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Lensless Schmidt camera..

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#1 nytecam

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Posted 24 January 2007 - 04:54 AM

Hi - I moved this item from the Equipment Forum for advise. I'd like to use a freebie shortie Meade Newt [advised as spherical f/4 primary] for DSLR camera [23x15mm format] at Newt focus by removing the Barlow/corrector lens [=f/8.8], shifting the primary mirror forward to reach focus and adding an aperture stop at mirror radius. The existing secondary is 40mm diameter. Any advise :question: TIA

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#2 theoUK

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Posted 24 January 2007 - 08:15 AM

Hi,

I think that if you remove the corrector, the coma and aberations on that f4 spherical primary will be very bad. Worth a try though, if its free!

Theo

#3 DAVIDG

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Posted 24 January 2007 - 08:29 AM

Ntyecam,
I did this with old Coulter Optical lensless Schmidt kit from the 70's. It uses an 8" f/3 spherical mirror to make a 5.5" lensless Schimdt. I used it with a webcam. With the larger CCD in your DSLR, there are two things to consider, the field curvature and the size of an image of the star it produces. In my case the pixel size is 10 microns and star images are slightly smaller then that. I'm also using a small CCD so the field curature over the area of the CCD is small and the image is at focus across the CCD. There is simple way to flatten the field so you can use it with larger CCD's One just uses a plano convex lens. The FL of the lens is given in "Telescope Optics" in pages that talk about the lensless Schmidt, I believe it's equal to the FL of the mirror.

- Dave

#4 jgraham

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Posted 24 January 2007 - 08:42 AM

The idea of a lensless Schmidt is to limit the off-axis coma by using an aperture stop at the radius of curvature of the mirror to limit the off-axis rays; different parts of the image plane use different parts of the mirror. This is the same principal used in short focal length SLR lenses where the iris is located within the lens stack. This particular design might be fun to experiment with. Another option to consider is using an Orion StarBlast. The StarBlast has a 4.5" f/4 parabolic primary and with the tiny type 1/3 CCD array used in the Meade DSI there's no noticeable coma.

Have fun!

-John

#5 wh46gs

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Posted 24 January 2007 - 08:42 AM

The only aberration would be spherical. The wavefront error is given by w=0.89Dmm/F^3 in units of the 550nm wavelength, which means you'd have 1/4 wave PV error at 72mm diameter. For photographic purposes, it can be more than that, but you probably don't want to go over 1/2 wave (86mm diameter).

Vlad

#6 nytecam

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Posted 24 January 2007 - 10:19 AM

snip... Another option to consider is using an Orion StarBlast. The StarBlast has a 4.5" f/4 parabolic primary and with the tiny type 1/3 CCD array used in the Meade DSI there's no noticeable coma. Have fun! -John

Thanks John for the theory :rainbow: I should have stated that as the scope was a freebie the finished product should be, essentially, likewise :smirk:

#7 nytecam

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Posted 24 January 2007 - 10:23 AM

snip...With the larger CCD in your DSLR, there are two things to consider, the field curvature and the size of an image of the star it produces... snip... There is simple way to flatten the field so you can use it with larger CCD's One just uses a plano convex lens. The FL of the lens is given in "Telescope Optics" in pages that talk about the lensless Schmidt, I believe it's equal to the FL of the mirror - Dave

Thanks Dave - I did wonder about field curvature synonymous to the Schmidt camera design and the simple field-flattener tip is great :rainbow: When relocating the primary I'll have to allow for the slight reduction in the overall focal length by a weak positive plano-convex field-flattener. Perhaps it can replace the current negative Barlow/corrector or does it have to be nearer the focal plane - without being inside the DSLR camera housing hopefully :question: Won't a plano-convex, unless in near-contact with focal plane, introduce CA :question:

#8 nytecam

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Posted 24 January 2007 - 10:49 AM

The only aberration would be spherical. The wavefront error is given by w=0.89Dmm/F^3 in units of the 550nm wavelength, which means you'd have 1/4 wave PV error at 72mm diameter. For photographic purposes, it can be more than that, but you probably don't want to go over 1/2 wave (86mm diameter). Vlad

