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24" noob dob

beginner DIY dob moon mirror making
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#1 bobsorenson

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Posted 10 November 2019 - 12:46 PM

Some 20 years ago I bought these two mirrors from a stars wars garage sale here in central Utah. I do not know what the original purpose for them was but they are Al and as far as I can tell, although tarnished the figure is still good. After discouraging interactions at another forum about Al mirrors I found the CN peeps, some of whom have experience using metal mirrors. So initially, I am looking for feedback to enable me to make a plan to construct a dobsonian, starting with an evaluation of the mirror(s). I have a pic of the mirrors but it is too big to post. I'll make adjustments and then post the file next. TIA

#2 Achernar

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Posted 10 November 2019 - 01:21 PM

There are optical shops such as Optic Wave Labs that could test the mirrors with an interferometer and let you know what the state of their figure is. It sounds as though you're not even sure of the focal length, a big consideration in building a telescope around the primary mirror. Unless the glass's manufacture is defective, even if the figure is poor it could be fixed and if the figure is fine it can simply be re-coated. A 24-inch Dob is an enormous telescope, you will need at least a van, a large SUV or a pick-up truck to transport it, and you really won't be satisfied using it in the front yards inside of a city. There's a number of construction techniques that work well for building mirror cells and the structure needed to support the mirrors that won't be overly heavy and bulky. If you do find you have useable optics and choose to proceed building a telescope, plan of installing and using digital setting circles, you will be glad you did. It's a very ambitious project, but the rewards will be great.

 

Taras


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#3 TOMDEY

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Posted 10 November 2019 - 01:23 PM

Yes, evaluating the optics would be the first and only thing to do. Starting with the obvious: Focal length and diameter... whose ratio is the F#. If that is not between three and five... the mirror is not worth further consideration. Any candidates then need wavefront test. If that is no good... then those are no good. Building a telescope around a pre-existing, but otherwise unknown mirror can be a real bargain --- provided it is right-sized, good, and not too fast or slow.

 

Most salvaged aerospace mirrors are way too fast.    Tom


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#4 bobsorenson

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Posted 10 November 2019 - 01:34 PM

Thanks Tom, "fast" meaning a short focal length? Told ya I was a noob!

I am attaching a pic and you can see that the mirrors are quite tarnished, so I am unsure as to how to test without some preliminary polishing.



#5 bobsorenson

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Posted 10 November 2019 - 01:45 PM

Here are the mirrors. They weigh 80lbs each, are SOLID Al 3" thick at center and 23.75" across.20191104_105602.jpg


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#6 wells_c

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Posted 10 November 2019 - 01:48 PM

Just to clarify, you’re saying the mirrors (substrate and all) are metal, not glass correct? Also, does the tarnish look like silver tarnish, dark grey and yellowish where it’s thinner? Or more like white powdery areas with pits?

As for testing, the easiest starting point is focal length. Should work through tarnish even if it’s pretty bad.

Turn the flashlight on on your smartphone and starting around 12 feet from the mirror try to pick up the reflection of the light on the back of the phone. Use a white card if you’re phone is black. Move back and forth until the image of the light is focused, I.e. as small as it gets. The distance from the mirror is twice it’s focal length. Then you can carefully measure diameter (use the edge of the reflecting surface) and divide fl by diameter and get the f/ number.

I think a few ppl here will be interested in learning more about them if they are metal.

Thanks for sharing, and keep us posted!

Wells
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#7 bobsorenson

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Posted 10 November 2019 - 01:50 PM

Taras, thanks for your reply,

The mirrors are solid Al, quite spectral when first obtained but now not. I don't believe they were coated (although I don't know for sure) but that the machined surface was polished.

 

Because of the weight of the scope, my present plan is to site it on my property and not transport. I live in a very small town in central Utah at 6,000 ft.



#8 bobsorenson

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Posted 10 November 2019 - 01:56 PM

Here is a pic of the back/side of the mirrors.20191109_164610.jpg



#9 bobsorenson

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Posted 10 November 2019 - 02:01 PM

Okay Wells, thanks for the directions.

Yes, the corrosion/oxidation is grey patchy with small pits but the units have been stored away, so not much mechanical damage to the surfaces.

Since it's daylight here, I will try pointing one at the sun and measure the distance to the focus.

Back in 1/2 hour



#10 bobsorenson

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Posted 10 November 2019 - 02:36 PM

The distance from the surface of the mirror to the sharpest image of sol on a paper is 70.5".

 

Wells said "The distance from the mirror is twice it’s focal length. Then you can carefully measure diameter (use the edge of the reflecting surface) and divide fl by diameter and get the f/ number."

 

So 70.5 in half is 35.25" (fl?) then divide by 23.75 = f/1.48?

doesn't sound right somehow....so is this right?

70.5 (fl)/23.75(dia)=f/2.9684

 

Sounds like a pretty "fast" mirror....Does this factor make it harder to focus or track?

 

thanks!


Edited by bobsorenson, 10 November 2019 - 02:40 PM.


#11 Jon Isaacs

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Posted 10 November 2019 - 02:45 PM

Okay Wells, thanks for the directions.

Yes, the corrosion/oxidation is grey patchy with small pits but the units have been stored away, so not much mechanical damage to the surfaces.

Since it's daylight here, I will try pointing one at the sun and measure the distance to the focus.

Back in 1/2 hour

 

Bob: 

 

The surface of a mirror needs to be accurate to within about 70 nanometers. This is about 1/10th of a wavelength of light. A human hair is about 1500 thicker than that.

 

 The entire surface should not deviate from a parabola more than about 70 nanometers.  Glass is hard and does not corrode so once the mirror is figured, the coating can be applied and when the mirror needs to be recoated, the coating can be removed chemically and the surface has not changed. 

 

Aluminum does corrode and it is soft.  What that means is it cannot be easily repolished when there is corrosion damage. Any visible corrosion represents damage far greater than the allowable surface error of the mirror.  Essentially, the mirror will have to be remade and that is almost the entire cost of the mirror.

 

There is a group of amateur astronomers in South America that makes aluminum mirrors, maybe someone reading this can provide a link.  There are also members here like Mike Jones who are experienced with military and space optics where they do use metal mirrors who could tell you more. 

 

https://br.groups.ya...etalmirror/info

 

I imagine this is what the previous forum also told you.

 

And yes, F/1.5 is very fast.  It is very possible this was not a mirror the visible spectrum or for optics. 

 

Jon


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#12 bobsorenson

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Posted 10 November 2019 - 03:11 PM

Thanks Jon,

Yes, I am hoping to get feedback from the people on this forum whom you mention.

I have read the threads on this site re: solid metal mirrors and it is clear that while many say that Al mirrors are no good, there are some who have actually made them and think differently.

 

Those with direct experience in doing are those who I hope will chime in.



#13 Bill Lee

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Posted 10 November 2019 - 03:14 PM

If you’re using the sun, the measured distance IS the focal length! If you’re holding the light source next to your eye, then you’re measuring the radius of curvature. The focal length is half the ROC.

 

BTW, there’s a reason why military and NASA satellites use a glass substrate and not aluminum, because it’s well neigh impossible to make a mirror accurate enough for visual use. Longer wavelengths yes, optical, no.



#14 Steve Dodds

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Posted 10 November 2019 - 03:24 PM

Aluminum doesn't tarnish but does oxidize and if these mirrors have been sitting outside you can bet what ever surface they had is ruined.



#15 bobsorenson

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Posted 10 November 2019 - 04:04 PM

Thanks for your replies.

 

Bill, thanks for the explanation on focal length. So my mirrors are f3/24. Also, from my reading, the military and NASA DO use some metal mirrors and there ARE people, some of whom are on this forum, who have successfully made and used optical grade metal mirrors.

 

Steve, yes, I understand. The reason I joined the CN community is because there are some here who I believe have direct knowlege and experience in methods needed to repair the surface. If you have experience at to how an ATM can do such a thing, then I would like to hear about it. Thanks for your input.



#16 TOMDEY

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Posted 10 November 2019 - 04:05 PM

Unsing the sun as you did... that's ~F/3... which is probably too fast for any eyepiece to gracefully tolerate. The surfaces look completely shot... so would have to be figured from scratch, much like the classics used to do with their speculum mirrors... only starting from a far worse condition in your case. The back looks like it was just literally screwed down to the mount aka "hard-mounted" with no particular consideration to maintaining wavefront.

 

I'm guessing that the original application may have been Infrared or even longer. Probably not the near infrared, but longer.

 

At this point --- could investigate further... but enters the realm of "just for fun". The likelihood that any one of these would be useful for a telescope is... asymptotically approaching... zero!

 

Many scrap yards will pay cash for scrap aluminum!  Tom



#17 Bill Lee

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Posted 10 November 2019 - 04:50 PM

AFAIK, the metal mirrors are used in long infrared by the military. Most certainly not for optical. Even on space probes, where masses everything—like Cassini and Voyager—were glass. They don’t make large optical telescopes out of metal, etc. Also, the CTE will be a killer.

I’m not saying it can’t be done (IDK the state of diamond turning these days, but is it accurate enough to produce a surface that doesn’t need polishing?),but I don’t know of one that has been made? Can you name an optical telescope made out of aluminum? Can you name a maker who has done it? Not trying to be argumentative, I just don’t recall anyone who has done it.

 

Finally, this is a common noob question, so you’re not alone.



#18 bobsorenson

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Posted 10 November 2019 - 05:16 PM

Tom, the project never left the realm of "just for fun".
There are people in this community who make telescopes using solid Al mirrors. That is why I joined. You can do a search on this site and can see for yourself.
I am hoping to get direct knowledge from someone who has done this.

#19 BGRE

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Posted 10 November 2019 - 05:24 PM

CTE is not an issue with metal mirrors due to their high thermal conductivity.

ESO considered metal substrates for the VLT as a backup should casting the primary in zerodur fail.

NASA have used india ink to optically polish aluminium.

All 4 ESO VLT 8m telescopes use 1.1m diameter beryllium secondaries. 



#20 bobsorenson

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Posted 10 November 2019 - 05:25 PM

Bill, you can do a search on this site and find several threads discussing the making and use of solid Al mirrors in optical telescopes.
In these threads you will see opinions from all sorts of educated people that are diametrically opposed.
I am looking to connect with and learn from one of those people who have done the work.

#21 wells_c

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Posted 10 November 2019 - 06:04 PM

Sorry for the disjointed post and reply, I didn’t see the pics when I posted, and didn’t check back until now.

Yes, the cell phone trick involves the object being coplanar with the image which would occur at twice the distance of an image of an object at infinity.

It looks consistent with Al oxidation, and you werent kidding about it being bad... because of the pitting I believe others are correct about having to refigure; material will have to be removed to get a clean polish... from what I understand (not a lot...) if they were far IR, the surface wouldn’t have needed a super fine polish, and may have done ok even looking like that. That being said one interesting exercise would be to hand wet-sand it up through, say, 2000 grit. This would produce a decently reflective surface. It would probably result in a pretty awful figure but if you’re in it just for fun, why not? It would also make for a heck of a solar concentrator...

For one I would be fascinated to follow an attempt to figure and polish it (them?) to something halfway decent, and even in the case of abject failure useful knowledge would undoubtedly result.

#22 bobsorenson

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Posted 10 November 2019 - 06:46 PM

Thanks so much Wells.
A moderator moved the thread and I also had to modify the pics to post so I was confused too for a minute.

I have assumed to have to do some refiguring and polishing on them so it's a little disconcerting to get comments like "you can sell them for scrap" or "they're shot, don't waste your time".

So to do some preliminary polishing up to 2000 grit would I use a tool or pad of some kind? I've read that on Al, it's possible that SiO2 particles can get trapped under/in the Al substrate and cause problems later. So before proceeding I want to make sure that I "first, do no harm".
Thanks again

#23 wells_c

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Posted 10 November 2019 - 07:19 PM

Yeah my suggestion should be researched for problems first... however my thinking is that a gentle sanding up through the grits would remove a tiny fraction of what’s needed to get to the bottom of the corrosion—you’d still have pitting, etc. and a serious effort to figure and polish would get rid of any damage or problems from sandpaper...

If I were to do it, I’d start with 320 wet, and wrap the paper around a large sponge in a way that produced a soft round contact point with no paper edges or creases to scratch. A drop of dishsoap helps keep the motion smooth... I’d go around the surface with a round buffing motion, and do a several entire passes around with each grit, rinsing everything when switching. I feel like you would get a pretty bright mirror-like surface, albeit with pitting, without removing very much material. Then when you study up on figuring and polishing metal, you’ll probably take off much more. The sanding would be a very quick rough treatment that wouldn’t contribute to the main effort, but might provide some insight, get you used to working with the shape and material, and seeing the surface sparkle (and who knows maybe even producing an image of some sort with a secondary and eyepiece) would probably give you inspiration to put the effort needed to do it right.

All that being said, I don’t think sanding will cause problems, but I don’t know.

Finally don’t let yourself be discouraged by metal scrap talk but do your research. And maybe ask those who say it won’t work what they have tried, what problems they encountered, and if they have pics and or a build thread so you can learn from their experiences :)

#24 bobsorenson

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Posted 10 November 2019 - 07:28 PM

Super super super. Thanks Wells

When I first got the mirrors they were so bright you had to be careful looking at them. Highly polished.
I am hoping that someone who has actually done the hands on work with Al will join in the discussion. He is Brazillian. Perhaps when I get to my desktop I'll tag a few and see if that works.

#25 wells_c

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Posted 10 November 2019 - 07:56 PM

Ah. Maybe start with 800 or 1000 in that case :)

Good luck and I hope to see this go far.


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