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Refractor Anti Reflection Coating longevity.

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#1 Plane_Guru

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Posted 21 February 2013 - 07:10 AM

Hello everybody,

I-am interested to know your opinions regarding the longevity/lifespan of Anti-Reflection Coatings on Refractor Telescopes.

In your Opinion would UV Radiation from Sunlight have a detrimental effect on the Anti-Reflection Optical Coatings in Refractor Telescopes? Does continual bombardment by Solar Radiation from the Sun degrade Optical Coatings(like Anti Reflection Coatings) over time?

I am of the belief that Magnesium Fluoride is still the mainstay of Anti-reflection Coatings, and that this material has almost zero solubility in water and all common organic solvents. Another material commonly used in Anit-Reflection Coatings is Silicon Dioxide (Quartz) which is an extremely hard and tough material. I believe both Magnesium Fluoride and Silicon Dioxide (Quartz) are highly transparent to UV light, and so will have little interaction with Solar UV Light due to the transparency of these Materials.

I presume that the Solar Radiation ( Light Energy) that passes through Refractor Anti-Reflection Optical Coatings does not damage or degrade the Optical Coatings over time due to the light having little interaction with the Optical Coatings.

I also learned that Optical Coatings were originally developed to retard the migration of Fluoride out of Glass.

Many Thanks for your input.

#2 Starhawk

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Posted 21 February 2013 - 09:23 AM

The coatings are really glasses of various types. They should last indefinitely. The main coatings for cheap optics are magnesium fluoride. The nicer stuff has all moved on to much more sophisticated multicoated systems, which give themselves away with colored sheens, or in the really high end ones, by completely vanishing except for very careful illumination.

With that said, surface prep and processing are paramount for getting them on there properly. And if you ever deal with a coating vendor, you will discover they only warranty them for 1 year in most cases.

So what happens if anti reflection coatings fail or are damaged? I asked Roland about that, and his answer was it is more work to polish the coatings off, recorrect the figure, recoat, and put an optical set back together than it is to build a new telescope.

So take care of your coatings.

-Rich

#3 Plane_Guru

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Posted 21 February 2013 - 09:33 AM

Hi Rich,

I am purchasing a new Televue NP-101 and Televue 60 IS Guidescope,I would hope the coatings would last indefinitely.I hope for the two Televue Telescopes to outlive me.

#4 Binojunky

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Posted 21 February 2013 - 11:18 AM

Providing you don,t get silly with constantly cleaning every minor blemish and mark off they will last a lifetime,DA.

#5 jrbarnett

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Posted 21 February 2013 - 03:06 PM

Only diamonds are forever. All else is subject to the ravages of time and misuse. :grin:

Seriously though, why the fixation on coatings? A well-cared-for refractor used and stored sensibly will outlast you, whether or not its coatings remain exactly as they were the day you took delivery of the scope.

In the grand scheme of things, the coating type and durability is way down the list of things to worry about when choosing a refractor. Coating type and integrity are not crucial for achieving excellent performance from a refractor. You can't say that about mirrored scope coatings, but you can about refractors.

Instead of worrying about coatings, I would worry about mechanical design, robustness of the optical alignment retention design (i.e., the lens cell), and above all optical figure quality. An uncoated excellent quality refractor will outperform a perfectly coated, poorly figured refractor every time.

Seriously, worry more about the optical quality and how well the scopes you're considering maintain optical alignment rather than coatings. Those are far more relevant concerns affecting enjoyable "lifetime" use and performance of a refractor.

Best of luck with your choice.

Regards,

Jim

#6 KWB

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Posted 21 February 2013 - 03:58 PM

Only diamonds are forever. All else is subject to the ravages of time and misuse. :grin:

Seriously though, why the fixation on coatings? A well-cared-for refractor used and stored sensibly will outlast you, whether or not its coatings remain exactly as they were the day you took delivery of the scope.

Jim

I think Jim makes a very good point here,just like the one he made to the identical question asked in another thread. :waytogo:

To the OP,in the future,please just ask a specific question one time, in one thread. :)

#7 Plane_Guru

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Posted 21 February 2013 - 04:04 PM

Hi jrbarnett ,

Thanks for the input.I was just curious as to the estimated longevity of Anti-Reflection Coatings because I am not wealthy and naturally I would hope that my expensive Televue NP-101 Telescope that I just invested in would outlive me .

