Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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There is a new experimental refractor concept, 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, 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, the color reduction is many more times than it should be, it can be reduced to 0.4mm by addition of a second lens. It is known as a "Wall Hypochromat", it is not a dialyte or a schup' as there is no flint lens in the system, the color is suppressed not corrected for. There is a little color left in the system and this can be ignored if you are not too fussy, it will make a superb RGB filter scope. The main thing it is very cheap, I have made a 5inch version.The system is in it's infancy and needs to be developed, how about it you ATM's out there. Ref to the Journal of The British Astronomical Association June 2009 vol  119 No3. Wanna know more, discuss it with me.
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John Kemp
journeyman
Reged: 03/09/09
Posts: 8
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Hi Crayfordjon - I was just about to email you to say that I think your suggestion of hypochromat is a better one than my arcane suggestion of elakhistochromat. You will no doubt have spotted that hypochromat(ic) already exists with another technical meaning, but this does not matter! Best wishes, John
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Hi John, I am sorry not to use your beautiful description of the new system, but I felt that it would,be a bit of a mouthful,so after much brain damaging cogitation the option of Hypochromat seemed fairly apt, the 'Hypo' means below, "hence Below color" You have no doubt heard of Hyper chromatic effect where "hyper" means above, this is found in some systems where over correction happens, I found this on my research into dialyte systems where the blue and red foci are reversed. 
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Post deleted by Crayfordjon
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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The system was called a Parachromat, the name has been changed
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robboski2004
member
Reged: 01/14/08
Posts: 19
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Quote:
There is a new experimental refractor concept, 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, 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, the color reduction is many more times than it should be, it can be reduced to 0.4mm by addition of a second lens. It is known as a "Wall Hypochromat", it is not a dialyte or a schup' as there is no flint lens in the system, the color is suppressed not corrected for. There is a little color left in the system and this can be ignored if you are not too fussy, it will make a superb RGB filter scope. The main thing it is very cheap, I have made a 5inch version.The system is in it's infancy and needs to be developed, how about it you ATM's out there. Ref to the Journal of The British Astronomical Association June 2009 vol 119 No3. Wanna know more, discuss it with me.
Hello John,
could you supply the perscription for you lens design ?
when you mention there is no flint in the system.......
what type of glas makes up the second part of the bino
objective ??
also if the final focal ratio of you system is F6 then
colour error has to be less than 0.4mm to even be as good as
a standard achromat.
look forward to further details.
all the best
Ian Robinson.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Diagrams of the hypochromat principle
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Yes Ian. The objective lens is BK7 but a low dispersion glass would be better. It is plano convex and need only be ground and polished, figuring is unescessary. It is F:40 for reduction of lateral chomatic aberration The bino achromat is just that, no special features 7X50 binos are OK but higher powers are obtained with focal length of 330mm Coliour is not so bad as calculation would predict, the images are brilliant lit and the resolution is that for the objective not the bino lens as so is the resolution. there is some edge color due to lateral CA, so the weak lens must be just that.I have seem worse secondary residual color in a commercial grade 5inch refractor! Make one and try it, you will be surprised.. easy! I fold my Hypos down to one third of half the focal length of the weak og. My current Hypo is only 32 inches long in spite of a focal length of 6000mm.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Further info Ian, 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, it will even focus on anything inside the objective lens, in this case the ob lens is out of play and only the reduction lens is doing the focusing, however there is a serious loss of aperture if the focus is on an object less than the focal length of the ob lens away. I fold the optics in three by using two flats, but the flats do not have to be high precision, within one wave is quite adequate, easy to make.
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GJJim
scholastic sledgehammer
Reged: 09/09/06
Posts: 907
Loc: Western CO
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Amazing - the objective doesn't have to be figured, and the EFL is 6 meters!
 
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1390
Loc: Highland Park, CA
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I can't find anything on the internet about this design. Any pointers?
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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George Kiger
member
Reged: 05/19/06
Posts: 23
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"it will even focus on anything inside the objective lens, in this case the ob lens is out of play and only the reduction lens is doing the focusing, however there is a serious loss of aperture if the focus is on an object less than the focal length of the ob lens away..."
I assume that focus can also be achieved past both the primary objective and the intermediate lenses, but in this case the aperture is reduced even further to the diameter of the observer's pupil!
Edited by George Kiger (06/24/09 01:30 PM)
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1962
Loc: Hockessin, De
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I played around with OSLO EDU and setup the design as described, that being a 100mm x 4000mm plano convex lens as an objective and achromat place 2000mm from the objective. 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
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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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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1962
Loc: Hockessin, De
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Here is the performance graphs.
- 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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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8226
Loc: Bremerton Washington
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Quote:
Further info Ian, 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, it will even focus on anything inside the objective lens, in this case the ob lens is out of play and only the reduction lens is doing the focusing, however there is a serious loss of aperture if the focus is on an object less than the focal length of the ob lens away. I fold the optics in three by using two flats, but the flats do not have to be high precision, within one wave is quite adequate, easy to make.
Very interesting for sure. What if you wished to use a bit stronger plano convexed lens...and its focal length was shorter...will the system still work accoring to your descriptions?
