telescope build

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OleCuss
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Re: telescope build

#21

Post by OleCuss »

You might consider visiting a farm supply store. You might find something intended to feed or water animals which is big enough?

Another thought would be to check in a store which sells water heaters and the like (some place like Lowe's). They sometimes have a rather large diameter "pan". They typically have a hole cut in the side but you may simply be able to tape over the hole and use it for that purpose - but they may not be as deep as you might want.

Worst case you make a wooden frame and line it with plastic sheeting.
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AntennaGuy
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Re: telescope build

#22

Post by AntennaGuy »

OleCuss wrote:
Thu Nov 07, 2019 1:35 am
You might consider visiting a farm supply store. You might find something intended to feed or water animals which is big enough?
Perhaps one of these might just be on the edge of roughly large enough?
https://www.agrisupply.com/sheep-tank-2 ... /p/110052/
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Re: telescope build

#23

Post by yobbo89 »

Just put it in a tub/tank of water to measure it's volume with displacement if the shape is to complex?
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bobsorenson
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Re: telescope build

#24

Post by bobsorenson »

Okay, so.
Each mirror (24") displaced 15,150 ml water and they each weigh in at 90 lbs. So I calculate the density at 2.69 gr/cc which means they are most certainly Al.

Now I need to do the foucault test to see about the curve and also start researching how to grind and/or polish Al.
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Re: telescope build

#25

Post by bobsorenson »

Is 90 lbs a reasonable weight for a mirror? I hoped to be able to go up the mountain here (9500 ') and set up.
I'm going to start reading about dobsonian designs.

Thanks so much all you searchers for sharing your knowledge and experience. What a great thing the internet can be!
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DeanD
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Re: telescope build

#26

Post by DeanD »

bobsorenson wrote:
Thu Nov 07, 2019 9:14 pm
Is 90 lbs a reasonable weight for a mirror? I hoped to be able to go up the mountain here (9500 ') and set up.
I'm going to start reading about dobsonian designs.

Thanks so much all you searchers for sharing your knowledge and experience. What a great thing the internet can be!
I hope you don't plan to hike up the mountain with it! ;)

That is quite heavy: in my mind I was thinking that aluminium is light-weight, but 1cc of glass weighs around 1.6g: a lot less! An equivalent glass blank would weigh 55 lbs...

This will impact on your mirror-cell design in order to avoid flexure. There are a lot of ideas out there about mirror cells and supports to try to minimise flexure, but they all relate to glass mirrors. I would assume that a multi-point support with 9 points spread over 3 triangles should be OK, but I'll leave that to the experts.

At least you wont have to worry about adding counter-weights to offset a heavy secondary cage!

Have fun, and all the best.

Dean
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Marcelo F.
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Re: telescope build

#27

Post by Marcelo F. »

The biggest problem in polishing I think will be the temperature (it can warp the curvature). I know that for example for stainless steel there is already chemical reaction polishing (soaking in a chemical bath) instead of mechanical action (rubbing with diamond paste).
It is a question of whether something similar already exists for aluminum.
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Re: telescope build

#28

Post by bobsorenson »

Marcelo, thank you.
I have been looking for info from people who have experience grinding/polishing Al mirrors without sucess so far, but I will continue.
Lots and lots of knowledge available about glass etc but no hands on experience with metal mirrors. Same situation with design and build of a 24" dob with a 80 lb mirror.
Not complaining here of course, just searching for know how and trying to make a sensible plan.
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Re: telescope build

#29

Post by bobsorenson »

after corresponding with a person who has years of experience making mirrors for the military, I've come to the conclusion that the mirrors I have will not be suitable for an optical telescope. They were probably made for lasers and cannot be made to be accurate enough for visible light.

thanks to all for the input and information. I will likely use them for some solar concentrator project.
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Re: telescope build

#30

Post by OleCuss »

Darn it! I had hopes for this potential build!
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Marcelo F.
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Re: telescope build

#31

Post by Marcelo F. »

bobsorenson wrote:
Sat Nov 09, 2019 6:34 am
after corresponding with a person who has years of experience making mirrors for the military, I've come to the conclusion that the mirrors I have will not be suitable for an optical telescope. They were probably made for lasers and cannot be made to be accurate enough for visible light.

thanks to all for the input and information. I will likely use them for some solar concentrator project.
Sooo....

Can we still build a death star?

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Telescope: Dobson N 76/300 FirstScope DOB w/ DIY tripod adapter, 5x24 Finderscope.
Eps: 4mm Ramsden, 6mm Huygenian, 12.5mm Huygenian, 20mm Huygenian, 4mm Plossl, Solomark LP Filter, Dark Green Moon Filter.
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bobsorenson
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Re: telescope build

#32

Post by bobsorenson »

I have come across some information as to how grinding and polishing solid Al mirrors can be done (and has been done) by amateur makers. So I will be proceeding with the project after all!

I am in hopes that the original figure of the mirrors is somewhat intact and that if I am careful, I may be able to avoid significant refiguring. This approach will require testing of the mirror concurrent with the work on the surface, so I will be building a Foucault/Ronchi unit to enable progress in the right direction.
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Re: telescope build

#33

Post by JayTee »

Hi Bob,

This is great news. Perhaps you could start another thread over at The DIY Corner -- ATM forum. I know a lot of us will be very interested in your progress reports, don't forget pictures. Also I know we're all rooting for you to successfully accomplish this Herculean task.

Cheers,
JT
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Re: telescope build

#34

Post by bobsorenson »

JT, thNks for the encouraging words.
I'll be spending a few days getting testing equipment ready and procuring materials for polishing etc. Plus a test stand for an eighty lb mirror!
When I get closer I'll start posting pics and making reports.
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SkyHiker
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Re: telescope build

#35

Post by SkyHiker »

Do you have any idea of the focal length? Just curious how long a ladder you will need.

If you look at Mike Clements' 70" mirror that was also meant to be used in space, his focal length is about 60' so he built a flat secondary about half the size to reflect it back to the base because a traditional Newt would have it 60' in the air. Do you plan on doing the same?

The mirror is awfully thin compared to glass mirrors, if it may behave well in space it may not, on Earth because gravity will flex it.
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Re: telescope build

#36

Post by bobsorenson »

Skyhiker, the finish is pretty oxidized so the focus wasn't great, but it's right around 70"...so pretty fast for a 24". A little under f3
The mirrors were for ground based lasers I think, not space. Part of the Star Wars program. The story I heard when I bought them is that they were bolted to a rather large control unit that was too heavy to lift with a forklift, so had to be taken apart and parted out.
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Re: telescope build

#37

Post by bobsorenson »

Alrighty,
I'm making some progress on one of the mirrors so I thought I'd make a report and post a pic.
I've got the beast on a turnable spindle and have started working the surface. First I constructed a wedge shaped tool to lap/polish with. This is made of hard maple and laminated with polyproplene. Since the mirror has its figure intact, I shaped the surface of the tool to the curve by rubbing on sandpaper taped to the mirror down to 1500 grit.
So far I have used short radial strokes to equalize abrasion and lubricated with water and dish soap, down to 600 grit. There is some pitting and I am deciding whether to grind it out or not.
mirror600.jpg
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Lady Fraktor
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Re: telescope build

#38

Post by Lady Fraktor »

Good luck with the restoration!
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SkyHiker
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Re: telescope build

#39

Post by SkyHiker »

Hi Bob, I really hope you succeed! But to be honest - I have never seen a pie-shaped tool, never heard of anyone lining a mirror with sandpaper to shape the tool, furthermore shaping and polishing a thin aluminum mirror seems to be a specialized discipline.

The accuracy you need, say 1/10 lambda, is about 1/20 of a micron. How much deflection will you get by the uneven pressure of your pie shaped tool?

I would make a mirror testing tool and become proficient at using it and use it a lot, that should tell you how well your progress is if this works at all. Most people start with a 6" mirror for practice before they move on to anything else, and many have failed with thin mirrors.
... Henk. :D Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK
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Re: telescope build

#40

Post by OzEclipse »

I have to agree with Henk. Small tools are often used on large mirrors, but they are circular just of a smaller diameter. A 6-10" circular will be fine. You could get a CNC machine shop to CNC machine the radius onto an aluminium tool then apply a thin layer of pitch or other compound to the surface and cold press to shape it. I don't know what a pie slice shaped tool will do to it. Like Henk, I've never seen one, but my gut instinct tells me nothing good will come from it. The curved edges of a circular tool are very important in the even cutting, shaping and polishing. Those sharp corners will gouge channels in the mirror surface, not visible to the eye but optically significant. For this reason, opticians don't even use square or polygonal tools because of the sharp corners.

I would suggest to give the surface a very gentle polish to remove oxidisation, this itself may change the figure. I would use cotton wool and a light polishing compound, nothing hard that will refigure it. See what you've got before you embark on a refiguring project.

If you can get hold of, or make an artificial star. Set up a star test. A ball bearing in the sun up the street will do it.
I'm sure there are plenty of instructions / videos on line for interpreting a star test.

Then consider your next move. If the figure is not great, you might still be able to use it, literally as a light bucket.....no high mag resolution limit views of planets or double stars but amazing lower power views of galaxies and faint nebulae. I'm not saying that it's as good as an accurately figured mirror, but if you've already got the mirror, make a cheap and easy dob mount for it and have a go.



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