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Fast newtonian with spherical mirror as astrograph?


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Here is interesting idea.

Would you use fast newtonian  - like 6" F/5 with spherical mirror as astrograph for wide field?

I know that there will be some spherical aberration, but how bad will it be if one samples at say 2"/px - resolution that is very fine for wide field work. No messing with coma corrector, not nearly as sensitive to collimation as fast parabolic.

Only remaining issue would be field curvature and I don't think there would be much of it with spherical mirror on 750mm of focal length?

What do you think?

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So this is like a Sky-Watcher MN190 without the corrector plate? It would certainly be a cheap scope to manufacture, I suppose the key issue is how bad would the aberration be?

Dare I say it, but software is now available that does a reasonable job in correcting star shapes, especially if these are only on the periphery of the FOV.

There has been a recent thread on here w.rt. how critical the corrector plate is on a RASA, but that’s a F2 system.

I have a very old F4 8” Schmidt Newtonian but unfortunately it would need quite a bit of work to try out the idea (the glued on secondary fell off and chipped the primary…)

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I just came to that idea because someone on our local forum made an inquiry about Omegon 150/750 on "EQ4" mount - it is this scope:

https://www.omegon.eu/telescopes/omegon-telescope-n-150-750-eq-4/p,22465

I did a quick search online and found several sources that claim it is fast spherical mirror.

Omegon does not state it is parabolic mirror - something they do for their other newtonian scopes - like this one:

https://www.omegon.eu/telescopes/omegon-advanced-telescope-150-750-eq-320/p,61021

image.png.c985bbc0e26f55b287db47af37419466.png

And according to this resource - it is most likely fast spherical mirror:

https://star-hunter.ru/en/black-list/

In any case - it seems that 6" scopes with fast spherical mirrors are available - there is one from TS as well (looks very much like that Omegon one on EQ3 mount):

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5546_TS-Optics-Beginner-Telescope-150-750-mm-with-mount-EQ3-1.html

That got me thinking about performance of such scope.

According to https://www.telescope-optics.net/reflecting.htm

P-V spherical aberration is given by this term:

eq225n.PNG

That calculates to 1.0656 waves of P-V error, so quite a bit of spherical aberration - but I can't tell how much that is on MTF diagram until I run some calculations / simulations.

My gut feeling tells me that it won't be as bad when we take into account seeing and tracking errors. Maybe it will be possible to shoot 2"/px images with such scope.

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It looks like effect might not be that significant as long as Airy disk is significantly smaller than seeing induced blur.

This shows 0.75 waves of spherical:

sa075.PNG

with different focus positions. I think that from graphs we can approximate resolution of the telescope alone as being 1/2 - 1/3 of aperture, so about 70mm scope. Maybe 2"/px would be a bit far fetched, but 2.5"-3"/px certainly possible with spherical mirror without corrector.

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Telescope-Optics.net page 10.2.2.1 gives minimum blur diameter for a lensless Schmidt as D/(128f^2) or 47 microns in the case of a 150mm f5 so pretty bad. 

Interesting to see how it turned out but spherical mirrors are said to be disappointing unless small or slow.  E.g 150mm f6.8 giving reasonable 25 micron star images.

David

Edited by davidc135
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11 hours ago, davidc135 said:

Telescope-Optics.net page 10.2.2.1 gives minimum blur diameter for a lensless Schmidt as D/(128f^2) or 47 microns in the case of a 150mm f5 so pretty bad. 

Interesting to see how it turned out but spherical mirrors are said to be disappointing unless small or slow.  E.g 150mm f6.8 giving reasonable 25 micron star images.

David

Same page gives formula that I was using (although I found it on page about spherical aberration of one mirror systems) which is:

W = 0.89 * D / F^3

(See above post - actual formula used was 0.888 * D / F^3).

This formula results in one wave of spherical aberration. I'm not sure what blur diameter represents but 47 microns does seem too much (blur resulting from spherical aberration does not have definitive diameter).

It does however go on to say this about 150mm mirror:

Quote

Doubling the wavefront error by further reducing the F number by a factor of 0.51/3, to f/5.4, results in the 84% energy circle over six times the Airy disc diameter, with less than 10% still contained within the Airy disc. Performance of such system is comparable to an f/21.5 system speed-wise on stars and small extended objects. Its first MTF resolution limit is reduced to about 1/5 of that in a perfect aperture (which effectively makes it only ~20mm in aperture in this respect), with some faint resolution windows possible at higher resolution levels. Needles to say, with nearly 1 wave P-V of spherical aberration, the contrast level is very poor; it is comparable to that in a system with 0.83D central obstruction.

Estimate that they give is that it acts as 1/5th of perfect aperture rather than 1/2-1/3rd (although 150 / 5 is 30 and not 20mm).

I do need to run actual tests to see impact when combined with average seeing.

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I seem to remember from long ago  that a design for a photographic scope consisted of a spherical mirror with a sub-aperture mask placed at the centre of curvature. IIRC the mirror was 8" (200mm) dia and the sub-aperture mask was 4" (100mm). This, of course, results in a slower effective system ( so less SA ) than the unobstructed main mirror but gives a much larger field of view. I can't remember the name of it now.

Anyway, an idea to play around with.

Nigel

Edited by Astrobits
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Aberrator to the rescue!

image.png.7cebc638b3ef5b4251cdeca067ca38b7.png

Right one is 150mm F/5 system with 34% CO, while right left one is the same system with 1 wave of spherical aberration and -1 wave of defocus (to balance spherical).

Not as drastic difference as would seem from above equations, but still quite significant when compared to 2" FWHM seeing blur:

image.png.fd32d20a022c7ca6d0898caf32acc64d.png

I guess it is not as feasible as I expected - too much blurring.

