Light weight conical mirrors: re-coating???

[mental4astro]

Hi all,

Light weight conical mirrors are becoming popular. One question I have is when it comes to re-aluminizing these, won't there be problems with the epoxy used in the anchor points? Re-aluminizing happens under vacuum, and as the epoxy will not be able to be removed totally, the expoxy will be continually contaminating the process. Is this point being overlooked, or have I missed something? Epoxy quality also varies tremendously too, and is not the safest material to remove too.

Mental

[MrB]

There are epoxies made for high vacuum use. Not sure if anyone uses these but they're available.

[Wavytone]

Hi Alex, The only reason to recoat a mirror IMHO is because the cost of a replacement would be prohibitive vs recoating, and I suspect the chaps at Bintel would know the answer to this.

Yes, as you correctly point out most epoxies will be a no-no when it comes to recoating, if the owner wanted to have this done. While there are vacuum epoxies there is no evidence these have been used. When I posed a question to Royce a couple of years ago they replied most mirrors are not recoated in their useful life, and they don't consider it warranted, particularly if they have had an SiO2 overcoat applied. Hence they glue the central stud in the back of some of their conical mirrors.

Secondly the cost of the chinese Newtonian scopes is so low that it raises a big question about the cost of replacement mirrors from the factory - if one was available. When you compare this vs the cost of freight, plus the cost or recoating (assuming it is done in Australia by a credible optical shop) it would be cheaper to toss the mirror and install a new one from the factory. In some cases its probably cheaper to replace the telescope than recoat the mirror.

It does make you wonder what the average life of a commercially-built dobsonian telescope is. My guess is the vast majority sold are used perhaps a dozen times, after which the owner loses interest and stashes them in a garage or storage until either it disintegrates and is trashed, the remaining bits & pieces flogged off or disposed of as junk.

[Satchmo]

Sounds like you need to spend some money on R and D then :)

[Wavytone]

Ah...

A little digging revealed the problem with epoxy isn't just the epoxy itself - microscopic air bubbles entrapped in it, plus small voids formed between it and the items glued when it is applied are also issues. These may take a very long time to out-gas, effectively preventing the vacuum chamber from quickly reaching the pressure required for vacuum coating.

[SkyViking]

Interesting question. I got my new 12.5" conical mirror exactly because of the prohibitive cost of re-coating. Not many coating facilities are available in NZ, and those that do exist seem to not offer protective overcoating. Without overcoating I doubt a mirror would last very long especially here in our relatively humid climate!
I would have had to pack it up, ship it overseas, wait several weeks probably, and pay for the shipping both ways as well as for the coating itself. Given that my previous mirror was a run-of-the-mill 10" which I could likely replace with an equally well performing GSO for a few hundred dollars I just didn't think it was worth the trouble.
So I decided to upgrade and got a 12.5" conical Royce mirror. I did consider the issue with the epoxy, but I'd agree that once this mirror needs a re-coat I'll likely have moved on to a different/bigger setup anyway.

[Satchmo]

The concept of 'disposable' conical mirrors is a new one on me - if you are told by the maker that your mirror will not be realuminisable don't buy it . As I said it all about R and D when you are making a good product.

[mental4astro]

Your R and D question is a valid one Mark, only if you are accepting a naked mirror. But if you drop several thousand dollars on a large scope, like the 18" Skywatcher, and you look to recoat, you are in for a world of pain. And if you buy one second hand, and also look to recoat, you will get a very nasty surprise.

This is a significant problem. Conical mirrors are designed to be fixed onto the anchor points with epoxy resins. Great idea, but a terrible option for recoating. "Built in redundancy" gone mad here. I would never consider an 18" mirror as disposable. I wonder if this point was considered by the mirror producers?

On another point, it strikes me as funny that these conical mirrors are made to safe weight, and to a lesser extent faster cooling, but the new scopes using the mirrors also sport bloody big counter weights :lol:

[Wavytone]

Mark, Alex,

There is, I suggest, an obvious answer - if the mirrors are over coated with SiO they shouldn't need recoating for many years, and there's no point trying to recoat them as it can't be chemically stripped and has to be polished off, meaning the mirror would have to be refigured before recoating. Which will cost more than replacing the mirror if its a straightforward Newtonian. Hence the reply I got from Royce.

