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Lensless Schmidt camera - updated

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#1 nytecam

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Posted 03 September 2014 - 08:44 AM

I've posted previously to ATM on my LSC - here updated. 

 

Today I made [out of a pizza box!] an 'aperture mask' for my piggybacked Meade 11cm f/4 quasi-Newtonian [with a spherical mirror] and converted it into a LSC.  I reused the ali strut from an earlier incarnation @  http://home.freeuk.c...gavin/lscmg.htm and await the next clear night to test.  I use the new  SX Lodestar-Cx2 OSC camera in brief exposure in near-live display to laptop.

 

The pics below show the set-up plus schematic of the LSC.  The fixing of the aperture mask to the strut is slotted and can be rotated around this axis to provide precise alignment of the 'hole' to the optical axis.

 

I have in the past tested various 'aperture masks' from 70mm to 90mm diameter - in theory the smaller the aperture the sharper the images in minimising spherical aberration eg the primary optical defect.  But this makes a 'slow' system when the central obstruction is taken into account - hence my preferred 90mm aperture and nominal f/ratio of f/5. :grin:

Attached Thumbnails

  • meadelsc.jpg
  • lsc1mask.jpg
  • lsc2.jpg


#2 starcanoe

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Posted 03 September 2014 - 09:26 AM

A lensless Schmidt has a curved focal plane.

 

You can also have a lensless Wright camera (stop at or near the focal plane I believe. You do get some abberations off axis, but not as bad as coma from a naked parabola. But the field is flat.

 

Might want to investigate that too.

 

But cool little project. Gotta love the simplicity and elegance of the idea.



#3 nytecam

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Posted 03 September 2014 - 10:43 AM

A lensless Schmidt has a curved focal plane.

 

You can also have a lensless Wright camera (stop at or near the focal plane I believe. You do get some abberations off axis, but not as bad as coma from a naked parabola. But the field is flat.

 

Might want to investigate that too.

 

But cool little project. Gotta love the simplicity and elegance of the idea.

Thanks - I presume you've not read my linked article which addresses these issues :)


Edited by nytecam, 03 September 2014 - 10:55 AM.


#4 starcanoe

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Posted 03 September 2014 - 10:54 AM

Your resume' shun is correct :)



#5 nytecam

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Posted 03 September 2014 - 11:00 AM

Your resume' shun is correct :)

Sorry - posted wrong address -should read http://home.freeuk.c...gavin/lscmg.htm



#6 nytecam

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Posted 04 September 2014 - 03:56 AM

Last night's tests on the sky proved very successful -  below a M57 montage of raw [pre-colour extraction] of quasi-Newt [left] and LSC [right] shows a marked improvement of star images with minimal coma via the LSC with 90mm aperture mask.  The illumination of each star image at focus is of course an annulus via the central obstruction of the secondary [absent from a true LSC but replaced by the camera!].

 

Also a processed M27 field with hint of tracking error but good overall and moon pic via the LSC.  Might try a larger aperture mask in 95mm - 100mm but only if it keeps coma in check. ;)

Attached Thumbnails

  • m57raw newt-lsc 20sx.jpg
  • m27pn140903x30mg.jpg
  • moo140903x1msmg.jpg


#7 Ravenous

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Posted 04 September 2014 - 06:07 AM

I've always admired the concept of the lensless schmidt (yes it's a bit long but it's a fun configuration to look at.)



#8 nytecam

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Posted 05 September 2014 - 04:35 AM



I've always admired the concept of the lensless schmidt (yes it's a bit long but it's a fun configuration to look at.)

Thanks Rav - decided to leave scope aperture mask at 90mm diam at the moment and just take more images like M27 below plus split frame - LSC [left] + 'Newt' [right] show the improved star quality via the LSC with 90mm aperture mask. 

 

I suspect the extra light grasp through a larger aperture mask will be offset by poorer images but perhaps someone can run this through their opty prog - TIA :grin:

Attached Thumbnails

  • ap-mask2.jpg
  • m27pn140903x1mg2x.jpg
  • m27pn140902-3x1mgcn.jpg

Edited by nytecam, 05 September 2014 - 11:49 AM.


#9 starcanoe

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Posted 05 September 2014 - 10:44 AM

Nice picture.



#10 nytecam

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Posted 09 September 2014 - 12:36 PM

An enlarged [x1.5] shot [click for full size image!] of Vega fov from last night via my LSC in 15s exp that shows good coma-free star images across the whole field in marked contrast to those taken without the aperture mask.  No darks/flat used   :grin:

Attached Thumbnails

  • vega.15s_2014.9.8_21.01.36neg2note.jpg

Edited by nytecam, 09 September 2014 - 12:38 PM.


