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Idea for reflector design with no diffraction spikes

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#1 Tangerman

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Posted 05 June 2020 - 05:41 PM

I work in a laser lab, and we use off-axis parabolic mirrors. Essentially, these are parabolic mirrors, but instead of the incoming light and focused light going more or less along the same path, the ones we use are offset by 90 degrees. So, in a top-down view, our laser comes in from the left, hits this off-axis parabolic mirror, and is focused to a plasma below. Naturally, I wondered about using this for a telescope. The largest off-axis parabolic mirrors I could find are from edmund optics, a company that makes lots of optics for spectroscopy and laser systems. You can peruse and find the 4" ones here: https://www.edmundop...Mirrors/39488/ 

Essentially, what I'm envisioning is a tube or shroud in one direction to shield the mirror from stray lights and/or dew, and another tube to the focuser. No other mirrors or diagonals necessary. An attachable focuser at the sky end could hold a laser collimator, and an attachment of circular rings centered in the normal focuser would help you achieve collimation. Since most of the mirrors listed here are very short focal length (f/1 or f/1.5 for the 90 degree off-axis parabolic mirrors), you could get a huge area of the sky if you can correct for the coma - perhaps by stacking Paracorrs? Some of the other angles they offer are longer focal lengths, so could require just one Paracorr, or none if you find it's not too bad of coma. 

One of the advantages would be no more silly arguments about whether a central obstruction is limiting your reflector. And you still get the advantage of a mirror: you don't have to worry about chromatic aberration. Of course, a large disadvantage is how difficult these mirrors are to produce. There's a reason a 4" one is the largest offered. There's a quoted surface roughness of 50-100 Å (turned on my Norwegian keyboard for that character, even though that measurement is named for a Swede), which is very good (and needs to be for laser applications). But I have no idea how one would do standard tests to determine the quality of the mirror without just trying it out. 

Any thoughts? Concerns I haven't addressed? I haven't thought about the mount at all, and I won't, but feel free to suggest good ways to mount such a design. 



#2 D_talley

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Posted 05 June 2020 - 05:55 PM

Sounds like you are talking about the Schiefspiegler telescope design:

 

https://en.wikipedia.../Schiefspiegler



#3 kathyastro

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Posted 05 June 2020 - 06:03 PM

Sounds like you are talking about the Schiefspiegler telescope design:

 

https://en.wikipedia.../Schiefspiegler

I don't think the OP is talking about a Schiefspiegler.  It uses an ordinary symmetrical primary mirror.  I believe he is talking about an asymmetrical  mirror where the rotational axis of the paraboloid is not in the centre of the mirror disk, perhaps not in the disk at all.



#4 siriusandthepup

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Posted 05 June 2020 - 06:19 PM

OP might want to peruse this website for the old standard of off axis telescopes.

 

DGM Optics Off-Axis Newtonian Telescopes

 

I have owned a couple - they work quite well.


Edited by siriusandthepup, 05 June 2020 - 06:21 PM.

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#5 Tangerman

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Posted 05 June 2020 - 06:20 PM

So I've attached two pictures, one from Edmund optics and the other I made to help illustrate what I'm thinking about. 

Attached Thumbnails

  • off-axis with light.png
  • off-axis.PNG


#6 Tangerman

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Posted 05 June 2020 - 06:22 PM

I'm not really thinking of the old standard, I'm wondering if this could be simpler



#7 TOMDEY

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Posted 05 June 2020 - 07:39 PM

Hi, Tangerman; Cool Idea!

 

But the specific 90o folded mirror that you are describing won't work for imaging because it is way too far off-axis, so has essentially no field whatsoever, other than the miniscule region that goes through the parent focal point. If you do the analysis, you will find that out in the field, even slightly... it is massively dominated by asymmetric Zernike Astigmatism, with other nonstigmatic stuff blended in. As a laser collimator, that is of course fine, because... the used field is essentially zero!

 

On the other hand, slow, slightly off-axis paraboloids are wonderful for astronomical use. I use these for unobstructed true binoscope. They are 4-inch F/15 cored from the same parent, and made of ULE (Ultra Low Expansion) material.