Alex if those mirrors are SiO over-coated then OK, but users should be warned they can't be recoated and should treat them with care to get a long life (i.e. protect from dust and cleaning).

Mark - I guess it depends on what you think the useful life is. From an engineer's view everything has a finite life and the choice of repair/replacement is a simple decision that comes down to lowest cost, and unfortunately the chinese mass-produced optics are absurdly cheap compared to what it takes for an artisan like yourself to make them. While I'm well aware a good mirror should be "forever", like many of yours, when you consider how many cheap dobs are sold commercially every year/month it implies (like so many other things) a hefty percentage end up in landfill sooner than later courtesy of someone who neither cares about such things, nor knows any better. A small fraction appear on Cloudynights.

Frankly for a consumer scope I'm not sure what is best. If you told a newbie customer (one with little idea and no experience) buying a 12" netownian for the first time that every 2-3 years the mirrors need recoating and the likely cost I suggest they'd balk. Alternatively tell them an over-coated mirror has perhaps 10 years life, then trash it ? Conversely an old-time ATM would be perfectly happy with a plain metallic coating (possibly BerAl or one of the other high-reflectivty blends). One with deep pockets might stretch to multilayer dielectric coatings (effectively lasting forever).

I'm more intrigued by the optics that are matched pairs and exposed to air, such as the Vixen/Tak cassegrain astrographs or the GSO RC's. On one hand it would make some sense of these mirrors were plain aluminium (no SiO coating) and had to be recoated every 2-3 years. But I suspect they're SiO over-coated, too. Would be interesting to know.

Might make an interesting chat with the guys at Bintel one day.

[SkyViking]

Not exactly disposable :lol:, but reality is that my old mirror lasted 17 years before needing a re-coat. I must say that while this 12.5" Royce mirror I have now is absolutely wonderful I cannot imagine that I will not have invested in an even larger one by the time it needs re-coating. :shrug:

[Satchmo]

Alex, the 18 inch conicals may be just held down to the cell with blobs of silicone just like any other Chinese mirror . Conicals are about fast cooling.and convenience of mounting.Ive never seen one any lighter than a standard 1.6 inch plano blank in the same size and conicals usually quite thick in the middle.

Nick, theres no issue removing overcoated coating from a mirror as long as it borosilicate. Coaters have to tread more carefully working with softer glasses, removing the coating requires more care and patience but it can be done. .It does require a breif scrub with a felt pad to break through the overcoating, which doesnt harm the optical surface before the mirror is soaked. (It is easy to see when the overcoat breaks up from the appearance of the aluminium coating. If the coating is allready breaking down due to age this step is not necessary. I have cleaned off coatings many times without harming the optical surface and thats been verified with optical testing.

[Wavytone]

Glad to hear. I found out the hard way some years ago that there's a manufacturer trying to pass off tempered lime soda (i.e. flint) glass as "Pyrex" (my old 12" was one BTW), not borosilicate. The former has a slight bluish tint whereas the latter is water clear to slightly sand-coloured or green.

[mental4astro]

Silicone is not that much better to clean off too.

Thanks for the tech details too. Will be interesting to see how low profile the mirror box can be for these conical mirrors to with their thicker profile.

[Steffen]

There are commercially available solvents for cured epoxy resins, such as ACL-23 Epoxsol among others, but I guess it depends on the particular resin and needs to be tried.

Cheers
Steffen.

[SkyViking]

Hi Mark,

I have to disagree re the weight.
I have a 12.5" conical, and while I don't have a similar sized 1.6 inch standard mirror to compare with I cannot see how the conical can possibly be heavier?

Assuming the mirrors are made from the same type of glass, and that the parabolic indentation is the same, we can just calculate the two volumes.

Here are the dimensions for my conical:
Outer diameter: 12.5"
Edge thickness: 0.5"
Max thickness: 1.75"
Diameter of flat back: 4.2"

I then get:
Volume of conical mirror = 110.55 cubic inches
Volume of standard mirror= 196.35 cubic inches

The weight of my conical is under 5kgs (I think ~4.5kgs from memory) and this light weight, compared to a standard mirror, is what allows me to keep the whole setup riding on a cheap Losmandy G-11 mount.

