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Does a mirror diagonal degrade after X number of years?

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#1 jokrausdu

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Posted 27 October 2021 - 09:25 AM

Hi all,

 

I've had a 2 inch mirror diagonal from Tele Vue for about 30 years now. Should I ever get it re-mirrored? Is it not worth the hassle, and just run out and get a new one for $200 or so? Is their Everbright ™ really for ever?

 

I also wonder about how flat the flat should be. Some advertise 1/10th wave or better. But, since it is so close to the eyepiece, the reflection is not going to redirect the lightwaves very far, so a quarter wave flat may look the same as a tenth wave flat. 

 

Thoughts? Any links to research with data?

 

Thanks, Joe


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#2 c2m2t

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Posted 27 October 2021 - 10:22 AM

Hi Joe!

Very interesting question!! This has never crossed my mind. I look forward to any follow-up opinions/thoughts. I've been looking at myself in mirrors similar in age and I would be hard pressed to say there has been any noticeable degradation of the mirror! I am guessing that it has much to do with the environment that the diagonal find itself in. After use, I always replace caps or make sure the mirror is not exposed to continuous air changes. I suspect that areas close to salt water may have some deleterious effects if the mirror is left uncovered. 

 

Now that I think about it, household mirrors are protected by a layer of glass...quite a big difference from optical mirrors. Now I am really looking forward to the experiences/knowledge of others!!

 

Cheers, Chris.



#3 astrokeith

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Posted 27 October 2021 - 10:44 AM

Since the diagonal has probably had a 'protected' life (ie no dew etc), it will probably be fine. If it looks Ok then leave it.

 

Distance to the eyepiece isnt a factor I'm afraid. You need a flatness at least as good as your primary objective. 1/10 wave is good and typical.



#4 ETX2112

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Posted 27 October 2021 - 10:49 AM

House mirrors are only protected by a layer of paint on the back, they start to fail usually starting at the edge and working it's way in.

Diagonals have a pretty strong coating and I would imagine last a very long time, but how long?

Everything man made breaks down adventuley.
30 years is a long run!

#5 MitchAlsup

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Posted 27 October 2021 - 03:43 PM

My old C11 with overcoated silver optics (sold) is still giving great images 32 years after being bought.

I have a 26 year old enhanced coating on my 20" F/4 (which has never seen dew*).

 

So, protecting the optics from dew (i.e., water) goes a long (LONG) way in reducing the degradation coating suffer.

 

(*) there is something about living in the central Texas and observing the west Texas desert.



#6 SeattleScott

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Posted 27 October 2021 - 04:14 PM

Dielectric coatings are extremely durable and shouldn’t ever really degrade. Enhanced aluminum or silver coating will fail, typically losing significant reflectivity after 10-20 years. For a 30 year old diagonal it probably isn’t dielectric, and could probably use a recoat.

Some will talk about SCT coatings lasting a long time, especially in dry environments. Yes a 30 year old SCT can still give good views. But typically when people recoat their ~40 year old mirrors does make a detectable difference in brightness. Or when they compare them side by side with a newer model. Generally it isn’t a night and day difference, but every 10% helps. Also, SCT is basically a sealed tube. It would be hard for dust to get up in there even when changing eyepieces, unless you are observing the ground. A diagonal is exposed and dust/pollen/dirt can settle in there every time you swap eyepieces. Not a ton of exposure but it adds up, and the diagonal will typically need cleaning long before a SCT primary mirror.

Consider that the enhanced aluminum coating was probably 95-96%. Dielectric are 99%. So if the coating even degraded a mere 5-6% reflectivity over 30 years, that would be 10% difference compared to a new dielectric.

Consider how many times you have cleaned the diagonal. I hear a good rule of thumb for reflector primary mirror is recoating the mirror after every three cleanings. Maybe this is excessive, especially if you are very careful how you clean the mirror. But if you have cleaned your mirror once every three years, that would be ten cleanings.

Once I had my nearly new VX10 with 97% coating out next to an old Coulter. My scope gave much brighter views. Coatings do matter. Granted the Coulter probably was around 88% originally, and a primary mirror has much more exposure than a diagonal mirror. I expect the difference would be more subtle in your case. But I expect there would be a detectable improvement in transmission.

Scott

Edited by SeattleScott, 27 October 2021 - 04:27 PM.


