Eyepiece coatings

[jetstream]

Since it has been cloudy,raining or snowing now for a few weeks,I have been reading quite a bit on optics etc.Others have pointed out some interesting articles on various topics including eyepiece performance.Roland has some very interesting thoughts on glass coatings including this,http://geogdata.csun.edu/~voltaire/roland/coating.html.I have just checked out my eyepieces in this manner and I am wondering if others have or want to?The results may be of value to us.

[bingevader]

I buy what I can afford. It's then of little relevance to me whether the image is yellow compared to a better corrected EP. Yep, I know it's not as good as a TV! 

Sorry. 

[Lee M]

Interesting read. I'll give that a go sometime.

[jetstream]

I buy what I can afford. It's then of little relevance to me whether the image is yellow compared to a better corrected EP. Yep, I know it's not as good as a TV! 

Sorry. 

I am glad you buy what you can afford-why wouldn't you? If you read the essay it points out a good test for all eyerpieces,and I am interested in a review of one that is not a TV or Zeiss.....Sorry

[bingevader]

But to what ends? You'll know what you already knew, or if not, depending on whether your eyepiece is half empty or half full, either wish you didn't know, or make no difference what so ever.

No need to apologise.

[Qualia]

I have just checked out my eyepieces in this manner....

Thanks for bringing this to our attention, Jetstream. I guess Bingevader is correct in claiming that our findings will only ground what was already suspected but I feel such a practice might be of interest especially when reviewing eyepieces. It gives a little more detail to the reader, as such. Anyway, what were your findings, Jetstream. Did you find any surprises in your own experiment?

[Moonshane]

my only criticism is (for the first comparison) that surely he's looking down the wrong end to test reflectivity of coatings? light travels up the eyepiece not down.

[Moonshane]

actually I have another criticism. surely it would be more objective to photograph each eyepiece individually from the same angle?

[Moonshane]

sorry, I have another, the length of the eyecup would surely also affect the result (if we are discussing coatings only, not design) so maybe they should all be removed?

[John]

The most interesting point he makes in that article for me is that there are a number of factors that go to make up the overall optical performance of a scope, not just the quality of the primary mirror or objective lens. The other components in the optical train all have an effect on the overall image quality. 

Common sense really, isn't it ?

[jetstream]

But to what ends? You'll know what you already knew, or if not, depending on whether your eyepiece is half empty or half full, either wish you didn't know, or make no difference what so ever.

No need to apologise.

Then why test eyepieces at all,in any matter if all testing shows is what you already new?

[jetstream]

Thanks for bringing this to our attention, Jetstream. I guess Bingevader is correct in claiming that our findings will only ground what was already suspected but I feel such a practice might be of interest especially when reviewing eyepieces. It gives a little more detail to the reader, as such. Anyway, what were your findings, Jetstream. Did you find any surprises in your own experiment?

Yes I did find some surprises, the most interesting is that my most inexpensive eyepiece is as good as my most expensive one,when viewing them from the top,flooded with indirect light.While I realize this test is not the most scientific one,it does show some differences in the coatings,glass polishing and internal stray light control.It can help pick eyepieces for certain uses.I understand Roland Christen has a fair idea about telescopic systems.

[YKSE]

actually I have another criticism. surely it would be more objective to photograph each eyepiece individually from the same angle?

I agree with this coment. When I checked my EPs under last night, the EPs look different darkness in coating in groups than I watch them individually. But I'm sure that Roland has worked out in his picture. I'd like to think that there're so many different coating technics, maybe not every producer use very blackish coating, some may be of greenish.e.g. As John has pointed out, coating is just a part of optical train, other thing like quality of glass, manufacturing precision, designs of a eyepiece play also very important roles, just my two cents.

[Moonshane]

very true - your eyes also play a part. I'd wager that it's more likely your eye will have slight astigmatism than will an eyepiece of even mediocre quality.

[John]

very true - your eyes also play a part. I'd wager that it's more likely your eye will have slight astigmatism than will an eyepiece of even mediocre quality.

Also true but it's a lot easier to upgrade glass mirrors and lenses than your eyeball

[Moonshane]

LOL true - although not if you believe the laser ads.

[jetstream]

From optical coatings to astigmatism,interesting

[AlexB67]

The most interesting point he makes in that article for me is that there are a number of factors that go to make up the overall optical performance of a scope, not just the quality of the primary mirror or objective lens. The other components in the optical train all have an effect on the overall image quality.

 

Common sense really, isn't it ?

 

I'll throw in my couple of pennies from what I learned so far.  What interested me is that some of the really expensive Multi element eyepieces can actually have a lower transmission in the sense of the amount of light the  optics allow through. In fact some of the more expensive eyepieces can cut quite a bit of light compared to simpler less element designs, BUT,  often that more expensive eyepiece will still let you see more due to better contrast and less scattering and light loss inside the optical train, this is not taken into consideration in that article but an important point too that results in the nice contrast laden views we like.