Rob
--------------------
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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nytecam
Postmaster
Reged: 08/20/05
Posts: 5717
Loc: London UK
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Hi CrayfordJon - your new scope got a mention a few weeks ago on this forum but thread got hijacked by a Schupmann fan  You're the best to describe - hope to see you maybe this Saturday at Greenwich for annual BAA ExMet 
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+e-finder+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9+Lodestar CCDs/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-my videos
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Hi Tim 53, you will not find anything on the net because it is a brand new conception, and was published in the Journal of the British Astronomical association this year. I discovered the principle about two years ago and have been researching it since, this is it's first exposure to the ATM community.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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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.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Hi George Kiger, yes you have hit the nail on the head, the aperture becomes very small when the system is focussed on any object a meter or so away.
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2258
Loc: Kent, UK
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Hmm, I started a 150mm x 4m heliostat lens a couple of years ago, maybe I'll finish it and try your idea.
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
PST
DSI 1
and a curious mind
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
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: 356
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: 356
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: 1394
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: 217
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: 863
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: 356
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: 217
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: 1962
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: 356
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: 8226
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
--------------------
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: 356
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: 356
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: 356
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: 356
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: 356
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: 863
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: 1962
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: 217
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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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 863
Loc: Easton, PA, USA
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John - Would these be the articles:
Volume 117 (2007)
Wise, P. & Wall, J.
- The retrofocally corrected apochromatic dialyte refracting telescope
Volume 112 (2002)
Wall, J.
- Building a 30-inch refractor
Gary
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8226
Loc: Bremerton Washington
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Quote:
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).
Well hello John Wall..inventer of the Crayford focuser.
Your new refractor concept looks interesting for sure.
So I can get a nice piece of float glass and grind a shallow convexed polished curve on one side(inside)leaving the outer surface flat and polished..and then figure the focal length(preferable long) insert a 50mm binoc objective in the light path and shuzam!
It sounds too simple to be even remotely possible.
Got any images of one of these set up so we can evaluate the train....?
Rob(Interested)
--------------------
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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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1962
Loc: Hockessin, De
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Here is the spot diagram from my OSLO file showing the center spot. The small black circle is the airy disk.
- 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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Dick Parker
sage
Reged: 08/17/07
Posts: 241
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Dave -
I put your numbers in to my OSLO last night and got the same spots. Plus lots of coma off axis. (Thought I did something wrong.)
Dick Parker
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8226
Loc: Bremerton Washington
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Quote:
Here is the spot diagram from my OSLO file showing the center spot. The small black circle is the airy disk.
- Dave
Yikes!
Looks like spherical prismatisism....
I'm assuming that's not good.... 
Rob
--------------------
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: 356
Loc: UK
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Gary from John Wall. yes they are the two seminal papers I published a couple of years ago, they are the result of many years of patient on off research into dialytes and retrofocal dialytes, which resemble a Schupmann but are trictly not so, as a Schup uses a mangin mirror making it a catadioptric refractor, as my conception uses all refracting optics it more resembles an over corected terrestrial scope. The investigations finally resulted in me realising a great ambition, that is, to build a very large refractor. The objective lens is a single plano convex made from Chance Pilkinghton float glass 25mm thick which I purchased from a builders merchant, edge ground for £250 (Sterling),float glass has very good optical properties, and is BK7. Optical test with a twenty two inch mirror I was making at the time showed only one slight striation in the corner of the disc. The design was taken up by Peter Wise who refined it into the Zerochromat.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Yeah it is badly corrected as your OSLO diagrams predict, but the system is new and highly experimental, I make no claims at all that it will replace an apo objective, there is coma and colour in the system but this is an order of magnitude smaller than that for a singlet having the same aperture and F:ratio as the hypo. The coma can be corrected by redesigning the reducing lens as these tend to be comatic anyway, most of the color is lateral chromatic ab' however computor ray traces are just that, the thing is to make one and look through it!!!, you see brilliantly lit and crisp images which are slighly marred by a little color fringing, if you can put up with that then it is a very good cheap refractor for beginners. The challenge is for the ATM community to do this and develop it into a serious scope. The hypo will make a first class deep sky imager's refractor using filters, eg O111 forbidden line of oxygen. I have been using the hypo with a cheap red acrylic filter and the image definition is perfect, and very bright.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Rob ( interested),Hi, yes it is a simple plano convex, flat on one side, and curved on the side facing the object, that is, the front surface. Get hold of a bino OG 7X50, but if you can a 10X60 and stick it half way down the tube on a test stand, it would be helpful to use a flat to fold the long focal length in half, this need not be 20th wave,but commercial grade 6 waves will do nicely. The back focus will now be slightly less than the f/l of the bino OG. It is as simple as that. John.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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I have two optical line diagrams of the system which I have been trying to send since I opened the thread but they are being sytematically rejected ( yes I have compressed them), sorry about that folks John Wall.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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diagram of the Hypochromat.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Diagram showing the comparison of disoersions of the Hypo against a singlet lens of the same focal ratio.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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This is an image of a television antenna at a distance of 55meters, about X40, taken with a 5inch hypo
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Sorry about the blooper, here it is again.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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I had computor problems all solved now, the diagrams and piccie will clarify the system and show the actual image quality.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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This image taken with a five inch Hypo at night of a porch light, dist 55m. Note: I use terrestrial objects to test optics, I get more information about aberrations and field distortions etc,than by imaging stars.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Ignore this, but it is an image taken through a singlet lens having the same aperture and F:no as the five inch Hypo., note the considerable CA.I'm having problems again!!
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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OK then go with the flow!! This image is the same as the last only taken with the five inch hypo,Same F/L same F: No Note Difference!
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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The out of focus blur to the left is not the scope but the camera was hand held and tilted.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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The out of focus blur to the left is not the scope but the camera was hand held and tilted.
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Andy Taylor
twistin' by the pool
Reged: 09/24/08
Posts: 384
Loc: t=0 UK
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OK,
It's a bit slow this afternoon so I had a rummage in the parts bin...
My first find is a 41/2 inch .25 diopter used to achieve close focus with motion picture lenses. (plano convex)
Focal length is 2000mm. I make this F19 ish. Sooo - if I only use the center 50mm I get my F40.
Next find is a 25mm dia x 180mm focal length achromat.
Using vee blocks and blue tack I then set it up with the achromat at 1000mm from the objective.
I used a movie camera eyepiece (about 35mm focal length) and focussed up on my sharpness indicator chart at about 45 feet away...
Yes it works!
Considering that collimation was rough at best the image was very bright, sharp and clean. CA looked minimal.
I will bring in some astro eyepieces next week and try to rig up something more accurate.
I also need to find a larger diameter achromat.
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Equipment list of shame:
A strange heap of assorted junk that when thrown together and dragged out into the dark shows me the wonders of the universe...
And then dews up...
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Gene Baraff
sage
Reged: 03/22/09
Posts: 217
Loc: Berkeley Heights, N.J.
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Quote:
OK,
Using vee blocks and blue tack I then set it up with the achromat at 1000mm from the objective.
I used a movie camera eyepiece (about 35mm focal length) and focussed up on my sharpness indicator chart at about 45 feet away...
Considering that collimation was rough at best the image was very bright, sharp and clean. CA looked minimal.
Would you be willing to compare the images you got with the long focal length lens in place - with the images you get with everyting else the same but without the front lens? You'd have to adjust the spacing to achieve focus, but do the comparison after you have made that adjustment. Without the front lens, your image scale would be larger of course, but I'm curious about the sharpness and color correction of the image, with and without the front lens.
The effect of that front lens, as far as I can tell, is to up the aperture, but in your case, you have stopped it back down so that there is not much aperture enhancement.
Thanks in advance.
Gene Baraff
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nytecam
Postmaster
Reged: 08/20/05
Posts: 5717
Loc: London UK
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Hi John - if you click edit button on YOUR posts - there's a delete button [at bottom of page] to remove your duplicate posts - but only for a day or so at most then they get locked.
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+e-finder+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9+Lodestar CCDs/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-my videos
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Andy Taylor
twistin' by the pool
Reged: 09/24/08
Posts: 384
Loc: t=0 UK
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Quote:
Quote:
OK,
Using vee blocks and blue tack I then set it up with the achromat at 1000mm from the objective.
I used a movie camera eyepiece (about 35mm focal length) and focussed up on my sharpness indicator chart at about 45 feet away...
Considering that collimation was rough at best the image was very bright, sharp and clean. CA looked minimal.
Would you be willing to compare the images you got with the long focal length lens in place - with the images you get with everyting else the same but without the front lens? You'd have to adjust the spacing to achieve focus, but do the comparison after you have made that adjustment. Without the front lens, your image scale would be larger of course, but I'm curious about the sharpness and color correction of the image, with and without the front lens.
The effect of that front lens, as far as I can tell, is to up the aperture, but in your case, you have stopped it back down so that there is not much aperture enhancement.
Thanks in advance.
Gene Baraff
Last post before packing up for the day...
Without the 1st lens everything still looks good, as you say - with a different scale.
Hard to say with a 25mm F7 mini refractor with a 1500 dollar eyepiece on the end..
I also tried just the 1st lens and the eyepiece - yuk.
The setup I have here is (I suppose) a half scale mock up.
Other questions/suggestions I'll try to deal with on Monday...
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Equipment list of shame:
A strange heap of assorted junk that when thrown together and dragged out into the dark shows me the wonders of the universe...
And then dews up...
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KI4YUN
sage
Reged: 04/21/08
Posts: 490
Loc: Satsuma, Florida, USA
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I must say that this design seems very promising on view of my wallet. I can't wait to possibly try out this design.
So does this mean that I could just get a 6" lens like this from http://www.surplusshed.com/pages/item/l3919d.html $25 for the 1st lens. Now with this lens being larger than what has been talked about, would it then be best to use an 80mm objective http://www.surplusshed.com/pages/item/l3911.html $32, instead of lets say a 50mm binocular objective? After all we wouldn't want vignetting.
Would be something to think about. I am just thinking and don't know how to use the ray tracing programs *got confused* so am not sure if it would work.
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-Tristan
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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K14YUN. Hi there, sounds good what you are about to do, now you must observe the F:40 rule for the main lens, you can drop it to F:35 but no more, and yes use an 80mm lens, so if you do your sums right, you will find that you can move the reducing lens closer to the primary, this will suppress the lateral chromatic aberration still further and shorten the scope. Now do not have a long focus reduction lens, it should be about the same F:No as for a bino OG not a refractor. the final configuration should not be more than F:6 but F:3 would be ideal, if there is two much color in the system then resort to the duplex version. Re the optical diagram, a 7X50 at half the back focus (BF) distance will give a much more corrected field and low color. A tip for you guys out there, try the scope out on terrestrial objects with plenty of detail, you will see clearly the residual aberrations in the system.
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grendel
sage
Reged: 04/12/09
Posts: 217
Loc: Canterbury, Kent, UK
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would an etx 80 lens work here, its about f5, but I have a spare one that I might be able to use with a surplusshed 6" lens. I love the idea though and its something I might well try - maybe surplushed could be persuaded to make a batch of 150mm f40 lenses if we got together enough requests?
Grendel
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Charl
member
Reged: 02/08/07
Posts: 81
Loc: Hants, UK
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I must admit that with only limited knowledge of optics, I found the idea confusing at first. Your supplied diagram has made the concept clear to me though. In effect you have an achromatic system with slightly converging rays entering the system in place of parallel rays. It would be interesting to see if figuring of the achromatic lens can further improve the quality. Maybe we'll finally find a good use for projector lenses!
Keep up the good work and thanks for sharing it with us. I will be following this thread with great interest.
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Gene Baraff
sage
Reged: 03/22/09
Posts: 217
Loc: Berkeley Heights, N.J.
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Quote:
I must admit that with only limited knowledge of optics, I found the idea confusing at first. Your supplied diagram has made the concept clear to me though. In effect you have an achromatic system with slightly converging rays entering the system in place of parallel rays. It would be interesting to see if figuring of the achromatic lens can further improve the quality. Maybe we'll finally find a good use for projector lenses!
Keep up the good work and thanks for sharing it with us. I will be following this thread with great interest.
Yes!!!!!
I had exactly the same thought. It was for that reason that I had asked Andy Taylor to report back on the difference between views he got using the original concept and views without the big lens in front.
The achromat and the eyepiece together constitute a stand alone scope. It has a certain image scale, aperture, and also has the various aberrations that experts know how to calculate and observe - spherical aberration, longitudinal chromatic abberation, lateral C.A., off axis coma, astigmatism,... whatever.
Place a large but weak lens way out in front of your scope. Clearly you end up with a scope having a slightly smaller image scale and (assuming that the diamter of the achromat is large enough) a much enhanced aperture.
The question then is "what is the effect of that added lens on the errors inherent in the back scope"?
I was about to ask David G. if he would be willing to redo his ray trace for the back scope alone, much as I had asked Andy to observe through the back scope alone. Then I saw your post which captures the concept far more concic=sely than I would have been able to.
David: Are you still willing to run the numbers? If you are, could you interpret them for guys like me that are ignorant of what they mean?
The bottom line, I suspect, is how much gain you get from the extra aperture relative to the (I am guessing here) the slightly greater error relative to that of the back scope alone.
Gene Baraff
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1390
Loc: Highland Park, CA
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Charl, Gene:
I feel similarly limited in my knowledge of optics, but I was wondering the same thing myself.
And one more: If the front lens were a doublet, would the system be a petzval? Or do the lenses in a petzval differ enough from a pair of achromats that simply putting two achromats together doesn't make a petzval?
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1962
Loc: Hockessin, De
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As per Gene's request, I used OSLO to look at a Parachromat system made with a 100mm f/40 plano convex lens and Melles Griot 75mmx 150mm achromat. The achromat was place 2000mm from the objective.
Here is the result for that system. If one looks at the Long. Spherical Abberation graph, this tells were the colors come to focus. I had OSLO focus the system for green light. Red focus long by 0.25mm and blue short by 0.37mm.
- 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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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1962
Loc: Hockessin, De
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Here is the 75mm Achromat by itself. Red and blue focus about equally on each side of green by 0.25mm. Also note that the red,green and blue curves do cross each other so one can refocus the image to achieve a better image. With the parachromat there is no crossing of the curves.
- Dave
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1962
Loc: Hockessin, De
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To get a feel for the performance of this system I compared the spot size it produces vs a 4" f/15 achromat of the Baker design that uses common BK-7 and F2 flint. Note that the black circle is the size of the airy disk.
- 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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GJJim
scholastic sledgehammer
Reged: 09/09/06
Posts: 907
Loc: Western CO
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Seems to me the Parachromat is roughly equivalent to using a binocular objective and looking through a wavy piece of plate glass.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Hey Guys, compute an objective lens that has a low dispersion glass such as Schotts FK54 Nd 1.437 Vd 90.7 you just might reduce the LCA to a minimum, for the objective lens. I might be right or wrong, are some opthalmic glasses with a Nd 1.9 having a very low dispersion available, this would give very shallow front curve and the low dispersion would have very little prism effect at the edge of the OG reducing the LCA to vanishingly small amount. I found that its LCA in the Hypo is the big problem, I think Corning make it.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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GJJJim. Hi. Build a Hypo (Para has been dropped), and you will find that the scope is far from that, the image is crisp and bright with a little color at the edges of the images, thats all,remember an important point, we are talking a cheap and very simple refractor here, and not a high performance APO!
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Gene Baraff
sage
Reged: 03/22/09
Posts: 217
Loc: Berkeley Heights, N.J.
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Quote:
As per Gene's request, I used OSLO to look at a Parachromat system made with a 100mm f/40 plano convex lens and Melles Griot 75mmx 150mm achromat.
- Dave
Thank you, both for the calculation and for the interpretation. The issue (I still think) is whether the loss of image perfection is a price that those who WANT the extra aperture without bankrupting themselves will be willing to pay.
It does call into question one of the assumptions of the original posting: Although one does get greater light throughput from the larger aperture, it is not clear that one gets the extra resolution as well. I say "not clear" because I am not skilled enough to tell, from Dave G.'s work, what the effect on the overall resolution is. I get the feeling that it is even worse than the achromat alone would have beeen, but that is only an impression - unsuported by any knowledge on my part.
Comments?
Gene
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 863
Loc: Easton, PA, USA
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Dave, Gene - I'm sure my understanding is less than Gene's, probably by a good deal, but I think I'm wondering the same thing. Leaving aside color, isn't resolution partly a function of contrast, and if the light energy is so dispersed the contrast must be quite poor - and wouldn't that pretty much negate the benefit of the larger aperture? Or am I getting it mixed up?
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Try this for size folks. Taken through a five inch F:40 OG using a 70mm bino lens at half the focus.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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This about what you see visually with a 15mm Plossl, seeing is believing. House 55m away.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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A night shot of a christmas tree at 55m
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Another night shot of a porch light at 55m, see much color?
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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I have been all through the OSLO analysis two years ago,and it belies what you actually see. What we are doing with OSLO is examining a tree, the Hypochromat looks at the forest! We have a newly born infant here folks, noisy smelly and I keeps you up at nights, what do we do? we train it love it and develop it into an upstanding adult that will stand apart from all others. So we must do for the Hypo. I think that what I have done is to do to telescope optics what Dobson did to telscope mountings, He built scopes out of tea chests with a flare for genius, and look at Dobs today, so I have put up a discovery for all to exploit that will hopefully follow in Dobsons footsteps. Remember the parameters of the Hypo, and keep to them, it is a low power high field angle scope, the power should not be pushed beyond X40, and it works beautifully at X25 to X30, no more. This is a scope that teenagers can build as a school project and aspiring refractors owners can make. The scope will work well for deep sky observations with and without filters, we look at fuzzies and star clusters with the Hypo, not planetary detail, and lastly, have you guys tried the scope with filters, the performance is top notch, and is a candidate for the imagers usage. So start building Hypos and stop analysing them, and evaluate the scope by looking through it for its quality!
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Andy Taylor
twistin' by the pool
Reged: 09/24/08
Posts: 384
Loc: t=0 UK
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I'm not convinced. Sorry John.
Now we are talking about special order low dispersion glass and two acromats. The price is going up as we speak.
This is going to be a massive 10ft long beastie requiring a rigid tube and a hefty (expensive) mount...
All this for 40x max...?
--------------------
--------------------------------------------------
Equipment list of shame:
A strange heap of assorted junk that when thrown together and dragged out into the dark shows me the wonders of the universe...
And then dews up...
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Andy, I mentioned earlier that the system has to be folded using two 6 wave flats..yes that is good enuff!. My own five inch F:40 is 30 inches long in a box tube. Breakdown is: first the focal length is cut in half by the placement of the red lens, then the remaing distance is folded three times using two flats, easy to collimate with a laser and masks during construction, the system is very forgiving where collimation errors are concerned... give it a chance now!
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Andy again, I agree with you concerning the glass, I would not use it, but it would be useful to try an OSLO analysis.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Andy, be my guest! I am interested too.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Last images for this thread. taken with: Helium interference filter @ 589nm, and green acrylic filter.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Ditto.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Comments about image quality, the scope is pointed through an unopenable double glazed window, this has taken the edge off the crispness.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Some of you perhaps are wondering about the long space between the OG and the Reductions lens, and I have made some mention of this previously, but I will elaborate on this now for those ATM's who would like to build one for fun. I fold my Hypo's three times using low grade flats: around one to six wavelegths of Helium light, these are easy to make, in fact it is sufficient to grind two plates together turn and turn about and polish, and as long as the spherometer does not register a sagitta they will be good enough, no need to test even, I have done this and it works! there is no degradation of the image. The OG is a similar case , after grinding and polishing there is again no need to figure. It is these attributes that allow the Hypo to be made by a fairly unskilled ATM. The attached diagram tells it all for folding, and no you will not need a tube about ten feet long, the scope is cut down to a meter for the box tube ( around 40 inches) the tail piece holds the reduction lens and focuser etc and sticks out a few inches more as a tube from the back of the box tube. This is pretty compact for a low power six inch refractor! Collimation: The optics can be lined up during construction using a pen or pointer laser, aviable for a few $'s for you. I attach n masks over the flats with a two mm hole at the exact center; if the flats are already alumized, this will also protect the coating but if the flats are uncoated I use a reflex camera mirror taped on with carpet tape for each, with a central ring inked on to the mirrors to locate the centers. I mount the laser in the OG cell with a diapragm locating the laser centrally on the optical axis, from this setup the flats can be built into the scope accurately.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Erratum,on diagram, D/2 should read F/2= 3000mm.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Well folks it looks as if this thread is over, thanks for taking an interest, I was pleasantly surprised that it got such a good response, I hope some of you try it and not analyse it: "Think outside the box." Aufwedersehen.
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starquake
member
Reged: 02/02/08
Posts: 172
Loc: Nádasdladány
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Hmm, the concept is not bad but I'm not an optician to judge if it will have many optical aberrations or not. Why I write is, because I just found out that the layout reminds me of the Yolo-telescopes, except that those are reflectors. But apart from the front lense the Yolo layout is very much the same (of course the primary is not flat in the Yolo, but is a very complex shape to avoid aberrations, that is a pain to grind).
Most spectrographs also look very similar to this layout, where the reason for such a solution is to make the spectrograph shorter, so it will not stress the focuser that much.
I'm happy that I'm not the only one who dares to try something that's not mainstream. 
--------------------
"At night astronomers agree." /Matthew Prior/
"Astronomers, like burglars and jazz musicians, operate best at night." /Miles Kington/
10x50, 114x900, 300x1500
My astronomical sketches: Graphite Galaxy
Don't take my words too seriously, I might be wrong. And sorry for my English.
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grendel
sage
Reged: 04/12/09
Posts: 217
Loc: Canterbury, Kent, UK
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I will certainly give it a go sometime, maybe with a f27 lens from surplusshed.
grendel
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omnivorr
member
Reged: 04/09/09
Posts: 27
Loc: horsetrailer (Downunder)
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I'm no optician, but I can see the basic idea of this system, I think... at f/40 the rays are "close" to parallel where they arrive at the reduction 'lens', and that lens has an opposite abberation to that the primary has induced in those rays.. thus nulling that abberation..
at least sofar as some CA is concerned..
I gather that f/40 in the primary objective is essential, otherwise a different kind of reduction 'lens' would be needed..... (what would 'abberate' equally opposite to that tempting f/27 SurplusShed lens???)
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1390
Loc: Highland Park, CA
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It still sounds to me like something on the lines of a 17th-century singlet with a focal reducer added.
-Tim.
--------------------
"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
Edited by tim53 (06/30/09 10:18 AM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1394
Loc: Sin-sin-atti
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Of course we'll analyze it. If you are using a 7X50 objective lens then you are vignetting a lot , even on axis making it closer to a 4 inch aperture working at f/60. I had to move the binocular lens farther back to get full aperture. I tried a number of different achromats from Newport, OptoSigma and Thor Labs but the color error was huge running between 8 to 15 waves of error. I don't see anything to recommend, a really poor performing 4 inch.
Far better in a 4 inch would be to buy f/10 or f/20 mirrors from Surplus Shed and a couple of lenses to make one of several unobstructed designs to get a color free, unvignetted high power scope in a smaller package. You might have to touch up the mirrors but it's very little work.
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Ed Jones
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Hi Ed Jones you are right I cannot fault your argument. I am airing a novel approach that could be improved upon if people played around with it not minding that everything is not 1/200wave accurate. I will point out once more, a point that everyone is ignoring at the moment, although I have stressed it, and that is, it is a very good filter scope. Start thinking on those lines.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Tim in a way it is, the early guys were inventing the telescope, and a number of solutions were tried, although I am unaware of the scope you have decribed,maybe the guy who experimented with it saw the correction it gave too, but the power was too low,And in those days they wanted as much power that could be obtained to see the brand new frontier. Nowdays it does not matter if a scope is dedicated to low power, we are spoilt for choice.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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omnivorr, Hi. F:27 is the bottom limit, the big problem with the scope is the lateral chromatic aberration caused by the outside edge of the lens where the prism effect is greatest, you have to cut the angle between the back surface and the curved front of the objective lens to a minimum, and this means at least F:40, more than this and the scope becomes inpractical, however I amd getting reasonable results at F:23. so your lens should be OK..just.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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omnivorr, Hi again, yes at F:40 the rays have a very small included angle, but if the reduction lens at half the focus, the aperture of the red lens will become the same as the OG, in fact, doubled. The focal length of the reduction lens will be increased to as the Effective Focal Length comes into play. Because the objective lens has a very shallow curve on the front surface, the spherical aberration is very small and will not matter in this case, more important, the red lens does not like converging rays coming at the front surface, and some coma will result, this can be corrected by careful design.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 863
Loc: Easton, PA, USA
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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
Could you explain exactly what you mean by "depth of focus". The term seems to have at least two definitions.
Also, is the difference between your Hypo and the 1828 design described here mainly the use of the achromat?
Gary
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Starquake. Hi, you are right there are parallels with completly different applications and configurations, look at the reduction lenses that are now used on Schmidt Cass's The Hypo has it's parallel look alikes, but none are used to suppress Chromatic aberration!.
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starquake
member
Reged: 02/02/08
Posts: 172
Loc: Nádasdladány
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Btw, talking about long focuses, how about Huygens' 210 foot air telescope? That had about f/290. Guess it was aberration-free. I also remember that I've read about a guy who competed with Huygens and built a 400 foot air telescope. But I'm unsure.
On the other hand, is there enough contrast in such a long focus telescope?
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"At night astronomers agree." /Matthew Prior/
"Astronomers, like burglars and jazz musicians, operate best at night." /Miles Kington/
10x50, 114x900, 300x1500
My astronomical sketches: Graphite Galaxy
Don't take my words too seriously, I might be wrong. And sorry for my English.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Hi Gary, by the depth, or should I have said 'range of focus' I meant that the telescope could be focused on an object at infinity or on an object stuck to the front surface of the objective lens, a ball of blue tack in this case,but the OG aperture will be cut down to around 18mm if the scope is focused on an object at 1meter distance, and zero for the ball of blue tack; the focal shift of the eyepiece is around 50mm or so.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Gary the Hypo uses an achromat reducing lens the other design does not. Actually I was unaware of the previous design until you brought itm up.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Starquake, Hi, The huge refractors of that time had enormous F: numbers and the CA was adequately reduced enough to see Saturns rings, however if you lived long enough to find such an object with a scope that size, I feel that you would be grateful to get any image at all!I worked out some time ago from the data on these long focus scopes that the magnification was around X400 An interesting point, the OG, only had an apeture of three inches or so,it looks as if scope was in fact nothing more that a glorified pinhole scope, as the depth of focus is collossal!, what if the lens is omitted, would you still get an image?? also you would still get an image using an eyepiece, you can prove that for your self by placing an eyepiece about 100mm from a pinhole and pointing at an object, the image is fuzzy and dim, but it proves the point.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Starquake, some figures for you. The long telescopes that I know of use a single lensed eyepiece of five inches focal length. so the 210 foot scope would have: 210X12= 2520 inches f/l, divided by the focal length of the eyepiece at 5 ins would yeald a power of X504. I forgot to qualify the diameter of the pinhole for thr long scope, it is of course 3 inches IE remove the three in lens and would you still get the same results from the slighly less than three inch hole in the support plate.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 863
Loc: Easton, PA, USA
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Thanks John,
Could something like this be used as an objective?
Gary
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George Kiger
member
Reged: 05/19/06
Posts: 23
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Great idea - less CA with higher f/#, so go all the way and make it f/infinity! You then get only the CA of the reducer lens, which is much reduced from the Hypo design.
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 863
Loc: Easton, PA, USA
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(I meant the filter as a blank to be ground of course...)
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Gary, just fine! make sure they dont send you a coloured one!. One side of the filter will be flat enough, so you only need to grind the front surface.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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George Kiger. Hi.If you make the OG at F: infinity, then it would be a plane parallel plate with no magnification, and the aperture would be the same as the red lens, and the EFL would also be the same as the red lens: the OG would act as a window. Any aberrations would be found only in the red lens. You have stated a limiting case. The F:40 criterion is the lowest permissable ratio that would give acceptable residue lateral chromatic aberration. You could go for F:50 of course, or even higher, but the reduction would slow down asymptotally, so it would be ever decreasing returns.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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A Hypo that I built and tested in the smaller aperture range, 3inch, one that could be made by enthusiasts and as a school science project. D=76.2. F/L= 2436mm, at F=31. focal reduction bino OG f/l= 330mm. d bino OG=50mm, (can be smaller), L, sep main OG from bino og 780mm. back focus results as 255mm, can vary. final F/ratio 5.1. Power using 25mm Plossl,= X17. with 10mm plossl = X51. The image quality using the low power is color free, the higher power LCA very small and faint, it compares very well with a standard 3inch refractor. There are no folding flats, just straight transmission, the total scope length from eyepiece to OG lens cell= 46 Inches!! The high quality of this refractor belies the OSLO analysis. See images.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Another.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Another
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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This is about X60, the image is just starting to break down, but the Hypo is not meant to operate at this power.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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A message from a professional lens and telescope manufacturer: Quote: Thinking about it, what you have achieved is to make a system at f4 or thereabouts, which gives almost identical performance to an f21 system ( except for slightly worse LCA). Not a mean achievement. The purists actually have missed the whole point of the Hypo! unquote. The said pro carried out an OSLO analysis.
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nytecam
Postmaster
Reged: 08/20/05
Posts: 5717
Loc: London UK
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Quote:
A message from a professional lens and telescope manufacturer: Quote: Thinking about it, what you have achieved is to make a system at f4 or thereabouts, which gives almost identical performance to an f21 system ( except for slightly worse LCA). Not a mean achievement. The purists actually have missed the whole point of the Hypo! unquote. The said pro carried out an OSLO analysis.
Nice one John - you may have been CN-OSLOed but it's those who rub glass that really count 
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+e-finder+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9+Lodestar CCDs/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-my videos
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 813
Loc: Waco, TX
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How far does the focal point shift between using this with and without the OG? If it doesn't shift too much, one could make this by putting a smaller refractor in front of a larger lens, or even a pair of binos in front of a pair of larger lenses.
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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arpruss, Hi, Re focus shift. One of my systems that I tested had a focus shift of 38mm. The system is this: D=127 for the Objective lens, f/l=6000mm, separation distance of reduction lens from Ob Lens= 2220mm, D reduction lens= 80mm, f/l=400mm, back focus=362mm. The scope was folded three times using two 6 wave flats to 740mm ( 29 inches appx). Image quality very good, LCA slight, but noticable , works well at X40, excellent at X20. Yes you can place the weak lens in front of a telescope, at less than half the focal length of the weak lens--from the focus that is, the aperture of the telescope will be doubled, but the F:no of the system will be less than that for the telescope, the EFL will be less than that for the telescope. You can do it for binos, but watch out for inter ocular dist constaint. If you place an F: 15 telescope in front of a weak lens the LCA and residual CA will be magnified, the idea of the system is to have a short focal length for the red lens: no more than 400mm, 330mm is a nice figure, that works well, and the focal ratio should not be greater than F:5.
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 813
Loc: Waco, TX
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Quote:
If you place an F: 15 telescope in front of a weak lens the LCA and residual CA will be magnified, the idea of the system is to have a short focal length for the red lens: no more than 400mm, 330mm is a nice figure, that works well, and the focal ratio should not be greater than F:5.
Sounds like the 68mm F/5.1 finder I'm going to be making should work here.
Out of curiosity: What would happen if one used a reflector for the smaller scope?
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Using a small reflector would work fine, and the system would become a cat refractor, however you have a central obstruction re introduced, and it is no longer a pure refractor, so the very finest resolution and detail is knocked off.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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I tested the 3inch version last night on the full moon, the disk is white surrounded by a thin blue ring, the Maria are steel grey against the white disk, and sharply detailed. I then tried an He filter 598nm (yellow ), suddenly the moon was transformed, the contrast between the Maria and the highlands is amazing, the detail minute, and the Maria seemed to stand out from the background, in high relief, this vindicates my statement that the Hypo is a filter scope of some clout. You can get a 25mm Dia He interferance filter from Edmonds for about $50, £33 UK.
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 813
Loc: Waco, TX
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Quote:
I tested the 3inch version last night on the full moon, the disk is white surrounded by a thin blue ring, the Maria are steel grey against the white disk, and sharply detailed. I then tried an He filter 598nm (yellow ), suddenly the moon was transformed, the contrast between the Maria and the highlands is amazing, the detail minute, and the Maria seemed to stand out from the background, in high relief, this vindicates my statement that the Hypo is a filter scope of some clout.
Please excuse my ignorance--I know next to nothing about refractors. With a filter, is the advantage over a fast singlet scope lower spherical abberation in the hypo? (It's a tempting thought to experiment with putting together a largish and cheap singlet scope--7.7" planoconvex lenses are sold by Edmund for about $80--for use with filters only.)
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Yes,the very shallow curve of the front surface of the Hypo will have very little spherical aberration, and the reduction lens does not seem to be corrupted much by the entrance rays being converging instead of parallel. As you raise the point: you can use a narrow passband filter with a plano convex lens by itself, to make a single lens scope of say around F:16, but the spherical aberration will be strong and you will have to figure the lens to an ellipsoid against a flat at the prime focus, this will be a null test; I have made several OG's this way, you will have to use say, a helium filter at the knife edge when figuring. To avoid figuring the OG must be around F: 20 to 30. The scope will be long, even if you fold it. The Hypo shortens the scope by 50%, before folding You quote Edmunds lens to be 7.7 ins dia, make sure it is at least F; 20 to F:25.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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A further point I should have mentioned, if you propose to use a helium filter say,with a long focus single element OG, then the field of illumination at the focal plane will attenuated to the point that it will be dim, even for the full moon (not the sun), In the Hypo, the rays from the OG are concentrated to a smaller focal plane by the red lens, and the images are very bright, even for a narrow pass filter, that is why the Hypo would make a very good filter scope. I made a six inch F:15 filter scope just using a singlet OG and used an He filter, it gave very disappointing dim images of he moon, so take note, whereas, the 3inch hypo gave brilliant images with an He filter.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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More tests with the 3inch Hypo, powered it up to X200 using a X2 barlow, and a 4mm Plossl, the images needle sharp with the He yellow filter, sharp with not a lot of color without, moon still fantastic. So the Hypo can take high power!! using filters.
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nytecam
Postmaster
Reged: 08/20/05
Posts: 5717
Loc: London UK
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Hi John - for all your newfound friends on CN-ATM I've found this pic of you with your latest Crayford mount, that I took at last year's BAA exhibition meeting in Leicester
--------------------
Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+e-finder+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9+Lodestar CCDs/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-my videos
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Goes well with the five inch Hypochromat.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Addition for those who are still viewing this thread, any imagers out there?, who would like to use the Hypo as a filter imaging scope, I have been having real success using the system with F: 15 OG's ( instead of the F:40), using single pass band filters, the image quality is high and very sharp detail can be had at X100 or more. See diagram of the filter scope.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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The OG in this system has a F:15 lens and the final focal ratio is F:3 to F:5, back focal length around 500mm, a wide angle field of about 5 degrees.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Erratum for back focal length read Effective focal length.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 356
Loc: UK
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Test trans for another thread please ignore
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