Protected Aluminium vs Enhanced Aluminium coatings - durability differences?
Hi all,
I've been looking at some new mirrors, but I have come to a brick wall when it comes to understanding the difference in durability between quartz protected aluminium vs enhanced aluminium coatings.
What I also have not been able to find out is if enhanced coatings are overcoated or not. If not, why and does this affect the longevity of the coating?
Which mirror finish will last longer?
Of course here I need to assume two mirrors, both kept under the exact same conditions of use, clean and storage.
I've been looking at some new mirrors, but I have come to a brick wall when it comes to understanding the difference in durability between quartz protected aluminium vs enhanced aluminium coatings.
What I also have not been able to find out is if enhanced coatings are overcoated or not. If not, why and does this affect the longevity of the coating?
Which mirror finish will last longer?
Of course here I need to assume two mirrors, both kept under the exact same conditions of use, clean and storage.
Thanks for looking,
Alex.
Great question! Looking forward to seeing responses.
Protected Aluminium has a half wave coating of Silicon Oxide. Approx. 85% reflectivity.
Enhanced Aluminium has multi layer coating of dielectrics Approx. 95% reflectivity in a selected band. Both are durable.
The method of depositing the Aluminium also has a bearing on the durability. Some form of ion assist being superior to evaporative. Evaporative is less dense, and can absorb water.
That's the thing - how durable/long lived? 5 years? Ten years? 15? And then how do the two compare side by side over the same period of time? If the enhanced deteriorates sooner than the standard protected coatings, where is the gain?
With no one now in Australia offering a recoating service, I am not finding the necessary information to make an informed choice.
Apparent reflectivity differences means little if over time the performance is not there.
Alex all the alloys used for “enhanced” coatings are vulnerable to salt corrosion so I’d say not a good idea in your location. The only thing that stops this is an SiO2 overcoat. And as you know with SCTs and maks the overcoated mirrors in a closed OTA can last many decades.
I think in general the so-called “enhanced coating” are pointless unless they are overcoated as well - as done in the SCTs in recent years (Starbright, XLT etc).
The reason is that if you want the ultimate reflectivity it’s now feasible to DIY coatings using the Angel Gilding kits to coat with silver and there’s a way to overcoat it to make it last ~2 years.
Given the situation here for anyone seeking to recoat a large mirror > 12” I’d suggest this is probably the only viable option short of sending it to the US to be done there.
OMI used to have some data on there website though it seems to be gone,have you emailed for some info ?
My 12 " was coated by them 6 + years back IBAD96 and was overcoated as well from memory
I used to live 1 km from the beach and gso coatings + inversion layers
( sound of the beach right at your back door) didnt work out well 5 years they really needed doing again .
The Omi coatings are holding up well I'm further from the coast now and a little fussy when i go out weather wise but theres little or no degradation i can see so far .
I thought the Saintech mirror coating business changed its name to Palmway optical and was still operating for mirror recoating based in Tuncurry NSW, this is what the about us page says on their web site, i must be behind the times as i was not aware they closed the business or stopped offering mirror recoating, very sad indeed if they have stopped trading. If they are still trading they would be good to contact for more info.
I know Saintech do, or does, a Silicon Dioxide overcoat. I had a discussion with Wayne about coatings a couple of years ago when I got my Galaxy 18 inch mirror recoated. I went to Bunnings with Wayne to buy some pool acid to strip my old coatings and spent the day watching the whole process. He spent 8 hours stripping the old coatings and cleaning the mirror and 10 minutes putting the new coatings on! I suspect Wayne Sainty knows more about coating mirrors than anybody else in Australia. Not sure if he is still in business though.
I just had my 18” Suchting primary recoated by Wayne a few weeks ago.
Aluminium coating with SiO2 and TiO2 IAD layers over the top for >96% reflectivity, Wayne also tested my Enhanced coating 4.5” secondary and it too was better than 96%.
