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mirrors for folded refractor

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#1 Jason B

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Posted 10 August 2012 - 02:04 PM

I have a buddy that wants to build a folded refractor but he is having trouble finding quality flat mirrors for the scope. Any thoughts on where I could direct him?

Thanks,

#2 MKV

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Posted 10 August 2012 - 03:06 PM

I have a buddy that wants to build a folded refractor but he is having trouble finding quality flat mirrors for the scope. Any thoughts on where I could direct him?

Besides the obvious "what size" and "what quality", there's another (minor but crucial) question that pops up: what price range is he willing to consider?

If money is no object, then he can try Melles Griot or Edmund Scientific.

Otherwise, eBay or CN Classifieds may have what he needs. Of course, every now and then, Surplus Shed may have a sufficiently large flat for a folded refractor at a bargain price.

Regards,

Mladen

#3 DAVIDG

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Posted 10 August 2012 - 03:18 PM

Surplus Shed as couple that might work. I bought a couple of these http://www.surplussh...tem/pm1114.html for the same purpose. I haven't had a chance to test them but if they are 1/10 wave they are a very good deal. You need that quality of flatness or better since the closer the flat is located to the objective lens the flatter it needs to be to not introduce astigmatism into the image.

- Dave

#4 Gene7

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Posted 10 August 2012 - 03:59 PM

Yes David, I am building a "Z" folded 70mm ED scope for digiscoping purposes. Purchased some thick 1/10 wave mirrors from the Shed that look good. Not tested yet. Will use two mirrors. Good price. Gene

#5 plyscope

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Posted 10 August 2012 - 05:38 PM

You might like to try Nova.

web page

#6 siriusandthepup

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Posted 10 August 2012 - 07:58 PM

+1 Nova

#7 Pinbout

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Posted 11 August 2012 - 09:57 AM

Purchased some thick 1/10 wave mirrors from the Shed that look good. Not tested yet.



I got a couple from the shed I had to send back. they do have a 10 day return policy which is nice.

I wouldn't wait to test them.

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#8 Jason B

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Posted 11 August 2012 - 10:28 AM

Thanks everyone. I have forwarded the link on to him.

#9 wiseone

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Posted 12 August 2012 - 03:37 AM

I have a lot of experience in folded refractors. You do not need 1/10 wave or anything close to that! 1 wave of flatness is perfectly fine. However, there must be no astigmatism in the flats.

#10 MKV

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Posted 12 August 2012 - 09:10 AM

Purchased some thick 1/10 wave mirrors from the Shed that look good. Not tested yet.



I got a couple from the shed I had to send back. they do have a 10 day return policy which is nice.

I wouldn't wait to test them.

Maybe they are referring to surface error, not radius error of 1/10 wave (iow, you can have a long radius curvature and a 1/10 wave smooth surace). It's hard to tell from your example. You need to look at 4-5 fringes max to get a somewhat decent idea of these parameters.

Mladen

#11 DAVIDG

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Posted 12 August 2012 - 09:30 AM

I purchased 6 of the PM1114 flats and checked all of them last night. They all test out to be good to at least 1/8 wave if not better using my semi coated master flat which is good to 1/10 wave.
But as Danny points out, with any optics you need to test each and everyone to be sure you getting what you think.

- Dave

#12 MKV

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Posted 12 August 2012 - 09:35 AM

I have a lot of experience in folded refractors. You do not need 1/10 wave or anything close to that! 1 wave of flatness is perfectly fine. However, there must be no astigmatism in the flats.

I am pretty sure they're referring to surface smoothness, not the radius of curvature. Any essentially zero-power mirror (optical power = 1/f) will be acceptable, as long as it's surface smoothness is 1/10 wave or better. Ignoring tilt (which worsens the error), 1/10 wave surface mirror will produce at least 1/5 wave wavefront error reaching the eye under ideal conditions - and that's close to the allowed maximum error of 1/4 wave as you want to go. So David is absolutely right that a mirror must be at least 1/10 wave, preferably
better.