[mental4astro]

Rolf, for what it's worth, a standard 12" mirror from GSO comes in at 6.1kg, Skywatcher a little lighter as they are a little thinner, though larger in diameter! 10mm larger in diameter. Skywatcher mirrors have a 2.5mm bevel going all the way around, and the effective reflective surface is also 2.5mm wider in radius all the way around.

[Satchmo]

Hi Rolf- I guess the desihn varies according top the size - I made a 14.5" conical with blank from same source as Royce and was surprised to find it similar in weight to the 14 " X 1.6" I had on hand . I will check your calcs tomorrow- that would be good news if you are right .
The 16" Is a beast with the central flat nearly 8" dioameter and 60mm thick and the edge about 20mm ..it was pretty heavy .

[SkyViking]

Hi Mark,

I have put it all in a spreadsheet and also calculated for your 16" conical. Based on the dimensions you gave it would have a volume of 243 cubic inches. A 16" flat at 1.6" thick (can at 16" be that thin?) would have a volume of 340 cubic inches, so is still much heavier.

The philosophy of conical mirrors is that most of the glass towards the edge serves no purpose for support and hence it is removed. A conical is typically only a little thicker in the centre than a corresponding flat, but all the shaved off glass more than makes up for this so conicals are much lighter.
Further to this, because the conical has only a relatively small flat back in the middle (where it is comparatively thick) it can be mounted flat against a plate with a bolt. This saves further weight and complexity for mounting hardware and also makes collimation extremely stable.
In addition, the air is free to move around the conical mirror because it's not mounted in a cell. So it reaches thermal equillibrium faster and also keeps it very vell.

I think the above is what exactly makes a "good product".
IMHO it's the perfect mirror design and I really don't understand why there are not more conical mirror makers out there.

While I'm no expert it did sound like you had handled a fair number of conicals in order to come to this conclusion:

Is that based on empirical testing or is it gut feel? I'm asking because I just can't get the math to stack up in favour of flat mirrors at all.

[Satchmo]

I'm not arguing against flat mirrors. I know for a fact that the 16" Conical design that is available is pretty hefty in the central half ( I've put his up on the test stand ) and Anthony Wesley's actual cooling data show it is a poor candidate for planetary as he can't get it to equalize quickly as other 16" that he has .

My experience on the 12.5" to 14.5" re weight is just that I have made a number of them - I felt the 14.5" was not too much different in weight than a standard 14' but that was just my feel. Next time I will put one on the scales :)

They are definitely more fiddly to make due to their propensity to rock on the polishing turntable - very hard to hold properly without twisting and the design that has been floating around for 30 years - as sold by Royce is pretty thin on the edge and can easily become astigmatic or irregular distortion during polishing and needs close monitoring. Only careful monitoring of the star image under optical null during manufacture gives this information - not something that most manufacturers do. I found 14.5" quite susceptible to distortions even during final figuring so polishing load has to be kept to a bare minimum.

So in essence they are far from ideal to work on from an opticians point of view which is probably one reason why everyone does not offer them .

[MrB]

Just for a bit of fun and to satisfy my own curiosity, I modeled the mirrors discussed above in my 3D package.

Both are 317.5mm diameter, with a 2.5mm edge bevel (312.5mm effective aperture) and f4 (2500mm COC, 4.8876mm sagitta)

The flat mirror was 1.6" thick (rounded to 41mm) thick, the conical as per Rolf's dimensions converted to millimetres and rounded up to the nearest 1/2 mm.
I guessed at a bolt hole of 1/2" or 12.7mm clear through the centre of the conical. This may not be correct, but should have minimal effect on the overall weight.

Volumes according to the 3D modeling:
The 12.5" x 1.6" flat has a volume of 3055.5 cm3
The 12.5" conical has a volume of 2024.3 cm3 (pictured)

For Pyrex, that translates to:
6.8 kg for the 1.6" flat,
4.5 kg for the Conical

Note that the mass in the screenshot for the conical is given as 5.26kg, the only material option given to me in the package is 'Glass' with a mass of 2.6g/cm3. I haven't worked out yet how to customise the material list (never needed to before)
Pyrex is less dense at 2.23g/cm3