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wyundiv
journeyman
Reged: 01/24/08
Posts: 7
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Hello,
I have a rather unusual problem, and it may require an unusual solution. We have a 5-inch Alvan Clark refractor that came with original eyepieces, probably Ramsden or Huygens design. We've been trying to use a modern diagonal with the telescope, but the image will not focus with any of the eyepieces, as the system requires more in-travel (because of the added half-length of the diagonal). My question is this: Are there modern eyepieces that have more out-travel than a simple Huygens or Ramsden? Would a barlow help? Any novel solutions will be welcome. 
Best,
Walter
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3226
Loc: "Metrowest" Boston
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It has to do with the way these eyepieces are mounted, not so much their optical design. The old-style mounting placed the flange of the eyepiece up very high, near the top, so the focal plane of the eyepiece was perhaps an inch below that. The modern arrangement places the flange about even with the focal plane of the eyepiece.
I don't know how close you are to reaching focus. If it's not too far, and if your diagonal is the 2" size, you may be able to reach focus with a "negative profile" 2" to 1.25" adapter, like these
http://www.scopestuff.com/ss_npea.htm
This is not an endorsement for any particular vendor--I just did a google search to find a photo. Anyway, that sort of adapter may help. As you can see it's intended to be used one-adapter-per-eyepiece, since the eyepiece is held by recessed set screws.
You may also find some eyepieces on the used market that are mounted the old way, i.e. all barrel and just a flange about even with the top of the eyepiece, e.g. old Brandons, Clave, and Gailand eyepieces.
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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rickertk
sage
Reged: 06/14/07
Posts: 217
Loc: Outer Philadelphia suburbs
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Are you using a mirror or prism diagonal? I'm told that mirror diagonals tend to add more path length and thus require more in-travel than with a prism diagonal.
Keith
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Oberwerk 11x56
Orion 8XTi
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wyundiv
journeyman
Reged: 01/24/08
Posts: 7
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Mirror diagonal, 1.25 inch. I have a Unitron prism diagonal, and it is smaller, but not by much.
Thanks for your help so far!
Walter
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Lawrence Sayre
Post Laureate
Reged: 10/16/04
Posts: 3677
Loc: N.E. Ohio
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The 8 mm Ethos apparently focuses way outward from the norm.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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rickertk
sage
Reged: 06/14/07
Posts: 217
Loc: Outer Philadelphia suburbs
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Quote:
Mirror diagonal, 1.25 inch. I have a Unitron prism diagonal, and it is smaller, but not by much.
Thanks for your help so far!
Walter
From my limited understanding of it, it's not just the length of the diagonal that makes a difference, it's that the light is going through a higher refractive index material rather than just air. I'm sure there's someone else out there more up on the optics of this than I (I'd have to dig out the old physics texts to even to begin to explain it). If nothing else, it might be useful as a comparison.
Keith
--------------------
Oberwerk 11x56
Orion 8XTi
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3226
Loc: "Metrowest" Boston
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Quote:
From my limited understanding of it, it's not just the length of the diagonal that makes a difference, it's that the light is going through a higher refractive index material rather than just air.
Yes, a prism will require less in-travel, all other things being equal. You can see a slight "outward" focus shift when using just thread-on filters. Focus an eyepiece, remove it without touching the focuser, thread on a filter, put it back. Voila! Needs a touch of outward focus travel.
--------------------
Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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wyundiv
journeyman
Reged: 01/24/08
Posts: 7
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Thanks! I do remember something about the index of refraction. A diagram that I'm sure we've all seen previously is of Newton looking through a prism, and we see the colors separating. But the light is not bouncing off the extreme end of the prism.
I'll try the Unitron diagonal to see if it works.
Thanks again!
Walter
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