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Dielectric and Metal-Coated Mirrors and an Alternative to Phase Coating? (1 Viewer)

Tringa45

Well-known member
Europe
There was a recent discussion and some speculation on the Meopta forum as to the type of mirrors used in the Meostars.

Dielectric mirrors for binoculars with Schmidt-Pechan prisms were introduced around the turn of the century and because they can boast reflectivity in excess of 99%, and perhaps because it is the newer technology, there is a tendency, at least amongst binocular users, to regard them as superior to metallic coatings.

Though not an amateur astronomer, I have followed recent developments and here the trend seems to be going in the opposite direction. Amateur astronomers are concerned with obtaining the highest contrast and making the most of the resolution provided by their telescopes when observing moon or planetary detail at high magnifications. Most are unwilling to accept an upside-down image when viewing on axis and settle for an upright left/right reversed image provided by a 45° erecting prism or mirror.

A few years ago one of the contributors to Cloudy Nights conducted tests on various erecting prisms and mirrors. His conclusions were that erecting prisms (with total internal reflection) gave the best detail and contrast and that silver or enhanced aluminium mirror coatings were superior to dielectric coatings. In contrast to binoculars, where the dielectric coatings are behind the glass surface, an erecting mirror has up to 140 (!!!) layers on a quartz or sittal substrate, so it's not really surprising that tolerances in the coating thickness can accumulate to degrade the flatness of the top surface. In addition, and this would also apply to binoculars, there is a lateral shift in the partial reflections from different levels as here: https://en.wikipedia.org/wiki/Dielectric_mirror#/media/File:Dielectric_mirror_diagram.svg.

Now the state of the art in erecting mirrors seems to be silver with a dielectric protective coating. Baader Planetarium have even gone so far as to put a silver coating on an erecting prism. Why they would want to do this escapes me, unless they for some reason wanted to avoid the polarization associated with total internal reflection.

The interference of polarized light from the two roof prism surfaces in binoculars results in a loss of contrast and can be compensated by phase coating. But what if the roof prism surfaces had a silver or enhanced silver coating? There would be a small sacrifice in transmission but an attendant gain in contrast as there would be little or no polarization at the reflecting surfaces.

Could it be that Meopta have taken this radical step? When checking out the phase coating on a TFT screen with a polfilter all my other monoculars, bins and scopes exhibit a colour shift across the roof edge (e.g. blue/yellow). Perhaps recent Meostars have a very effective phase coating but the tested example (2016 build) goes progressively from a clear view to an almost uniform violet.

John
 
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Most of the experienced astronomers use upside down images, for all observing.
I always did unless the angle was so difficult that a mirror was needed.

Also simple 3 or 4 element eyepieces are used for planetary work.

Horace Dall's telescopes nearly always gave upright images, as he chose to make them this way and to observe like this.

Many astronomical discoveries were made with uncoated refractor elements and uncoated simple eyepieces.
With long focus instruments a 2 element eyepiece is good enough, although 3 or 4 element eyepieces are also used.
However, most of my refractors and eyepieces were single coated.
My best eyepiece is a 3 element RKE 8mm with single coatings.

8 element multicoated eyepieces and multilayered coatings don't actually result in any real improvement.
The EWA Naglers etc. are useful for undriven scopes as they don't have to be moved so much and the fields are flat.

Most astronomers now are digital imagers, i.e. photographers.
There are not that many dedicated visual observers, reporting their findings to say the B.A.A., ALPO or AAVSO.
 
Binastro,

I too have an Edmund Optics RKE, but from the other end of the scale, the 28 mm. It offers the largest practical field stop for my spotting scopes (Swaro ATM 65 and Kowa 883) and with only 16,5x or 18x magnification makes a good finder eyepiece when using shorter focal lengths for my occasional views of the planets or the moon. Despite the rather narrow AFOV and some field curvature and pincussion distortion, the long eye reief and large exit pupils of 4 mm and 5 mm respectively make for a very relaxing view.

