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mak5
member
Reged: 03/09/09
Posts: 22
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Hello, everybody.
I'm trying to build a 6" f/20 Classical Cassegrain telescope and I have some trouble determining what back focal length should I choose. I will be using a Moonlite SCT focuser with a total length (fully racked in) of 3 inches, a 1,25" diagonal mirror and 21, 17 and 13mm Baader Hyperion eyepieces. The distance from the reflective face of the primary to the focuser flange is 1,35 inches.
The problem is I don't know where the focal plane of the eyepieces is located, therefore I can't determine what back focal length I need.
Can anyone help?
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 853
Loc: Waco, TX
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If you already have the eyepieces and have another working scope, you should be able to figure out the focal plane location of the eyepieces. Just check how far racked out the other scope's focuser needs to be to focus an image of the moon or Jupiter on some wax paper held across the focuser, and then compare with how far racked out the focuser needs to be to come to focus with the eyepieces.
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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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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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mak5 -
Looking at the eyepiece from the open end, if the first item you encounter is the aperture stop, that is where the focal plane is. Some one else may comment on eyepiece designs where the aperture stop is internal.
Keep in mind that, with a Cassegrain, relatively small changes in the inter mirror distance makes large changes in the location of the focal plane of the telescope system. I design my Cassegrain OTA's with a provision to move/tilt the primary mirror from the back so I can move the primary a bit and place the focal plane where I need it relative to the focuser travel. There is specific inter mirror distance where the correction for spherical aberration is optimum, but there is a useful range of movement.
Dick Parker
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mak5
member
Reged: 03/09/09
Posts: 22
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Thanks for the answers.
My two other scopes are a 5" mak and an 8" SCT. I will have to wait until I finish my 80mm refractor to try the paper trick.
I can move a little the primary in my design, but I left more adjustement range at the secondary to be able to tweak the focal plane position.
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jeffg
member
Reged: 02/13/07
Posts: 66
Loc: Irvine CA
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Dick is right, any lateral movement of the secondary will move the focus by the magnification factor of the secondary. For your F/20, that's 5X. So slight adjustments in the secondary should give you more than enough wiggle room. The bigger problem is getting the baffling right.
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Jeff
14", 10", & 4.25" Dobs
8" Schmidt Newtonian, C-8 SC, 8" LSC
6" Cave Student Model A, 6" Dall-Kirkham, 6" RV-6
5" Refractor & 80 mm Folded Refractor
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mak5
member
Reged: 03/09/09
Posts: 22
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I agree, the baffling already begun to give me headaches. I figured I can trade some edge of field illumination for a reduction of the secondary baffle diameter.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Gotcha covered on doing the baffle design. I'm assuming you're buying or making the mirror set, right? Your system focal ratio is f/20. Tell me your primary focal ratio and desired back working distance (BWD) and I can do an optimized baffle design for it.
Optimized baffles mean finding the minimum primary obscuration for a fully baffled field diameter. I wrote a simple macro in ZEMAX to do just that.
Standing by...
Mike
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mak5
member
Reged: 03/09/09
Posts: 22
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Thank you, Mike.
At the moment I don't have an exact value for the backfocus. I've just finished my 80mm refractor and I have to measure the focal plane position of the eyepieces I intend to use on the cassegrain to get the backfocus. I'm still waiting for clear sky.
The primary is f/4. For a 0.15° full illuminated circle and 200mm backfocus (I think 175mm is more likely), I get a 35mm secondary. CassDesign says I need a 41mm secondary baffle (27,3% central obstruction). I would like to keep the central obstruction at maximum 25%.
When I'll have an exact value for the backfocus, I will post it.
Thanks again for the help.
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