#11 DAVIDG

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Posted 09 September 2014 - 09:32 PM

 Here are two spot digrams of your lensless Schmidt with an aperture of 90 and 75mm. The field of view is 1.5 degrees. As you can see in both, the off axis coma is very well controlled. The black circle is the size of the Airy disk so in both cases the system is not diffraction limited but that is not problem since seeing see blots the stars to many times the Airy disk in a only a few seconds along with tracking errors.  Your system is producing wonderful images with a simple optical system !  I have old Coulter 8" f/2 LSC kit they sold in the 70's that I'm going to have to try out.

 

                              - Dave

 

LSC90.JPG

 

LSC75.JPG



#12 Chriske

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Posted 10 September 2014 - 01:58 AM

Very impressive what can be done with such a small scope...

Nice work,

:waytogo:



#13 nytecam

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Posted 10 September 2014 - 02:33 AM

Thanks David for your post and spot diagrams for my LSC - these are very illuminating and greatly appreciated. :bow:

 

The actual diagonal field on my Lodestar-Cx2 [new Sony ICX829 sensor 6.4 x4.75mmm 752 x 580 p] is precisely 1 degree so I could technically, from your spot diagrams, use a larger sensor !

 

Do the spot diagrams take into account [or need to] the central obstruction?    I ask a when I defocus the image [focus is extremely critical] a star image become obviously an annulus but the central obstruction shadow of the secondary flat seems disproportionally larger than its 40mm diameter!

 

Thanks again for your efforts and I hope your experiments with your Coulter Schmidt kit prove rewarding - I do remember those ads in S&T :grin:  



#14 DAVIDG

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Posted 10 September 2014 - 08:29 AM

 The spot diagrams were generated without the central obstruction but it would make  little difference in the size of the spots.  When I looked into this design a couple of years ago, it came down to balancing the size of the stars vs the how many pixels they would cover, vs the aperture of the diaphram to find the shortest exposures. I was also looking at binning the pixels to reduce exposure time since  the size of the stars would cover a couple pixels and I wouldn't really be losing any resolution.  One thing you do have to watch is the diameter of the field of view vs the size of the primary mirror. If you were going to try say a 2° field, you would need a larger primary or you would not have a fully illuminated field of view.

  Back in the film days, you wanted the size of the stars produced by these system to be no smaller then the grains in the flim since that  set the  resolution of the system and in reality 2x large spots made no difference because seeing  and tracking error would blur them to this size or larger. So the same goes with CCD and matching the star size to the pixels size.

  Very nice work !

 

                                    - Dave     



#15 nytecam

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Posted 20 September 2014 - 04:50 AM

Yesterday I made and tested on the stars a larger and reinforced card aperture mask - upping the 90mm aperture to 100mm and nominal f/ratio raised from f/5 to f/4.5  eg f/6 and f/5 respectively allowing for 40mm secondary obstruction. 

 

My target field was Vega and dispite drifting cloud preliminary images where pretty good with nice round tight stars images free from coma.  A clearing cold front approaches UK tomorrow so time to try in depth. 

 

Here's a montage of yesterday's work. :grin:

Attached Thumbnails

  • lsc100all.jpg


#16 nytecam

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Posted 22 September 2014 - 02:39 AM

The clearing front wasn't that clear!  Here's a sampler from last night in two PNe M27 M57and two GC M15 M56 via modded 10cm f/4.5 Lensless Schmidt - I'm very pleased with these full size OSC images which are pretty good and coma-free to the corners.  Stars to mag 14.5/15 are recorded in 30s exp - no darks or flats used - data on images 

 

It seems optical theory can be relaxed slightly in practise at the telescope  :grin: .

Attached Thumbnails

  • m27pn140921x30mgx.jpg
  • m57pn140921x30mg.jpg
  • m15gc140921x30mg.jpg
  • m56gc140921x30mg.jpg

Edited by nytecam, 22 September 2014 - 02:51 AM.


#17 mclewis1

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Posted 22 September 2014 - 07:37 AM

Maurice,

 

Great images and a good example of what a budget of well less than $1000 USD (camera and scope) can do. Plus it's a simple setup, with no reducers and such. By staying within 30s exposures I could also see someone putting a similar scope on something like a Celestron SLT or iOptron cube alt az goto mounts and still staying in budget.

 

A 2nd hand scope and mount together should be well under $300. The extras would be the sub aperture cut out and mounting, and perhaps a vixen dovetail bar along with a few other minor bits and pieces.



#18 DAVIDG

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Posted 22 September 2014 - 01:01 PM

Optical theory shows the coma but in the real world, seeing, tracking errors and the resolution of the CCD all add up to smear out the star images so the coma becomes invisible and you get wonderful images. Very nice work !

