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Lensless Schmidt Camera for Astrophotography

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#26 marcosbaun

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Posted 28 January 2020 - 05:57 AM

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

 

attachicon.gifHonders_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

Very interesting !
First I will study to be able to evaluate the technical aspects.
A first analysis I had an excellent impression.
Thank you very much.



#27 MKV

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Posted 28 January 2020 - 01:10 PM

You're welcome. Just FYI, the circles in the Spot Diagram section represent the Airy disc. So, as you can see, all the light is geometrically calculated to fall inside that area over the entire 3-degree FOV. The other one, the calculated interferogram shows PV wavefront error of 1/66 wave correction.

 

 



#28 luxo II

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Posted 30 January 2020 - 05:01 AM

And one who did, from go to woah and some initial images

http://www.iceinspac...ad.php?t=151040


Edited by luxo II, 30 January 2020 - 05:03 AM.


#29 bridgman

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Posted 30 January 2020 - 05:35 AM

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.  

Hey, I resemble that remark. Started an (IIRC) 8" f/2 lensless Schmidt camera back in the early 70's and only gave up on it because I could no longer find the (ground but not polished) mirror blank. Seemed like a good idea at the time but the curved film holder turned out to be a blocker for me. In hindsight I probably could have made one out of wood, but that didn't occur to me at the time.

 

Fortunately the Celestron FASTAR came along and gave me the option of pretending it all never happened :D



#30 MKV

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Posted 30 January 2020 - 09:23 AM

Seemed like a good idea at the time but the curved film holder turned out to be a blocker for me. In hindsight I probably could have made one out of wood, but that didn't occur to me at the time.

No need for a curved film holder. A flat one was fine with a coated PCX lens of correct radius of curvature (readily available stock item). This field flattener lens would be placed in contact with the film or a fraction of a mm in front of it; the flat side facing the film. Antirefleciton coating is needed to minimize/eliminate ghost images. The radius of curvature needed to flatten the image field = f(n-1/n), where n is the refractive index of the lens , and f is the focal length of the Schmift camera mirror.

 

Here's an example: 100 mm f/6 lensless Schmidt camera with a lens field flattener.

 

Diameter of primary 154 mm f/4 sphere.

Aperture stop diameter 100 (f/6)

f = 600 mm

n = 1.5168 (Schott N-Bk7 crown)

Field flattener's curved surface radius of curvature = 600 (0.5168/1.5168) = 204.4 mm, round it off to 200 mm.

 

Here's a raytrace analysis.

 

4IN F6 lNSLS sCHM cAM W FFL LNS.png

 

Notice the images remain in focus over a wide flat field.


Edited by MKV, 30 January 2020 - 09:25 AM.


#31 TOMDEY

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Posted 30 January 2020 - 11:07 AM

Hey, I resemble that remark. Started an (IIRC) 8" f/2 lensless Schmidt camera back in the early 70's and only gave up on it because I could no longer find the (ground but not polished) mirror blank. Seemed like a good idea at the time but the curved film holder turned out to be a blocker for me. In hindsight I probably could have made one out of wood, but that didn't occur to me at the time.

 

Fortunately the Celestron FASTAR came along and gave me the option of pretending it all never happened laugh.gif

If I ever seriously go back into imagery, that FASTAR has me intrigued... turn-key package... buy it and use it. But... so many people are primarily doing imagery now (more than half of us here?) that I want to keep on paths less traveled by, which makes all the difference. My current obsession is increasingly becomig... Night Vision...   Tom

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#32 bridgman

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Posted 30 January 2020 - 03:15 PM

No need for a curved film holder. A flat one was fine with a coated PCX lens of correct radius of curvature (readily available stock item). This field flattener lens would be placed in contact with the film or a fraction of a mm in front of it; the flat side facing the film. Antirefleciton coating is needed to minimize/eliminate ghost images. The radius of curvature needed to flatten the image field = f(n-1/n), where n is the refractive index of the lens , and f is the focal length of the Schmift camera mirror.

 

...

 

Notice the images remain in focus over a wide flat field.

Well that's pretty neat - thanks !

 

If I ever seriously go back into imagery, that FASTAR has me intrigued... turn-key package... buy it and use it. But... so many people are primarily doing imagery now (more than half of us here?) that I want to keep on paths less traveled by, which makes all the difference. My current obsession is increasingly becomig... Night Vision...   Tom

If it helps, back when I purchased the rig (2001-ish IIRC) it was definitely the path less taken and everyone said it was a dumb idea. I just remind myself of that whenever I'm feeling a bit too mainstream smile.gif

 

The original FASTAR rig needed a fairly small camera which in turn limited resolution, and cameras back then were pretty slow in terms of download speed. I still have the ST-237A and am trying to rustle up a PC sufficiently old to run it, but as far as I can see it should be possible to run a newer/faster camera with much higher resolution in the same small sensor area. I picked up a ZWO 290MC as a gateway camera to try it out, but haven't had time yet.

