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Lensless Schmidt Camera for Astrophotography

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#1 marcosbaun

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Posted 26 January 2020 - 01:55 PM

I found this interesting article about Schmidt camera without a correction plate.
I was interested in making this camera using a 12 inch mirror
Question: Has anyone done this camera, and can say something about its performance?

https://archive.org/...ge/n47/mode/2up



#2 MKV

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Posted 26 January 2020 - 02:23 PM

It's all pretty much summarized in your reference. For a reasonable setup, you'll need at least an f/8-f/9 sphere with an aperture stop placed at between 5 and 5.5 meters (almost 16 feet in front of the mirror). That will make a very long and cumbersome instrument. 


Edited by MKV, 26 January 2020 - 06:51 PM.

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#3 markb

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Posted 26 January 2020 - 02:32 PM

Oh no, not the darn internet archive copy of Sky & telescope!

 

Every time I do this I can never remember which one is sharp enough to read. And I spent over two hours downloading and unpacking various formats last time I did this.

 

I just checked the current downloads on this particular issue, and the Comic Book Rar is crystal clear, unlike the PDF.

 

I know many of us are older, but only some of us (old OR young) are comfortable with filetypes etc.

 

You will, unfortunately, need a free  CBR file reader. The latter was designed for showing crisp clear copies of comic books and works well for the illustrations and even text in Sky & telescope. The PDFs are made from massively over compressed jpegs as far as I can tell. It looked like a free RAR (like a ZIP) file decrypted / unpacker might be needed as well, but not here, see below.

 

I have tried several CBR readers, but currently use  Astonishing Comic Reader (couldn't have made that one up if I tried) for CBR files, for Android devices. I'm not sure what I last used for my Windows laptop. Use a google search for ' CBR reader free gnu ' which should pop up a couple of choices that are free to use. The 'gnu' will get you true freeware programs written under a GNU license.

 

I use WinRAR IIRC on the laptop for ZIP and RAR files, but the internet archive page had a direct download of a CBR file, no RAR required.

 

They have all the older sky and telescopes up on the internet archive, an awesome resource. The old ads are blast to read as well.

 

And the CBR version actually shows the ads in all their glory.

 

I'm looking forward to reading the Schmidt  camera article (MKV nailed it), I hope this information helps other people get to it as well.


Edited by markb, 26 January 2020 - 02:35 PM.


#4 Richard O'Neill

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Posted 26 January 2020 - 03:11 PM

In the seventies I bought the Coulter Schmidt camera kit with the matching correcting plate but never got around to assembling the optics. I still have the whole kit and kaboodle packed in it's original shipping cartons. So many projects, so little time. flowerred.gif

 

Richard


Edited by Richard O'Neill, 26 January 2020 - 03:12 PM.


#5 davidc135

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Posted 26 January 2020 - 03:14 PM

In 'Telescope optics' on page 78, Rutten and Venrooij have a short account including a useful graph linking apertures, blur size and minimum f ratios.

They aren't that feasible; either large aperture, slow and huge or faster but with small apertures. Better to parabolise and add a paracorr or make a Schmidt plate.   

 

Why not have a go at a Schmidt plate? Say 10'' to go with a 12'' mirror. I think you could succeed.   David


Edited by davidc135, 26 January 2020 - 03:28 PM.


#6 John Rose

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Posted 26 January 2020 - 03:33 PM

I have one of those kits Richard has. I believe instead of the "matching corrector plate' he means the matching film holder. A lensless schimdt has a curved focus plane. So the film holder needs to the machined to mqtch the curvature of the mirror. Sized for 35mm film which had to be cut and installed. A digital detector will need a corrector of some sort placed before it.

 

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#7 TOMDEY

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Posted 26 January 2020 - 04:09 PM

Yeah, that would be a fun project!