Thanks Vlad for the calcing PV errors - it does seem the effective aperture is getting smaller :bawling: Do your calcs allow for the 40mm obstruction of the secondary where the illuminating field is effectively an [ever reducing] annulus :question:

#9 wh46gs

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Posted 24 January 2007 - 02:37 PM

Do your calcs allow for the 40mm obstruction of the secondary where the illuminating field is effectively an [ever reducing] annulus



The obstruction actually slightly increases encircled energy with large wavefront errors for spherical aberration, but it doesn't make much difference. You can go with the full aperture, which would have the smallest blur ~10% larger than 0.025mm (a lousy raytracing criteria for the maximum acceptable aberration for the photography). But it would be a ~1.5 wave PV system, and would deliver accordingly.

You can try different apertures, and see how it works with what you do. It is as easy as replacing the aperture mask.

Vlad

#10 nytecam

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Posted 24 January 2007 - 04:21 PM

...snip..You can try different apertures, and see how it works with what you do. It is as easy as replacing the aperture mask. Vlad

Indeed any aperture can be tested with minimal effort :jump: Here's one we did earlier

#11 CNposter

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Posted 24 January 2007 - 11:34 PM

Is this a potential use for the 16" Meade "freespace" mirrors that show up on eBay regularly? Such as 160077985467
right now.

#12 wh46gs

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Posted 25 January 2007 - 12:49 AM

Here's one we did earlier



You lose on resolution and limiting magnitude, but even at 3 waves PV the stars still pretty much look like stars :)

Vlad

#13 jgraham

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Posted 25 January 2007 - 10:42 AM

The fun part is always dealing with the curved focal plane. I visited an 18" Schmidt at Ohio State many yars ago and saw this nifty film holder they had that forced the thin glass film plates into the correct shape.

-John

#14 DAVIDG

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Posted 26 January 2007 - 08:11 PM

Hi Nytecam,
I finally found a few minutes to look up the design for single lens field flattener for your lensless Schmidt.
To find the convex radius of the plano convex lens use
R = r x n-1 / n were r is the film plane radius which is eqaul to the focal length of the camera and n is the refractive index of glass you make the lens from and at the color you wish to correct. If you use BK7 and the index of 1.52237 for blue light, red and green focus pretty well. I calculate that for your 114mm, f/4 mirror the lens would need a radius of 156.46 mm on the convex surface. The flat side is placed as close as possible to the CCD. This information can be found in "Telescope Optics" by Rutten and Venrooji.
There is also a graph in the book that relates focal length and aperture to the blur size. It looks like your system will produce a star image that is about 0.065mm in diameter.

- Dave

#15 nytecam

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Posted 28 January 2007 - 11:27 AM

Thanks Dave for your data :bow: but do you mean stars are 0.065mm diameter as shown circled on attached crop from M45 via ETX-70 with 0.0075mm pixels :bawling: or should there be another nought in there :rainbow: There's not much point proceeding with a LSC if the stars are so large :tonofbricks:

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#16 DAVIDG

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Posted 28 January 2007 - 01:34 PM

Nytecam,
I took a look at the May '74 Sky and Tel article on making a lensless Schmidt camera. It stated that the size of a star is given by d = 0.008 a / N^2 were "a" is the telescope aperture ( stop at the front) and "N" is the focal ratio.
If you invert this you can find the diameter of the aperture stop needed to fit the pixel size in your DSLR. I calculate it would be a very small 15mm !
Back in the days of slow film and long guiding, it was difficult to obtain star images on film smaller then 0.004 inches because of guiding errors, and atmospheric blurring so star images that large were acceptable.
For the project I was planning to use my LSC for I wasn't worried about tight star images but keeping exposures short and I wound bin the image which farther increased the pixle size to better match the star images. A LSC with a webcam or DSLR can be used to do Super Nova search and varibale star work.

- Dave

#17 nytecam

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Posted 28 January 2007 - 03:19 PM

snip... the size of a star is given by d = 0.008 a / N^2 were "a" is the telescope aperture (stop at the front) and "N" is the focal ratio...snip..the diameter of the aperture stop needed to fit the pixel size in your DSLR I calculate would be a very small 15mm!

