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Are Questar mirrors and BB coatings from the late 1970's at risk of failure?

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#1 alwilder

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Posted 07 December 2021 - 07:59 PM

Hi all, I need some input from experienced Questar users. I own a 1978 Duplex with a Cervit mirror and BB coating that had a Questar standard service over 3 yrs ago. The BB coating looks flawless and the likewise with the primary mirror showing no signs of chips or spots all the way out to the edges. No mention from Questar or C7 of any impending issues once the service was completed. If the scope is continued to be well cared for and kept in dry evironment, is the coating or ceramic mirrors at risk of failure due to it's age? Someone interested in buying the scope was told that Questars optics from the 60's & 70's are on "borrowed time". Thanks in advance.



#2 pbealo

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Posted 07 December 2021 - 09:26 PM

The cervit itself is bulletproof.

 

The potential issue is coating degradation. But if you really keep it dry (desiccant in the case, etc) you could be good to go for some long time. 

 

Peter



#3 alwilder

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Posted 08 December 2021 - 12:16 AM

If the Cervit mirrors are "bulletproof" (assuming no pre-existing damage) could the BB coating sometimes fail over time if well cared for, stored in a dry place and not physically compromised by improper cleaning? I would like to think these coating are stable enough not to fail by simple aging (as we humans are prone to do). Rather is it more likely some form of physical (micro-scratches) or enviromental (oxidation, excessive moisture) stress be required to precipitate the failure?


Edited by alwilder, 08 December 2021 - 12:18 AM.


#4 Mike Allen

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Posted 08 December 2021 - 10:08 AM

The issue of mirror coating degradation is problematic, in my opinion.  My approach is that mirror coatings, standard or broadband, are living on borrowed time from the moment they leave the coating chamber.  Well cared for mirrors last longer than those that are not.  I would suspect that a perfect looking 50 year old broadband Questar could have a lot of useful life in it before noticeable degradation.

 

I feel there is another issue that a potential buyer should consider.  It has been stated by others in this forum that broadband optics cannot be recoated.  A complete replacement of the optics in a Q 3,5 is very expensive.  I know, because I have done this.  I upgraded my scope from standard to broadband.  It was worth it for me, and I would not hesitate to do it again,  in the unlikely event that I outlive them.



#5 Optics Patent

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Posted 08 December 2021 - 03:17 PM

If the Cervit mirrors are "bulletproof" (assuming no pre-existing damage) could the BB coating sometimes fail over time if well cared for, stored in a dry place and not physically compromised by improper cleaning? I would like to think these coating are stable enough not to fail by simple aging (as we humans are prone to do). Rather is it more likely some form of physical (micro-scratches) or enviromental (oxidation, excessive moisture) stress be required to precipitate the failure?

I assume every major BB coating failure is just from the natural course of things, and not some specific abuse you can avoid.  I assume that the substrate material has nothing to do with it.



#6 alwilder

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Posted 08 December 2021 - 05:15 PM

So as I understand it, BB coating failure on the corrector is a separate issue from mirror failure. The only tie-in is that if one of these components fails before the other, both components must be replaced as a matched set since a corrector with BB coating cannot be stripped and re-coated.



#7 Optics Patent

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Posted 08 December 2021 - 05:24 PM

So as I understand it, BB coating failure on the corrector is a separate issue from mirror failure. The only tie-in is that if one of these components fails before the other, both components must be replaced as a matched set since a corrector with BB coating cannot be stripped and re-coated.

Yes, and I note that "mirror failure" is unknown to me other than coating failures and breakage.



#8 alwilder

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Posted 08 December 2021 - 06:26 PM

Yes, and I note that "mirror failure" is unknown to me other than coating failures and breakage.

 

Other than actual mirror breakage, does the factory protective overcoat on the mirror's silver fade away over time putting the silver at risk of failing from oxidation or moisture?



#9 Spikey131

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Posted 08 December 2021 - 06:31 PM

A Questar is not a diamond, as beautiful as it may be.  It will not last forever.

 

It is a complicated optical-mechanical device.  It is normal to expect that it will need maintenance to keep it working as new.  And all reflecting telescopes will face degradation of the coatings over time,  It is to be expected.

 

When I got my 1963 Standard, I fully expected that I would need either renew the optics or live with dimming views.  And I think that a Questar is worth saving, so when the time comes I plan to renew the optics and fix whatever breaks.


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#10 Gregory Gross

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Posted 08 December 2021 - 06:36 PM

I have to confess that I had to remind myself about exactly what "broadband coatings" constituted insofar as Questars are concerned. From our sponsor's listing for a new Standard Questar:

 

This broadband-coated Questar includes ultra-high transmission/low reflectivity broadband dielectric multicoatings on both sides of its objective lens for a light loss of less than 1/10th of 1% per surface for the brightest possible images. This compares with a light loss of 1% per surface with standard magnesium fluoride antireflection coatings. This multicoatings package also includes high reflectivity silver mirror coatings with a protective overcoating of thorium fluoride instead of standard aluminum coatings with a silicon monoxide overcoat. The broadband coatings package gives you a full 22% overall gain in light transmission and contrast that's very useful for photography and when viewing faint deep space objects.

