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cosmic camper
member
Reged: 06/12/08
Posts: 21
Loc: Santa Cruz, CA
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Hi,
I'm new to these forums. Just wondering if anyone knows a good affordable source for a first surface mirror to build a binocular mirror box. I have 20X80 standard Oberwerks. One source I know, High Reflective Mirrors says their 6mm (1/4")mirror is <1 wavelength, which they tell me is .8 wavelength. Is that adequate for 20X binos?
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12341
Loc: Cumberland, R I , USA42N71.4W
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Read the recommendations from FSmirrors.com
http://www.fsmirrors.com/
also read what Trico Machine has to say about the Skywindow.
http://www.tricomachine.com/skywindow/
see also
http://www.bigbinoculars.com/mirror.htm
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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cosmic camper
member
Reged: 06/12/08
Posts: 21
Loc: Santa Cruz, CA
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Thanks, Ed. As you notice, the info at these sites gets you close, but fails to answer my question. When I asked these vendors for some specifics, they were silent. So what I know is that 1/4 wave works for 20X binos, but fsmirrors' 1 wave doesn't. I may have to gamble on the .8 wave, or spend several hundred more dollars to be sure.
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Paul,
the Cosmic Camper
__________________________________
2005 Ford Econoline camper van
16" Astro Sky/Swayze truss tube dob
DGM 5.5" off-axis dob
Celestron Nexstar 5SE
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OBERWERK
Vendor
Reged: 06/24/03
Posts: 80
Loc: OH
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Quote:
Thanks, Ed. As you notice, the info at these sites gets you close, but fails to answer my question. When I asked these vendors for some specifics, they were silent. So what I know is that 1/4 wave works for 20X binos, but fsmirrors' 1 wave doesn't. I may have to gamble on the .8 wave, or spend several hundred more dollars to be sure.
Our site does answer the question- our mirrors are 1/4-wave (which is why they are 1/2" thick).
Kevin Busarow
Oberwerk Corp.
Edited by OBERWERK (06/14/08 01:56 PM)
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GlennLeDrew
sage
Reged: 06/18/08
Posts: 467
Loc: Ottawa, Ontario, Canada
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Here's an option... instead of one big mirror, how about *two* smaller mirrors, which I'm sure are more readily available, at reasonable prices and decent quality. Moreover, you can then tweak the tilt of one to get better collimation than most binoculars typically offer.
Also, if dewing is a concern, you might want to buy (or better yet, make on the cheap) dew heaters to attach to the back surfaces.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Wes James
Carpal Tunnel
Reged: 04/12/06
Posts: 2354
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I brought up the idea of individual mirrors some months ago: Quote:
Re: Sky Window [Re: Frank Dement]
#2283853 - 03/26/08 03:39 PM
I still think the answer to a "Sky Window" mirror type device for the larger bino's lies in 2 seperate mirrors mounted side-by-side in individually collimatable cells.
here.
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Wes
Atlantic Beach, FL
Some bino’s from Miyauchi 5x32 Binon's up through Garrett 20x110 Signature's,
Some telescopes from a Stellarvue 80mm NHNG up through a couple of 8” reflectors…
And a wonderful 4.25" Delmarva Shiefspiegler!
Some good friends, made here on C/N.
Oh- several cats and a wonderful wife!
Anyone want a cat???? :-O
"When your work speaks for itself- Don't Interrupt" -Gamble Rogers
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12341
Loc: Cumberland, R I , USA42N71.4W
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I would think one would want to eliminate any secondary potential for another possiblity at misalignment. Seems most of the manufacturers agree, given most (if not all) binocular mirrors are one-piece. Just my 2 sense.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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cosmic camper
member
Reged: 06/12/08
Posts: 21
Loc: Santa Cruz, CA
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All interesting ideas. But back to my original question - what is the minimum flatness in waves of light required for 20X80 binos? From all I have read, it is under 1 wave. But is it .25, .50, .75? And is there any affordable source (under $100) for a single unit of such mirror? Someone must know the answer, perhaps a formula, for the flatness question. 
