Question:
Is a design like this possible? Practical?
I have a very high fidelity paint image here. And on the left is what I believe reflector telescopes do with the light received.
On the right is what I’d see as possibly an alternative solution that will maybe eliminate the risk of donuts from the image? At the expense of reducing the speed of the telescope.
I’m assuming there’s a reason why the solution on the right is not done. It may simply be impossible to get the right mirror geometry to produce this “zig zag” light path folding pattern without distortion. Though I suspect that is not the case.
It may be that for astrophotography, with a fixed single, infinity focal length, the donut patterns are relatively easy to resolve and so it’s not worth the reduction in light capture or complexity of design.
It may be that calibrating and lining everything up would be too complicated.
Not sure but I’m curious what thoughts others have.
📷 image.png
Alternative reflector telescope design to eliminate diffraction patterns
Beats me. Why don’t you build one and find out?
Make something brand new. 😉
Ahkilleux · Mar 3, 2026, 11:54 PM
Question:
Is a design like this possible? Practical?
I have a very high fidelity paint image here. And on the left is what I believe reflector telescopes do with the light received.
On the right is what I’d see as possibly an alternative solution that will maybe eliminate the risk of donuts from the image? At the expense of reducing the speed of the telescope.
I’m assuming there’s a reason why the solution on the right is not done. It may simply be impossible to get the right mirror geometry to produce this “zig zag” light path folding pattern without distortion. Though I suspect that is not the case.
It may be that for astrophotography, with a fixed single, infinity focal length, the donut patterns are relatively easy to resolve and so it’s not worth the reduction in light capture or complexity of design.
It may be that calibrating and lining everything up would be too complicated.
Not sure but I’m curious what thoughts others have.
📷 image.png
You will only see donuts in the image from a reflector when it’s out of focus. You won’t see that when you are in focus. All the stars would be round with a gaussian distribution of intensity. You’re looking for a solution to a problem that doesn’t exist. Now if you’re asking about reducing diffraction spikes, that’s a different thing entirely.
Question:
Is a design like this possible? Practical?
I have a very high fidelity paint image here. And on the left is what I believe reflector telescopes do with the light received.
On the right is what I’d see as possibly an alternative solution that will maybe eliminate the risk of donuts from the image? At the expense of reducing the speed of the telescope.
I’m assuming there’s a reason why the solution on the right is not done. It may simply be impossible to get the right mirror geometry to produce this “zig zag” light path folding pattern without distortion. Though I suspect that is not the case.
It may be that for astrophotography, with a fixed single, infinity focal length, the donut patterns are relatively easy to resolve and so it’s not worth the reduction in light capture or complexity of design.
It may be that calibrating and lining everything up would be too complicated.
Not sure but I’m curious what thoughts others have.
📷 image.png
In a way it has been done, it is called a schiefsplieger or off-axis newtonian. It avoids having an obscuration of the exit pupil therefore avoiding the reduction in contrast at mid-frequencies in a typical newtonian design. Not easy to make at all and even less friendly in setting up and it is quite slow for all practical designs so good for visual but not so good for imaging.
I wanted to say thank you Andrea, that was incredibly helpful and informative. I figured it had to be a thing. Sorry for my delayed reply, I’ve been away for a minute.
YES, this is it. This is what I was envisioning in my mind. This “off axis” or “tilted component” telescope. “TCT”. The image on wikipedia looks exactly like what i had in mind for a simpler version.
https://en.wikipedia.org/wiki/File:Off-axis_optical_telescope_diagram.svg
This is incredibly exciting to me.
Despite the slower speed and practicality drawbacks you mention.
Because in theory, it allows you to compose a large reflector telescope out of smaller more portable components, and add to it over time.
You could build an array of off axis reflectors, and gather as much light as a much larger, single component monolithic, reflector.
Which is very interesting for backyard astro where size and portability matter.
If you need to be more portable, just take 2 or 3 of your modules with you.