Sadly these Days we live in a Throw away Society.I look after what I own .I love Science too and the Science behind Anti-Reflection Coatings.I love learning,in general.

All my best

#8 la200o

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Posted 21 February 2013 - 04:38 PM

yer NP101 will outlive you, no problemo

Bill

#9 EdZ

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Posted 21 February 2013 - 07:59 PM

Post deleted by KWB

#10 Starhawk

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Posted 21 February 2013 - 08:17 PM

The only real failure I've seen in TV OTAs is the flocking coming off in the interior. In everyday life, coatings die from over cleaning and using the wrong materials.

So:

Plan A keep it clean- no touching, think before you take it out in blowing dust or pollen, and store horizontally.

-Rich

#11 Plane_Guru

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Posted 21 February 2013 - 10:32 PM

Hi Starhawk,

Thank-you.I hope the Flocking Paper peeling is not a major issue with Televue NP-101 Refractor Telescopes?.Is Peeling Flocking Paper rare in Televue Refractor Telescopes?

I would hope that this issue has been rectified by now? Has Televue changed to better stronger Adhesive technologies by now?

Peeling Flocking Paper is a concern of mine due to the fact I reside in a Humid locale.

#12 Plane_Guru

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Posted 21 February 2013 - 10:37 PM

Hi,

While I am thinking of this..I also have to ask abut Dielectric Coated Star Diagonals? I read that Dielectric Star Diagonals supposedly never go bad due to the fact that Dielectric Coatings never oxidise and so should last "Forever" with no loss of brightness( performance)?

#13 jrbarnett

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Posted 21 February 2013 - 11:47 PM

Brightness (reflectivity) of a diagonal is pretty much irrelevant. What are key are flatness, mechanical precision and smoothness of the surface. While a dieclectric coated diagonal won't need a recoat, they do tend to be rougher than standard coated mirror diagonals. Also, not all dielectric diagonals are created equally. Some have rougher surfaces than others. The A-P Maxbrights and TV Everbrites are in the middle in terms of surface smoothness. The GSO Quartz diecletrics are smoother, but mechanically the housings are a little flimsy. For my most critical observing (planets and double stars) is use a conventional mirrored turret.

Again, only diamonds are forever. There is nothing else under the Sun that will last forever. These items are tools. They are meant to be used, not put on display. A scope used every clear night for 10 years that then fails is still a better scope than one that lasts fifty years but isn't used by the owner for fear of it losing its "newness".

Refractors don't fail catastrophically. Wear items include the focusers and the retractable dew shade retention materials. Regular maintenance items include occasionally cleaning the front of the objective. Mishandling related maintenance might include recentering and/or collimating the objective. NP-101s are sensitive to decentering as all four elements need a common center and squaring. If you whack it hard on accident, you will either need to build your own jig and recenter the front elements relative to one another and the rear group or else send it off to Televue for service.

It's nice that Televue offers service. They will repair, clean, colimate, etc., Televue refractors for a fee. Of course, nothing lasts forever and one day Televue will fold or be acquired, and the days of readily available service may end.

Are you planning on imaging or observing visually, or both? What makes you think the NP-101 is your best option? There are many nice 4" and larger refractors available that cost much, much less than the NP-101. have you considered other makes and models?

Perhaps if you didn't spend quite so much, you'd be less worried about it lasting "forever" which is an unrealistic expectation, though pretty much any refractor is likely to outlast you and me. :grin:

- Jim

#14 la200o

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Posted 22 February 2013 - 09:22 AM

Deflocking and loss of collimation are rare problems. Get your NP101 and don't look back. It can do things no other 4" refractor can do. Superb wide field views and great at high power. Wouldn't be without one, myself.

Bill

#15 Paul G

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Posted 22 February 2013 - 10:36 AM

Hi,

While I am thinking of this..I also have to ask abut Dielectric Coated Star Diagonals? I read that Dielectric Star Diagonals supposedly never go bad due to the fact that Dielectric Coatings never oxidise and so should last "Forever" with no loss of brightness( performance)?