Great - the secondary is 40mm diameter so the exposures would be infinite :bawling: :bawling: :bawling: I think I'll forget all the advise and just experiment :grin: :grin: :grin:

#18 refractory

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Posted 28 January 2007 - 05:48 PM

After reading your thread it occurred to me that the flexible electronics being developed now might have application here- you could have a curved CCD and then no need for any correction.

Jess Tauber

#19 nytecam

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Posted 29 January 2007 - 03:46 AM

After reading your thread it occurred to me that the flexible electronics being developed now might have application here- you could have a curved CCD and then no need for any correction. Jess Tauber

Indeed Jess - I'm sure it will come oneday but probably not from Canon or Sony [=Nikon/ Pentax/ Sony SDLR sensors] etc :bawling:

I just collimated the primary and viewed down the garden with 40mm/ 20mm and 9mm eyepieces. The SA is grossly evident - essentially the image refuses to focus and I'm amazed a scope could be marketed that can't produce a sharp view at the lowest power x25. So much for the so-called x2.2 Barlow/corrector!

OK on x25 it's stopped-down slightly for me in daylight [so should be sharper] due to the over-large exit 4.5mm pupil size. By stopping-down with a card before scope from max 114mm to 77mm[=3"] the view much better but not perfect.

#20 DAVIDG

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Posted 29 January 2007 - 10:58 AM

Nytecam,
For what it is worth, I've seen a couple of these type 'scopes were the corrector was install backwards when someone took it apart. This type of design is a Bird/Jones system and in theory they work very well especially in 100 - 125mm aperture range. A friend has a Tasco version and it star tests very well and easily comes to sharp focus. He made the mistake of assembling the correct backwark after cleaning it one time and of course the image was terrible.

- Dave


[
I just collimated the primary and viewed down the garden with 40mm/ 20mm and 9mm eyepieces. The SA is grossly evident - essentially the image refuses to focus and I'm amazed a scope could be marketed that can't produce a sharp view at the lowest power x25. So much for the so-called x2.2 Barlow/corrector!

OK on x25 it's stopped-down slightly for me in daylight [so should be sharper] due to the over-large exit 4.5mm pupil size. By stopping-down with a card before scope from max 114mm to 77mm[=3"] the view much better but not perfect. [/quote]

#21 nytecam

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Posted 29 January 2007 - 12:43 PM

For what it is worth, I've seen a couple of these type 'scopes were the corrector was install backwards when someone took it apart. This type of design is a Bird/Jones system and in theory they work very well especially in 100 - 125mm aperture range. A friend has a Tasco version and it star tests very well and easily comes to sharp focus. He made the mistake of assembling the correct backwark after cleaning it one time and of course the image was terrible. - Dave

Thanks Dave for the info - perhaps I condemed too soon :p I need to devise a tool to unscrew the lens locking sleeve which looks undisturbed and note the corrector lens surface nearest to primary is ~50mm radius and the rear surface ~200mm radius. Does this sound as if the lens is reversed :question:

#22 DAVIDG

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Posted 29 January 2007 - 02:01 PM

Nytecam,
I have a design for 200mm Jones/Bird using a spherical f/4 primary and the resulting 'scope is f/6 In it the radius of the lens facing the primary is 331.66 and the surface is concave and the one facing the diagonal is 178.12mm and also concave. At least in this design the longer radius surfaces faces the primary.

- Dave

#23 nytecam

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Posted 29 January 2007 - 03:34 PM

I have a design for 200mm Jones/Bird using a spherical f/4 primary and the resulting 'scope is f/6 In it the radius of the lens facing the primary is 331.66 and the surface is concave and the one facing the diagonal is 178.12mm and also concave. At least in this design the longer radius surfaces faces the primary. - Dave

Thanks Dave - yes I should have quoted both corrector surfaces are concave and in apparently the same order as your design but mine are stronger radii for f/8.8 final f/ratio. Doesn't look good if I get this poor focus as is :bawling:


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