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magic612
professor emeritus
Reged: 09/30/08
Posts: 548
Loc: Somewhere south of Chicago, IL
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Okay, a couple dumb question(s):
Where should one figure the approximate placement of an eyepiece when figuring a raytrace for a telescope? I'm making an OTA for a very fast f4.4 refractor lens. And I don't know where I should place the eyepiece in the raytrace.
Does the light focus from the converging light cone? Or does it go "past" the focal point, and does the eyepiece then focus based on the re-emerging (and therefore expanding) light cone on the other side of the focal point? Or does the focal point effectively hit the spot where the first air-to-glass lens surface is? The field stop?
Sorry if these are really stupid questions.
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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: 609
Loc: Iasi, Romania
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Hi magic!
In an astronomical telescope (focused to infinity) the two focal points of the objective and eyepiece coincide.
In raytraceing a major concern would be the focal plane and FOV, vignetting. So yes, the field stop of the eyepiece is within objective's focal plane.
For practical purposes you can consider the longest FL eyepiece you intend to use to get your FOV. Also you have to consider the FOV of the eyepiece. So you can start with the widest and longest focal eyepiece in order to see in your raytrace if you have vignetting issues or not.
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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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jg3
sage
Reged: 05/27/07
Posts: 328
Loc: near Auburn, CA
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neo is right. The telescope has a focal plane somewhere in the focuser, the eyepiece has a focal plane somewhere in the barrel, and when observing, all we need to do is move the eyepiece up or down a little until they coincide.
But where exactly is the eyepiece's focal plane? Few makers publish this. (Notable exception: Televue.) Usually it's at the field stop within the barrel, explaining why the edge of the field looks sharp rather than blurred when in use. But some eyepieces have a lens near the end of the barrel, concealing an internal field stop; in these, the focal plane is inside the eyepiece.
Usually an eyepiece's focal plane is a little below the shoulder (that rests on the focuser's front surface). The distance varies among eyepieces; if it were the same, then all eyepieces would be parfocal.
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magic612
professor emeritus
Reged: 09/30/08
Posts: 548
Loc: Somewhere south of Chicago, IL
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Okay, so basically if I set the focal point at the spot where the eyepiece shoulder meets the end of the focuser, I'll be within 1/4" to 1/2" of the right spot, yes? I realize that most eyepiece manufacturers don't post where a given eyepieces focal plane is (and I don't own any Televues).
But I just want to make sure I'm "close" as I design my OTA. I can adjust the location of the lens cell to some degree; I just want to have the focuser in the right spot so whatever cell adjustments I might need to make would be minor.
Thanks for the info guys. 
Oh, one other question: Is a refractor's focal length measured from the front of the outer lens element, or the back of the inside lens element?
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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: 609
Loc: Iasi, Romania
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It's a bit more complicated...because of refraction the plane from where you can measure the true focal length is somehow between the lenses, I mean in a simple doublet is somewhere in the middle o the glass.
But you don't need to bother with this things. Just take the lens, project the sun on a piece of paper with it and measure the back focal length from the last element to that paper. When you do the raytracing just draw the focus plane according to the back focal length of the objective.
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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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jg3
sage
Reged: 05/27/07
Posts: 328
Loc: near Auburn, CA
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CAUTION!!! Be super careful with any telescope optic in sunlight at all! Focused sunlight is not visible until it's too late for somebody's eyes, or something gets scorched. Better if you can focus the moon, or a distant radio antenna, water tower, etc.
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magic612
professor emeritus
Reged: 09/30/08
Posts: 548
Loc: Somewhere south of Chicago, IL
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Hehe - I was going to make a wisecrack comment to neo about focusing "the sun on a piece of paper". Knowing me, I'd set the paper on fire long before I'd ever get to measure the focal length!
No worries - I won't burn anything down, or anyone's eyes (including my own). 
--------------------
- 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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Sean Cunneen
Let Me Think
Reged: 08/01/07
Posts: 1221
Loc: Blue Island Illinois
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Also remember that you focus in for infinity and out for closer objects, so your highest mm eyepiece should focus with your focuser just about raced all the way in. Your smaller focal-length eyepieces will then focus racked out a bit.
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Sean Cunneen
Blue Island, IL
12.5" Ultralight Strut Dob
127mm f/9 refractor
35Pan, 17T4,
10XW, 7XW, 5XO
Complete set of Orthos.
Member of the Calumet Astronomical Society
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neo
professor emeritus
Reged: 02/12/08
Posts: 609
Loc: Iasi, Romania
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Quote:
Hehe - I was going to make a wisecrack comment to neo about focusing "the sun on a piece of paper". Knowing me, I'd set the paper on fire long before I'd ever get to measure the focal length!
No worries - I won't burn anything down, or anyone's eyes (including my own).
Oh thank you very much  !
On the level now guys....if you project the sun on a paper, now in automn the sun's much weaker so sure will not have any problems (you can do this early in the morning ore in the evening before sunset, if you want). I believe it's ovious you'll NOT look at the sun through the lens, God forbid! But projecting the sun on wite paper it desn't actually set the paper (...or house) automatically on fire. You can wear some sunglasses if the image of the sun is to bright.
I've done the same measurements for my Rodenstock lens too, in complete safety.
When I gave the indications I presumed I give them to a grown-up persone, so some minimum safety issues are simply ovious...you'll not gone project the sun onto your hand or leg...you can use a wall, or other straight suport where you can see a clear image.
So don't prepare to throu away this method yet...it's actualy the most direct and simple way to measure, with a little care of course.
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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: 1250
Loc: Ottawa, Ontario, Canada
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The general rule of thumb I've encountered when laying out a telescope is to position the focuser so that when it's fully racken *in*, the objective's focal surface lies about 1/2 inch *outside* the end of the focuser's drawtube. Of course, this could be modified when the focuser's range is short, not to mention the specifics of the field stop placement in each of the eyepieces you plan to use.
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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
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