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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
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: 838
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?
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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: 368
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: 368
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: 838
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.)
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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: 368
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: 368
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: 368
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: 5741
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 
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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: 368
Loc: UK
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Goes well with the five inch Hypochromat.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 368
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: 368
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: 368
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: 368
Loc: UK
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Test trans for another thread please ignore
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