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Discover the Power of Optical Coatings: A Guide for Backyard Astronomers

When choosing telescope eyepieces or binoculars you’ll likely come across these different types of optical coatings. If like me, what these mean for the view you get through your instrument will escape you as a novice backyard astronomer.

optical coatings explained

The following is what I’ve since discovered about the different types and what each have to offer…

What are Optical Coatings?

Optical coatings are thin layers of materials applied to the surface of optical components like lenses, mirrors, or prisms. These coatings are designed to reduce internal light loss and glare, ensuring even light transmission, resulting in greater image sharpness and contrast.

Without optical coatings, each lens may lose up to 5 percent of the light passing through it. This can go up to about 35 percent if the optics are of poor quality. The coatings are applied to the surface of the lens, and they have a better light transmission resulting in greater image sharpness and contrast.

Optical coatings are a key technology in the field of optics, and they have become an essential part of modern binoculars. The coatings can be applied to a single surface or to multiple surfaces of the lens.

The number of coatings and the type of coating used can affect the quality of the image produced.

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

The anti-reflective (AR) coatings reduce light 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).

Different Types of optical coatings Used in eyepieces

There are quite a few levels of optical coating used on eyepieces, including fully coated (FC), multicoated (MC), fully multicoated (FMC), and more (see below).

The different types of optical coatings described for optical instruments:

  • Coated
  • Fully (FC)
  • Multi (MC)
  • Fully multi (FMC)
  • Fully broadband multi
  • Ultra-wide band (UWB)
  • Other proprietary coatings

Fully coated means that all air-to-glass surfaces have received at least a single layer of anti-reflection coating, which is good.

Multicoated means that one or more surfaces of one or more optical elements have received multiple layers of anti-reflection coating.

Fully multicoated means that all air-to-glass surfaces have received multiple layers of anti-reflection coating.

Optical coating types Compared

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.

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.
– Loses 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 coatingPer Pentax, seven layers on every lens element canceling an ultra wide band of reflections in the visible spectrum. Optimizes the optical through-put across this spectrum.
– Aka Super-Multi Coated (SMC)
– Cuts reflection to 0.2% per glass surface

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.

diagram showing how light reflects and transmit through glass lenses with different types of optical coating
Uncoated, single coated, and multicoated lenses (L to R) with light passing through increasing from 96 to 99.5%. Source: Astrosurf.com

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).

eyepiece showing greenish tinge of a fully multi-coated eyepiece
Greenish purple reflection of fully multi-coated eyepiece. Source: Astrosurf.com

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

Best performers with optical coatings

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