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Silver enhanced mirrors

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#1 pao

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Posted 21 December 2019 - 10:55 AM

Hi everybody.
sorry for my ignorance but I would like to ask you this question: is it true that silver enhanced mirrors degrade after about 10 years (or so)? If so, why does Questar offer this expensive option for its telescopes? Why did they produce even the 3.5” 50th anniversary top edition with this type of mirror if it can be ruined after a few years? Since a Questar telescope is for life, it is better to choose other solutions for the primary mirror (pyrex or zerodur, not silver coated)? Thank you.

pao


Edited by pao, 21 December 2019 - 10:57 AM.

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#2 J A VOLK

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Posted 21 December 2019 - 11:12 AM

Pyrex and Zerodur is the substrate. The reflective coating is deposited on the substrate. Aluminum is most common, silver can last if well done with a proper overcoating. In the 1980's both Meade and Celestron tried silver coating and it was a disaster. Many coatings failed within months, some lasted years, mostly depending on the environment. They lost a lot of money recoating customer scopes. Techniques have improved, I would presume that Questar is confident that their silver coating methodology gives long lasting results.
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#3 pao

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Posted 21 December 2019 - 11:24 AM

Thank you for your answer.
In reference to the 3.5” scope, as far as I know only the 50th edition is offerered with silver coatings (and quartz substrate mirror).

pao



#4 TOMDEY

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Posted 21 December 2019 - 11:36 AM

That is a tradeoff that will go on forever. When I worked at a coating house, I of course always put enhanced silver on all my stuff, because it was minor and free to recoat. Satellite mirrors are most often ultra-enhanced silver and can really take a beating and still hang in there... so it can be done right, just not that commonly... especially among the smaller shops. If your mirror is going to see dew, humidity, salt, temp-cycling, dust, pollen, bugs... might be best to go with EnAl.    Tom



#5 Optics Patent

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Posted 21 December 2019 - 12:46 PM

There is a thread here (search broadband) where this has been helpfully discussed. My take is that in the long run the cheapest standard coatings are best.
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#6 pao

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Posted 21 December 2019 - 07:20 PM

Satellite mirrors are most often ultra-enhanced silver and can really take a beating and still hang in there...

Yes, but in space there are not dew, humidity, salt, dust, etc...lol.gif

Thanks anyway. Maybe, as in real life, simple is better...

pao


Edited by pao, 21 December 2019 - 07:24 PM.


#7 pao

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Posted 21 December 2019 - 07:24 PM

There is a thread here (search broadband) where this has been helpfully discussed. My take is that in the long run the cheapest standard coatings are best.

Thanks for the advice, even if in that topic the discussion was focused on broadband coatings, not silver coatings...
Anyway, congratulations for your extraordinary collection of Questar telescopes. I am envious! bow.gif 
pao


Edited by pao, 21 December 2019 - 07:28 PM.


#8 TOMDEY

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Posted 21 December 2019 - 07:59 PM

Yes, but in space there are not dew, humidity, salt, dust, etc...lol.gif

Thanks anyway. Maybe, as in real life, simple is better...

pao

Yep! But the space environment turns out to be harsh in other ways... especially ~hard radiation~ and a kind of dust that is rare, but the few particles that contact surfaces are explosively abrading... literally! Also ionizing chems and the upper atmosphere gets up there just a bit, for a lot of our lower orbit sats. We sent up lots of optical coating specimens on  >>>

~

"The Long Duration Exposure Facility (LDEF), which was retrieved in 1990 after spending 69 months in LEO, revealed that space environments are very hostile to many spacecraft materials and components. Atomic oxygen, which is the most prevalent atomic species encountered in LEO, is highly reactive with plastics and some metals causing severe erosion. There is also extreme ultraviolet radiation due to the lack of an atmospheric filter. This radiation deteriorates and darkens many plastics and coatings. The vacuum in space also alters the physical properties of many materials. Impacts of meteoroids and orbiting man-made debris can damage all materials exposed in space. The combined effects of all of these environments on spacecraft can only be investigated in space. On Earth, a material can only be subjected to one environment at a time...."

 

The ISS crew are also running materials exposure experiments.    Tom

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#9 pao

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Posted 21 December 2019 - 08:03 PM

Yep! But the space environment turns out to be harsh in other ways... especially ~hard radiation~ and a kind of dust that is rare, but the few particles that contact surfaces are explosively abrading... literally! Also ionizing chems and the upper atmosphere gets up there just a bit, for a lot of our lower orbit sats. We sent up lots of optical coating specimens on  >>>

~

"The Long Duration Exposure Facility (LDEF), which was retrieved in 1990 after spending 69 months in LEO, revealed that space environments are very hostile to many spacecraft materials and components. Atomic oxygen, which is the most prevalent atomic species encountered in LEO, is highly reactive with plastics and some metals causing severe erosion. There is also extreme ultraviolet radiation due to the lack of an atmospheric filter. This radiation deteriorates and darkens many plastics and coatings. The vacuum in space also alters the physical properties of many materials. Impacts of meteoroids and orbiting man-made debris can damage all materials exposed in space. The combined effects of all of these environments on spacecraft can only be investigated in space. On Earth, a material can only be subjected to one environment at a time...."

 

The ISS crew are also running materials exposure experiments.    Tom

Interesting... 

Thanks for sharing.

pao



#10 Terra Nova

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Posted 23 December 2019 - 03:56 PM

Something that has not been mentioned (forgive me if I missed it), is that silver has a higher reflectance than aluminum.

Edited by Terra Nova, 23 December 2019 - 04:12 PM.

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#11 NC Startrekker

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Posted 24 December 2019 - 10:30 AM

Terra is correct. This is a timely discussion. Jerry Oltion's article in the January issue of S&T discusses silvering telescope mirrors. He reports 98% reflectivity for silvered mirrors compared to 89% for aluminum. At 3.5 inches of aperture, this provides a distinct boost in throughput for the little Q, even more so than the difference between standard and BB coatings. The downside is silver's propensity to tarnish. Questar's silver mirrors are coated which theoretically protects it from the oxidizing elements of the atmosphere. I can't speak authoritatively to the degree of effectiveness or potential degradation over time. My 2013 50th anniversary edition shows no signs of dimming. But, I suspect any change would be so minute as to be imperceptible to the eye.  Alan


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#12 pao

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Posted 25 December 2019 - 07:48 PM

Well, hope the silver coatings are good enough and highly protective because I'm going to acquire a 3.5" silver enhanced primary  mirror soon.... praying.gif


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#13 Erik Bakker

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Posted 26 December 2019 - 07:48 AM

Well, hope the silver coatings are good enough and highly protective because I'm going to acquire a 3.5" silver enhanced primary  mirror soon.... praying.gif

You will love it! And those coatings do provide a noticeably better image. Generally, they last very long, though some samples age quicker under harsh acidic/salty conditions, easily noted as the edge of the primary mirror darkens to spots with a brownish color. Only then do you need to consider what to do. But even at that moment, they can still have many years good performance ahead of them.


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#14 Wigg

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Posted 29 December 2019 - 01:20 AM

In 2013 I was in the process of ordering a 50th Anniversary 3.5 scope direct from Questar, when they discovered that I lived by the sea they recommended that I buy the standard aluminium coating.  This left lots of cash for extras! I must say I greatly respect Questar for this advice.  It goes to show that they are genuinely interested in the individual client's satisfaction with their product.  


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