Telescope design; eliminating mirror-support diffraction

This is a question primarily for those of you who build or use telescopes for amateur (or professional) astronomy, but since it's a magnetics question, I figure there's others on here with motor design experience that might also have information, ideas, etc.

EDIT (added for clarity): the idea itself is actually for huge reflector telescopes (like Hubble, which is about the size of a bus), where things like a prefocusing lens around the secondary mirror as a support is impractical for various reasons, and where a number of them don't even have a tube around them, just an open frame, with more framework or strut(s) to support the secondary mirror.

(the idea might be applied to smaller, portable telescopes, like those many amateur astronomers use, but it is not intended for them because there are other simpler solutions, some of which solve other problems at the same time.)

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In astronomical photography, when imaging bright "point" sources, like stars, quasars, etc., vs galaxies, nebula, and so on, a "cross" or other visual artifact caused by diffraction of light around the secondary mirror supports can distort/alter the image being recorded.

https://en.wikipedia.org/wiki/Diffraction_spike
https://en.wikipedia.org/wiki/File:Comparison_strut_diffraction_spikes.svg

There are ways to deal with the problem after it occurs, and by moving the secondary mirror out of the center of the light path, but they all have their drawbacks or complications. (At least some of them are discused in the wikipedia article linked above.)

One problem with this is the "cross" blocks out other things that may be the actual focus of the image, stuff one would like to see instead of the cross.

Another problem the idea below might solve is that the supports themselves block a bit of the light that could've been used to image with.


The "obvious" way to eliminate the cross is to eliminate the supports that cause the diffraction.

But something must still be used to hold the secondary mirror in place, that holds it completely steady in relation to the primary mirror and the receiving lens or instrumentation.

The only "invisible" thing I can think of is a magnetic field. I'm not sure how well this would work over the distances needed for large scopes; even small amateur scopes of a few inches across still have quite a large gap between the main structure and the secondary mirror.

Permanent magnets could be used on both the main structure and the secondary mirror, but I don't know how practical that would be given the gap size; for large scopes it almost certainly wouldn't be.

Another issue is inertia delaying motion transfer via the field from main structure to the secondary mirror. That could be a problem not only from vibration, but possibly also when fixing on a single object over long periods, as the scope is traversed. (my guess is that is slow enough to not be an issue).

Vibration is probably not much of a problem on larger astronomical scopes, as they likely already have damping mechanisms to prevent it from reaching the main structure in the first place, but small portable amateur scopes definitely have issues with that.


Computer-controlled electromagnets on the main structure, and permanent magnets on the secondary mirror might be able to generate a large enough field, and compensate for vibration/etc. (presuming some sort of sensor system to detect vibrations, secondary mirror position, and compensate for various things, including temperature changes, etc. )



Thoughts?

I'd be super concerned about having a secondary magnet drop down onto the main mirror if it was suspended by just magnetism. It would need some kind of locking mechanism to secure it until everything was set up and ready for use.

Of course, I'm sure you are aware of the various catadioptric designs that use a front lens element to hold the secondary lens. The Maksutov design is pretty popular among astronomers. The front element that holds (or forms in the case of the Maksutov) also acts to correct for spherical aberration created by the main reflecting mirror.

I've used various catadioptric telescops or lenses for digiscoping (shooting ultra-telephoto with small digital cameras). I used this rig for a long time. Before that I used a 90mm Lomo Maksutov.

You would have to do the math but to eliminate arms and have a meaningful aperture between the smaller mirror and its magnetic support would probably require a superconducting electromagnet.

Me and math don't really get along, and I have no idea how to calculate out anything like this.

But...you could be right. I have seen a few old harddisk systems the size of washing machines that used a magnetically-suspended and positioned arm/head mechanism, but the distances between outer an inner coils was only a few to several mm at most. For the kind of distance from outer casing/frame to secondary mirror the scopes I imagined this for, it probably would need a pretty big field.

I've looked around the web and talked to others via a friend and nobody's seen the magnetic thing done, so there is probably a good reason for it (I can't imagine nobody ever thought of it before).

wturber said:

I'd be super concerned about having a secondary magnet drop down onto the main mirror if it was suspended by just magnetism. It would need some kind of locking mechanism to secure it until everything was set up and ready for use.

Click to expand...

Hadn't thought of that, but that would be a serious concern.

Wouldn't be quite as big an issue for a space telescope, though, once it was in it's orbit or station (like a lagrange "point").

Of course, I'm sure you are aware of the various catadioptric designs that use a front lens element to hold the secondary lens. The Maksutov design is pretty popular among astronomers. The front element that holds (or forms in the case of the Maksutov) also acts to correct for spherical aberration created by the main reflecting mirror.

Click to expand...

When I thougth of the magnetic thing I hadn't heard of the above, but have been referred to it by others since then. It seems to be used only for smaller telescopes, because of weight concerns primarily, and difficulty and cost of making large lenses for this purpose.

(I don't actually know very much about telescopes, or astronomy, though I may know more about bits and pieces of both than the average. I got interested in the designs of telescopes a little bit when an acquaintance, Pete Manly, showed off his book about his design(s) some years back, but I had so many other interests and stuff to do then that I never really got into it...nowadays I end up with more nighttime time when I can't sleep or do much of anything useful, so occasionally I point the cheap 'scope I have at things I can see in the city sky and ponder...).

Anyway, I was thinking of the magnetic thing for MUCH larger telescopes--those too large to make such lenses practical, like Hubble-sized ones and the like. I might've missed info, but I couldn't find any that don't have some sort of physical support strut for the secondary mirror, which thus interferes with getting a pure image.