Regards,

Mladen

#13 wiseone

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Posted 12 August 2012 - 10:08 AM

Excellent surface smoothness is a requirement of any optical surface, but I repeat that 1/10 wave flatness is overkill. OSLO or ZEMAX will tell you that 1 wave of flatness is perfectly adequate. The tilt with a long refractor OG is very small, so astigmatism is not a problem. Even a Newtonian flat need not be 1/10 wave flat! If the departure from flat is elliptical in shape so that the projected error is purely spherical, you can get away with a surprising lack of flatness. It only changes the focal position very slightly. See Norman Oldham's web site on this point.

#14 MKV

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Posted 12 August 2012 - 11:07 AM

Excellent surface smoothness is a requirement of any optical surface, but I repeat that 1/10 wave flatness is overkill. OSLO or ZEMAX will tell you that 1 wave of flatness is perfectly adequate. The tilt with a long refractor OG is very small, so astigmatism is not a problem. Even a Newtonian flat need not be 1/10 wave flat! If the departure from flat is elliptical in shape so that the projected error is purely spherical, you can get away with a surprising lack of flatness. It only changes the focal position very slightly. See Norman Oldham's web site on this point.

Industry standard of excellence for surface smoothness is 1/2 wave. Obviously this is not "excellent" for astronomical optics. That's why optical quality must be stated in terms of surface finish. The radii of curvature are specified in linear terms, not in terms of sagitta depth in wavelengths.

If you're talking radius of curvature in terms of sagitta then even 1 wave is an overkill. Such "flats" can be substantially curved and still be more than adequate for folded optics. In fact C. R. Burch gives the formula for acceptable curvature of these "flats" in terms of interference rings of allowable sagittal depth: 3.4*(F/D)^2, where F/D = focal ratio of the optics tested. So, even for an f/2 (!) system the acceptable curvature for a folding flat (assuming minumum tilt) is 3.4*(4) = 14 interference rings of curvature, or 7 waves!

At any rate, knowing that surface errors are doubled on reflection is more important in these "flats" than the nominal radius of curvature.

Regards,

Mladen

#15 MKV

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Posted 12 August 2012 - 11:56 AM

I got a couple from the shed I had to send back. they do have a 10 day return policy which is nice.

The mirror offered by Surplus Shed is a Melles-Griot catalog item 02 MPG 016, which was either discontinued or rejected, but that category of flats is listed as being flat to 1/10 wave. Maybe they produced a whole batch that turned out to be something other than 1/10 wave flat - but the important thing to remember is that Melles-Griot no longer lists that item in their catalog.

Regards,

Mladen

#16 Crayfordjon

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Posted 12 August 2012 - 12:18 PM

I have used flats in folded refractors for several years now, including a twelve inch refractor, I found through actual experience and not "recieved textbook knowlege", that the flats need only be smooth with no ring zones, and that they need not be more than 1/2 wave error of any kind. There is too much waffle about optics having to be ultra accurate before they will work at all.

#17 Gordon Rayner

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Posted 12 August 2012 - 05:16 PM

Is there an easily accessible program/information about:

The effect of non- 45 deg, or non-0 deg. incidence angle upon dielectric stack mirror reflectivity, or upon "enhanced" silver or aluminum mirror surfaces?

For example, consider the mirrors in the virtual roofline 2-mirror pseudo Amici "prism" binoscope featured in the April Sky and Telescope ( the one with Titanic on the cover). There, the beamprints are 2:1 ellipses( so some of the 1.414 major/minor axis mirror width is wasted), and the incidence of an axial ray is 60 degrees. The overall deviation of the line of sight is 90 degrees in this case.

The flatness requirement is increased by using 60 deg. incidence, rather than 45 deg.

What is the change in the wavelength of maximum reflection, and the change in the reflectivity performance, in the use at 60 deg. axial incidence, of a dielectric stack or enhanced metal mirror optimized for 45 deg.?






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