dongchina
member
Reged: 08/19/08
Posts: 10
Loc: Tianjin, CHINA
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hello~~ everyone.i am from Tianjin,CHINA. i am so happy to find here by Google.and i feel that it is like a big family.Now i have a Newtonian: F8 D114 f900.I've read the paper written by Peter Fawdon & Maurice Gavin.But I can't understand fully. Especially I think that I do need a diagram,then maybe I could do. I DO HOPE FRIENDS HERE GIVE ME SOME GREAT HELP~~~~~ Additionally, usually I take photos by film,perhaps i will use ccd camera,but maybe not SBIG ST series,it is too expensive for me.  Maybe just a little popular ccd camera.
 please help me~~~~
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I am an amateur astronomer in Tianjin, CHINA. I hope that I could make good friends with you here. And to enjoy the beautiful sky & universe together......
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1937
Loc: Boechout, Belgium
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This could give you a start.
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Chris
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 612
Loc: Easton, PA, USA
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Welcome!
Gary
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dongchina
member
Reged: 08/19/08
Posts: 10
Loc: Tianjin, CHINA
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 Thank you ~~~Chriske & Gary! I have already read the link and gotten some useful knowledge.
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I am an amateur astronomer in Tianjin, CHINA. I hope that I could make good friends with you here. And to enjoy the beautiful sky & universe together......
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dongchina
member
Reged: 08/19/08
Posts: 10
Loc: Tianjin, CHINA
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Who can tell me what they are?? [the red "?" part]
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I am an amateur astronomer in Tianjin, CHINA. I hope that I could make good friends with you here. And to enjoy the beautiful sky & universe together......
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nytecam
Post Laureate
Reged: 08/20/05
Posts: 4755
Loc: London UK
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Quote:
Who can tell me what they are?? [the red "?" part]
They are two alternative aperture stops that can be swung into the light path precisely. The aperture stop is essential to the lensless Schmidt design as fully described on my lensless Schmidt camera homepage 
BTW - the primary mirror MUST be spherical in figure - not a Newtonian parabola but perhaps, like me, you have a short tubed Meade DS-2114s clone with spherical primary  Good luck in your project and keep us posted
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+DS-2090+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9/Lodestar/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-spectro page
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Kobayashi
sage
Reged: 07/10/08
Posts: 291
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Quote:
Who can tell me what they are?? [the red "?" part]
A "Lensless Schmidt" is a telescope with a spherical primary mirror and an aperture stop at the center of curvature of the primary. The part you pointed to is the aperture stop.
The idea is, with this configuration there is no "optical axis" to the telescope. A spherical mirror is symmetric around any point. So the image quality is the same everywhere in the field of view. Though the image plane is also a spherical surface with the same center as the primary.
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-- Ken Kobayashi
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Kobayashi
sage
Reged: 07/10/08
Posts: 291
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p.s.
With a conventional Schmidt camera, there is a corrector plate at the aperture which corrects spherical aberration. A "lensless Schmidt" omits the corrector, which means you have a huge amount of spherical aberration.
I have to say, I'm still not convinced that the whole exercise is worth the trouble. Here are spot diagrams of a 75mm f=456mm telescope with a spherical primary with no aperture stop (i.e. aperture limited by the primary mirror size):
And here's a "lensless Schmidt" with the same specs (75mm f=456mm):
The RMS spot size is only improved by 10% or so, and in either case it's nowhere near diffraction-limited (the solid black circle is the Airy circle).
Edited by Kobayashi (09/02/08 04:53 PM)
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Kobayashi
sage
Reged: 07/10/08
Posts: 291
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And for what it's worth, here's a conventional Newtonian (parabolic mirror, no aperture stops) with 75mm aperture and f=456mm:
I've ignored central obstruction in the above diagrams but it shouldn't make much difference.
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-- Ken Kobayashi
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dongchina
member
Reged: 08/19/08
Posts: 10
Loc: Tianjin, CHINA
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thank you! Nytecam & Kobayashi.now I show my questions below:
1. why are there 2 kinds of aperture stops in front of the tube? what is the difference between them in using?
2.Nytecam, i have seen "MG@...." Are you Mr. Maurice Gavin?
3.my friends & I have no special & professional apparatus to test the LSC, what & how can we do for setting up the spherical focus plane?
4.Kobayashi,you say that your LSC is 75mm f456mm, then its F=6.08. As I know that Schmidt telescope has very little F, such as 1.2-----3.3 etc., why is yours so big? I myself think that F is relative to the envirenment of observing, don't I?? If it is polluted seriously by light, then F shall become bigger. Is this correct??
5. For a LSC,is the theory that the corretor or the aperture stop is 2/3 to the primary mirror in size, Kodayashi, why is yours not 2/3???
6. how shall I deal with the 45 degree 2nd little mirror? Remove it out of the tube?
Please help me~~~~
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I am an amateur astronomer in Tianjin, CHINA. I hope that I could make good friends with you here. And to enjoy the beautiful sky & universe together......
