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pdfermat
super member
Reged: 11/12/07
Posts: 142
Loc: Wisconsin
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I was thinking about anti-reflection coatings, and maybe someone can set me straight. When we get an eyepiece, we all like to look at that top lens and see that nice green/purple reflection that the coating provides. But isn't the idea of the coating to reduce reflections? Wouldn't it be ideal to look at an eyepiece and not see this green/purple reflection? If the eye lens has a green sheen to it, wouldn't that mean that the light at that wavelength is being reflected by the lens, and not passing through it? (I'm assuming that a "fully multi-coated" eyepiece has uniform coatings on each surface. So the reflection that I see from the "user end" of the eye lens is also present on the opposite side of it.)
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Pat
Orion XT8i
TV 32mm, 20mm, 15mm Plossls
9mm Planetary
Orion Shorty-Plus Barlow
DGM NPB Filter
Lumicon Deep Sky Filter
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3226
Loc: "Metrowest" Boston
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Because of the way AR coatings work, it's not possible that they work identically from every angle of incidence. What should happen is that they should seem to disappear when viewed head on, and when viewed obliquely they should appear. When coatings can be seen head on, then, yes, there is some inefficiency there. I once bought a low-cost finder scope made from cheap Russian binoculars which had an obvious green reflection from any angle. The images through it were slightly ruddy. You may find these essays informative.
http://geogdata.csun.edu/~voltaire/roland/coating.html
http://geogdata.csun.edu/~voltaire/roland/coating2.html
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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BillP
Carpal Tunnel
Reged: 11/26/06
Posts: 2209
Loc: Vienna, VA
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Also to consider, is that coating anti-reflectance varies by the angle of the light ray. So when you read that a particular multicoating has 99.25 transmission, it means it is that efficient only when the light ray is intercepting the lens at an angle 90 deg to the surface (ie, when you look at it straight on). So as you tilt it and look at an angle, reflections will appear brighter as the coating does not do as well at other than 90 deg to surface. An excellent source to read is this: AR Coatings
Coatings are also optimized for different spectra, so depending on how efficient they are at various spectra, the "color" you see will be impacted. IMO, you can't really make any determinations on the quality or effectiveness of a coating by color, even though it is hinting for you what balance of spectra are leaking thru more or less.
It's also interesting to note in the link I provided, the chart that shows how differently the standard MgF2 coating behaves on different glass types (BK7 vs SF5) -- red and green lines on bottom left chart of page 1. So the coating really needs to be optimized for each glass type when your eyepiece or objective lens is made of varying glass types.
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250mm f/4.7 Orion XT10i Dobsonian
102mm f/8.0 Tak TSA Super-APO
66mm f/5.9 WO ZenithStar SD APO
40mm f/10 Coronado P.S.T.
Edited by BillP (10/10/08 01:23 PM)
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BillP
Carpal Tunnel
Reged: 11/26/06
Posts: 2209
Loc: Vienna, VA
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Forgot your other question...."Fully Multicoated." You will often see some EPs advertised as simply Multicoated and others as Fully Multicoated. *Generally* this means that if simply Multicoated that only the top eye lens is multicoated and the rest of the glass-air surfaces in the eyepeice are not multicoated and most likely just single-coated. The term "Fully Multicoated" is supposed to mean that every air-to-glass surface has a multicoating applied to it.
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250mm f/4.7 Orion XT10i Dobsonian
102mm f/8.0 Tak TSA Super-APO
66mm f/5.9 WO ZenithStar SD APO
40mm f/10 Coronado P.S.T.
Edited by BillP (10/10/08 01:27 PM)
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pdfermat
super member
Reged: 11/12/07
Posts: 142
Loc: Wisconsin
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Ah-ha, I see, good to know. I'm going to try that "test" on my eyepieces that is described in the first essay that Mike linked to. I'm hoping my TV plossls hold up well.
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Pat
Orion XT8i
TV 32mm, 20mm, 15mm Plossls
9mm Planetary
Orion Shorty-Plus Barlow
DGM NPB Filter
Lumicon Deep Sky Filter
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Jim Rosenstock
Carpal Tunnel
Reged: 07/14/05
Posts: 2361
Loc: MD, south of the DC Nebula
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Quote:
Forgot your other question...."Fully Multicoated." You will often see some EPs advertised as simply Multicoated and others as Fully Multicoated. *Generally* this means that if simply Multicoated that only the top eye lens is multicoated and the rest of the glass-air surfaces in the eyepeice are not multicoated and most likely just single-coated. The term "Fully Multicoated" is supposed to mean that every air-to-glass surface has a multicoating applied to it.
Yup. But many manufacturers and suppliers play so fast and loose with coating terminology, it's often hard to tell what is actully being sold.
Beyond that, I think it's more important how WELL the coatings are executed. I'd rather have a vintage eyepiece with sharp glass and well-done single-layer MgF2 coatings, than a poorly executed "fully broadband multicoated" modern-day "bargain" eyepiece.
Cheers,
Jim
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Dylan Gladstone
scholastic sledgehammer
Reged: 08/05/07
Posts: 955
Loc: Connecticut, USA
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I think Brandon eyepieces aren't multicoated. They claim multicoating reduces low contrast detail.
I'd love to try a Brandon eyepiece someday just to see for myself. (They have received VERY good ratings.)
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Orion SkyView Pro 127mm Maksutov
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