But if you have time and want to do a lot of light capture, assemble maybe 10 of these in an array.
EDIT: I suppose, combining them into a multi module unit requires another problem solved, which is how to re-combine the light streams, each which require a different correction to arrive flatly at the focal point, without obscuring the other contributing light paths.
BUT, allegedly even with just a single off axis, you can get sharper images. So perhaps it’s worth building one., even if it’s impossible to combine the flows of multiple units onto a single focal plane.
Though, if there is a way to shape the final reflecting mirrors for each off axis in a ring array, such that you can have multiple adjacent mirrors reflecting light evenly an flatly at a common focal point, then that could be some cool tech. Because it would be modular. You could just slot in modules. Swap them over time if one goes bad , without giving up all of your light capturing capability.
EDIT: Grok had this to say
“use relay optics, mirrors, beam splitters/combiners, or pupil relays to redirect and overlap these beams so they appear (from the perspective of the final focal plane) to originate from a single, larger effective aperture. “
:) It’s possible. Modular, scalable, reflector telescope arrays, that can have far more flexible geometry and space requirements. Though the phase needs to be synced up pretty tightly for sharp images if combining the beams so it’s no small engineering feat. But it can be done and is done, at least for large projects. The question is, can it feasibly be done for consumer grade equipment? Idk But off axis seems exciting either way.
There’s nothing new here as various off axis reflector designs have been around for a long time as Andrea pointed out. The thing is, it’s not as simple or easy solution. Yes, you get rid of the central obstruction but at the cost of higher complexity. For instance, while it is possible to grind, polish and figure an off-axis paraboloid, it’s not easy. Here’s how you do it if you want a 6” off-axis Newtionian:
Take a 15” mirror blank and pre-cut 4, 6” circles with a large diamond cutter. Place the cut circles back into the main blank and fill the gaps with plaster. Now, grind, polish and figure the reassembled 15” blank. Keep in mind that if you want the final 6” scopes to have a focal length of F/6 (36”), the large mirror will have to have the same focal length. That means you are making a 15” F/2.4 mirror, not a trivial task. I believe there was a company that made off-axis Newtonians this way back in the 90’s.
TCT type systems get around some of these shortcomings but have issues of their own. That’s why, even back in the hayday of amateur telescope making they were niche curiosity.
Going back to the original example. Which telescope do you think would give better performance. The single 15” F/2.4 Newtonian or 4, 6” F/6 Newtonians? The original 15” would gather more light and give higher resolution at a lower cost and much lower complexity. I also believe that the 6” scopes would have the same degree of field curvature and coma that the original F/2.4 mirror had.
The only approach that makes sense, which I believe as already been mentioned in this thread is to combine a number of small refractors on a single mount. People do this and there are gains to be had but it’s not as efficient as a single larger aperture. If you think you’re going to simply combine the light from all the separate apertures together, that’s not going to work without having a way to bring them all into phase. That means controlling spacing of components to within a fraction of the wavelength you’re working with.
It seems there was a really cool off axis guy who was making “DGM Optics” off axis telescopes. But it looks like business is more or less defunct. He may have an astro filter site up now but I don’t think he’s doing scopes anymore.
I’m sold on the design. I need one. In carbon fiber ideally. Something I could possibly put on a heavy duty equatorial. It seems I’m going to have to make one. But if anyone knows a place I can get one built, lmk.
Alternatively if anyone knows a good place to get some fast mirrors that I could use to build one, lmk.
“Going back to the original example. Which telescope do you think would give better performance. The single 15” F/2.4 Newtonian or 4, 6” F/6 Newtonians? The original 15” would gather more light and give higher resolution at a lower cost and much lower complexity.“
So, this is interesting actually.
It would take 6-7 ( 6 is a bit dimmer, 7 a bit brighter) 6 inch reflectors to match the light gathering capability of a 15 inch.
