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magic612
professor emeritus
Reged: 09/30/08
Posts: 552
Loc: Somewhere south of Chicago, IL
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I've got a couple Surplus Shed, short focal length 50mm achromats laying around, and although I've made a finderscope with one before (using a not-very-wide-field .965" cross-hair modified achromat eyepiece), I've not been happy with the field of view I'm getting. Obviously this is due to the MA that I'm using as the finderscopes eyepiece.
But that got me wondering - what is the typical configuration of most of today's finderscopes? Is it effectively using a Plossl eyepiece? A Kellner? Something else?
I rather enjoy making as much of my astronomy stuff as I can afford to, and finderscopes are one of those areas where I have the ability to make it, I'm just not sure of the "what exactly goes in this section" part for finders.
All thoughts appreciated!
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- Celestron C8+, Orion 90mm f/10, Orion ST-80, 5" f/8 Dob, 127mm f/9.4 refractor, 114mm f/8 on DS GoTo, 60mm Sears 6333-A, 127mm f4.4 refractor lens (current project), 12" f/5 mirror (future project)
- Orion Vista 10x50s (5 deg), Sears #6207 7x35 (7 deg), Jason #138 Statesman 7x35 (11.5 deg)
Yes, I'm addicted to telescopes and binoculars. I am getting help. Every time I look at the heavens, it helps.
http://www.eyesonthesky.com
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neo
professor emeritus
Reged: 02/12/08
Posts: 614
Loc: Iasi, Romania
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Hi magic,
well..I guess although the common sence tells that in a finderscope it would be nice a wide field eyepiece to get a big FOV, usually the plossl is the most used.
You could try an Erfle if you can find good lenses. The Erfl design fell in disgrace somehow nowdays but I believe it would be great in a finder. Beyond that I guess it get's to expensive for a finderscope.
I'm not familliar with the last offers from SShed but I know they have some nice small achromats you could combine. It won't be a scientific approach  on making eyepieces, more a trial an error thing, but a great fun for sure.
Good luck !
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Russian 15x50 binos
Home made 8" f/5 Newton on eq mount
Home made 70mm f/6 (Rodenstock Rotelar lens) Apo refractor
www.astronomy.ro
Edited by neo (09/29/09 07:09 PM)
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neo
professor emeritus
Reged: 02/12/08
Posts: 614
Loc: Iasi, Romania
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Hey, I forgot about Konig. It's quiet simple to build - 3 lenses in two groups, meaning an achromat and a single PCX lens with about 55 degrees Fov.
http://en.wikipedia.org/wiki/Eyepiece
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Russian 15x50 binos
Home made 8" f/5 Newton on eq mount
Home made 70mm f/6 (Rodenstock Rotelar lens) Apo refractor
www.astronomy.ro
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Most finder objectives are the same as those used in binoculars. So it stands to reason that a bino eyepiece would probably be the ticket. I'd go for a wide angle design taken from something like a 7X35 or 8X40. The focal lengths of these are often around 20mm or so, and so if your 50mm lens has a focal length between the typical values of 180-200mm, you'll get a magnification around 9-10X and a true field of 6-7 degrees.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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I think for most of the cheapie finders out there Huygen's are used. I modified my GSO finder into an erect image finder and so have played with different EP's. I think plossls are probably the best for a budget, I settled on a 20mm. I have a UO 24mm Konig and it was horrible. I'm sure a 24Pan would be optimal but I doubt it's worth the cost.
Glenn's suggestion makes a lot of sense... I think I'm going to have to try a bino EP or two.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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magic612
professor emeritus
Reged: 09/30/08
Posts: 552
Loc: Somewhere south of Chicago, IL
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Great info everyone - not sure I'll go the Huygens or Erfle route, but certainly the wide-field Kellner option of old binocular eyepieces sounds quite feasible, and likely the best overall direction.
Thanks for all the thoughts - much appreciated.
--------------------
- Celestron C8+, Orion 90mm f/10, Orion ST-80, 5" f/8 Dob, 127mm f/9.4 refractor, 114mm f/8 on DS GoTo, 60mm Sears 6333-A, 127mm f4.4 refractor lens (current project), 12" f/5 mirror (future project)
- Orion Vista 10x50s (5 deg), Sears #6207 7x35 (7 deg), Jason #138 Statesman 7x35 (11.5 deg)
Yes, I'm addicted to telescopes and binoculars. I am getting help. Every time I look at the heavens, it helps.
http://www.eyesonthesky.com
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