#7 Starman1

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Posted 27 October 2021 - 04:25 PM

Dielectric coatings are extremely durable and shouldn’t ever really degrade. Enhanced aluminum or silver coating will fail, typically losing significant reflectivity after 10-20 years. For a 30 year old diagonal it probably isn’t dielectric, and could probably use a recoat.

Some will talk about SCT coatings lasting a long time, especially in dry environments. Yes a 30 year old SCT can still give good views. But typically when people recoat their ~40 year old mirrors does make a detectable difference in brightness. Or when they compare them side by side with a newer model. Generally it isn’t a night and day difference, but every 10% helps.

Consider that the enhanced aluminum coating was probably 95-96%. Dielectric are 99%. So if the coating even degraded a mere 5-6% reflectivity over 30 years, that would be 10% difference compared to a new dielectric.

Consider how many times you have cleaned the diagonal. I hear a good rule of thumb for reflector primary mirror is recoating the mirror after every three cleanings. Maybe this is excessive, especially if you are very careful how you clean the mirror. But if you have cleaned your mirror once every three years, that would be ten cleanings.

Scott

I clean my primary mirror quarterly, and it's filthy every time I clean it--a cleaned line across the middle is shiny compared to the rest of the mirror.
If I recoated every 3rd cleaning, I'd recoat once a year, and that is ridiculous.

A good coating should last 10 years and anything beyond that is "bonus" for keeping the mirror clean and free of dew.

A 20 year old coating is really long in the tooth and likely very low reflectivity compared to new.

 

As for dielectric coatings, we don't have any statistical evidence.  First, because people aren't recoating them (they would need to be repolished and coated again),

and second because they haven't been around all that long.  Give it another 20 years and we'll know.

 

One thing about coatings: unless it is obvious the coating is failing, the fact it still looks shiny is NOT an indication the coating is good.

People won't notice a clean mirror with a 50% reflectivity has any problem at all, yet it is past-due for a recoat.  It is only in comparison with a new coating

the difference becomes obvious.


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#8 Frisky

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Posted 27 October 2021 - 04:30 PM

My 2 cents, based on 3.5 years and 611 nights out, but not 30 years. If you clean it gently, and it's clear and bright, you're good to go. When I bought my Meade, the company said the 2" mirror diagonal was meticulously hand-polished. I wanted to replace it with a dielectric, but I had never seen a mirror so clear and bright in my life! So, I kept it. It took 2 years to get dirty and 5 minutes to clean to get it back to like new brightness. Another 1.5 years later, I haven't touched it and it still looks like new! So, if it looks nice and bright, with no marks of any kind, I'd just use it.

 

Joe



#9 helpwanted

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Posted 27 October 2021 - 04:32 PM

Hi all,

 

I've had a 2 inch mirror diagonal from Tele Vue for about 30 years now. Should I ever get it re-mirrored? Is it not worth the hassle, and just run out and get a new one for $200 or so? Is their Everbright ™ really for ever?

 

I also wonder about how flat the flat should be. Some advertise 1/10th wave or better. But, since it is so close to the eyepiece, the reflection is not going to redirect the lightwaves very far, so a quarter wave flat may look the same as a tenth wave flat. 

 

Thoughts? Any links to research with data?

 

Thanks, Joe

Since its a TV, call and ask them, you will end up talking with Al or David. 


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#10 Frisky

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Posted 27 October 2021 - 04:33 PM

Of course, I also agree with Starman. You probably can't really tell if it has lost its luster. Unless you see obvious marks.

 

Joe



#11 luxo II

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Posted 28 October 2021 - 04:03 AM

Protected coatings can last a very long time. I’ve seen a Intes scope 25 years old with its protected coatings measured at 96% reflectivity and no flaws, but they’re inside a closed OTA.

Even more intriguing was an original ATM mak that appears here with simple aluminised coatings circa 1960 that were still in fair shape after 60 years, though the reflectivity was obviously down - they looked a bit grey or milky - which means the aluminium is oxidising.

Coatings can also fail in a way that exposes patches of glass about the size of a fingernail. These patches continue to grow and eventually most of the coating is gone. This suggests the adhesion of the coating to the glass is failing; apparently due to water molecules from the air (or moisture ie. dew) finding a way in through pores in the coating, at the molecular scale.

Edited by luxo II, 28 October 2021 - 04:12 AM.