Maybe only feasible for very low resolution work like 4"/px and below (instead of 50-200mm camera lens - but one would need to bin pixels and use mosaics).

Edited by vlaiv
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I think that by the time all the parts of the 150mm f5 or any lensless Schmidt astrograph setup are bought and put together you might as well have spent a bit more on a parabola or aspherised a window to make a Schmidt camera or SN. Field curvature and (where it occurs) coma seem modest at f5.

David

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17 minutes ago, davidc135 said:

I think that by the time all the parts of the 150mm f5 or any lensless Schmidt astrograph setup are bought and put together you might as well have spent a bit more on a parabola or aspherised a window to make a Schmidt camera or SN. Field curvature and (where it occurs) coma seem modest at f5.

David

Once you have a fast spherical mirror your work is about halfway to a parabolised surface, hence a fast parabola would likely be about twice the cost of a spherical one. 

Figuring an aspherical corrector plate would be another ball-park probably doubling the costs again. The corrector plate needs both surfaces to be polished, Either 1/2 the correction on each side or full correction on one side and optically flat on the other. Float glass does not have a good enough surface without attention. I have tried to find bits of float to use as flats for Newtonians and had no success.

Nigel

 

 

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33 minutes ago, Astrobits said:

Once you have a fast spherical mirror your work is about halfway to a parabolised surface, hence a fast parabola would likely be about twice the cost of a spherical one. 

Figuring an aspherical corrector plate would be another ball-park probably doubling the costs again. The corrector plate needs both surfaces to be polished, Either 1/2 the correction on each side or full correction on one side and optically flat on the other. Float glass does not have a good enough surface without attention. I have tried to find bits of float to use as flats for Newtonians and had no success.

Nigel

 

 

Yes, working and figuring the aspheric corrector would be of interest to an ATM whose set up for it but hardly an option otherwise. I'd have thought most would prefer moderately priced corrected mirrors to either blurry images or slow systems.

If reasonably priced 6'' f5 paraboloids are available, that is.

David

Edited by davidc135
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2 hours ago, davidc135 said:

At £259 from FLO the TS-Photon 6'' f5 OTA claims to have a paraboloid mirror.  David

Yes, almost all 6" F/5 scopes are in fact with parabolic mirrors - but there are a few exceptions.

Interestingly - one can get 6" F/5 spherical mirror of AliExpress for something like 25e :D - so it might not be nearly as expensive for a test project?

I wonder if simple sub aperture corrector / reducer can be made to reduce spherical aberration to acceptable levels for something like EEVA?

Will have to play around with some optical design software to see what I can come up with that can be cheaply ordered online (simple lens and mirror combinations).

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1 hour ago, vlaiv said:

Yes, almost all 6" F/5 scopes are in fact with parabolic mirrors - but there are a few exceptions.

Interestingly - one can get 6" F/5 spherical mirror of AliExpress for something like 25e :D - so it might not be nearly as expensive for a test project?

I wonder if simple sub aperture corrector / reducer can be made to reduce spherical aberration to acceptable levels for something like EEVA?

Will have to play around with some optical design software to see what I can come up with that can be cheaply ordered online (simple lens and mirror combinations).

Fun project.

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I think that if anyone is going to try this they will need to grind their own mirror or commission one from a reputable source. A finished 6" for 25euro from China is unlikely to be of sufficient quality judging by my experience ( admittedly 25+ years ago ) of Chinese mirrors in that bracket. I do not know if we could even purchase a mirror blank for that price and the coating alone would also exceed 25euros.

Nigel

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13 minutes ago, Astrobits said:

A finished 6" for 25euro from China is unlikely to be of sufficient quality judging by my experience ( admittedly 25+ years ago ) of Chinese mirrors in that bracket.

You think it will suffer from spherical aberration? :D

I don't know, I think all of those mirrors on AliExpress come from the same factory / factories as those mirrors in cheap newtonians.

If whole scope with EQ3 mount (not really EQ3 they just like calling it that):

https://www.teleskop-express.de/shop/product_info.php/info/p5546_TS-Optics-Beginner-Telescope-150-750-mm-with-mount-EQ3-1.html

can cost 235e prior to tax - that is with shipping from China, tube, mount, counterweights, finder, eyepieces and of course profit margin of TS - I don't see why it could not cost 25e for mirror alone.

We can see a lot of small refractors in same price bracket or even cheaper - and sure enough you can purchase achromatic pair (even select blue or green coating for small price premium) in 20-30euro range.

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25+ years ago a local importer and retailer of astro goods ( no longer in business) arranged with the Chinese manufacturers to receive a batch of telescopes with the mirrors to be uncoated and packaged separately for final finishing here ( by me).  Of the 25 5" mirrors I received I had to discard one for a large crack. One I accepted as near enough parabolic ( they were supposed to be spherical ). The remaining 23 needed various amounts of correction to smooth out severe zones and grinding marks that had not been properly polished out. None were what I would call optically spherical. Thus on top of the CA from a good spherical mirror there would have been all sorts of image artifacts from these gross defects.

The mirrors at 25euro might be better than they were those years ago or they might be suitable for shaving. At 25 euro it will be cheaper than buying the blank here to grind your own so why not give them a go.

A locally sourced 6"mirror kit here in the U.K. is priced at £122. and you still have to get the finished mirror coated... more £££££

Nigel

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I think spherical mirrors, as an optical shape are too easily dismissed.  If not for the fact that a sphere, on a Foucault test presents a null result to a knife edge rather than the harder to judge parabolic shadows, it would be almost as difficult to produce a high quality sphere.  At a suitable focal ratio, I'd sooner have an excellent spherical mirror than a poor parabolic one.    🙂  

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