Jump to content


Photo

Eypiece transmission

  • Please log in to reply
130 replies to this topic

#51 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12486
  • Joined: 01 Feb 2006

Posted 12 December 2012 - 03:13 PM

Sorry, no way. Human vision has a logarithmic response.


I agree that the likelyhood that even the very very best observer can reliably differentiate between much less than 10% brightness variation is unlikely.

On the other hand, I think that I have to interject this. I believe that some of the discrepency is they way we look at the numbers, and we tend to do this with an "Absolute" scale of "100%" so that any eyepeice that is between 90% and 100% would appear as bright.

Since the highest transmission is 97.9%, you kind of want none of your eyepieces to fall below a loss of 0.1 magnitude, or 10%, which says it should be at least 87.9%



But that is not the way it works. You really have to start the differential from the lowest transmission eyepeices and add 10% from the bottom.

In other words, if you started with an eyepice with 88% transmission, you would have a "Baseline" brightness as compared to the 100% absolute.

But to see a difference, you would only have to be 10% brighter than this baseline.

So, if the eyepeice started with 88% transmission, and you added 10% to that, you would add 8.8% transmission.

Now my math may not be perfect so this is more about the idea. So, this means that if I can see a difference of 10%, if I use an eyepeice with 88% + 10% of that baseline brightness, I would come up with a relative transmission of the sum of the two. So, a 10% brightness increase from the baseline would be 88% plus 10% of that (8.8%) for a total of 96.8%. So, the step between an eyepeiece with 88% transmission and one with 96.8% transmission would perhaps be at the edge of the range of detection for some individuals, but perhaps not all.

Anyway, we can't look at it as a fraction of 100%, but as the amount of light increase from the baseline eyepeice and add 10% to that number.

And using this, I could indeed see people detecting a tiny brightness difference. But these two eyepieces would have to be seperated by between 8.8% or more in "absolute" transmission.

My own experience is that the difference in brightness is so tiny and difficult to see that it is basically meaningless and no one should worry about this with modern eyepcies.

But some older eyepecies did seem dim to me. Most notably, the Meade 8.8 UWA and a Meade 6.4mm Plossl (Contamination in the cementing between the lenes on the plossl. In other words, a defective unit. I think the Meade 8.8 just didn't have modern enough coatings).

Mostly, modern wide field eyepces though seem to be on par with simpler types to me in brightness. If there is a difference, it is very, very subtle.

Anyway, this explains has at the extremes of transmission, it could be possible to see without 10% seperation on an absolute scale, but I still believe that while there may indeed be individual cases where we see an eyepeice with below 90% transmission compared to some of the eyepieces with the highest possible transmission, most eyepecies today have better than this. Not all, but most.

But using the logic above, I can see how an 88% transmission eyepeice could be detected when compared to a 96.8% transmission, and clearly some of the better eyepcies have this level of transmission.

If somone measured a T6 Nagler and it had 88.7% transmission, then yes, an eyepeie with (88.7 + 8.7) 97.4% transmission should be able to be discerned by someone doing very careful comparisons.

Will it make a difference in viewing? Maybe. I an not inclined to worry about it though. Given that every eyepiece design in production today is diffraction limited at the center of the field, to me personally, all of the meaningful difference in eyepiece performance is to be found off axis. Here, differences are titanic and easy to see, and for me personally have a huge impact on how I perceive the field. But a barely detectable change in brightness (.1 magnitude) provokes a big yawn from me personally.

But someone saying that they can see the difference between a .92% and a .97% transmission? I for one don't believe it.

I do feel as if seeing the difference between an eyepiece with 88% transmission and 97% is indeed possible even thought they are not seperate by 10% of the total brightness because it is the relative brightness differece starting with the lower transmission unit.

#52 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12486
  • Joined: 01 Feb 2006

Posted 12 December 2012 - 03:47 PM

Here is an example from this thread that corresponds to the post I just made.

If the TV Nagler 13 mm II has 87.3% transmission, to be seen as 10% brighter, you would add 10% to that figure, or 8.73% in relative terms. So adding 10% of the 87.3% means that any eyepice with 96.03% transmission would be 10% brighter on a relative scale.

And I have no doubt that someone looking for it could see this difference.

But this is an old eyepeice, and modern eyepeies tend to be better than this.

I would put the cutoff at 89% though. If add 10% to this, you get 97.9%. Still possible that there are some eyepieces out there with this level of transmission difference, but for the most part, most modern eyepices will indeed have transmission that is within 10%, but you are looking for a 10% improvement in brightness over the one with the lowest transmission, and not as an absolute of the highst possible transmission.

In other words, If I started with 50% and added 20% transmission to this, I would still have only 60% of the total.

This is important for marketing because sometimes you rea that "Transmission has been improved by 8%!"

But suppose you have a Baader Mark V bino that already has 93% light transmission. If you improved it by 8%", then it would have 101% light transmission, and of course that can't happen.

But if it starts with 88% and baader improved it by 10%, then the unit passes 10% more light than it did in the past. But it is only working at 96.8% transmission.

It didn't go from 88% to 98%, but the transmission could very honestly be said to have improved by 10%.

And this was common in the great SCT coating wars. People thought that a 7% transmission inprovment meant on an absolute scale, but it was relative to the amount of light that was going through the system. From a marketing standpoint, it sounds better to say that "Transmission has improved by 10%" rather than say "we went from 70% transmission to 77% transmission. To someone not thinking about it, it would sound like 80% of the light is getting though, but that is not what a 10% increas means at all. It means 10% more from where you startetd.

Hope this example helps.

#53 slack

slack

    Ranger 4

  • *****
  • Posts: 372
  • Joined: 02 May 2012

Posted 12 December 2012 - 03:58 PM

I didn't see information about the current ES series in this thread, but regarding the Nagler T6 line (and 13mm in particular), info here may relate to my experience evaluating them against comparable focal lengths in the ES line. I have posted my observations before.

I could not claim that there were faint stars that I could not see in the T6 but did see in the ES. However, I could definitely discern faint stars more quickly in the ES than the T6.

#54 Jim Curry

Jim Curry

    Surveyor 1

  • *****
  • Posts: 1551
  • Joined: 29 Oct 2007
  • Loc: STL

Posted 12 December 2012 - 07:59 PM

I subscribe to the physiological, not psychological theory.

Talking about diffuse, threshold objects. When you observe the Flame Nebula do you look for detail and veining with Alnitak in the field? If you do you're missing a lot of detail. Move Alnitak out of the field to get rid of that "flashlight" shining in your eyes and it's a whole new object you're viewing. Perhaps an extreme example so let me continue.

Now for me a threshold subject would be Stephan's Quintet. Tackle that with a Nagler, Delos or any other wide field and there's not a prayer of detection. I know, I've tried. Drop in a 40-45 deg. ortho and they will shimmer into view. Why? Because many, many "flashlights" dispered across the wide field ep are shining in my eyes. The ortho cuts the view down to a couple dozen, maybe(I've never counted). When your eyes are fully dark adapted even a 9-10 mag star in the field will seem bright and make a difference between detection or not.

Jim

#55 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16173
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 12 December 2012 - 08:18 PM

Jim,

Yes, exactly. The physical eye plays a larger role in visual astronomy than many observers seem to realize. Why wouldn't it? The telescope is important. The eyepiece is important. But what about the eye of the observer?

Just try to detect a faint fuzzy at the limiting magnitude of any telescope, even with an eyepiece which has the highest possible light transmission, when the eye is not optimally dark adapted. Good luck with that.

Mike

#56 Dave Ittner

Dave Ittner

    Messenger

  • *****
  • Posts: 403
  • Joined: 29 May 2012
  • Loc: SF Bay Area, California

Posted 12 December 2012 - 08:52 PM

Since I have been getting out observing with others I have noticed a big difference among folks in what each can see (or say they can see) with their naked eye. It's interesting as there are quite a few reasons as to why that happens. One thing that bugs me is the folks that refuse to get glasses or corrective lenses.

#57 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22561
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 12 December 2012 - 08:54 PM

I subscribe to the physiological, not psychological theory.

Talking about diffuse, threshold objects. When you observe the Flame Nebula do you look for detail and veining with Alnitak in the field? If you do you're missing a lot of detail. Move Alnitak out of the field to get rid of that "flashlight" shining in your eyes and it's a whole new object you're viewing. Perhaps an extreme example so let me continue.

Now for me a threshold subject would be Stephan's Quintet. Tackle that with a Nagler, Delos or any other wide field and there's not a prayer of detection. I know, I've tried. Drop in a 40-45 deg. ortho and they will shimmer into view. Why? Because many, many "flashlights" dispersed across the wide field ep are shining in my eyes. The ortho cuts the view down to a couple dozen, maybe(I've never counted). When your eyes are fully dark adapted even a 9-10 mag star in the field will seem bright and make a difference between detection or not.

Jim

Well, I see the point and I agree. That's not the ONLY explanation, though it works in the case of NGC2024 or NGC404 or even the Horsehead. It also partially explains why raising power can make something more visible--it narrows the field of view.
But it doesn't explain why sometimes the reverse occurs (e.g I see fainter things with the 100 degree 13 Ethos than I did with the 82 degree 13 Nagler) or why some observers claim to see fainter in one 45 degree eyepiece than another.

But I'll grant there may be both physiological and psychological factors at work.

#58 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22561
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 12 December 2012 - 08:57 PM

Since I have been getting out observing with others I have noticed a big difference among folks in what each can see (or say they can see) with their naked eye. It's interesting as there are quite a few reasons as to why that happens. One thing that bugs me is the folks that refuse to get glasses or corrective lenses.

In some cases, as my optometrist and I have discussed, it's simply impossible to correct the person's vision because there are multiple cylinders of astigmatism and variable correction needed across the retina.
But vanity, poverty, and convenience all play roles in human lives.

#59 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16173
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 12 December 2012 - 09:08 PM

Dave,

Since I have been getting out observing with others I have noticed a big difference among folks in what each can see (or say they can see) with their naked eye. It's interesting as there are quite a few reasons as to why that happens. One thing that bugs me is the folks that refuse to get glasses or corrective lenses.


... or wear good sunglasses or clip-ons during the day to protect their eyes. Can folks really expect to have their eyes scoured by full sunlight over a lifetime and not have it affect their ability to see the faint stuff at night?

Mike

#60 Dave Ittner

Dave Ittner

    Messenger

  • *****
  • Posts: 403
  • Joined: 29 May 2012
  • Loc: SF Bay Area, California

Posted 12 December 2012 - 09:19 PM

yup, something I now try to bring to everyone's attention more often.

#61 Paul G

Paul G

    Aurora

  • *****
  • Posts: 4932
  • Joined: 08 May 2003
  • Loc: Freedonia

Posted 13 December 2012 - 07:23 AM

Here is an example from this thread that corresponds to the post I just made.

If the TV Nagler 13 mm II has 87.3% transmission, to be seen as 10% brighter, you would add 10% to that figure, or 8.73% in relative terms. So adding 10% of the 87.3% means that any eyepice with 96.03% transmission would be 10% brighter on a relative scale.


That's not how our visual system works. If you increase transmission 10% the difference would be barely detectable, some would see a slight difference, others wouldn't see a change. It's a physiological phenomenon called response compression. For example, for a point light source or an extended object to be seen as 100% brighter its intensity would have to be increased about 900%.

#62 tomharri

tomharri

    Messenger

  • *****
  • Posts: 465
  • Joined: 19 Sep 2008
  • Loc: USA

Posted 13 December 2012 - 08:12 AM

I think you guys are just looking at your eyepieces and not looking thru them. The 6mm Delos shows so much more on Jupiter than the latest ortho, the Kasai, that it is no contest. When you put the Delos in, it's like you just put your glasses on. Jupiter is sharper-clearer-more contrasty, put in any positive adjective you can think of, it is so much better, plus you have 70 degrees to play with. Ever since I bought my first Pentax XW, and now the Delos, they both are superior visually, just like the Zeiss, over anything else out there.

#63 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16173
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 13 December 2012 - 08:41 AM

I don't think Jupiter - or any other bright planet - is a good test of transmission. For that you need to look at deep sky objects. IMO, the best light-transmission field test would involve detecting objects near the limiting magnitude of the telescope.

Even detecting faint moons of a bright planet is not the best test of transmission. In that case, it's too easy to confuse the effects of light scatter with light transmission. Best to keep it simple.

Mike

#64 ThomasM

ThomasM

    Mariner 2

  • -----
  • Posts: 297
  • Joined: 19 Apr 2009

Posted 06 April 2013 - 11:32 AM

A short update, here are transmissions for 532 nm:

Kasai HC Ortho 12 mm, 97.8%
Baader Classic Ortho 10 mm, 97 %
Paradigem 8 mm, 90,5 %

in addition I measured the transmission at 405 nm

Kasai HC Ortho 12 mm, 89%
Baader Classic Ortho 10 mm, 95 %
TMB ED 1.8x barlow, 94,5%

regards

Thomas

#65 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22561
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 06 April 2013 - 12:49 PM

A short update, here are transmissions for 532 nm:

Kasai HC Ortho 12 mm, 97.8%
Baader Classic Ortho 10 mm, 97 %
Paradigm 8 mm, 90,5 %

in addition I measured the transmission at 405 nm

Kasai HC Ortho 12 mm, 89%
Baader Classic Ortho 10 mm, 95 %
TMB ED 1.8x barlow, 94,5%

regards

Thomas

What this points out is that if the eyepiece is inexpensive, the absolute state-of-the-art coatings are unlikely to be applied.
If they were, the transmissions at 532nm would have read 98%, 98%, and 96% respectively.
The very high-end brands also multi-coat the lens surfaces that are cemented to increase transmission and reduce internal scatter.

This also shows that transmission spectra differ from eyepiece to eyepiece.

One of the things I found remarkable about TeleVue's newer Ethos and Delos eyepiece types was the improved red transmission. Whether this was simply due to improved transmission over the Nagler series, or actually represented a different curve of transmission (my suspicion), I don't know, but it points out how somewhat different transmission levels at the extremes may influence one's perception of "tint" in the eyepiece.

#66 Shneor

Shneor

    Viking 1

  • *****
  • Posts: 835
  • Joined: 01 Mar 2005
  • Loc: Northern California

Posted 06 April 2013 - 03:25 PM

It would be interesting to test Explore Scientific 68" and 82" eyepieces :)

Not to mention the ES100° and 120° eyepieces...

Clears,

#67 ThomasM

ThomasM

    Mariner 2

  • -----
  • Posts: 297
  • Joined: 19 Apr 2009

Posted 06 April 2013 - 04:21 PM

A short update, here are transmissions for 532 nm:

Kasai HC Ortho 12 mm, 97.8%
Baader Classic Ortho 10 mm, 97 %
Paradigm 8 mm, 90,5 %


Thomas

What this points out is that if the eyepiece is inexpensive, the absolute state-of-the-art coatings are unlikely to be applied.
If they were, the transmissions at 532nm would have read 98%, 98%, and 96% respectively.
The very high-end brands also multi-coat the lens surfaces that are cemented to increase transmission and reduce internal scatter.

This also shows that transmission spectra differ from eyepiece to eyepiece.

One of the things I found remarkable about TeleVue's newer Ethos and Delos eyepiece types was the improved red transmission. Whether this was simply due to improved transmission over the Nagler series, or actually represented a different curve of transmission (my suspicion), I don't know, but it points out how somewhat different transmission levels at the extremes may influence one's perception of "tint" in the eyepiece.


Actually, I think the simple rule of 1% loss per single element does not apply for short focal lenght Orthos, typially the transmission is less by 1-2%, I have no idea if it is an artefact of the measurement or if it is real, but this trend can also be seen here:

http://www.amateuras.../tips/tips3.htm

The transmission of a 12 mm Ortho with an excellent Barlow, such as the TMB ED is better than that of an 7 mm Orho, while at the same time it is much more comfortable to use.

Thomas

p.s. I was not aware of the high transmission of the Ethos eyepieces in the red, are there any results published?

#68 Alan A.

Alan A.

    Mariner 2

  • -----
  • Posts: 256
  • Joined: 08 Feb 2007
  • Loc: San Francisco

Posted 06 April 2013 - 05:15 PM

Hi Thomas,

Thanks for posting your transmission measurements.

Regarding multicoatings there is a nice article by Rodger Gordon who was an optical engineer for unitron.

The article is here:

Multicoatings


Multicoatings can not only absorb some of the spectrum, but can also cause narrow angle scattering, which has already been mentioned above in this thread.

It would be interesting to shine lasers of different wavelenghts through each eyepiece and have the resulting spot projected onto a distant wall to see how much scattering there is, and to see if it varies between eyepieces.


Alan

#69 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22561
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 06 April 2013 - 05:37 PM

Sigh.
This old canard again.
If you truly appreciate HOW a multicoating enhances transmission, then you would appreciate that IF the multicoating layers are chosen correctly (and they are in tandem with the refractive index of the glass they coat), then this so-called "narrow angle light scatter" cannot occur (or, if you will, what level of it occurs could not possibly be visible to the eye). It will be the axial ray that has the highest transmission, least reflection and scatter, and it is the more oblique rays that are more likely to scatter and/or be changed in spectrum.

Of course, it's certainly possible that inexpensive multi-coatings may not be the proper materials to produce the best transmission (leading to more reflection), and it may be that, applied to poorly-polished surfaces, they don't help enough to reduce what scatter would have been already there.

