Mirror coatings: durability??

[Insomniac]

Anyone got any objective or practical experience to shed light on the relative durability of normal aluminium plus quartz overcoat versus enhanced coatings (aluminium, plus quartz plus overcoat of titanium dioxide or pentoxide).

Since no one has probably used 2 identical mirrors with the 2 coatings under identical circumstances to test this out, I suppose it'll boil down to anecdotal experience, unless there are some good scientific reasons why one should be superior. Isaac from C Qin Pty LTd says the enhanced coatings are subject to greater stress & don't last as long. But I guess those offering the enhanced would not jump to agree.

Some say the few percentage points gain in reflectivity of enhanced coatings can't be easily appreciated by the eye anyway, and are not therefore worth the extra expence. Since I'm shipping a large mirror from N.Z, my main concern is that I have to re coat as infrequently as possible!

[Roger Davis]

Straight aluminium around 2-5 years, depending upon storage conditions.
Aluminium + quartz around 10 years again dependant upon storage.
Aluminium + quartz + overcoat 15 years plus, again storage dependant
I just had a small 5" mirror come in which is aluminised and quartz overcoated, because it has been stored in a bedroom, not dried properly before storage, the dew has eaten in under the edge of the coating and the couple of pin holes that were through the overcoating have allowed oxidization of the aluminium.

[skies2clear]

You'll probably find examples of long life from both standard Al + quartz Vs Enhanced + quartz. It depends how well the coatings are done. It's probably easier to get reasonable standard coatings than enhanced though. My own personal experience hasn't been good with enhanced coatings with one 8" mirror I had done lasting about 6 months before large spots started to show on it. It was redone by someone else and is much better the second time round. But I've never has problems with standard Al coatings, unless not overcoated with anything. No doubt others will have their views too.

Clear skies

[bojan]

I have Coulter Optics 10" mirror that I bought 1982. It was coated wit SiO, I think
Still OK... just minor blemishes on edges, but this does not (visibly) affect the image quality.

[astroron]

anyone know if C Quin or Astro Optical do 16" mirrors, if so how much?
Astro Optical only quote upto 12.6" mirrors and C Quin has no web site.
As anyone had a mirror done by either of these two companies?
PS are there any other companies in Australia who do coating , other than LaserDine?

[Satchmo]

I've had many mirrors done by CQin Co and can recommend them. Astro Optical do not do overcoating . CQin are about $530 for a 16" mirror with Quartz overcoat.

All the 10 year old Quartz overcoated coatings on large mirrors I know of are still going strong.

[astroron]

Thank you Mark:)What is the average time frame for a mirror from go to woa?

[Insomniac]

Optical Coatings Associates in Adelaide advertised in the Nov/Dec S&T. They offer enhanced coatings over Al or silver. But I've not heard hardly any reports about the quality of their work as very few people seem to have heard of them despite living in the same country!

[astroron]

Thanks I will check them out:thumbsup:

[Insomniac]

Thought I'd just posted a reply, but it doesn't seem to have registered on the forum. To repeat: Optical Coating Associates mentioned a rough figure of $800 to re coat my 20", plus a small extra charge for stripping off the old coatings. At this distance I need piece of mind when paying the huge shipping costs to get a re coat. Bottom line? These coatings by this company seem relatively untested & unkown in the amateur community. & offer an improved reflectivity which one might never be able to discern. C Qin seems to have lots of happy clients and there's enough experience based negative comment out there to give me grave reservations about dialectric coatings. If living in Oz, I might not care. But with my shipping costs I don't want any early coating failures!