In a recent mailing from Edmund Optics there was a graph of typical reflectivities of metallic surface mirrors: protected aluminium, enhanced aluminium, UV enhanced aluminium, protected gold and protected silver. Above 450 nm the protected silver was in the high 90% range and was only marginally bettered by protected gold well into the inviible IR.

I suspect that Meopta may not be alone in their use of silver coatings. Dielectric and metallic coatings do not mutually exclude one another, so this could be an explanation for the extended transmission of Leica Ultravids well beyond 700 nm.

John
 
Just a quick follow-up on the silvered Baader prism mentioned above, which is described on their German site as being made of BaK4.
I should mention that, unlike prisms in binoculars or spotting scopes where they are part of the optical design, erecting prisms or mirrors for astronomical scopes are accessories and the additional glass path of an erecting prism is likely to introduce disturbing chromatic aberration at the faster focal ratios.

I posted the question on Baader's German site why they used a silvered coating when the prism would provide total internal reflection anyway. When a prism, as opposed to a mirror, only makes sense at focal ratios "slower" than f/6 would not BK7 be the better choice, as it has better transmission and lower dispersion than BaK4?

After over a month they replied that the hignly reflective silver coating on the "hyperthenuse" (sic) protected the glass surface from aging! And what about the other two optical surfaces? A further exchange led to them retreating behind "trade secrets".

I have always regarded Baader Planetarium as a responsible company, but here I can't escape the impression that they have no clear idea what they are doing.

John
 
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Ihave always regarded Baader Planetarium as a responsible company, but here I can't escape the impression that they have no clear idea what they are doing.

John

John
I have no knowledge of Baader but looking back over the years of visiting Birdforum and the posts reporting contact between members and different optical companies it often seems to be the case that the spokespersons at these companies don't have a clue about their own products and if they bother to obtain tech info from more tech-savvy colleagues they misunderstand it and pass on to the enquirer a garbled mess of an explanation. This certainly doesn't apply to all contacts with optical companies but it happens often enough. Then of course there can be a natural reluctance to discuss in public a process that took a lot of work to get right as they don't want to give clues to competitors.

One of my contacts in optics in Germany has regular contact with Baader and over the years I have got the impression that Baader (or at least some of the people in Baader) do indeed know what they are doing but this is secondhand and I don't have any first-hand experience myself.

Lee
 
[...]but here I can't escape the impression that they have no clear idea what they are doing.
I've worked for companies with superb levels of technical knowledge among their engineering staff, represented by completely clueless PR types. It's often only the PR staff who are allowed to speak in public ;)

...Mike

P.S. It is worth noting, though, that sometimes the smartest engineers aren't the best public speakers available...
 
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Lee, Mike,

I know what you mean. The classic example of the marketing guys getting their knickers in a twist is when they try to convert fields of view from imperial to metric or vice versa.

Here though I'm questioning the whole concept. Baader wrote that the silver coating ensures highest reflectivity with short focal lengths (they probably meant short focal ratios), but if BaK4 can provide total internal reflection in binoculars at focal ratios around f/4, the silver coating is not required and one would be better served by one of their mirror diagonals on a "fast" scope.

The penalties of course are not that great, just the additional cost, a loss of about 2% reflectivity and CA in the wrong application.

I'm a bit of a watch fan but the industry is full of absurdities like wrist watches with tourbillons, for which some customers are prepared to pay six or even seven figure sums for the attendant increased wear and imagined accuracy gains.

John
 
Hi,

to put some things right:

- a Schmidt Pechan erection system needs at least the 2nd face mirrored as the angle of incidence is not sufficient for total internal reflection with usual glass like BaK4. Here either dielectric or metal coatings are possible.

- total internal reflection on the roof edge will lead to a phase shift between the two half-beams and destructing interference leading to loss of contrast. This can be fixed by applying a dielelectric coating (usually called phase coating) on the roof faces to adjust the phase shift so not destructive interference takes place. It has been speculated that metal-mirroring the roof edge will also fix the phase shift at the price of two reflections on metal with a few % less transmission.