Wayne’s laboratory premises are no longer available, I do hope that he decides to continue in 2020.
As of a couple of days ago Wayne may well be re opening for business in February. He could change his mind between now and then of course.
There are a couple of coating options still available for smaller mirrors under 20 inches. Alan Fry in Sydney can do mirrors to 20" and David Baker can do the same in QLD. They both also work closely together, as David owns Alan's coating Chamber in Sydney.
Enhanced Die Electric Coatings are very difficult to strip and to ensure a good substrate to recoat. really need to be polished off.
Graham secondaries aren’t worth recoating - buying another will be cheaper.
Dielectric coatings are rare on primary mirrors - the thickness of the stack of layers in the coating is several wavelengths thick, which is problematic vs surface accuracy better than 1/8 wavelength. It’s easy enough to achieve for small secondary mirrors and diagonals, but much harder for a primary mirror. OTOH a dielectric coating is very durable and should last a lifetime - even with washing - unless scratched. They’re also impossible to strip - the coating has to be removed by fine grinding and the mirror repolished and figured.
Yeah i read that with parabolics to get it right costs a big box of $ does anyone coat this way .?
My secondary flat at a few differant angles has a werid purple swirl thing going on coating wise , the degradation looks enough and deep enough to bin it i think or definatelly cost out what to do , especially that bit away from the edge,, but i did live in hope .
Yeah i read that with parabolics to get it right costs a big box of $ does anyone coat this way .?
My secondary flat at a few differant angles has a werid purple swirl thing going on coating wise , the degradation looks enough and deep enough to bin it i think or definatelly cost out what to do , especially that bit away from the edge,, but i did live in hope .
Wayne's price for the enhanced coating (>96%) for my 4.5" secondary was $140. A new secondary of the same quality is about $1K......
I have a 10 year old protostar secondary that needs recoating anyway you can tell how its coated John ?
Do you have a any contact info for the qld company
Hi Graham,
Is your secondary a Protostar "Quartz" or "Pyrex" and what size is it? I assume you are talking about the one in your 12"/F5 scope, which should be either 2.14" or 2.6". 2.6" quartz was the largest quartz secondary Protostar made, which is what I have in my 14" SDM. Matched with a 14"/F4.5 Zambuto primary, the scope has stunning optics.
I have 2 x Protostar Quartz Secondaries. They are extremely high quality and you don't "bin them". You might be able to buy a replacement GSO pyrex secondary for the cost of recoating, but you won't buy a high quality one for the same price. The Protostar Quartz secondaries have 10-5 scratch/dig, and 2-4 nm rms micropolish roughness, which is in a different league to the cheaper secondaries. The foregoing assumes the quality of your primary mirror is also high quality. If the primary is a mass produced one, or not top shelf, you may be better off just buying a cheap GSO replacement secondary.
All of the Protostar Secondaries use enhanced coatings (Both Quartz and Pyrex have 96% reflectivity). Note that some coaters will try to strip enhanced coatings. The problem is that it is almost impossible to get "everything" off to provide a substrate of the same quality and smoothness as the original optic. I discussed this with Alan Fry and he won't recoat optics with enhanced coatings unless the old coating has been polished off, as they don't have the durability or quality of the original coatings, unless you do.
Alan's Coating prices are pretty reasonable. Not sure what he would add to cover the cost of polishing off the old coatings. He will polish off optical flats, but not parabolic mirrors.
I would contact Alan and discuss it with him and get a price. He is a really nice helpful guy and I think he would be the one to polish off the old coatings anyway.
If all else fails, you can send it to me and if the coatings are strip-able I can recoat it with plain or protected aluminium at a much more affordable price then what you will find elsewhere. I work with low overhead costs but I don't have any industry certification for optical coatings. I use a DIY chamber.
Quote:
Originally Posted by GrahamL
I have a 10 year old protostar secondary that needs recoating anyway you can tell how its coated John ?
Do you have a any contact info for the qld company
