Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
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: 867
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: 370
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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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
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: 370
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: 370
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: 370
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: 867
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: 29
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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: 867
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: 370
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: 370
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: 370
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: 370
Loc: UK
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Another.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
Loc: UK
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Another
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
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: 370
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: 5742
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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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 838
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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