The benefits of a dielectric diagonal are:
-- coatings don't degrade over time
-- they can be kept clean without risking damaging delicate coatings; dielectric coatings actually become part of the glass and must be ground off to remove
-- much less scatter than aluminum mirror diagonals (on a molecular level the aluminum coating is quite rough)

Some cautions:
-- not all dielectrics are the same
-- there are some cases of inexpensive dielectrics flaking off, a sign of improper surface preparation or improper application of the coatings
-- the coatings are designed to have maximum reflection for light coming in at a 45 degree angle, can be less reflective and /or more transparent if the light comes from a nonoptimal angle (for instance, shining a laser straight down on the mirror)
-- how the mirror is supported is important
-- how the diagonal is baffled is important
-- how the diagonal is designed to dissipate the small % of light that isn't reflected is important, you don't want that light reflecting around inside the substrate and coming out the other end (there has been at least one post here on CN where this dissipation was incorrectly thought to be scatter; scatter of the transmitted light isn't the same in terms of affecting the image as scatter that is reflected off the surface)

A good dielectric is likely to be the most accurate surface in the optical train. The AP Maxbright, for instance, is guaranteed to be at least 1/20 wave, most are around 1/40 wave.

#16 Plane_Guru

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Posted 22 February 2013 - 10:42 AM

Hi Bill,

Good to hear that De-Flocking is rare.I have a new Televue NP-101 on order.

#17 jrbarnett

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Posted 22 February 2013 - 10:52 AM

"much less scatter than aluminum mirror diagonals (on a molecular level the aluminum coating is quite rough)"

Sadly, this is untrue. Having tested a host of diagonals, including the A-P Maxbright using a precisely spaced green laser and calipers to measure contact spot size on the mirror or prism first surface, even cleaned dielectric diagonals, on average, show larger contact spots (indicating greater surface scatter) than standard aluminum coated and quality prism surfaces. Theory is fine as far as it goes, but when it is accepted as fact without verification it becomes marketing hype.

Dielectric diagonals have more durable coatings, which is great because diagonals are rubbish bins collecting all kinds of junk and need to be cleaned, but you buy that virtue with the vice of higher scatter on average.

Now whether you can see the level of reduced scatter of a quality enhanced aluminum diagonal over a quality dielectric one at the eyepiece is debatable, but I hate to see the marketing myth of superior surface smoothness perpetuated as fact.

Regards,

Jim

#18 la200o

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Posted 22 February 2013 - 11:21 AM

Hi Bill,

Good to hear that De-Flocking is rare.I have a new Televue NP-101 on order.


You will love it.

Bill

#19 BCNGreyCat

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Posted 22 February 2013 - 01:47 PM

I have a tv-102 manufactured around 2002 and the flocking paper is still very good. No sign of peeling or bubbling. I once thought there was one but it turned out to be under the lens cell bolt, where the inside surface of the tube is not flat.

#20 Paul G

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Posted 22 February 2013 - 02:18 PM

"much less scatter than aluminum mirror diagonals (on a molecular level the aluminum coating is quite rough)"

Sadly, this is untrue. Having tested a host of diagonals, including the A-P Maxbright using a precisely spaced green laser and calipers to measure contact spot size on the mirror or prism first surface, even cleaned dielectric diagonals, on average, show larger contact spots (indicating greater surface scatter) than standard aluminum coated and quality prism surfaces. Theory is fine as far as it goes, but when it is accepted as fact without verification it becomes marketing hype.


What you're seeing was discussed some time ago:

Dielectric vs aluminum mirror diagonals

Back to the op, the primary reason to get a dielectric diagonal is the ability to clean it without causing sleeks in the coatings. A little dust on the diagonal mirror surface will have a much greater adverse effect on the image than any difference in reflectivity or scatter. Keep in mind the image occupies the central 1/4 inch or so of the mirror, a little dust that would cause zero degradation in the image if on the objective surface causes noticeable loss of low contrast detail if on the diagonal mirror (as does dust on the surface of an eyepiece where the image is also concentrated in a very small area).