 

On top of that, coatings have the property  that some wavelengths  cut more than other wavelengths, some eyepieces have fairly constant performance over the range, some other less so,  hence you get a contribution that results in a hue of slightly different colour in eyepieces.  Mirrors can also have coatings that have such a dependence. I forgot which type, but there is one type of coating  I saw some data for,  it had supreme reflectivity at shorter wavelenghts, but the reflectivity really dropped off at longer wavelengths.

 

Here is some useful data listing transmission performance for some eyepieces btw.

 

http://www.amateurastronomie.com/Astronomie/tips/tips3.htm

 

http://www.lsi.pt/Astronomia/EyepieceTransmissionSurvey.htm

 

All of these things play into limiting magnitude calculations/predictions and affect performance in the end of the day. The original article is a guide, but an approximate one  that of course neglects other factors how to calculate the drop off in performance, but all the same it is a useful one.  In my calculator I compute these things too, taking this into account, as well as other things, I still would not take it as gospel truth though, but it provides quite useful information all the same and gives some insight 

[jetstream]

Alex,great links and insight into the optical path.I think Christens point was that for some eyepieces light is 'absorbed" mostly & not reflected back at us,giving better potential performance-like you said here, some multi element eyepieces give better contrast at a lower light through put.I have seen this myself with 2 of my eyepieces.

[John]

....What interested me is that some of the really expensive Multi element eyepieces can actually have a lower transmission in the sense of the amount of light the  optics allow through. In fact some of the more expensive eyepieces can cut quite a bit of light compared to simpler less element designs,....

In some cases, yes but others, no, eg: the multi-element Naglers seem to do rather well against the simple orthoscopic designs, actually having a little more light transmission in one or two cases. I've seen some figures elsewhere that suggests the Ethos and Delos do a touch better again. 

Given their cost though, I guess you might expect that !

Some of the designs listed in those tables are a decade or more old now so an update would be useful. I expect Baader GO's Astro Hutech and Fujiyama orthos are a small improvement over the orthos listed too.

Fancy the humble Skywatcher 6.3mm plossl matching the Zeiss ortho for transmission though

[AlexB67]

That's the only data I could ever find and get my hands on unfortunately, and for some reason the mirror data I can no longer find for that one case study. GSO boast about their 94% reflectivity, and SW never say as an advertising point or is it anywhere to be found ( hazard a guess anywhere in the 88 - 92 mark) .

Agreed with regard to the mutli-elements. I suppose I should have emphasised in some cases, but not always true

[John]

That's the only data I could ever find and get my hands on unfortunately, and for some reason the mirror data I can no longer find for that one case study. GSO boast about their 94% reflectivity, and SW never say as an advertising point or is it anywhere to be found ( hazard a guess anywhere in the 88 - 92 mark) .

Agreed with regard to the mutli-elements. I suppose I should have emphasised in some cases, but not always true

These apply to 2 surfaces (primary and secondary) so thats 6%-12% per surface or 12%-24% per system before eyepiece losses.

FWIW Orion Optics claim 97% for their Hilux coatings.

I believe mirror coatings gradually loose their reflectivity over time so those figures are when new I guess.

Amazing we can see anything at all through a 10 year old newtonian with standard coatings and a low cost eyepiece isn't it ?  

[AlexB67]

Thanks for that Orion data John. I'll add it to my data for my calculator.  Shame on me   I have not one Orion UK model listed in there ( yet ) I'll add a few. All my presets are the cheap skate models, skywatcher, GSO etc. Lots of televue in there though.

Indeed as you say there are two mirrors in an newt, plus the secondary obstruction, to put that in perspective. when you calculate the theoretical light grasp by aperture it comes out at a 1000 or so, by the time you take the reflectivity into account and secondary obstruction size it comes out more like 800, that is even before it reaches the eyepiece, similarly taking into account the brightness factor including eyepieces and magnification it reduces still further.

In my calculations at least I invented a term called effective light grasp, or at least I have not heard anyone use it as opposed to just light grasp or light gathering power, a bit like an effective aperture accounting for obstruction effects but accounts for reflectivity also. Usually manufacturers/sellers quote only the former, but when you take all those little things into account you can find quite a bit of performance difference across brands with eyepiece telescope combos by the time light reaches the eye, even if on paper they are theoretically the same performance in terms of straight aperture neglecting all those things.  Interesting stuff

[John]

While you are updating your data, I've seen a range of claimed reflectivities for GSO mirrors from 89% to 94%. GSO themselves state an average of 92%. Summarian optics quote 89% for a standard GSO mirror or 93% for the extra cost enhanced "deluxe" coatings. Telescope Services claim 91%-94% depending on which part of their website you look !

It's very hard to find any information regarding the reflectivity of Synta / Skywatcher mirrors.  Having used both GSO and Skywatcher mirrored scopes my guess would be that they are about the same.

Of course this is nothing to do with eyepieces and this is the eyepiece section 

[bingevader]

Then why test eyepieces at all,in any matter if all testing shows is what you already new?

Absolutely!

I look at the stars with mine, not walls. 

Cheers