Comparative Coatings and
Obstructions

by Roland Christen

Shown here are 3 cats with 3 different coatings. The larger scope (at
bottom) has normal single layer mag-fluoride with about 1.5%
reflectivity per surface. The Mak on the left has a typical "high
reflectivity" multi-coating. The reflectivity varies with wavelength,
reaching 5% or more in the blue and red end, and dipping to an
estimated 1% in the green. This coating is made from several layers
of mag-fluoride and is relatively inexpensive. The Mak on the right
has been coated with a high efficiency oxide multilayer that averages
less than 0.5% per surface over the visual wavelengths.

Interestingly, there are many eyepieces that have similar variations
in coatings. When I do a side by side test I can see very large
differences between poorly coated and superbly coated eyepieces. In
the case of the above scopes, the difference of 5% vs. 0.5% may not
be readily seen by the average observer, but in eyepieces with many
coated surfaces, the poorly coated ones can have transmission as low
as 65%-70% in the blue end of the spectrum vs 97% for the really
outstanding ones. This would make a bit of difference in the ultimate
magnitude visibility.

Included here are two images of some of my eyepieces. The first image
shows a number of eyepieces with increasing amounts of backscatter due to
poor coatings. The test for a really good coating is to set it
outside in the shade with the eye end up so that light from the sky
can flood the optics. The other end should have a black end cap
attached. Look straight down into the eyepiece to see how much of the
light is coming back out at you. This is the light that is reflected
back from the various coated surfaces, light that should have gone
through and be absorbed by the black end cap. The darker the optics
look, the more light gets through to your eye, and the less is
reflected back or scattered. The really bad oculars literally glow
with reflected light.



The left most eyepiece is my standard, a 25mm Zeiss Abbe Ortho with a
proper multi-coating applied. Next to that is a high quality
multi-element Japanese ocular. Both these eyepieces represent the
state of the art in high transmission, low scatter optics.
Transmission over the visible range is on the order of 97%. The third
eyepiece is a popular wide field design. The coating is not too bad,
but there is definitely more reflection especially in the blue end of
the spectrum. This eyepiece is also the one that is compared in
transmission in photo # 2.

The 4th eyepiece is another similar wide field. The 5th is an
inexpensive 12.5mm Ortho. Just because it's an Ortho does not mean it
has good coatings or good transmission characteristics. #6 and #7 are
real stinkers, even more so because these are some of the most
expensive (and popular) eyepieces on the market today. They have many
elements, and a very wide field, but their coatings are really not
optimized. Another problem that can be readily seen is that there are
heavy reflections from internal spacer rings that appear to be black
anodized, but not blackened properly. At real low angles, a black
anodized surface can be very reflective, resulting in the shiny ring
seen in the eye end.

Although all the eyepieces in this picture are "multicoated", they
are not optimized. The different elements in a typical wide angle
ocular are made from different index glass. For each index there
exists an optimum coating design, one that will achieve the lowest
reflectivity for that element. For optimum efficiency, each element
must be coated with a different coating design according to its
refractive index. What is often done is to coat all elements with the
same formula. This results in some wavelengths getting attenuated
more than others, usually in the blue end of the spectrum. These
eyepieces will show a distinct yellowing of the image, as well as an
overall lower transmission efficiency. This can be seen in the second
image where I have compared the 25mm Zeiss Ortho with the 3rd ocular.
You can see a definite yellowing of the white wall behind the ocular,
as well as a faint reflection of my head and sky behind me, which is
completely missing in the Zeiss.



So here you have two simple tests for comparing two eyepieces. Look
for heavy glow or back reflections from the optics when aimed at the
sky, and look through the eyepiece at arm's length at a white wall to
see how white or yellowish the light looks as it passes through the
glass.

Finally,if you combine a scope with large central obstruction with a
poorly coated ocular, here's what you might get: