Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Hi DavidG, your understanding is OK, yes the red lens must be larger than 50mm to accomodate for the diameter of the field. The red lens does reduce the focal length from 6000mm to around the figure you have computed for, and the system is has a fast F: number, around F:2 to F:6 or so, depending which red lens you use: a note, you can put the reduction lens anywhere you want, the one half focal length of the OG is an optimum position, but I have placed the red lens at only one third the dist inside the prime focus, and conversely one third the distance away from the OG, it is a very loose system. Thanks for the analysis, I have only had one other ATM do one for me on OSLO.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Hi Rob, The F:40 option was arrived at because of the lateral chromatic aberration caused by the wedge effect of the periphial edge of the OG, acting as a prism, if you shorten the focus to say F:16, the CA will still be well suppressed, but the LCA will give unacceptable colour fringes to the edges of the images, the solution I arrived at was to make the OG very weak and at F:40 the LCA is hardly noticable. Now! if the Hypo is used as a filter refractor where images in only discreet wavelengths are required, eg, red blue green, He line,O111 etc, then the OG need only be F:16
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Hi Nytecam, I am afraid I now live in Coventry, and going all the way down to Greenwich is not an option, I have been down twice since I moved up here, and did a nostalgiasville tour of the ORO where I worked for seven years and I have used the 28 inch refractor many times.I was the optical engineer there, and built equipment for the planetarium and 28 inch. About Crayfordjon. very very dedicated ATM or 'TN' I have been making telescopes since 1953. lost count on how many I have built. I worked on retrofocal dialytes ( Schup deriviative)using all refracting optics, this enabled me to make the thirty inch refractor dialyte which is at the Hanwell Community Observatory near Oxford. I am also the English amateur so described who invented the Crayford focuser, go into the website of the Crayford Manor House Astronomical Society for further details; telescope making? is there anything else in this universe?
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GJJim
scholastic sledgehammer
Reged: 09/09/06
Posts: 907
Loc: Western CO
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Quote:
Hi GJJJim, The objective lens is so weak that there would be difficulty figuring it, the Hypo system is very insensitive to errors of figure, but the upside is that only a very shallow sagitta is required on the front surface, easy peasy to make.
Just curious, have you ever ground, polished and figured a mirror or a lens?
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1413
Loc: Sin-sin-atti
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How about a prescription for the skeptics?
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Ed Jones
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Gene Baraff
sage
Reged: 03/22/09
Posts: 245
Loc: Berkeley Heights, N.J.
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Quote:
I used an Edmund A32-886 50mm achromat which has 150mm focal length. This is a typical binocular achromat. I found the color correction to be fairly good but the effective focal length of the system is only 278mm which is not close to 6 meters. I also found that the 50mm diameter lens would be too small in diameter when placed at 2000mm away from the objective. Maybe I'm not understanding the design correctly ?
- Dave
Dave,
I re-read his first posting on this thread, and he was saying f/6 on a 100mm objective. That works out to 0.6meter, not 6 meters focal length, so I assume that your number above is a typo.
For vignetting, I would think that a 50mm lens placed at half the focal distance behind a 100mm objective would pass all the light that could have reached the on-axis spot on the focal plane. I assume then that your comment about the lens not being large ehough refers to off-axis vignetting?
Gene
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Gene wroteQuote:
I re-read his first posting on this thread, and he was saying f/6 on a 100mm objective.
Isn't that (f/6) the final f ratio, after the achromat? Wall says: "it uses a very weak objective lens which is a single plano convex 100mm aperture imager of around F:40" - or did I misunderstand?
John wrote: Quote:
The scope is unique in that it has an enormous depth of focus ranging from infinity to a spot of tape tacked to the front of the Objective
Am I understanding correctly that you mean you don't have to refocus once anything in front of the lens is in focus? Like a pinhole camera? If so, it sounds like you wouldn't need to focus at all as long as it was set initially to somewhere beyond the objective.
At the Hanwell site it looks like your 30" isn't covered. The page says the 12.5" reflector isn't covered but the optics are removed after viewing - I'm assuming that's not the case with the 30", or is it?
Is the 30" the same design or similar to your Zerochromat?
The page says "John did all the development of this wildly unconventional approach to large refractor building, the detailed design-work and all its actual fabrication, both mechanical and optical, single-handedly, in a tiny back-garden workshop and mostly using materials commonly available in the engineering and building trades." Could you tell us more about that project? Maybe in a new thread? I'd be especially interested in details of support and testing.
Thanks,
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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yes it has to be bigger than 50mm in actual fact
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Gene Baraff
sage
Reged: 03/22/09
Posts: 245
Loc: Berkeley Heights, N.J.
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Quote:
Gene wrote
Quote:
I re-read his first posting on this thread, and he was saying f/6 on a 100mm objective.
Isn't that (f/6) the final f ratio, after the achromat? Wall says: "it uses a very weak objective lens which is a single plano convex 100mm aperture imager of around F:40" - or did I misunderstand?
Gary,
That's the way I understood it - that f/6 on a 100mm objective meant 600mm effective focal length - that is, an image the same size as would have been produced by a 600mm focal length lens.
When I did the calculation, i got exactly the same result as Dave G - namely, that the image produced by the 4000mm lens followed by the 150mm lens with a spacing of 2000mm between them gives you an image the same size as would have been produced by a 279mm lens. So the system (as far as light collection is concerned) is 100mm objective working at f/2.79.
Clearly, Dave G and I are misunderstanding the meaning of the post in essentially the same way. The most straightforward reconciliation of all this, I suspect, would be something that the original poster Crayfordjon hinted at: Namely, that placing the reducing lens halfway down the tube means "somewhere towards the middle of the tube" the exact placement being unimportant to the concept he is proposing.
Gene
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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Gene,
My OSLO analysis showed that the achromat would need to be about 65mm in diameter to give around a 1/2 degree field of view which is unvignetted. With a 50mm lens placed 2000mm from the objective, the unvignetting field is around 0.15 degrees.
The effected focal length of under 300mm using a 150mm achromat is also something to keep in mind.
- 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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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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GJJJim Yes I have ground and figured quite a number of mirrors in my long ATM career, the second telescope I made was a 6inch refractor quite a bit to chew off for a beginner. I have made quite a number of achromats since, the largest were two 8 inch short focus jobs made together for Halleys comet. The largest objective lens I have ever made is a thirty inch plano convex for a Schup style all refracting scope, retro focally corrected dialyte.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8305
Loc: Bremerton Washington
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Quote:
Hi Nytecam, I am afraid I now live in Coventry, and going all the way down to Greenwich is not an option, I have been down twice since I moved up here, and did a nostalgiasville tour of the ORO where I worked for seven years and I have used the 28 inch refractor many times.I was the optical engineer there, and built equipment for the planetarium and 28 inch. About Crayfordjon. very very dedicated ATM or 'TN' I have been making telescopes since 1953. lost count on how many I have built. I worked on retrofocal dialytes ( Schup deriviative)using all refracting optics, this enabled me to make the thirty inch refractor dialyte which is at the Hanwell Community Observatory near Oxford. I am also the English amateur so described who invented the Crayford focuser, go into the website of the Crayford Manor House Astronomical Society for further details; telescope making? is there anything else in this universe?
A man after my own heart..... 
You're telling me that you are the Crayford behind the Crayford focuser design? 
I used that design when building my electric version for my 10"f/16 refractor. Cool!
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Hi Ed Jones. prescription for making a Hypo. get a piece of Float glass or BK7 100mm dia by 10mm thick,grind one side flat, and grind the front surface to produce an F:40 curve, polish and use, no figuring required obtain a 60mm bino lens and place it half way down the focal length, it would help to have a folding flat to reduce the length of the set up. mount it all on a bench made up from slotted shelving angle section. Search for the back focus and fit a Crayford focuser with a 20mm and a 15mm Plossl. Use it on terrestrial objects such as brickwork roughcasting or pebble ash and window frames, these images will tell you all about the quality of the system. You will see a small amount of residual color fringing on verticles and horizontals. Good to be a skeptic, I am! the telescope needs a lot of development to get rid of the LCA alltogether. Go on make one!!.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Hi Rob,I had better clear the air, Crayfordjon is my web page alias actually I am John Wall inventor of the Crayford focuser, named for the town of my birth ( like the Springfield mounting).
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Gene Baraff Hi, the objective lens is F:40, and is 6000mm-- 6meters focal length, the bino OG is place at 300mm from the the primary lens at half way down the tube. the resultant back focus will be around F:6 or less depending on the focal length of the red lens, and the efl is around 800mm.The the 50mm og can be shifted down towards the primary focus to accomodate the field diameter.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Gary Fuchs, Hi more on your query, the whole of the aperture is used and the scope is not a glorified pinhole camera, the effective aperture drops when objects are viewed at a distance less than the focal length of the objective lens, and continues to decrease to zero up to the front surface of the objective lens.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
Loc: UK
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Hi Gary again, the 30 inch is under a tarp. I could start a new thread on it when this present issue has exhausted itself yeah! sounds like a good idea. There are two seminal papers on the 30 inch I wrote in the BAA Journal archives. Yes the Zero is based on the 30 inch.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Quote:
the scope is not a glorified pinhole camera
Hi John,
Sorry! I didn't mean to suggest that at all - only the one point of apparent similarity that with a pinhole camera everything is in focus to infinity.
Gary
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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Attached is the OLSO file for the design. It uses a 100mm plano convex lens of 4000mm focal length, 10mm thick and made from BK-7. A placed a Melles Griot MG101CA0821 75mm achromat which has 150mm focal length 2000mm behind the objective and had OSLO focus the system for the smallest spot size.
- 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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Gene Baraff
sage
Reged: 03/22/09
Posts: 245
Loc: Berkeley Heights, N.J.
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The quotes below are both from John Wells's (Crayfordjon)'s very first post - the one which started this thread:
Quote:
... it uses a very weak objective lens which is a single plano convex 100mm aperture imager of around F:40, this is not corrected for color..
Then where did the 6 meters focal length you mentioned in your last response to me come from?
Quote:
but if a small achromatic 7X50 binocular objective is placed half way along the focal length, the resultant F:6 image is very free of colour, the difference between the red and blue focus is only about 1mm,
What would the difference between the red and blue focus be for that achromat binocular objective acting by itself be?
I'm trying to get my head around what's important here.
Gene Baraff
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