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#12 noisejammer

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Posted 28 October 2021 - 06:36 AM

I'll echo that I think protected coatings can last almost indefinitely.

 

As an example, I gave the Johannesburg observatory's Tinsley 12" DK a wash. At the time it was going on 35 years old and the mirrors - I think they are silver over-coated with quartz - look like new. I had similar experience with a Grubb Parsons mirror that became my first scope - even when it was 30 years old, the mirror looked great. The coatings on my RCOS all look fine (but it's only 16 years old.)

 

But.. if the mirror isn't protected, silver goes brown in a few months. Aluminium lasts a fair bit longer. Silver is easy to strip and re-apply.

 

Finally - dielectric coatings. My AP-diagonal is 18 years old and still works fine. So does my Tak diagonal which is about the same vintage.



#13 LDW47

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Posted 28 October 2021 - 07:59 AM

Hi all,

 

I've had a 2 inch mirror diagonal from Tele Vue for about 30 years now. Should I ever get it re-mirrored? Is it not worth the hassle, and just run out and get a new one for $200 or so? Is their Everbright ™ really for ever?

 

I also wonder about how flat the flat should be. Some advertise 1/10th wave or better. But, since it is so close to the eyepiece, the reflection is not going to redirect the lightwaves very far, so a quarter wave flat may look the same as a tenth wave flat. 

 

Thoughts? Any links to research with data?

 

Thanks, Joe

I have heard that under the proper care they will last for a thousand years



#14 ETX2112

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Posted 28 October 2021 - 09:09 AM

Something that comes to mind, I'm 60...in ten or especially twenty years my observing days are over.

With that in mind I'm not concerned about really any of my astro equipment going bad.

Everything thing I own will out last me...lol, can't take it with me.

Now you younger folks is a different story.

Good day.
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#15 SeattleScott

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Posted 28 October 2021 - 09:24 AM

Something that comes to mind, I'm 60...in ten or especially twenty years my observing days are over.

With that in mind I'm not concerned about really any of my astro equipment going bad.

Everything thing I own will out last me...lol, can't take it with me.

Now you younger folks is a different story.

Good day.

That’s all fine but if someone has a 30 year old diagonal that is only 80% reflectivity, compared to a new 96-99%, they are losing significant brightness. Even if one only has 10-20 years, don’t you want to get the most out of those 10-20 years? If all your equipment is brand new, then fine, you don’t have to worry.

Ultimately the question is if a 30-year old non-dielectric diagonal has lost significant reflectivity. While there are variables such as how often it was used and how dry the climate is, most likely it has probably lost significant reflectivity. Yes, it may still look good to the eye. Yes, the views may still look good through the scope. But like the frog in the pot as it heats up, are you really going to notice the views in your scope getting half a percent dimmer each year? But after 30 years that would be 15% dimmer.

Now is it worth it to the OP to bother and pay to recoat the diagonal, or buy a new one, to get another maybe 10%, 20%, possibly 30% brightness? That’s the real question now.

Scott
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#16 SeattleScott

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Posted 28 October 2021 - 09:30 AM

I'll echo that I think protected coatings can last almost indefinitely.

As an example, I gave the Johannesburg observatory's Tinsley 12" DK a wash. At the time it was going on 35 years old and the mirrors - I think they are silver over-coated with quartz - look like new. I had similar experience with a Grubb Parsons mirror that became my first scope - even when it was 30 years old, the mirror looked great. The coatings on my RCOS all look fine (but it's only 16 years old.)

But.. if the mirror isn't protected, silver goes brown in a few months. Aluminium lasts a fair bit longer. Silver is easy to strip and re-apply.

Finally - dielectric coatings. My AP-diagonal is 18 years old and still works fine. So does my Tak diagonal which is about the same vintage.

I saw the reflector in the Goldendale WA observatory and the coating looked like total crap. I think they replaced the scope a year or two ago. Apparently the original mirror wasn’t overcoated with quartz? I think it was about 50 years old.

As has been mentioned a mirror can look “fine” visually but still be 10%, 30%, maybe even 50% lower reflectivity than a new coating. You can only really tell by doing a comparison under the stars against a newer (or more recently coated) mirror.

Scott

#17 LDW47

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Posted 28 October 2021 - 09:38 AM

I saw the reflector in the Goldendale WA observatory and the coating looked like total crap. I think they replaced the scope a year or two ago. Apparently the original mirror wasn’t overcoated with quartz? I think it was about 50 years old.