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Paul,
the Cosmic Camper
__________________________________
2005 Ford Econoline camper van
16" Astro Sky/Swayze truss tube dob
DGM 5.5" off-axis dob
Celestron Nexstar 5SE
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GlennLeDrew
sage
Reged: 06/18/08
Posts: 467
Loc: Ottawa, Ontario, Canada
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Ok, I'll take a *preliminary* stab at this.
For diffraction-limited performance, 1/4 lambda is generally taken as the desired maximum peak-to-valley deformation of the wavefront. But that's for 0.5mm to perhaps 1mm exit pupil diameters, where diffraction effects become visible. Let's be conservative and use 1mm, which applies to sharp-eyed observers.
Because you're using the mirror in the full-aperture configuration, wavefront error (over that large surface) is more critical than when using a mirror in a diagonal. The peak-to-valley error mus apply to an area at least as large as the instrument's aperture * 1.41 (for 45 degree tilt). Beware mirrors which specify quality PER INCH, for example. Across larger dimensions the total deformation my be rather larger!
With the comparatively low magnification (read: large exit pupil) of binoculars, the wavefront error criterion is necessarily more relaxed. By how much? Simply as the ratio of your bino's exit pupil to the diffraction-limited pupil diameter of 1mm.
So here's my off-the-cuff formula:
Max wavefront error = exit pupil / 1mm * 0.25 lambda
Example:
20x80mm bino (effective aperture at 45 degrees = 80 * 1.41 = 91mm)
Exit pupil = 4mm
Max wavefront error = exit pupil / 1mm * 0.25 lambda
= 4mm / 1mm * 0.25 lambda
= 4 * 0.25
= 1 lambda
Now, that's the wavefront error *after* reflection. But a mirror of given *surface* quality doubles the wavefront error. So the mirror's surface *surface* quality must be 1/2 the allowable delivered wavefront error.
Therefore, if surface quality is specified, in this example for a 20x80 bino, a mirror having no more than 1/2 lambda deformation over 91mm is desirable.
I've not considered the impact of mirror tilt, but I do believe it further constrains mirror quality, such that it should be 40% better than the figure derived. (Lambda * root 2)
Again, this is a "back-of-the-envelope" treatment, and I can stand to be corrected.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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GlennLeDrew
sage
Reged: 06/18/08
Posts: 467
Loc: Ottawa, Ontario, Canada
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Oops! (Lambda / root 2) = (Lambda / 1.414).
So if 1/2 lambda is the required surface quality via the "formula", in the 45 degree tilted configuration this should be
1/2 / 1.414 = 0.35 lambda.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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cosmic camper
member
Reged: 06/12/08
Posts: 21
Loc: Santa Cruz, CA
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Thanks, Glenn. I follow your argument, and it's the best I've seen yet. I'm just thinking though that since there is only a single reflection there might be no doubling of the wavefront error as with a secondary, i.e. there is zero error to start. So does that then make it 1/1.414 = 0.71 lambda?
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Paul,
the Cosmic Camper
__________________________________
2005 Ford Econoline camper van
16" Astro Sky/Swayze truss tube dob
DGM 5.5" off-axis dob
Celestron Nexstar 5SE
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GlennLeDrew
sage
Reged: 06/18/08
Posts: 467
Loc: Ottawa, Ontario, Canada
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Paul,
All mirrors cause a wavefront error double that of its surface deformations. And yes, that applies to the single reflection, because the light is being "folded" back on itself.
Imagine a perfectly flat mirror with a bump 1 wavelength high. Light approaching the mirror will reach the top of the bump first and be reflected back while the adjacent light is still on its journey toward the rest of the mirror. By the time these adjacent light packets reach the surface, the light that struck the bump will already be 1 wavelength above the bump, or 2 wavelengths above the rest of the mirror, when the adjacent light is just beginning its reflection. So you can see how a mirror's surface deformations are doubled. A sketch will help to see this.
