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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 858
Loc: Waco, TX
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Alright, I'm getting close to done with my travel F/3.93 8" dob. It uses an effective 7.4" mirror (a good deal of the mirror is duct taped off--there was damage to the mirror due to a clamshell crack). It works--nice, pinpoint stars with my 13mm Hyperion (or at least that's how it was before I added the rockers--I haven't tried it since).
I am having balance problems--when it's at 70 degrees and up, it tilts upward, and when it's at 30 degrees and down, it tilts downward. I've almost got these licked by using rubber bands, but I need to upgrade the rubber bands to springs. But I don't know how to calculate what springs I need, and I don't want to be spending too much money buying spring after spring at the hardware store. Here's where I need help. I don't want to go with counterweights, as I don't want to add to the weight to travel with.
I am eventually going to make the struts (1" square poplar) disassemble into smaller pieces that fit in a suitcase (that's why they stick out past the focuser board).
The lightshield is not yet done.
Here's the almost finished scope:
Here's the bottom part. The very low profile rocker box has dimensions about 12" x 7" x 1" (or maybe even a bit less). It rotates on a 12" vinyl record.
The mirror box is 3/4" hardwood (that's what I had in my scrap pile) and fits in my backpack.
The focuser is a wooden push-pull Crayford:
Here's the mirror cell on tailgate. Because the mirror glass is damaged, it's a six point cell. The mirror is glued on (1/4" thick blobs of silicone).
The secondary mount collimates Coulter-style--it tilts in two axes. You can see one axis in the picture. It's now attached to a 5/16" threaded rod which is a single vane spider. The rod rotates, and that's the other adjustment axis. Also, the position is somewhat adjustable in the focuser board.
The whole thing was fairly inexpensive, except for the fact that I had to buy a drill guide and a jigsaw (but a friend with a table saw helped cut the hardwood for the mirror box; he also used a router for one hole in the focuser board). The most expensive single item was the primary, which I think was about $35. The secondary was $9. The focuser was a couple of bucks. The springs are valve springs from a junked motor at a mower repair shop--I gave the guy $5 for extracting them for me. I am a bit surprised at the cost of all the screws, bolts, nuts, fender washers, etc., which I haven't tallied up.
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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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Mirzam
super member
Reged: 04/01/08
Posts: 118
Loc: Lovettsville, VA
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If you increased the separation between the teflon pieces your frictional problems would be lessened. Of course then you would need short side walls to support the bearings rather than just strips. Still should not be a big deal modification.
Very cool nonetheless!
Jim
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 858
Loc: Waco, TX
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Quote:
If you increased the separation between the teflon pieces your frictional problems would be lessened. Of course then you would need short side walls to support the bearings rather than just strips. Still should not be a big deal modification.
The rocker box will take up a lot more space in the suitcase then. But you're certainly right that it would help. Maybe what I can do is add something that can be removed for transport.
The other option is to get rid of the teflon on the bearings, but that's obviously unattractive. :-)
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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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GaryS
super member
Reged: 10/30/06
Posts: 191
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Quote:
I am having balance problems--when it's at 70 degrees and up, it tilts upward, and when it's at 30 degrees and down, it tilts downward. I've almost got these licked by using rubber bands, but I need to upgrade the rubber bands to springs. But I don't know how to calculate what springs I need, and I don't want to be spending too much money buying spring after spring at the hardware store. Here's where I need help. I don't want to go with counterweights, as I don't want to add to the weight to travel with.
I had the same problem with my 8-inch travelscope. Pop over to my web page and have a look at the first image in the article "My 8-inch Travelscope." Notice how the spring is offset from the centre of the side bearing. This allows the spring tension increase when I aim near the horizon, have no effect when I'm aimed 45-degrees up, and then increase again as I aim towards the zenith. (I hope that makes sense.) The way you have your rubber band attached, it exerts the same force at all angles -- which helps a bit, but not enough.
The way to have an adjustable spring is to use a spring that's shorter than you need, and then attach a turnbuckle to one end. You can adjust the spring tension with the turnbuckle. (Also hope that makes sense!)
Regards,
Gary
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Sky & Telescope Contributing Editor
SkyNews Columnist & Blogger
www.GarySeronik.com
A place for stargazing enthusiasts.
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 858
Loc: Waco, TX
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Actually, the rubber band does pull more near horizon and zenith, but the differential is insufficient. I can optimize that placement (I just happened to have a nut in that place from how I attached the rockers to the mirror box).
Turnbuckle! Good idea. Or, more cheaply (and with a tiny bit less weight), a loop of string or wire should do the job.
Thanks!
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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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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 858
Loc: Waco, TX
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The balance is fixed for now. I replaced the rubber bands with springs, and removed one of the struts. Surprisingly, it's sufficiently stable at up to 200X with one strut removed. (It may not be, though, once I make the strut come apart into two sections for putting in luggage.)
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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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