 

Night vision is really interesting - I had a chance to view through a Gen3 device with a narrow-band (7nm ?) Ha filter... even with the filter in place images were spectacularly bright compared to naked eye. This was with a very small objective (maybe 2" ?) and hand-held but still impressive. I guess one could be a buzzkill and say that "it's just EAA without all the wires" but it really does seem like an unexplored field.

 

The ability to compensate for significant filtering seems like it could really be helpful now that light pollution is taking over the world. I have visions of big neon signs and searchlights being installed at dark sky sites so that people can find them more easily :(

 

I wonder if anyone has tried using two NV devices with different narrow-band filters on the front (eg Ha and O-III) and different phosphors on the back (reddish and greeny-blueish) then combined the outputs somehow to get a colour image in real time ?


Edited by bridgman, 30 January 2020 - 03:20 PM.


#33 TOMDEY

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Posted 30 January 2020 - 03:41 PM

Well that's pretty neat - thanks !

 

If it helps, back when I purchased the rig (2001-ish IIRC) it was definitely the path less taken and everyone said it was a dumb idea. I just remind myself of that whenever I'm feeling a bit too mainstream smile.gif

 

The original FASTAR rig needed a fairly small camera which in turn limited resolution, and cameras back then were pretty slow in terms of download speed. I still have the ST-237A and am trying to rustle up a PC sufficiently old to run it, but as far as I can see it should be possible to run a newer/faster camera with much higher resolution in the same small sensor area. I picked up a ZWO 290MC as a gateway camera to try it out, but haven't had time yet.

 

Night vision is really interesting - I had a chance to view through a Gen3 device with a narrow-band (7nm ?) Ha filter... even with the filter in place images were spectacularly bright compared to naked eye. This was with a very small objective (maybe 2" ?) and hand-held but still impressive. I guess one could be a buzzkill and say that "it's just EAA without all the wires" but it really does seem like an unexplored field.

 

The ability to compensate for significant filtering seems like it could really be helpful now that light pollution is taking over the world. I have visions of big neon signs and searchlights being installed at dark sky sites so that people can find them more easily frown.gif

 

I wonder if anyone has tried using two NV devices with different narrow-band filters on the front (eg Ha and O-III) and different phosphors on the back (reddish and greeny-blueish) then combined the outputs somehow to get a colour image in real time ?

Most of my Night Vision is true binocular: 1x, 3x, and 16-inch true binoscope. Some here have tried two different filters. And, at work (ITT Night Vision) one of our Premium Field-Use devices comprised both ~Regular~ Night Vision and Thermal into the same view. The NV displayed as white and the Thermal as Orange... both top quality and perfectly balanced and superposed, great resolution and contrast, very low noise. My recollection is that the low-list (to quantity certified customers) was a bargain at --- it was tens of thousands $$$ for a cute little helmet-mounted monoscope! I compared it side-by-side with my (old) nifty Collins Electro-Optics Select Gen 3 NV and ... no comparison ... sigh... The best is pretty amazing stuff.    Tom

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  • 78 COLLINS VISUAL CONFIG TOM.jpg
  • 79 toms old chart Collins NV device.jpg

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#34 ed_turco

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Posted 13 February 2020 - 10:59 AM

It must be remembered that lensless Schmidts were suggested in the era of Film.  Film couldn't resolve the bloated star images, being on the order olf 10 microns or so.   Try that with a ccd today.

 

Does anyone remember the Monty Python skit where "film" was mentioned and it was funny?

 

ed



#35 Todd N

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Posted 13 February 2020 - 10:29 PM

I contemplated building a lensless schmidt camera for several years but came to the conclusion it wasn't worth it which I nevertheless found disappointing since I found the project compelling because I have a particular interest in film astrophotography. An achromat refractor with a field-flattener and some filtration would produce far better images and I have played around with that. There is no point in employing such a bulky system like the LSC with it's spherical aberration where a telephoto lens of smaller focal length would produce superior images. Here is a link to Gavin & Fawdon's paper on a LSC if your interested:

http://adsabs.harvar...JBAA...99..292F

I contacted the late Maurice Gavin(Nyecam on CN) some years ago about this but he didn't have any of his LSC images on hand. He also converted a cheap Jones-Bird newt into a a small CCD imaging LSC.

https://www.cloudyni...camera-updated/



#36 coinboy1

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Posted 14 February 2020 - 03:24 PM

I experimented building a lensless schmidt from a broken C14 several years ago. The concept definitely works as my results show:

 

https://www.cloudyni...ject/?p=6685730




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