I'm not sure what you meant with your last question, but hopefully it's answered been here.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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KennyJ
Reged: 04/27/03
Posts: 10029
Loc: Lancashire UK
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Glenn ,
I have read every word of every post you've made to this forum since your sudden arrival the other night , and have thoroughly enjoyed reading every post .
Your explanation above , describing mirror wavefront error in simple layman's terms , is the best example yet of your gift for using analogy as a substitute for a diagram or sketch .
Although illustrations are indeed just about the only way to follow many technical aspects of optical behaviour , I for one always appreciate a writer who can succeed without them .
Long may your contributions continue here .
Kind Regards
Kenny
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Two eyes and a preference to use both
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Captain's Helmsman 7 x 50
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Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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cosmic camper
member
Reged: 06/12/08
Posts: 21
Loc: Santa Cruz, CA
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Yes, thanks Glenn for an explanation we non-scientist/engineer types can comprehend. Just curious how that plays out then in a newtonian. So a 1/10 wave P-V primary really delivers 1/5 lamda to the secondary. If the secondary is say 1/20 wave, then what's the final lamda?
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Paul,
the Cosmic Camper
__________________________________
2005 Ford Econoline camper van
16" Astro Sky/Swayze truss tube dob
DGM 5.5" off-axis dob
Celestron Nexstar 5SE
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GlennLeDrew
sage
Reged: 06/18/08
Posts: 467
Loc: Ottawa, Ontario, Canada
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Quote:
Yes, thanks Glenn for an explanation we non-scientist/engineer types can comprehend. Just curious how that plays out then in a newtonian. So a 1/10 wave P-V primary really delivers 1/5 lamda to the secondary. If the secondary is say 1/20 wave, then what's the final lamda?
Yes, a 1/10 lambda *surface* deformation delivers a 1/5 lambda wavefront. In fabrication, testing of an optic is based on the wavefront sent to the test apparatus, and therefore takes into account this doubling of error. That's why one must ascertain just what the specified quality refers to; the surface or the wavefront.
In assessing cumulative errors, the standard method is to take the square of the sum of the squares. To work with your specified values:
1/20 = 0.05
1/5 = 0.2
resultant lambda = SQRT (lambda1^2 + lambda^2)
= SQRT (0.05^2 + >0.2^2)
= SQRT (0.0025 + 0.04)
= SQRT (0.0425)
= 0.206
~1/5 lmbda
As you can see, the worst optic is the weak link in the chain, because its waveform error so dominates the better optic's. If one mirror was perfect and the other had lambda 1/5 errors, it's easy to see that the resultant error will also be 1/5 lambda.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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rodelaet
Carpal Tunnel
Reged: 04/28/06
Posts: 2533
Loc: 50°56' N - 4°58' E (Belgium)
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Quote:
As you can see, the worst optic is the weak link in the chain, because its waveform error so dominates the better optic's. If one mirror was perfect and the other had lambda 1/5 errors, it's easy to see that the resultant error will also be 1/5 lambda.
Can we include atmospheric turbulence also into the optical chain? And would an observer have any benefit of diffraction limited optics when the seeing is average? If so, the not so perfect mirror might not be the weakest link in the chain?
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Rony
My Astronomy Sketches
My Binocular Sketches
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GlennLeDrew
sage
Reged: 06/18/08
Posts: 467
Loc: Ottawa, Ontario, Canada
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Rony,
At typical bino magnifications (up to about 25X in most cases), atmospheric seeing has to pretty bad to be visible. Only on perhaps two occasions in my life did I see anything more than the usual scintillation (twinkling) in even a fairly large bino (unless looking very close to the horizon on so-so nights). So I would discount that as a reason to relax the mirror tolerances.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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