 

[I did some work characterizing targets for laser energetics... got a white-paper on that someplace.]    Tom

Attached Thumbnails

  • 53 Tom's Off Axis Parab Binoscope Objective Mirrors.jpg
  • 96 Laser Energetics Targets Metrology Tom Dey.jpg

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#8 Jon Isaacs

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Posted 06 June 2020 - 12:14 AM

I think Tom and Ed hit the nail on the head.  

 

The problem with the OA Newtonians is that they are limited to quite slow focal ratios, greater than F/10.  But they are not free of off-axis aberrations even though they're greater than F/10 because the OA section is actually cut from a much faster mirror. 

 

So, the question becomes, is a OA parabola F/11 Parabola without a CO but with the "coma" of an F/6 or faster parabola preferable to an F/11 parabola with a small CO?

 

The coma free field of an F/11 is 29mm.  Using Mel Bartels diagonal calculator, a 6 inch F/11 with a low profile focuser illuminates a 0.52 degree field, with a 17% CO. 

 

Which would you prefer?

 

Jon


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#9 stargazer193857

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Posted 06 June 2020 - 09:53 AM

So I've attached two pictures, one from Edmund optics and the other I made to help illustrate what I'm thinking about.

Your top picture needs to be flipped about the line y=x.

Does your lab use coma correctors?
The one in your picture would have to be very fast.

Edited by stargazer193857, 06 June 2020 - 09:58 AM.


#10 TOMDEY

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Posted 06 June 2020 - 10:56 AM

Your top picture needs to be flipped about the line y=x.

Does your lab use coma correctors?
The one in your picture would have to be very fast.

Impossibly-fast as in nonexistent. The OAP major aberration is local Zernike Astigmatism, because the reference coordinate system origin has shifted to the center of the circular off-axis pupil. When fishing around for the unique focal point of an OAP, what one does is to drive out the Axial Astigmatism, not the coma. I've got some pictures showing that, from lab work I was doing, using AC Interferometry as the arbiter. I'll (try to) dig those up, if anyone is interested. Also shows my Axis Reference Fixture (ARF) then used to tag that one unique aerial point.  Tom



#11 Tangerman

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Posted 06 June 2020 - 01:46 PM

We do not use coma correctors. A properly collimated light source comes in from the left, and then focuses down to a very small point. We've actually imaged the focus using a very low-power laser, and we're working on new knife-edge measurements to image our THz frequency light (wavelength about 1 mm) at the focus. A properly collimated light source that comes in at exactly 90 degrees does not exhibit coma (at least not the kind we talk about in spectroscopy), and we've achieved a very tight focus.

Of course, in using a telescope, we don't exactly have a collimated source. So there would be massive amounts of coma, and if the OAP isn't made properly, lots of astigmatism too. 

Also, can you tell me why my image needs to be flipped? From the left comes collimated light, and at bottom it's focused down. That's what I do in my lab. If you have experience with this sort of thing, I'm very curious to know.



#12 stargazer193857

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Posted 09 June 2020 - 11:16 AM

If you really want to get rid of coma and spikes, you could get an SCT. Only problems is the narrow field of view.

#13 siriusandthepup

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Posted 09 June 2020 - 12:14 PM

You can talk yourself to death about off axis detrimental aberrations.

 

Everyone seems to want to focus on the "terrible coma" inherent to the design.

 

It doesn't exist.

 

Don't believe me?  You can check it yourself.

 

Take your 12"+ fast Dob (f/4 - f/4.5) and put an off-axis 5" stop on it. Try and find the coma (yea, it's there and it's minuscule).

 

Cutting the little off axis parabola from the parent mirror doesn't carry the magnitude of the primary coma into the viewing experience.

 

You won't ever hear about owners of these off axis parabolas whining about the coma - that's a discussion for strictly non-owners.

 

Get one or make your own "tester" with your fast mirror.

 

Tangerman - angle of incidence = angle of reflection for mirrors. Your drawing looks OK for your top yellow line, but the bottom yellow line is incorrect. The reflection of the bottom line would be very close to the incoming line.

 

Exploring new designs is always fun. waytogo.gif

 

I encourage everyone to become an off axis parabola observing expert. Make your own with your Dob and spend some times observing with the off axis stop in place. Make your own judgement about the view and practical aberrations present. Enjoy the Airy disks!


Edited by siriusandthepup, 09 June 2020 - 12:26 PM.



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