I'm sure there are computerized image-processing capabilities that can "undo" some of this interference, but they can't truly restore the lost information caused by it.

I was hoping there might be a way to remove the interference completely, and improve astronomical imaging for everyone...but I'd guess probably not. Like bicycles, telescopes have been around a while, and it's difficult to find actual improvements that haven't already been tried. (Especially when I don't know enough about how to build such a magnetic suspension system to test one).

Forget magnets.

The obstruction of the secondary mirror creates diffraction all by itself, no spider required.

Look at the Chief design, it's an off axis telescope that is probably the easiest to implement zero diffraction reflecting telescope. The Cheif name is a takeoff on Schiefspiegler or something which is a different style of off axis reflector (without the corrector lens of the Chief).

I have a 12" f8 mirror that I'm thinking of using in a Chief design. It should perform about like a 12" apochromatic refractor except a lot more compact, somewhat portable and needing a lot less mount.

I think about 100 years ago Canada was pretty proud to buy a refractor like that for a national observatory somewhere so if you do it right, you'll have something pretty special, something a small nation could have used not all that long ago.

I appreciate the extra info about the secondary causing diffraction; that's something no one pointed out to me yet.

As my previous post states, though my original post did not (my mistake), the purpose of the idea is for large (huge) astronomical telescopes, not personal ones. Like the Hubble space telescope.

I myself have insufficient use for a telescope to be worth spend any resources on building my own; the cheap 4" reflector I got several years ago at Goodwill is probably about the best I'd ever be able to use, especially here in the city where I can't really see much anyway (despite the desert climate). I don't even see diffraction interference in my scope, cuz it's just used with my eyes, not cameras, so the magnetic thing or any of the other methods wouldn't make any difference to me.

While I'd love to have much better stuff...it's just not worth the time and money (that I don't have anyway) to build something better, and I definitely couldn't buy one--what income I have has to be spent either on the dogs, household needs, or my transportation (presently SB Cruiser) (if I did have more income, there's a lot of other stuff it'd go to before I ever got to telescopes :lol: ). I haven't even gotten to fix up the self-pointing/tracking tripod thing (busted gears) it came with, and am now using a different manual tripod I also got cheap at GW, which has it's own problems but at least functions. Just too many things I want, and not enough to do much of it. :/



Anyway, this idea isn't about me or my telescopes, it was just to help out others that could actually use it.

https://www.surreynanosystems.com/super-black-coatings/vbx-coatings/vantablack-vbx-2

Spray the mirror supports with that stuff, as well as inside the tube walls, and it will help a bunch. If you don't have reflective surfaces, just blocking light there isn't such a big deal, because any single point of the barrel has the whole of the image passing through it.

Smoke said:

Forget magnets.

The obstruction of the secondary mirror creates diffraction all by itself, no spider required.

Click to expand...

Yes, but the diffraction is evenly distributed and manifests itself mainly as lower image contrast.

Smoke said:

Look at the Chief design, it's an off axis telescope that is probably the easiest to implement zero diffraction reflecting telescope.

Click to expand...

You mean no diffraction from a central obstruction. All optics must deal with the diffraction from their entrance pupil.

liveforphysics said:

https://www.surreynanosystems.com/super-black-coatings/vbx-coatings/vantablack-vbx-2

Spray the mirror supports with that stuff, as well as inside the tube walls, and it will help a bunch. If you don't have reflective surfaces, just blocking light there isn't such a big deal, because any single point of the barrel has the whole of the image passing through it.

Click to expand...

The spikes are from diffraction, not reflection. I'm pretty sure that super-black will have no effect. The problem comes from the presence of a physical object in the light path.

Reflection definitely isn't the issue, because this happens to open-frame telescopes as well, so no kind of coating would help (nothing to coat).

Besides, if it was the issue, the large astronomical observatories would most certainly have fixed it one way or another long ago, since it's a relatively easy thing to do.

wturber said:

Smoke said:

Forget magnets.

The obstruction of the secondary mirror creates diffraction all by itself, no spider required.

Click to expand...

Yes, but the diffraction is evenly distributed and manifests itself mainly as lower image contrast.

Smoke said:

Look at the Chief design, it's an off axis telescope that is probably the easiest to implement zero diffraction reflecting telescope.

Click to expand...

You mean no diffraction from a central obstruction. All optics must deal with the diffraction from their entrance pupil.

Click to expand...

Well, yes but if you want to eliminate diffraction spikes, the central obstruction is what you need to get rid of.

There has always been a trade-off of less light gathering with a refractor or more light gathering with diffraction spikes with a reflector telescope. Until optical calculators became advanced enough to design scopes with more than 3-4 elements only a really hard core optical designer would attempt the more complicated designs.

The Chief and a few other designs let you have both but as I said, the Chief is probably the easiest to implement.

Of course if it's big telescopes you are thinking about, pretty much everything has been tried. All of the new improvements are a lot more esoteric than magnets.

Smoke said:

wturber said:

Smoke said:

Forget magnets.

The obstruction of the secondary mirror creates diffraction all by itself, no spider required.

Click to expand...

Yes, but the diffraction is evenly distributed and manifests itself mainly as lower image contrast.

Smoke said:

Look at the Chief design, it's an off axis telescope that is probably the easiest to implement zero diffraction reflecting telescope.

Click to expand...

You mean no diffraction from a central obstruction. All optics must deal with the diffraction from their entrance pupil.

Click to expand...

Well, yes but if you want to eliminate diffraction spikes, the central obstruction is what you need to get rid of.

Click to expand...

No. You need to get rid of the spider. The central obstruction does not cause diffraction spikes. Cassegrain/catadioptric designs have a central obstruction and no diffraction spikes.