Edit:

Also to the op, mag fluoride has its applications but has drawbacks as well. It would be an especially poor choice for ED glass elements since both the ED glass (including fluorite) and mag-fl are both very low refractive index materials and there wouldn't be enough difference between them to reduce reflections by more than a very small amount. On an appropriate type of high index glass mag-fl at best still leaves about 1.5% reflection, modern multicoatings are in the 0.25% range. Also, mag fluoride's transmission curve isn't flat over the color spectrum.

#21 Eddgie

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Posted 22 February 2013 - 02:37 PM

Back to the op, the primary reason to get a dielectric diagonal is the ability to clean it without causing sleeks in the coatings. A little dust on the diagonal mirror surface will have a much greater adverse effect on the image than any difference in reflectivity or scatter. Keep in mind the image occupies the central 1/4 inch or so of the mirror, a little dust that would cause zero degradation in the image if on the objective surface causes noticeable loss of low contrast detail if on the diagonal mirror (as does dust on the surface of an eyepiece where the image is also concentrated in a very small area).



I absolutely agree.

Serious planetary observers should buy a good quality dielectric.

For the most serious planetary viewing, the diagonal should be cleaned immediately before use (eyepecies too).

As the light cone converges to the diagonal, the intensity for unit area increases greatly. Dust on the diagonal will indeed scatter this light.

I find cleaning a dirty diagonal makes far more difference in scatter than changing from A T6 Nagler to an Ortho for example. In the latter case, it is hard to see a difference, but if there is dust on the diagonal, it is easy to see the difference in performance.

And this is why people should use dielectric. It is the most robust coating available. it will withstand even sand (silica quartz) abrasion.

Silica Quartz dust is the benchmark because it is hard enough to scratch MgF2 which is the hardest substance that most optics are ever exposed to.

This is why the Military went to Dielectric long ago. Based on my own observations, Marines are excellent shots, but not gentle with optics.

With Dielectrics, they don't need to be. Even desert sand won't scratch dielectrics when wiped with a shirt sleeve.

Hard. Very, very hard.

To be fair, Enhanced Aluminum is quite hard as well. Much harder than most people realize (harder than the glass it is bonded too).

But as far as I know, dielectric is the hardest currently available commercial glass coating technology.

#22 Plane_Guru

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Posted 22 February 2013 - 02:56 PM

Hi again Doug,la200o,BCNGreyCat, jrbarnett and Paul G

Thank you ever so much, once again.Lastly I need to seek a answer to this query..this may seem silly and minor,BUT... do the latest new Televue NP-101 Telescopes and their Eyepiece Holders use Rust free Alloy Screws throughout?

I read of Televue having an issue with Rusty Screws which is a pain due to the fact that having to replace Rusty Screws with Stainless Screws is actually quite an expensive endeavour.

#23 Plane_Guru

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Posted 22 February 2013 - 03:11 PM

Hi Eddgie

Thanks. Dielectrics are very tough,yes and there is also the longevity which is a serious positive,supposedly Dielectric Star Diagonals won't (dare I say ever) degrade or oxidise or lose brightness in-a human lifetime.

Secondly Anti Reflection Coatings are Dielectric.

#24 la200o

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Posted 22 February 2013 - 05:10 PM

Hi again Doug,la200o,BCNGreyCat, jrbarnett and Paul G

Thank you ever so much, once again.Lastly I need to seek a answer to this query..this may seem silly and minor,BUT... do the latest new Televue NP-101 Telescopes and their Eyepiece Holders use Rust free Alloy Screws throughout?

I read of Televue having an issue with Rusty Screws which is a pain due to the fact that having to replace Rusty Screws with Stainless Screws is actually quite an expensive endeavour.


Dunno the answer to this one, but none of my TV scopes show any sign of such rust, and my 102 is an early one that's been covered with dew, ice, and water from melting ice many times.

Bill

#25 jrbarnett

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Posted 22 February 2013 - 05:48 PM

I've never read about Televue scopes having rusty screw issues. There are a number of owners on this forum who have Televue scopes and live in maritime climates (in the UK). Perhaps one of them will read this and comment on the reality of the corrosion issue.

Televue scopes have very few exposed screws in actuality, compared to other refractors.

- Jim






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