.

A common AR coating material is magnesium fluoride (MgF2). Others are silicon (SiO₂) and titanium dioxide (TiO₂).

Optical coating types Compared

I wrote in Choosing The Best Telescope Eyepiece how some of these coatings compare in terms of light transmission.

Here is a larger list:

Untreated	No coatings on any of the lenses. Lose about 4% light through reflection on single glass surface.
Coated	At least one coating on one glass layer. With single layer of MgF2 down to about 1.3% in reflection on single glass surface.
Fully coated (FC)	Each glass surface has one coated layer.
Multi-coated (MC)	One lens surface or more treated with multiple layers of anti-reflective coatings. More light passes through. With multiple layers of AR coating on single lens showing reflection reduced to 0.25%.
Fully multi-coated (FMC)	All lens surfaces are treated with multiple layers of anti-reflective coatings. Much more light passes through.
Fully broadband multi-coated	Multi coating on all glass layers. Broadband meaning it cancels a wide range of reflections in the visible spectrum. Allows a lot more optical through-put across this spectrum. Used in binoculars and spotting scopes.
Fully multi-coated XLT coating	Limited to certain binoculars and spotting scopes of Celestron. A hybrid of of Starbright XLT and fully multi-coated.
StarBright XLT	Celestron’s own coating – used on their Schmidt corrector lenses. Said to increase transmission up to 97.4%.
Ultra-wide band coating	Multi layer on every lens element canceling an ultra wide band of reflections in the visible spectrum. Optimizes the optical through-put across this spectrum. Used in binoculars.

Out of the above, there are four primary categories of coating: single-layer, fully, multi-layer and fully multi.

Fully multi-layer coatings minimise reflectivity reduction and produce sharper images than uncoated and single layer coated lenses.

You’ll know you have a multi-coated eyepiece when you hold it at an angle in a lit area and see a tinted appearance (eg. greenish/purple reflection).

Overcoated is the alternative name for multi-coated used in Europe.

Best performers

If you’re not sure of the quality, you can run this simple test (thanks to Sky At Night Magazine):

• Fix a black cap to the base of your eyepiece
• Then in daylight look down the barrel
• “The darker the glass looks, the less light is lost and the better the eyepiece”

Coating performance depends on these characteristics:

• Material quality
• Thickness relative to glass thickness
• The layers of glass coated
• Amount of layers in the coating