telescope build

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Re: telescope build

#21

Post by OleCuss » 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?

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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Re: telescope build

#22

Post by AntennaGuy » Thu Nov 07, 2019 3:35 am

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 » Thu Nov 07, 2019 6:35 pm

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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Re: telescope build

#24

Post by bobsorenson » Thu Nov 07, 2019 9:09 pm

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 » 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!
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Re: telescope build

#26

Post by DeanD » Fri Nov 08, 2019 12:37 am

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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Re: telescope build

#27

Post by Marcelo F. » Fri Nov 08, 2019 9:16 am

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 » Fri Nov 08, 2019 3:51 pm

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 » 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.
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Re: telescope build

#30

Post by OleCuss » Sat Nov 09, 2019 11:51 am

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

#31

Post by Marcelo F. » Mon Nov 11, 2019 2:53 pm

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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Re: telescope build

#32

Post by bobsorenson » Wed Nov 13, 2019 4:51 pm

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 » Wed Nov 13, 2019 10:19 pm

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 » Thu Nov 14, 2019 5:31 am

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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Re: telescope build

#35

Post by SkyHiker » Fri Nov 15, 2019 11:51 am

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 » Fri Nov 15, 2019 7:58 pm

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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