See https://ebooks.wtbooks.com/static/wtbooks/ebooks/9781280234903/9781280234903.pdf page 25 ff.

Joachim
 
John,

Your original post sent me to this article:

https://www.baader-planetarium.com/en/blog/baader-bbhs-reflective-properties/

There I found to my surprise that Baader claims to have been using silver mirror coating on some of their diagonal prisms going back for 20 years. I got excited about that since I happen to have one of those prisms.(#2456095), which I bought 10 years ago in July 2008. I disassembled it and found no mirror coating on the prism. I'm not sure whether to be disappointed or relieved.

I participated in a Cloudy Nights discussion of prism vs mirror diagonals some years ago. After a series of tests I found the Baader and other prism diagonals had two effects on my two short focal length refractors (Takahashi SKY90-f/5.6 and A-P Stowaway-f/6.5). The prisms added spherical over-correction compared to mirrors, which was beneficial for the under-corrected SKY90, but caused the nearly perfect Stowaway to become slightly over-corrected. Chromatic aberration was increased with the prisms, but all the increase appeared to be in the red, so that the main visible change in the SKY90 was that CA switched from purple fringes to slightly wider red fringes and red fringes appeared in the essentially color free Stowaway. Since then I have always used a prism in the SKY90 because I judged the decreased spherical aberration to be more important than increased defocus in the red. I usually use a mirror with the Stowaway to preserve its corrections.

Henry
 
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Hi,

to put some things right:

- a Schmidt Pechan erection system needs at least the 2nd face mirrored as the angle of incidence is not sufficient for total internal reflection with usual glass like BaK4. Here either dielectric or metal coatings are possible.

- total internal reflection on the roof edge will lead to a phase shift between the two half-beams and destructing interference leading to loss of contrast. This can be fixed by applying a dielelectric coating (usually called phase coating) on the roof faces to adjust the phase shift so not destructive interference takes place. It has been speculated that metal-mirroring the roof edge will also fix the phase shift at the price of two reflections on metal with a few % less transmission.

See https://ebooks.wtbooks.com/static/wtbooks/ebooks/9781280234903/9781280234903.pdf page 25 ff.

Joachim

Joachim

This is not a contradiction to what I had already posted. If the angle of incidence does not allow total internal reflection, metallic coatings may offer some advantages over dielectric coatings at the expense of a some reflectivity. However, if the prism material allows total internal reflection from the field edges I don't see any justification for a metallic coating. A possible exception would be the complete elimination of polarization at the roof prism surfaces.

John
 
John,

Your original post sent me to this article:

https://www.baader-planetarium.com/en/blog/baader-bbhs-reflective-properties/

There I found to my surprise that Baader claims to have been using silver mirror coating on some of their diagonal prisms going back for 20 years. I got excited about that since I happen to have one of those prisms.(#2456095), which I bought 10 years ago in July 2008. I disassembled it and found no mirror coating on the prism. I'm not sure whether to be disappointed or relieved.

I participated in a Cloudy Nights discussion of prism vs mirror diagonals some years ago. After a series of tests I found the Baader and other prism diagonals had two effects on my two short focal length refractors (Takahashi SKY90-f/5.6 and A-P Stowaway-f/6.5). The prisms added spherical over-correction compared to mirrors, which was beneficial for the under-corrected SKY90, but caused the nearly perfect Stowaway to become slightly over-corrected. Chromatic aberration was increased with the prisms, but all the increase appeared to be in the red, so that the main visible change in the SKY90 was that CA switched from purple fringes to slightly wider red fringes and red fringes appeared in the essentially color free Stowaway. Since then I have always used a prism in the SKY90 because I judged the decreased spherical aberration to be more important than increased defocus in the red. I usually use a mirror with the Stowaway to preserve its corrections.

Henry

Thanks for the link, Henry, which I hadn't seen. They mentioned the aging aspect in one of their mails, but I don't buy that. Some 19th century scopes are still in use.

I hadn't considered spherical aberration and almost burst some of my few remaining grey cells trying to understand the influence of an additional glass path, but I think I've got it now ;).

John
 
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