 

This broadband coatings package is not recommended if you live full time on ocean-front property, or spend much of the year at the seaside. Constant exposure to salt air can adversely affect the silver mirror coatings. Occasional visits to the shore are not a problem, only extended stays (particularly if the scope is not packed away in its case when not in use). If prolonged exposure to salt air might be in your scope's future, consider the Questar with standard optical coatings (Questar model #Q3), rather than this broadband-coated version. You will lose some light transmission, but will gain a measure of optical coating longevity. Adding a few packets of desiccant (silica gel or similar, available at most camera stores) to the case of any spotting scope to absorb moisture when near large bodies of salt water would be a helpful preventative measure in any event.

 

So, to summarize, the broadband package includes special coatings on both sides of the corrector lens (Astronomics incorrectly refers to it as the objective lens) in addition to silver mirroring with a thorium fluoride overcoat on the primary mirror. Contrast this with the standard package of magnesium fluoride coatings on the corrector lens and the aluminumized primary mirror with silicon monoxide overcoat.



#11 Gregory Gross

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Posted 08 December 2021 - 06:43 PM

And from an historical perspective -- this is how Questar characterized broadband and VLR (very low reflection) coatings in its price list dated September 1, 1967 (Questar first introduced them in the spring of 1967).

Attached Thumbnails

  • Broadband and VLR Coatings - September 1 1967.jpg


#12 Mike Allen

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Posted 08 December 2021 - 07:22 PM

It would be interesting to know if the durability of broadband coatings have improved since they were introduced in 1967, to when the topic starter’s 1978 model was manufactured, to the present time.  I would hope that there have been improvements, perhaps in the chemicals and their application.



#13 Gregory Gross

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Posted 08 December 2021 - 08:40 PM

Well, insofar as Questar's warranty is any indication -- and it may be an indication either of how advanced broadband coatings got or of how confident Questar got about putting itself on the hook for covering failed broadband coatings -- one sees a gradual increase in that warranty.

 

In its September 1967 price catalog, the company noted that “broadband and VLR coatings are too new for us to know their life span, but we are guaranteeing them for one year.” In 1968, the warranty increased to two years. In 1971, it extended its guarantee to three years. And in the early 1980s, Questar adopted a five-year guarantee for broadband coatings.



#14 Mike Allen

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Posted 08 December 2021 - 09:40 PM

There is an excellent discussion in this forum about the useful lifespan of broadband coatings back on February 22, 2019. Notable posts were from DAVIDG, Optics Patent, and Erik Bakker.  A comment by Erik should be of particular interest to the topic starter.



#15 Moon-Watcher

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Posted 09 December 2021 - 11:24 AM

I spoke to Jim Reichart at Questar a couple of weeks ago about this very topic. He assured me that the BB coating is as stable as any coating you find on refractors. The debate on whether BB is less stable than a standard coating apparently came about due to the difference in warranty times: I think 5 years for the former and 10 for the latter if memory serves. However these are warranties provided by the two different, unrelated coating companies and are not due to some exhaustive side by side test of the two coatings by Questar. In summary, the word from Questar is that there is no reason to think that there is a difference in stability between the two. CAVEAT: unless you live by the coast in which case BB is not a good idea.
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#16 alwilder

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Posted 09 December 2021 - 09:25 PM

I spoke to Jim Reichart at Questar a couple of weeks ago about this very topic. He assured me that the BB coating is as stable as any coating you find on refractors. The debate on whether BB is less stable than a standard coating apparently came about due to the difference in warranty times: I think 5 years for the former and 10 for the latter if memory serves. However these are warranties provided by the two different, unrelated coating companies and are not due to some exhaustive side by side test of the two coatings by Questar. In summary, the word from Questar is that there is no reason to think that there is a difference in stability between the two. CAVEAT: unless you live by the coast in which case BB is not a good idea.

Good to know about BB coating. I don't know how BB coating compares to multi-layer camera lens coatings but I've never heard of or seen those coatings fail going back to their introduction in the early 1970's. OTOH, Leica camera lens coatings from the 50's-mid 60's were notoriously soft and more prone to failure especially on the inner elements.



#17 alwilder

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Posted 15 December 2021 - 04:52 PM

I too just spoke to Jim Reichart at Questar who confirmed what Moon-Watcher stated. Jim also made the following points about BB vs. standard MgFl models:

 

1. BB models have treatment applied to both the corrector and mirrors allowing the highest light transmission possible. About 17% higher than MgFl models.

 

2. Not only do BB models have higher transmission higher compared to MgFl, even a BB model showing some optics failure has a higher transmission than a healthy standard MgFl model. He also said if BB treatment begins to fails, the progession is slower compared to MgFl models where if they fail, progession moves a bit faster. However, MgFl is their most durable coating especially in adverse environments.

 

3. A well cared for Questar with BB coating "should" last a lifetime assuming it's not stored at the beach or a humid enviroment. A few packs of silica desiccant packs in the case was also highly advised to maintain optics longevity.

 

4. The optical surfaces of built-in prism and Barlow on BB models have MgFl coating, not BB coating. That's because if BB coating was applied to their optical flat surfaces, it doesn't do as well and wouldn't meet their 1/8 wave accuracy criterion.

 

Interesting information about their coatings.


Edited by alwilder, 15 December 2021 - 05:00 PM.


#18 Mike Allen

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Posted 15 December 2021 - 06:32 PM

Thanks to Moon-Watcher and alwilder, for this helpful information from Questar Corp.  Perhaps this should be pinned for future reference.



#19 Joe Bergeron

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Posted 15 December 2021 - 11:26 PM

The coatings on my 1976 broadband still look perfect. 




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