As has been mentioned a mirror can look “fine” visually but still be 10%, 30%, maybe even 50% lower reflectivity than a new coating. You can only really tell by doing a comparison under the stars against a newer (or more recently coated) mirror.

Scott

I think you are all wrong but I won't get into it



#18 ETX2112

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Posted 28 October 2021 - 12:15 PM

Lifes short..I would just buy a new diagonal or eyepiece if you can afford it? People are obviously passionate about this hobby and the equipment used to get the most from every photon that hits their eyes.

As for me most all my new equipment is less then 2 years old so I can put the worries aside and enjoy the hobby.

Good day
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#19 SeattleScott

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Posted 28 October 2021 - 12:49 PM

I think you are all wrong but I won't get into it

If you have experience doing side by side comparison with a decades old mirror compared to a newer one, I’m all ears. Generally people who have noticed an increase in brightness from the newer coatings.

Sure a lot of people are just like the mirror looks fine, so it’s fine. Which is fine, if you are fine with 80% reflectivity.

Scott

#20 wrvond

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Posted 28 October 2021 - 04:29 PM

What is the lowest magnitude star you can see with your thirty year old diagonal? Do you feel like you are missing something? Are you happy with the detail on the planets and moon?

If you're happy, then continue on and enjoy. Thirty years ago it was 1991 - with the exception of Baader BBHS coatings, I don't think the technology has changed significantly for mirror diagonal manufacture since then.

On the other hand if you just want a new diagonal, I think you've gotten your money's worth out of that one. 


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#21 Starman1

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Posted 28 October 2021 - 04:44 PM

What is the lowest magnitude star you can see with your thirty year old diagonal? Do you feel like you are missing something? Are you happy with the detail on the planets and moon?

If you're happy, then continue on and enjoy. Thirty years ago it was 1991 - with the exception of Baader BBHS coatings, I don't think the technology has changed significantly for mirror diagonal manufacture since then.

On the other hand if you just want a new diagonal, I think you've gotten your money's worth out of that one. 

Pure dielectric coatings on diagonals have appeared since then.


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#22 luxo II

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Posted 28 October 2021 - 04:53 PM

I think you are all wrong but I won't get into it

So.. you're buying a new one every year ?



#23 Jon Isaacs

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Posted 28 October 2021 - 05:25 PM

My two cents:

 

Coatings degrade because cleaning and the conditions. A mirror stored in a vacuum and never cleaned will last a long long time. A mirror cleaned frequently and subject to a humid, corrosive environment will likely be short lived..

 

This mirror is probably somewhere in the middle.. Diagonals generally get cleaned frequently but do not suffer from a harmful environment. I think it's wrong to assume just because it's 30 years old, the coatings are no longer good.  

 

I have five 2 inch TeleVue diagonals. 4 are Everbrite's, one, I bought used 5 or 7 years that is unlabeled.  I have no idea how old it is but comparing the brightness to an Everbrite by swapping them, I don't a see a difference.  The smallest difference that is supposed to recognizable is 0.1 magnitudes = 10%.

 

Jon

 

 

 


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#24 SeattleScott

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Posted 28 October 2021 - 05:37 PM

Yes, ultimately the OP has applicable information that we don’t have. Was it used regularly for 30 years? Or was it used regularly for five years, put in storage for 20 years until kids went off to college, and then used regularly the last five years? Is the OP in a humid environment (like me in Seattle, explaining some issues I have seen with mirrors)? Or is he in the desert? There are certainly factors at play that would impact the useful life of the mirror that the OP knows, and can take into consideration.

Now if the mirror has been regularly used and cleaned for 30 years, and if the OP doesn’t live in the desert, I would guess a new diagonal (or recoat) would make a detectable difference. Probably a fairly subtle difference though.

Current enhanced aluminum coating is typically about 96% reflectivity. Dielectric is 99%. I have not been stargazing long enough to know what coatings TV used 30 years ago. I know standard protected aluminum coating was common for primary mirror back then, and typically 87-88%. But for a diagonal maybe enhanced coating was already in use, at least for a premium brand like TV.

Scott
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#25 LDW47

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Posted 28 October 2021 - 05:46 PM

So.. you're buying a new one every year ?

I have at least 6 2" and 3 1.25" dielectric diagonals, I guess I am good from here to eternity




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