telescope build

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bobsorenson
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telescope build

#1

Post by bobsorenson »


Greetings. I would like to use a mirror I purchased years ago to build a Dobsonian telescope.
The mirror is quite unusual...it is surplus from the DOD "starwars" program and I bought it as aluminum scrap about 20 years ago. It is/was highly polished cast aluminum about 20" across and 2 to 3" thick and a focal length about 6 or 7 ft. It weighs about 40 lbs. My understanding is that this mirror was part of a laser assembly to blast satellites!!
I know this sounds kinda crazy, but what I need is some info on whether this would be a practical use for this mirror, and some guidance on grinding and polishing the solid aluminum alloy.

thank you kindly for any advice, Bob Sorenson
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Re: telescope build

#2

Post by Lady Fraktor »


It sounds like an interesting project, how shiny is the parabolic surface?
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
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Re: telescope build

#3

Post by bobsorenson »


When I first got it, it would ignite wood quickly but has oxidized somewhat since then. Has not been well taken care of so probably has some scratches etc. A friend suggested that it had a dielectric coating...I don't know anything about that.

thanks
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Re: telescope build

#4

Post by Lady Fraktor »


I imagine it can be re-coated if necessary as long as the aluminium has not pitted.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
Az/Alt: AYO Digi II/ Argo Navis, Stellarvue M2C/ Argo Navis
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
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Re: telescope build

#5

Post by DeanD »


I would strongly suggest doing a Foucault test of the mirror before you spend too much time and effort. (eg: See http://www.jlc.net/~force5/Astro/ATM/Fo ... ester.html for a few ideas). There might be some mirror makers in your local astronomical society who could help you with this (I have a home-made set-up that cost next to nothing and gives a great idea of the mirror quality: but it is a bit of a trip for you to South Australia!). A ronchi grating or even a razor blade will do the job nicely, and it is cheap and easy to construct.

My main concerns are: firstly that while the mirror may focus a beam to burn wood satisfactorily it probably wouldn't need to be anywhere near astronomical quality to be able to do this. I would very much doubt that it would be, say 1/4 wave or better which is necessary if you want it to function as a good telescope.

Secondly, while a piece of aluminium can be highly polished and figured (maybe better than the early speculum mirrors?), its coefficient of thermal expansion is around 2.5x that of ordinary glass, 6x pyrex glass, and 40x more than fused quartz (see: http://hyperphysics.phy-astr.gsu.edu/hb ... thexp.html ), and even around 24% higher than speculum. I have no idea as to whether it would maintain a good figure over a range of temperatures when it has such high expansion, particularly if there were any irregularities in the casting or its thickness. It could well expand more in one direction than another due to its inner structure if it is not completely homogenous.

Thirdly I suspect it could be not as smooth as (say) a glass mirror at a microscopic level due to imperfections and crystalline structure in the casting itself. You wouldn't necessarily see this by eye, but it could affect the ability to focus at higher power.

Good luck!

- Dean
Telescopes: 12" f5 dob, Celestron CPC800, 150mmf5 Celestron achro, Tak TSA102, TV76, ETX125...
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Re: telescope build

#6

Post by OleCuss »


I'm really not sure I'd put too much into that mirror although it sounds very appealing.

As best I can tell you get a lot of thermal expansion issues unless the rest of the OTA is made of the same alloy. Re-polishing is apparently difficult with heat control being a very sensitive issue. And then they seem to be putting a nickel coating on it in order to get a good surface - and I'm really not sure how the reflectance works at visual wavelengths for nickel.

One other thing? There is mention that some of the mirrors used for "Star Wars" applications aren't/weren't aluminum but beryllium. The weight difference was apparently the reason for going to the more expensive and much more toxic metal. While I'm guessing your mirror truly is aluminum it is still a thought.

And yeah, I can start fires with a singlet lens which would be pretty bad in an OTA. I won't generally do that as it is usually impractical for campfires and with a big lens it can be very unpleasant even with sunglasses. Probably wouldn't be so bothersome with welding glasses but then it would be difficult to see to direct that light/heat to the tinder for the campfire.


But still, a 20" aluminum mirror would be so cool that it might be worth trying to figure out what alloy was used and then build an OTA/truss system with that alloy and have fun building it even if it might not turn out as hoped?