 

The lensless Schmidt is probably the most planned but never executed camera. Next in line is the Houghton, only a few ever made. The curved focal plane make them even more novelty, now that small flat arrays have replaced film.    Tom



#8 marcosbaun

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Posted 26 January 2020 - 06:32 PM

The favorable impression I got from this Lensless camera (due to the analysis the calculations presented in the diagram in this article in Sky Telescopes ), is that the calculation is made from the opening of the diaphragm positioned in the mirror's CoC. As the opening of the obstruction diaphragm is smaller than the mirror diameter, the total length of the camera is more acceptable.
Let's see the camera presented
Diaphragm opening: 6 1/2 inch
Spherical mirror diameter: 8 1/2 inch
Mirror CoC: 63.2 inch
Mirror Fd 3.72
Diaphragm aperture: Fd 4.8
It therefore seems reasonable that with an 8 1/2 inch mirror the total length of the camera is 63.2 inches.



#9 davidc135

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Posted 26 January 2020 - 06:49 PM

I'm getting around 1.25 waves SA wf and image blur over 50 microns.  David



#10 clivemilne

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Posted 26 January 2020 - 07:04 PM

I think the real value of the lensless camera is as a conceptual learning tool.

It should teach you the relationship between the spherical aberration corrector (aspheric) or the aperture stop relative to the primary radius of curvature and how that influences coma.

When they coincide, the system is said to be symmetrical and coma disappears.

As others have noted, a simple Newtonian with a sub aperture corrector is so much more efficient that it makes the Lensless camera completely obsolete in a practical sense.

That's not to say that you shouldn't make one just for the fun of it.

#11 MKV

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Posted 26 January 2020 - 07:29 PM

Yeah, that would be a fun project!

 

The lensless Schmidt is probably the most planned but never executed camera. Next in line is the Houghton, only a few ever made. The curved focal plane make them even more novelty, now that small flat arrays have replaced film.  

optics_lowres_LR.jpg Houghton_1986_cr.jpg houghton_tested_LR.jpg

 

Some of us did. :o) From left to right: optics and tools, completed telescope (6-in f4), k-e and 85 lpi Ronchi tests without corrector, and a Ronchi test with the corretcor added (slightly misaligned)


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#12 MKV

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Posted 26 January 2020 - 07:36 PM

I'm getting around 1.25 waves SA wf and image blur over 50 microns.  David

Spot on. And the image blur will be about 55 microns.



#13 TOMDEY

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Posted 26 January 2020 - 07:42 PM

attachicon.gifoptics_lowres_LR.jpgattachicon.gifHoughton_1986_cr.jpgattachicon.gifhoughton_tested_LR.jpg

 

Some of us did. :o) From left to right: optics and tools, completed telescope (6-in f4), k-e and 85 lpi Ronchi tests without corrector, and a Ronchi test with the corretcor added (slightly misaligned)

Nice! Yeah, the Houghton. I worked on a 3-element corrector symmetric Houghton, all spherical surfaces so the concave and convex could be contact-tested upon each other. Got the glass done fine (all spherical surfaces, of course) and then never built the camera. Still fun, as far as I got. For some reason, lenses feel different (nicer?) than mirrors. I don't know if anyone else experiences that... just psychological, I guess?   Tom



#14 MKV

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Posted 26 January 2020 - 07:48 PM

It should teach you the relationship between the spherical aberration corrector (aspheric) or the aperture stop relative to the primary radius of curvature and how that influences coma.When they coincide, the system is said to be symmetrical and coma disappears.

It's a matter of simple geometry. An aperture stop at the center of curvature of a mirror, forces all incoming ray bundles to encounter the mirror surface squarely, that is -- orthogonally. One can say all incoming light into a Schmidt camera is technically "on-axis." And since coma is an off-axis aberration, it will be zero for all incoming angles of light. is zero There's no astigmatisms either since that, too, is an off-axis aberration. The only aberration remaining is its the spherical aberration and that depends on the size of the clear aperture. The smaller the aperture relative to the ROC the smaller the SA. An aspheric corrector is a zero-power optic and only affects the optical path difference across aperture. This permits larger apertures relative to ROC and increased photographic speed for which such cameras are well known.