- Dave

#19 nytecam

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Posted 22 September 2014 - 02:03 PM

Thanks Mark - I've yet to hear of a satified user of the original Meade DS-2114s OTA with the Jones-Bird corrector [JBC] - a classic lemon by all accounts :D  But with JBC removed the spherical f/4 primary + aperture mask it's clearly punching well above its weight as a LSC with the little Lodestar.  My scope was a dealer discard.   :grin:

 

As you state it could make a good low-cost intro for video/EAA even with an AltAz mount.  Paul [LodestarLive] uses an AltAz mount and his latested s/w LodestarLive v0.10 stacks a series of images that takes field rotation into account so more good news.  The slim-line Lodestar cam has a 32mm diameter barrel could in theory be placed at prime focus central within the the 'scope tube as figure below - a focuser could be fun!   In the past I've adapted a helical close-up devise that didn't rotate the camera at first focus.   The simplicity of the LSC does place limits in scaling it larger eg tube length and acceptable aberrations.    Just one [minor] downside - LSC has no visual application. :shocked:

 

best

Nytecam

 

 

 

 

Attached Thumbnails

  • LSCX.jpg

Edited by nytecam, 22 September 2014 - 02:19 PM.


#20 nytecam

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Posted 22 September 2014 - 02:05 PM

Many thanks David - your comments and expertise are greatly appreciated :bow:



#21 DAVIDG

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Posted 22 September 2014 - 04:59 PM

When Schmidt made his first cameras, he pulled a vacuum on the plate and ground them flat not spherical. He understood that the curve he needed was a polynomial that was made up of a couple of terms. The first squared term corrects for most of the spherical aberration and that is the curve that results when you pull the plate under vacuum and grind it flat. If you want better correction you need to include the second term in the equation and you get that by grinding the plate spherical to a calculated radius, while under vacuum.
The plus of the lensless design is that it is easy to make and the results are very good. The negative side is that you can't go really fast in F-ratio with that design, you need a Schmidt corrector to get there but as the lensless design has shown in the real world that even thought raytrace shows problems, you still get some pretty nice images. So then how good does a Schmidt corrector plate have to be to get good images ? I've played around with corrector plates and vacuum pans over the years. The construction and use of the pans is what stop most ATM's from trying to make a plate. Over the years people have experimented with pulling spheres into shapes that are close to parabola with a bolt glued to the back of the mirror. So I wonder how good of a Schmidt corrector do you need or putting it another way how bad can you get away with ? So say you made a corrector by deflecting the glass to the proper depth with a bolt glued to the back and ground and polished it flat ? Could you make a camera that would get you to F/3 or maybe F/2 and still give good images ?

- Dave

Edited by DAVIDG, 22 September 2014 - 05:02 PM.


#22 nytecam

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Posted 23 September 2014 - 02:29 AM

So say you made a corrector by deflecting the glass to the proper depth with a bolt glued to the back and ground and polished it flat ? Could you make a camera that would get you to F/3 or maybe F/2 and still give good images ?

- Dave

 

Sounds logical - and you're the one with the experience to test it Dave - keep us posted.  OSLO can compute the 'relaxed' potential.

Here in the London light dome such f/3 'speeds' would be impractical over the larger fov the Schmidt Camera can cover - I get a changing sky background gradiant over <1 deg!

 

Here from last night is PAL 8 in Sgr @ 14deg alt on my southern horizon via LSC under v poor transparency.  No flat used - the gradiant [darker corners]  probably due to enlarged [cylindrical] shadow of secondary mirror off-axis but interesting.  Stars to mag 14 recorded in the 30s exp and PAL 8 partially resolved.

 

Also included Pluto v.low in Sgr [ID Wikisky] in 4x30s summed stack then artificial flat [via original image Gaussian Blur 75% to remove stars] to minimise vignetted corners etc  :D

 

Nytecam

Attached Thumbnails

  • pal8gc140922x30mgx.jpg
  • plu140922x2mgx3.jpg

Edited by nytecam, 23 September 2014 - 05:40 AM.


#23 nytecam

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Posted 25 November 2014 - 02:22 PM

It's 2 months since I updated my post here.   Firstly, for now, I've dropped the option to 'speed' my 10cm f/4.5 [=450mm fl] Lenless Schmidt Camera with a focal reducer immediately before the sensor yielding f/2.8 & 280mm efl - the coma proved excessive. :blush:

 

These are some recent images from my LP London backyard in typically 60s or 180s via the piggybacked basic LSC [10cm f/4.5] + SX Lodestar-Cx2 camera - hope they are of interest and includes Vega [1s exp]/ M42 [20s exp ]/M1/Horsehead/M35/ and exotic grav-lensed quasar in Lynx @12BLY @ m15.2 [R].  Image quality proves good from a Meade 'Newt' with spherical primary + card aperture mask on the mirror radius  :waytogo:

Attached Thumbnails

  • vega141223x1smgx.jpg
  • m42 lsc141123X20szz2cut.jpg
  • m1 lsc141123x3msh2.jpg
  • m35 lsc141123x1mgx.jpg
  • lynxq141123x3mgz.jpg
  • hh lsc141123x4mgx.jpg

Edited by nytecam, 25 November 2014 - 02:37 PM.


#24 coinboy1

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Posted 17 June 2015 - 08:06 PM

Wow i am impressed with this! I just researched lensless schmidt camera's and this is amazing! I have a C14 with a broken corrector and have thought about doing this...Any spot diagrams available for a lensless C14?




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