And there has to be a re-coater around who will do nickel?
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Re: telescope build

#7

Post by bobsorenson »


Thanks
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Re: telescope build

#8

Post by bobsorenson »


DeanD, thanks for the info. I will look into the tests you suggest.

The more I read, the more I suspect the mirrors are beryllium, which is much more stable than Al. There are two of them and they are 24" across. I had planned to regrind and polish anyway, so the present condition isn't a huge factor.
The casting and machining appear to be of the highest quality and if this forum allows, I will post a pic here.
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Re: telescope build

#9

Post by Lady Fraktor »


Very interesting, good luck with the project.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
Az/Alt: AYO Digi II/ Argo Navis, Stellarvue M2C/ Argo Navis
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
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Re: telescope build

#10

Post by OleCuss »


bobsorenson wrote: Mon Nov 04, 2019 9:19 pm DeanD, thanks for the info. I will look into the tests you suggest.

The more I read, the more I suspect the mirrors are beryllium, which is much more stable than Al. There are two of them and they are 24" across. I had planned to regrind and polish anyway, so the present condition isn't a huge factor.
The casting and machining appear to be of the highest quality and if this forum allows, I will post a pic here.
An amateur scope with a beryllium mirror would be simply awesome! Everyone would want to see it and/or see through it - including me!

But still, I'd just be sure to research how to safely set the thing up. It'd be lousy to get lung issues. I don't think you'd have health problems since I believe the Beryllium issues are more of a problem in people who have a lot of exposure during the various aspects of their employment working that metal.

To me it sounds like you have the makings of a very special scope. Lightweight with lots of light-gathering and a high-quality manufacture. I'd still check on the reflectance at visual wavelengths but even if it turns out not to be ideal I'd seriously consider building it out anyway because with that much aperture you'd likely get great views even if not ideal. But I'm betting the reflectance will be good!
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Re: telescope build

#11

Post by AntennaGuy »


You said it was DoD surplus.
1. Do you know what range of wavelengths it was intended for? A mirror designed for long-wave IR (e.g., a CO2 laser) would not require as much geometric surface perfection (in absolute terms) as one intended for visible or (especially) UV.
2. Not to sound negative, but any chance it was surplus specifically due to being rejected as unsatisfactory for a good reason?
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Re: telescope build

#12

Post by DeanD »


bobsorenson wrote: Mon Nov 04, 2019 9:19 pm DeanD, thanks for the info. I will look into the tests you suggest.

The more I read, the more I suspect the mirrors are beryllium, which is much more stable than Al. There are two of them and they are 24" across. I had planned to regrind and polish anyway, so the present condition isn't a huge factor.
The casting and machining appear to be of the highest quality and if this forum allows, I will post a pic here.
Two mirrors! Binoculars!!! :banana-dance:

Have you done mirror grinding before? This could be a fascinating project. I would assume because of the light weight that the mirror cell wouldn't have to be as sophisticated as for a thin glass mirror.

Make sure you let us know how it is going!

All the best,

Dean
Telescopes: 12" f5 dob, Celestron CPC800, 150mmf5 Celestron achro, Tak TSA102, TV76, ETX125...
Binos: Steiner Wildlife XP 10x26, Swarovski 8x30 Habicht, Zeiss SFL 8x40, Vanguard Endeavour 10.5x45, Fuji FMTR-SX 10x50, Tak 22x60, Orion Resolux 15x70
Eyepieces: way too many (is that possible?), but I do like my TV 32mm plossl, 13mm Nagler T6, 27mm Panoptic and 3-6mm Nagler zoom, plus Fujiyama 18mm and 25mm orthos and Tak 7.5mm LE
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Re: telescope build

#13

Post by bobsorenson »


antennaguy, they were part of a large lot of decommisioned equip from the Toole Army Depot in west Utah. There were 10" scopes for $200 and my friend got a "shed" for his yard made of titanium. There were about 20 of these mirrors selling for $30 each but I couldn't see buying them all! Although, today on a whim I looked up the price of Beryllium and it's about $340/lb so if they are beryllium, then they are worth $13,000 each. Something isn't right there.
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Re: telescope build

#14

Post by Lady Fraktor »


Beryllium should weight about 9.63 lbs per square foot at 1" thick, Aluminium weights 14.04 lbs for the same size piece.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
Az/Alt: AYO Digi II/ Argo Navis, Stellarvue M2C/ Argo Navis
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
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Re: telescope build

#15

Post by DeanD »


I'd get out the scales if I were you: sounds like you might be able to sell them as scrap and get enough to buy a nice big dob... (and then berate yourself for not buying all 30!!!) :)
Telescopes: 12" f5 dob, Celestron CPC800, 150mmf5 Celestron achro, Tak TSA102, TV76, ETX125...
Binos: Steiner Wildlife XP 10x26, Swarovski 8x30 Habicht, Zeiss SFL 8x40, Vanguard Endeavour 10.5x45, Fuji FMTR-SX 10x50, Tak 22x60, Orion Resolux 15x70
Eyepieces: way too many (is that possible?), but I do like my TV 32mm plossl, 13mm Nagler T6, 27mm Panoptic and 3-6mm Nagler zoom, plus Fujiyama 18mm and 25mm orthos and Tak 7.5mm LE
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Re: telescope build

#16

Post by bobsorenson »


So, the first thing to do is weigh the mirrors to try to determine the composition. I will start tomorrow on that unless someone knows of a test I could perform to distinguish between AL and BE.
I don't know what the original purpose was...probably laser. Turns out that polished beryllium is only 50% reflective for visible light but 98% for IR. Now I'm going to read all about IR telescopes!
Thanks all for the great info
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Re: telescope build

#17

Post by Lady Fraktor »


IR telescopes can be quite expensive as they usually have gold coated optics.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
Az/Alt: AYO Digi II/ Argo Navis, Stellarvue M2C/ Argo Navis
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
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Re: telescope build

#18

Post by OleCuss »


bobsorenson wrote: Tue Nov 05, 2019 11:58 pm So, the first thing to do is weigh the mirrors to try to determine the composition. I will start tomorrow on that unless someone knows of a test I could perform to distinguish between AL and BE.
I don't know what the original purpose was...probably laser. Turns out that polished beryllium is only 50% reflective for visible light but 98% for IR. Now I'm going to read all about IR telescopes!
Thanks all for the great info
It may be worth remembering, however, that even if you have only 50% reflectance in the visible spectrum? You've got 20" of aperture so you'd still be getting as much visible light as a 14" mirror would be collecting so you should still have some really good views. Yeah, a bigger structure which is harder to handle but still a very pleasing scope!
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Re: telescope build

#19

Post by OzEclipse »


OleCuss wrote: Wed Nov 06, 2019 10:33 am
bobsorenson wrote: Tue Nov 05, 2019 11:58 pm So, the first thing to do is weigh the mirrors to try to determine the composition. I will start tomorrow on that unless someone knows of a test I could perform to distinguish between AL and BE.
I don't know what the original purpose was...probably laser. Turns out that polished beryllium is only 50% reflective for visible light but 98% for IR. Now I'm going to read all about IR telescopes!
Thanks all for the great info
It may be worth remembering, however, that even if you have only 50% reflectance in the visible spectrum? You've got 20" of aperture so you'd still be getting as much visible light as a 14" mirror would be collecting so you should still have some really good views. Yeah, a bigger structure which is harder to handle but still a very pleasing scope!
Light gathering of a 14" but still with the resolution of a 20"!


You can easily differentiate between beryllium and aluminium by density. Measure the diameter of the mirror and its edge thickness expressed in centimetres and calculate the volume of material in cubic centimetres.
Volume = pi x radius squared x height

Providing it isn't a soup bowl(big deep curve), ignore the depth of the curved surface just calculate volume as a cylinder. Weigh the mirror - convert the weight to grams.

the density is the weight in grams divided by the volume in cubic centimetres.

berillium is ~1.85grams per cubic centimetre
Aluminium is ~2.7grams per cubic centimetre

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Re: telescope build

#20

Post by bobsorenson »


Thank you ozeclipse, the mirrors have a one inch edge but then the back slopes to a smaller radius on the back. The whole thing is about 3 inches thick, So the shape is more complex.
For this reason I've been looking for a 24 inch wide basin I could do a displacement measurement in and then I can use your numbers.
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