#15 marcosbaun

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Posted 26 January 2020 - 07:58 PM

I think the real value of the lensless camera is as a conceptual learning tool.

It should teach you the relationship between the spherical aberration corrector (aspheric) or the aperture stop relative to the primary radius of curvature and how that influences coma.

When they coincide, the system is said to be symmetrical and coma disappears.

As others have noted, a simple Newtonian with a sub aperture corrector is so much more efficient that it makes the Lensless camera completely obsolete in a practical sense.

That's not to say that you shouldn't make one just for the fun of it.

Analyzing for practical sense, I must agree with you.
There is not even much practical sense for example to manufacture a 12 inch F3 spherical mirror and then to manufacture a Schmidt correction plate to correct the spherical aberration of that mirror.
It seems more logical to me to manufacture a 12 inch F3 parabolic mirror and then install a diaphragm. Everything suggests that the result would be the same or better than the camera with a corrector.
Furthermore, removing the diaphragm would have a 12 inch Newtonian F3 RFT
Well, here is a good discussion anyway!



#16 MKV

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Posted 26 January 2020 - 08:06 PM

Nice! Yeah, the Houghton. I worked on a 3-element corrector symmetric Houghton, all spherical surfaces so the concave and convex could be contact-tested upon each other. Got the glass done fine (all spherical surfaces, of course) and then never built the camera. Still fun, as far as I got. For some reason, lenses feel different (nicer?) than mirrors. I don't know if anyone else experiences that... just psychological, I guess? 

No it's not psychological, Tom. Making the lenses was like making four extra Newotnian telescope. I had to figure out and then make necessary tools to enable grinding, polishing and figuring the lenses without creating astigmatism, allowing them to freely rotate, etc. 

 

LENS_ed.jpg

 

Porperly supporting the lenses is the key. In my younger days with all the mirror blanks being full thickness,m astigmatism wasn't a big deal but now with thinner primary blanks it is, so current mirror makers know more about this then I did when making the Houghton corrector lenses. 

 

My 2-element corrector is also symmetrical: R1 = -R3, and R2 = -R4. Grinding them together is possible but not recommended. There's always going to be a small difference in radii of curvature and this is a killer when ti comes to figuring the corrector by contact interference. It's best to grind all lens radii of curvature separately to their exact profile, the figure the concave and match the convex by nulling them. 

 

The Houghton is a lot of work (5 surfaces) and a lot of grinding and polishing and figuring. The other issue is its physical weight. The telescope eights twice as much as an equivalent size Newtonian.



#17 MKV

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Posted 26 January 2020 - 08:12 PM

It seems more logical to me to manufacture a 12 inch F3 parabolic mirror and then install a diaphragm. Everything suggests that the result would be the same or better than the camera with a corrector.
Furthermore, removing the diaphragm would have a 12 inch Newtonian F3 RFT

Except parabolizing an f/3 12-inch mirror is no easy task. 



#18 marcosbaun

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Posted 26 January 2020 - 08:33 PM

Except parabolizing an f/3 12-inch mirror is no easy task. 

Making a good sphere is not as easy as it seems (I suppose, someone who can make a good sphere will be able to do the parabola without any problem) and  also :
Making a Schmidt corrector with precision and that really corrects is also not so simple.
As I said, this is a good discussion



#19 MKV

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Posted 26 January 2020 - 09:03 PM

Making a good sphere is not as easy as it seems (I suppose, someone who can make a good sphere will be able to do the parabola without any problem) and  also :
Making a Schmidt corrector with precision and that really corrects is also not so simple.

I never even mentioned the Schmidt corrector -- especially not for an f/3 mirror! That should be a separate topic.

 

However you're right, making a good sphere is not easy, but making a fast sphere is easier than making a fast paraboloid because for the sphere you have a very sensitive null test which can be done with a small light source and a knife-edge. For a paraboloid you'd need either a large flat for a double pass autocollimation test, or a trustworthy interferometer. 

 

For a good f/3 paraboloid must be no more than 0.01% short of a true paraboloid. 