But note that all such "reports" are anecdotal, and they are usually suspect for the same reason global climate change reports from coal companies are suspect--the principals have a stake in the end result of such an "examination of evidence".

And it is interesting to note that many of the eyepieces that are the most highly-regarded eyepieces made are and were multi-coated throughout. In fact, several such eyepieces are known for having NO light scatter, either on or off axis.


As for the previous point that you can count on 1% light loss per element, this is no longer true. Some 8-element eyepieces with 10 air-to-glass surfaces transmit more than 96% and some 9-element eyepieces are in the same range. It does, however, depend on the coatings chosen, and those particular eyepieces would have lost prodigious amounts of light were it not for the use of expensive multicoatings on every glass surface. Nikon even has some professional 20-element photographic lenses that transmit somewhere in the 96-97% range. We could never afford those coatings in telescope eyepieces, but it goes to show that modern multi-coatings are a far cry from the multicoatings of yesteryear.

#70 Scanning4Comets

Scanning4Comets

    Markus

  • *****
  • Posts: 13508
  • Joined: 26 Dec 2004

Posted 06 April 2013 - 08:46 PM

As for the previous point that you can count on 1% light loss per element, this is no longer true. Some 8-element eyepieces with 10 air-to-glass surfaces transmit more than 96% and some 9-element eyepieces are in the same range. It does, however, depend on the coatings chosen, and those particular eyepieces would have lost prodigious amounts of light were it not for the use of expensive multicoatings on every glass surface. Nikon even has some professional 20-element photographic lenses that transmit somewhere in the 96-97% range. We could never afford those coatings in telescope eyepieces, but it goes to show that modern multi-coatings are a far cry from the multicoatings of yesteryear.


:applause:

So true Don! In this day & age, we have a wealth of them to choose from as well.

:cool:

#71 Alan A.

Alan A.

    Mariner 2

  • -----
  • Posts: 256
  • Joined: 08 Feb 2007
  • Loc: San Francisco

Posted 06 April 2013 - 09:12 PM

I have no doubt that the ZAO IIs have multicoatings which cause close to zero scatter. I'd like to to see data on other eyepieces before I assumed their multicoatings were as good as the ones on the Zeiss. There's a big difference between knowing how to do it well and being able to do it well, and within price constraints that the amateur market will bear.

When I purchased a ball eyepiece from Harry Siebert, we were talking about coatings, he mentioned to me that there are eyepieces currently on the market that do not transmit evenly across the full visual spectrum due to their multicoatings. One would think that should be a thing of the past, but it isn't. Optics is a nuanced field, I wouldn't take anything for granted.

If you feel narrow angle scatter is a canard, I would appreciate a good link or a reference with data showing otherwise. My intention in making my previous post was to be helpful to Thomas. As far as I know Rodger Gordon was extremely well respected for his work.

#72 buddyjesus

buddyjesus

    Vanguard

  • *****
  • Posts: 2236
  • Joined: 07 Jul 2010
  • Loc: Davison, Michigan

Posted 07 April 2013 - 12:11 AM

very informative thread. thanks guys

#73 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22561
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 07 April 2013 - 01:45 AM

I have no doubt that the ZAO IIs have multicoatings which cause close to zero scatter. I'd like to to see data on other eyepieces before I assumed their multicoatings were as good as the ones on the Zeiss. There's a big difference between knowing how to do it well and being able to do it well, and within price constraints that the amateur market will bear.

When I purchased a ball eyepiece from Harry Siebert, we were talking about coatings, he mentioned to me that there are eyepieces currently on the market that do not transmit evenly across the full visual spectrum due to their multicoatings. One would think that should be a thing of the past, but it isn't. Optics is a nuanced field, I wouldn't take anything for granted.

If you feel narrow angle scatter is a canard, I would appreciate a good link or a reference with data showing otherwise. My intention in making my previous post was to be helpful to Thomas. As far as I know Rodger Gordon was extremely well respected for his work.

Follow this thread:
http://www.cloudynig...3457819/Main...
and this one:
http://tech.groups.y...s/message/14857
There is no agreement among very respected sources.
I mentioned this once to a friend of mine who designs lenses for the military and he mentioned he'd heard of it but that the effect was "irrelevant" when "coating materials were properly chosen for the index of refraction".
And the conversation I had one afternoon with the Nikon engineers who design coatings on lenses to maximize transmission only mentioned the positive effects of superior coatings for transmission, reduction of scatter ,and improved contrast.