But they offer you much greater flexibility in terms of the space and weight carrying capability you need to deploy them.
Interestingly if you can beam combine these, you can actually get greater angular resolving resolution with just two 6 inch newts than a single 15 inch.
So I believe that means you’ll actually have the potential for a more detailed image, with 6 newts than you would with a 15 inch.
Also it may be easier to get 6 high quality 6 inch mirrors than one high quality 15 inch mirror.
EDIT: Price may be a consideration. With refractors fore example, price scaling is not linear. Meaning it’s probably far cheaper to buy six 6” refractors than a single 15” refractor of the same quality. If you can beam combine those, there may be significant price benefits to deploying an array. Reflectors don’t have nearly the same scaling penalty, but there may still be some price benefit to using an array.
Ahkilleux · Apr 6, 2026, 07:44 PM
It seems there was a really cool off axis guy who was making “DGM Optics” off axis telescopes. But it looks like business is more or less defunct. He may have an astro filter site up now but I don’t think he’s doing scopes anymore.
I’m sold on the design. I need one. In carbon fiber ideally. Something I could possibly put on a heavy duty equatorial. It seems I’m going to have to make one. But if anyone knows a place I can get one built, lmk.
Alternatively if anyone knows a good place to get some fast mirrors that I could use to build one, lmk.
That’s the company I was talking about in my post.
Ahkilleux · Apr 6, 2026, 08:06 PM
“Going back to the original example. Which telescope do you think would give better performance. The single 15” F/2.4 Newtonian or 4, 6” F/6 Newtonians? The original 15” would gather more light and give higher resolution at a lower cost and much lower complexity.“
So, this is interesting actually.
It would take 6-7 ( 6 is a bit dimmer, 7 a bit brighter) 6 inch reflectors to match the light gathering capability of a 15 inch.
But they offer you much greater flexibility in terms of the space and weight carrying capability you need to deploy them.
Interestingly if you can beam combine these, you can actually get greater angular resolving resolution with just two 6 inch newts than a single 15 inch.
So I believe that means you’ll actually have the potential for a more detailed image, with 6 newts than you would with a 15 inch.
Also it may be easier to get 6 high quality 6 inch mirrors than one high quality 15 inch mirror.
EDIT: Price may be a consideration. With refractors fore example, price scaling is not linear. Meaning it’s probably far cheaper to buy six 6” refractors than a single 15” refractor of the same quality. If you can beam combine those, there may be significant price benefits to deploying an array. Reflectors don’t have nearly the same scaling penalty, but there may still be some price benefit to using an array.
And that’s the whole problem and it’s not trivial:
https://taas.org/wp-content/uploads/2024/07/Optical-Interferometer-Telescopes.pdf
andrea tasselli · Mar 5, 2026 at 11:29 AM
In a way it has been done, it is called a schiefsplieger or off-axis newtonian. It avoids having an obscuration of the exit pupil therefore avoiding the reduction in contrast at mid-frequencies in a typical newtonian design. Not easy to make at all and even less friendly in setting up and it is quite slow for all practical designs so good for visual but not so good for imaging.
Is that the same as a Kutter telescope?
“Is that the same as a Kutter telescope?“
Yes, that’s what i’m reading. A Kutter is a subtype or implementation of the schiefsplieger design. Also a “Yolo” I think it’s called.
You don’t need off-axis elements to make an unobstructed telescope. Ed Jones designed what he called the ChiefSpiegler (or just the “Chief” for short) telescope using a simple parabolic primary combined with off the self components. You can read more about it here:
https://opticaleds.com/unusual-telescope-designs/chiefs/
The advantages of an unobstructed telescope over an obstructed telescope are relatively minor. A lot of folks go to great lengths to make unobstructed telescopes that have an awkward configuration and added optical complexity for very modest gains in image quality. In my view, improving optical quality, reducing scattered light and eliminating strays might go further to improve image quality than worrying about the effects caused by an obstruction in the pupil.
John