#20 marcosbaun

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Posted 27 January 2020 - 05:52 AM

Thank you all for the analysis and feedback.
It was clear that for medium and large mirrors it is necessary to build a corrective plate for the construction of the Schmidt camera.
In this case I think it is more logical to do the parabola in the mirror.
My option will then be a 12 inch F3 mirror.



#21 davidc135

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Posted 27 January 2020 - 01:33 PM

A 12'' parabola is probably the way to go but, for fun a comparison between it and the Schmidt alternative, which belongs here just as much.

 

There's no comparison between figuring a 12'' F/3 parabola and a sphere. My 10'' F/2 sphere seemed to be pretty well automatically produced.

 

One approach would be to buy a Celestron 14 corrector plate for 1200 eu (Toscanoptics) or much less if second hand and stop it down to 10ins. It will correct a sphere of 30ins f.l(approx.).  A field flattener would also be needed.

A wide field large paracorr for the Newtonian would cost over 1000 USD, much less for the smaller version.

The Schmidt would be twice as long as the Newtonian.

 

So, overall, it's the Newtonian!

 

David


Edited by davidc135, 27 January 2020 - 01:45 PM.


#22 MKV

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Posted 27 January 2020 - 04:51 PM

If you're looking for a 10 to 12 inch f/3 or thereabouts system that is fully corrected and has a flat field, which can also be made by an experienced mirror maker, I suggest a Busack or Honders-Riccardi type dialytes. Here's an example of one

 

Honders_Riccardi 300 f3_6.ntitled.png

 

All surfaces are spherical so the configuration can be readily scaled, the glass substrate used for the corrector, the primary Mangin mirror and the field flattener lens is the affordable N-BK7 (Schott) or BSC7 (Hoya), S-BSL7 (Ohara) or similar crown. The system has a flat, diffraction-limited, Schmidt-camera-like wide photographic field of view of 3, and is essentially color-free, better even than much slower APO objectives.The absence of aspheric surfaces greatly eases production and testing.

 

The convenience of being able to bring the focus outside the tube is a great advantage over the classic Schmidt camera. The added bonus is that such systems are visually excellent as well, save for the large central obstruction, which is unavoidable in very fast configurations.

 

Notice also that the corrector is almost a plano-convex lens. In fact, it can easily be designed as a pcx lens if needed. This is by far much easier to make than a Schmidt corrector. The field flattener ubaperture lens could even be an off-the shelf stock lens of commercial quality. What this lens does is flatten the field by a judicious redistribution of astigmatism as seen in the Wright telescope and the Linfoot camera configuration of the late 1930's and 1940's. http://adsabs.harvar...MNRAS.109..535L

 

A similarly well corrected all-spherical  configuration can also be designed in a Cassegrain mode. Officina Stellare in Italy and AstroPhysics in the US have offered (or still offer) such instruments in sizes of 300 mm (12-inch) aperture operating at or near f/3 to f/3.5 photographic aperture. No Newtonian+Parracor will ever rival these configurations.

 

Unfortunately these solutions remain relatively unknown in the amateur world. Be the first to break the mold, and make one. You will surely get a Stellafane award. :o)

 

Mladen


Edited by MKV, 27 January 2020 - 04:53 PM.

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#23 davidc135

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Posted 27 January 2020 - 06:19 PM

It's certainly a great design for anyone up for the challenge. Higher performance versus a lot more surfaces, weight and cost.

 

David 



#24 MKV

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Posted 27 January 2020 - 07:24 PM

A good 12-inch f/3 paraboloid will cost pretty penny, and so will a large Schmidt corrector. Of course, a wide-field Paracorr won't be a bargain. BK7 glass is cheap. You should be able to get two 1.25" thick blanks for less than the price of the Paracorr and the paraboloid. The field flattener can be an off-the-shelf lens if so designed, or it can be cut out of a 3 or 4 inch lens blank. If money and skill is a speed bump, then there is no point in even discussing a 12-inch f/3 system.



#25 davidc135

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Posted 27 January 2020 - 07:59 PM

Well, good luck for whatever the OP decides.  David




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