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David Hall is an Electrical Engineer designing electronic circuits for almost 30 years. Most of that time was in the aerospace industry with Boeing/McDonnell-Douglas in St. Louis, MO. Currently, he designs laser electronics for Cutting Edge Optronics, Inc. in St. Charles, MO.
His primary astronomy interests are observing, ccd imaging, and astronomy related software development.
The Authors – John Duchek
John has a doctorate in Organic Chemistry. He has been a professor, pilot plant manager, plant troubleshooter, and laboratory research chemist in his over 30 years in the chemical industry. He currently works for Tyco-Healthcare in St. Louis, MO.
His primary astronomical interests are observing and photography (ccd and 35 mm).
An Eyepiece Coating Experiment
Program suggests eyepieces can be very important in light loss in your telescope.
The coating effectiveness
Depends on its thickness
varies with wavelength
What do the terms mean?
Coated or single-coated: one air-to-glass surface(ATGS) is coated with a single layer of antireflection material, usually MgF2
Fully Coated: All ATGS are single-coated
Multicoated: at least one ATGS surface has been coated with multiple layers, the rest are single coated or not coated.
Fully Multicoated: all ATGS are covered with multiple coatings.
Single Coating (MgF2)
Transmission dependent on wavelength, substrate's index of refraction, thickness of the coating, and angle of the incident light
Coating is designed so the relative phase shift between beam reflected from upper and lower surface destructively interfere (cancel)
Optical thickness must be ODD NUMBER OF (l/4) where l is the wavelength being optimized. (gives ½ l path difference)
(Discussion from www.edmundoptics.com/TechSupport/)
“The principle of coatings is to reduce the light reflecting on the glass surface. If the coating has a ¼ wavelength thickness and the glass an index of refraction higher than the coating the 2 reflections are in phase opposition and cancel. For 1 multicoated surface, the transmission can increase up to 3.5%”
How do you know your coating is working as advertised?
Light Losses (if coating working properly)
No Coating ~ 4% lost/surface
Fully Coated ~2% lost/surface
Multicoated ~ 1.5% lost/surface
Fully Multicoated ~ <=1% lost/surface
¼ of 550 nm is 0.000000137 meters or 1370 angstroms. One MgF2 molecule is ~2.5 Angstroms.... coating should be 550 molecules thick.
If the coating is twice as thick, the waves will reinforce, not cancel. The thickness is critical!!!!
Using an optical bench, equipped with a green laser (550 nm), measure the % transmittance of a variety of eyepieces.
Dave Hall had access to the necessary equipment at his job at CEO lasers.
Laser is on the right.
The eyepiece to be tested is just in front of the detector.
Dave is adjusting the detector to collect the whole light beam.
We found that we were unable to get consistent results on any eyepieces under 10mm, probably because we couldn't get the lens close enough to the detector to get all the light
12 mm results should be taken with a grain of salt.
The method for each eyepiece (1)
Measure the dark voltage Vd and the light voltage Vl with no eyepiece in place.
The total voltage Vt=Vl-Vd
Put the eyepiece in.
The method for each eyepiece (2)
Take 5 voltage readings for each eyepiece after rotating the eyepiece 90 degrees. (verify that 1st and last are the same)
Average the 4 readings and subtract Vd from the average to get (Va)
% transmittance = (Va/Vt)*100
1.25” Eyepiece Results
2” Eyepiece Results
The 12mm results are suspect…they are clustered at a low value. Transmission overpowered by light losses (light missing detector).
Fully multicoated did not fare well in this test (85-90%)
Multicoated orthoscopics, and particularly UO orthoscopics are superior at high light transmission (95-98%)