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dongchina
member
Reged: 08/19/08
Posts: 10
Loc: Tianjin, CHINA
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And Nytecam, I can not make link to your LSC homepage and spectro page. Don't why..... I've tried many times, it all defeated.
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I am an amateur astronomer in Tianjin, CHINA. I hope that I could make good friends with you here. And to enjoy the beautiful sky & universe together......
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Kobayashi
sage
Reged: 07/10/08
Posts: 291
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Quote:
4.Kobayashi,you say that your LSC is 75mm f456mm, then its F=6.08. As I know that Schmidt telescope has very little F, such as 1.2-----3.3 etc., why is yours so big?
I took the numbers from nytecam's telescope. The lensless Schmidt needs to be very slow to suppress spherical aberration. Even at F/6 the spherical aberration is pretty big, as you can see above. A conventional Schmidt camera can be much faster because the corrector plate corrects for spherical aberration.
Quote:
5. For a LSC,is the theory that the corretor or the aperture stop is 2/3 to the primary mirror in size, Kodayashi, why is yours not 2/3???
The relative size depends on the desired field of view. I don't recall this theory that calls for a 2:3 ratio though, where did you get that from?
Quote:
6. how shall I deal with the 45 degree 2nd little mirror? Remove it out of the tube?
Either leave it and use it the way it was designed for, or remove it and put the detector in the middle of the tube.
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-- Ken Kobayashi
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isramirez
scholastic sledgehammer
Reged: 11/04/05
Posts: 759
Loc: Mexico City
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Welcome to CN!... interesting project by the way 
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Santel MK91 & MK6
IntesMicro 715
Celestron SCT6 & ONIX80 EDF
Meade SN6 & AR6
Chinese 6in MakCass.Gregory
DBK + DMK + NexImage + LPI
CG-5 & LXD75 Mounts
many EP's, Barlows and Binoviewers
Great astronomic passion 
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mattyfatz
Pooh-Bah
Reged: 12/27/06
Posts: 1126
Loc: R E N O
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I'm loving this thread... Can't wait to see the outcome!
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*****MATTY******
All sorts of stuff
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nytecam
Post Laureate
Reged: 08/20/05
Posts: 4755
Loc: London UK
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Quote:
I have to say, I'm still not convinced that the whole exercise is worth the trouble....
Thanks Kobayasi - I love your computer raytracing but, as always, you can't beat practical experience as I've demonstrated and even as a lo-res spectrograph
Also remember I'm using a portable Meade DS-2000 series altaz mount to track the camera so each recorded frame is not perfect but results an order of magnitude better than expected
BTW-I've imaged my LSC without the aperture stop [eg 114mm f/4 spherical primary] and spherical abberation is very pronounced but well controlled even with the larger 90mm aperture stop [=f/5 nominal] thus and x2 enlargement of centre of fov
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+DS-2090+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9/Lodestar/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-spectro page
Edited by nytecam (09/03/08 04:21 AM)
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Kobayashi
sage
Reged: 07/10/08
Posts: 291
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nytecam, what would you say is the FWHM of the star image? It does look better than the Zemax model, any idea why? Maybe it's just the co-adding of multiple frames (I assume you did that)...
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-- Ken Kobayashi
Edited by Kobayashi (09/03/08 09:02 AM)
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nytecam
Post Laureate
Reged: 08/20/05
Posts: 4755
Loc: London UK
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Quote:
nytecam, what would you say is the FWHM of the star image? It does look better than the Zemax model, any idea why? Maybe it's just the co-adding of multiple frames (I assume you did that)...
Don't do much theory just practise. The times I've be advised [in the 60 yrs since my first mirror grind] "it won't work" when a small test proves conclusively otherwise 
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+DS-2090+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9/Lodestar/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-spectro page
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Kobayashi
sage
Reged: 07/10/08
Posts: 291
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OK here's a more careful analysis... I should have included the central obstruction to begin with, it does make a difference - if you think about it, an annular lens has less spherical aberration than a full circle. An infinitely thin annulus has no spherical aberration.
The RMS spot size is about the same as a Newtonian (parabolic primary) at 0.5 degrees off-axis, but the central core of the spot looks tighter, and more symmetric. So I expect it does produce better-looking wide-field images than a Newtonian. Cool.
Lensless Schmidt:
Spherical mirror, aperture stop at primary mirror:
Newtonian:
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-- Ken Kobayashi
Edited by Kobayashi (09/03/08 11:42 AM)
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1424
Loc: Hockessin, De
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Are you plotting the spots on a curved focal plane for the Lensless Schmidt vs a flat plane of the Newtonian ? With the smaller area of a CCD it will not make much difference but just wondering.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Kobayashi
sage
Reged: 07/10/08
Posts: 291
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Quote:
Are you plotting the spots on a curved focal plane for the Lensless Schmidt vs a flat plane of the Newtonian ?
I used a flat image plane for all configurations.
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-- Ken Kobayashi
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