As for Siebert's comments, he's right. Also, uncoated eyepieces don't transmit flat across the visible band, either. And neither do simple MgFl2 coatings. or aluminum coatings on mirrors, etc. I have done literally thousands of hours of observing through hundreds of scopes and hundreds of different eyepieces over 50 years. And the only "narrow angle light scatter" I've ever seen was color in refractors (aka chromatic aberration).
Oh, I've seen scatter all right--that caused by haze in the atmosphere, dew on the optics or lenses, rough optical surfaces, light reflection internal to the scope and eyepiece, uncoated lenses, dust on optical surfaces, spherical aberration, uncorrected fields, aberrations of multiple kinds, etc. etc.

I side with Roland Christen on this one. Superior multi-coatings make possible superior contrast, not poorer contrast.

#74 Alan A.

Alan A.

    Mariner 2

  • -----
  • Posts: 256
  • Joined: 08 Feb 2007
  • Loc: San Francisco

Posted 07 April 2013 - 03:22 AM

I have no doubt that the ZAO IIs have multicoatings which cause close to zero scatter. I'd like to to see data on other eyepieces before I assumed their multicoatings were as good as the ones on the Zeiss. There's a big difference between knowing how to do it well and being able to do it well, and within price constraints that the amateur market will bear.

When I purchased a ball eyepiece from Harry Siebert, we were talking about coatings, he mentioned to me that there are eyepieces currently on the market that do not transmit evenly across the full visual spectrum due to their multicoatings. One would think that should be a thing of the past, but it isn't. Optics is a nuanced field, I wouldn't take anything for granted.

If you feel narrow angle scatter is a canard, I would appreciate a good link or a reference with data showing otherwise. My intention in making my previous post was to be helpful to Thomas. As far as I know Rodger Gordon was extremely well respected for his work.

Follow this thread:
http://www.cloudynig...3457819/Main...
and this one:
http://tech.groups.y...s/message/14857
There is no agreement among very respected sources.
I mentioned this once to a friend of mine who designs lenses for the military and he mentioned he'd heard of it but that the effect was "irrelevant" when "coating materials were properly chosen for the index of refraction".
And the conversation I had one afternoon with the Nikon engineers who design coatings on lenses to maximize transmission only mentioned the positive effects of superior coatings for transmission, reduction of scatter ,and improved contrast.

As for Siebert's comments, he's right. Also, uncoated eyepieces don't transmit flat across the visible band, either. And neither do simple MgFl2 coatings. or aluminum coatings on mirrors, etc. I have done literally thousands of hours of observing through hundreds of scopes and hundreds of different eyepieces over 50 years. And the only "narrow angle light scatter" I've ever seen was color in refractors (aka chromatic aberration).
Oh, I've seen scatter all right--that caused by haze in the atmosphere, dew on the optics or lenses, rough optical surfaces, light reflection internal to the scope and eyepiece, uncoated lenses, dust on optical surfaces, spherical aberration, uncorrected fields, aberrations of multiple kinds, etc. etc.

I side with Roland Christen on this one. Superior multi-coatings make possible superior contrast, not poorer contrast.



As BillP lists in the thread, there are many scientific papers explaining the phenomenon of low angle scatter. A canard is a false and baseless story, it doesn't seem like that word should be used to describe a phenomenon that has backing from scientific publications as well as some serious experts in optics.

Thomas Back specifically mentioned he measured low angle scatter on his coatings for his monocentrics and thought they were the lowest scatter he had seen on multicoated eyepieces.

You mention you haven't seen any scatter, but Chris Lord reports that the effect would be a reduction in contrast between contrast boundaries.

How is it that a simple uncoated bk7 ball lens performs as well on the planets on axis as the best planetary eyepieces available?

At the very highest levels of performance, I will guess that unless significant expense in getting multicoatings applied using the best technology, there is the potential for degradation of the image.

Our problem as amateurs is that we don't have the equipment to objectively measure things like narrow angle scatter for ourselves.

#75 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5573
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 07 April 2013 - 10:52 AM

I side with Roland Christen on this one. Superior multi-coatings make possible superior contrast, not poorer contrast.


Indeed, doing a search on the Yahoo AP turns up many postings by Roland Christen on eyepiece coatings, contrast, and performance from the point of a optician and purist. Coatings of course being only one element of eyepiece performance. Extrapolating from "superior coating" to "superior eyepiece" would be a logical error. But it's certainly a good place to start.

It would appear that RC was of the opinion that while a few commercial eyepieces might do it well, only Zeiss was doing it correctly.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics