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Bino Mirror Mount - DIY

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#1 mike_k

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Posted 19 August 2013 - 01:40 PM

I've decided to try and construct a binocular mirror mount, but as the supply of tank periscope mirrors from surplusshed has dried up I need to source a large first surface mirror that won't run me $300.

My question here is - what surface accuracy of the mirror do I need for this project? I definitely plan on using 10x70s with it. And, if I am able to swing it, maybe a pair of 15x70s.

Thanks

#2 GlennLeDrew

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Posted 19 August 2013 - 04:19 PM

At such a large exit pupil, you can get away with a wavefront error of perhaps 2 waves, and certainly 1 wave, across the objective aperture. This implies a *surface* flatness of 1/2 that, or 1/2-1 wave over 70mm.

I know that float glass mirrors are typically rated as having 4-6 waves per inch error, on the surface I'm reasonably certain, which if so is a wavefront error of 8-12 waves per inch on reflection.

It's entirely possible to find float glass with portions rather better than this. To illustrate... An older friend observes through 2 sheets of window glass with 100mm binos up to 30-40X. The image leaves much to be desired, but the view for him is serviceable.

Perhaps the best approach is to obtain not one monolithic mirror, but two smaller ones of, say, 80X110mm. You're more likely to get flatter specimens, and you can mount one so as to refine collimation.

#3 Pinbout

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Posted 19 August 2013 - 06:43 PM

Google first surface mirrors, there use to be 2companies, now only one site works. They state 2-4waves/in.

The other site would provide 1~, but now the site is totally different.

#4 StarStuff1

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Posted 20 August 2013 - 10:31 AM

Years ago the company I worked for threw away some old microfiche projectors (the computer age had arrived). I was allowed to cannibalize them for optics. One piece was a first surface mirror roughly 8-in X6-in. I used it to make a mirror mount for my then new 14x70 Fujis.

This worked fine but after 5 or 6 uses I gave up on it. The mirror image orientation of the sky was confusing but the big problem was the dewing up of the mirror. Dew is something you have to deal with where I live and at the time I didn't have a dew gun.

Have you tried American Science and Surplus?

#5 Jeff Morgan

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Posted 20 August 2013 - 10:54 AM

Perhaps the best approach is to obtain not one monolithic mirror, but two smaller ones of, say, 80X110mm. You're more likely to get flatter specimens, and you can mount one so as to refine collimation.


I love my SkyWindow for my Fuji 10x50's. But if I wanted to go high-power I would definitely get a pair of Newtonian diagonals. Pricey yes, but the comfort is unbeatable, and SkySafari solves the mirror reversed image "problem".

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#6 mike_k

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Posted 21 August 2013 - 04:15 PM

I love my SkyWindow for my Fuji 10x50's. But if I wanted to go high-power I would definitely get a pair of Newtonian diagonals. Pricey yes, but the comfort is unbeatable, and SkySafari solves the mirror reversed image "problem"


Unfortunately Sky Window isn't being made anymore. Also, I contacted Trico Machining (the manufacturer) and they had no "spare" mirrors left either.

What do you mean "SkySafari solves the 'problem'"?

#7 mike_k

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Posted 21 August 2013 - 04:22 PM

Perhaps the best approach is to obtain not one monolithic mirror, but two smaller ones of, say, 80X110mm. You're more likely to get flatter specimens, and you can mount one so as to refine collimation.



I thought of that, but then I realized I'd have difficulty with the collimation, and thought if someone did know a source for a single piece it would be oh-so much easier. I guess I need to go back to that idea. Now to plan how I'll make one of the mirrors adjustable - without requiring machining a fancy part. :(

#8 RossSackett

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Posted 21 August 2013 - 04:37 PM

That's funny. I was rummaging through my attic this morning and found my old SkyWindow. It seemed like a good idea at the time and really helped with neck comfort, but it wasn't quite flat enough across it's width to keep acceptable bino-collimation with my 14X70 Fugis--no neck ache, but major eye-strain. I never noticed a problem with 10X50s, though, so the concept was sound.

#9 GlennLeDrew

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Posted 21 August 2013 - 04:54 PM

No mirror, however bad its optical figure, can be so non-flat as to affect collimation. A single mirror which introduces mis-collimation must be so warped that awful astigmatism is expected.

No fancy machining is required to build in tilt adjustment on one mirror. The square (or rectangular) plate the tiltable mirror is mounted on is 'anchored' at one corner. The two corners which form the ends of an 'L' (the anchored corner being the vertex of this 90 degree angle) have adjustable, fine-pitch screws for mutually perpendicular adjustment of tilt.

Note that it's not required to have the two mirrors share a common plane. One can be higher than the other. All that's important is that each mirror not have an edge fall inside the circle defined by the entrant light bundle.

#10 RossSackett

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Posted 21 August 2013 - 05:34 PM

It was definitely warped--can see it in shallow reflections. It was worse on one side than the other, causing both bad stig in one eyepiece and overall miscollimation that wasn't present with direct views. Having a fixed and an adjustable mirror would have been a much better solution.
Conventional binocs with star diagonals would have been even better.

#11 mike_k

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Posted 21 August 2013 - 05:48 PM

That's funny. I was rummaging through my attic this morning and found my old SkyWindow. It seemed like a good idea at the time and really helped with neck comfort, but it wasn't quite flat enough across it's width to keep acceptable bino-collimation with my 14X70 Fugis--no neck ache.

Willing to sell?

The main reason I want the mirror mount is a neck injury that gives me a migraine if I crank my head in the extreme "looking up" position for more than 7-10 minutes. I just cant afford the $230 for the Oberwerk mirror mount at the moment.

Plus, I like the idea of the moveable mirror and stationary binocular (in regards to inclination), vs. Oberwerk's fixed binocular/mirror angle. Which is what I plan to do if I end up making my own.

#12 mike_k

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Posted 21 August 2013 - 06:00 PM

Note that it's not required to have the two mirrors share a common plane. One can be higher than the other.

Ah! That's one of the things I was worried about.


All that's important is that each mirror not have an edge fall inside the circle defined by the entrant light bundle.

Hunh? :confused:

#13 mike_k

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Posted 21 August 2013 - 06:35 PM

At such a large exit pupil, you can get away with a wavefront error of perhaps 2 waves, and certainly 1 wave, across the objective aperture. This implies a *surface* flatness of 1/2,that, or 1/2-1 wave over 70mm.

I know that float glass mirrors are typically rated as having 4-6 waves per inch error, on the surface I'm reasonably certain, which if so is a wavefront error of 8-12 waves per inch on reflection.

It's entirely possible to find float glass with portions rather better than this. To illustrate... An older friend observes through 2 sheets of window glass with 100mm binos up to 30-40X. The image leaves much to be desired, but the view for him is serviceable.

I found that firstsurfacemirrordotcom sells 1u (I assume that's 1 lambda) 6mm thick mirror for a reasonable price - 6"x10" costs $33. that's definitely in my price range. The best I found for 1/4 wave was 5"x10" for $300.

#14 GlennLeDrew

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Posted 21 August 2013 - 09:04 PM

Mike,
The statement of mine which caused confusion was made primarily in the context of one mirror being set higher than the other, and where the mirrors are not safely oversized. Because the bino is looking toward the mirrors at an angle, if one mirror is higher it must be moved laterally a suitable distance dictated by geometry toward that bino's objective. This is only of concern if one mirror is notably higher and if it's near minimum size.

If a mirror is spec'ed as 1u, I take it to mean 1 micron, which is about 2 waves of visible light. That's most likely the surface error, meaning 4 waves of error on the reflected wavefront. This seems to be *selected* float glass, which the 1/4" thickness supports, and which may or may not have significant variations over short distances.

I would say that at that price it could well be worth trying. If found dervicesble, you can make a heater of about 10 Watts to place under, thus reversing its state as a dew magnet.

#15 Jeff Morgan

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Posted 22 August 2013 - 12:09 AM

Unfortunately Sky Window isn't being made anymore. Also, I contacted Trico Machining (the manufacturer) and they had no "spare" mirrors left either.

What do you mean "SkySafari solves the 'problem'"?


The "problem" I was referring to was the image being flipped left to right (mirror reversed). Some binocular users don't like that aspect of a mirror-fed solution because it does't match star charts (although telescope users have been dealing with it for generations).

SkySafari can be set to flip the sky to whatever orientation your optical device provides.

#16 mike_k

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Posted 26 August 2013 - 01:40 AM

Hi

You mentioned the other day your SkyWindow was not up to snuff for you. Would you be willing to sell it?

mike k






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