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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 854
Loc: Waco, TX
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Having found it difficult to find a cheap 1.25" drawtube, I did the following. I took some 1 1/4" PVC conduit. The stuff is super cheap (I think I paid about $2 for about eight feet). Its ID is just under 1 3/8". I then cut three narrow strips of bondable 1/16" teflon (from amazon's Industrial and Scientific store). Each strip was less than 1/4" wide, and about 2" long (I haven't measured). I beveled the top edges of the strips and JBWelded them at the top of the PVC tubing, and waited for the epoxy to set. This gave me a drawtube with ID just under 1.25". I then used a flat file to file the teflon strips slightly, until eyepieces would slide in and stay snugly inside. I expected filing teflon might produce an unusable mess (I tried filing PVC on an earlier occasion--ugh!), but it nicely thinned it down while keeping the friction apparently just as low before.
The result is a drawtube that keeps eyepieces, Barlow and my laser collimator very snugly with zero play (I may eventually add a thumbscrew, but right now there is no need), but yet they can be easily pulled out. (I haven't tried this at night yet, though.)
Another bonus is that one doesn't have to use anything special for painting the inside of the PVC black--any matte black paint that sticks to PVC will do (I used DecoArt acrylic black, which is $1 for a 16 oz bottle that was more than enough for the whole scope that the draw tube is a part of) since one doesn't have to worry about the eyepieces rubbing off the paint, as the eyepieces never touch the tube.
One could even flock the drawtube with some cloth that's 1/32" thick or less.
One worry is thermal expansion of the PVC. I am hoping that the Teflon can compress a little to compensate, and that the low friction of the Teflon will ensure the eyepiece doesn't get stuck. If it is a problem, I'll have to file the Teflon down some more, and add a thumbscrew. Currently, the paint is drying. After that, I'll try it in the freezer.
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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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Nathan F
sage
Reged: 10/10/08
Posts: 221
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Very cool! As for the expansion issue, teflon is amazing stuff and I suspect it will compensate nicely if the tube expands significantly.
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12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 854
Loc: Waco, TX
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Quote:
Very cool! As for the expansion issue, teflon is amazing stuff and I suspect it will compensate nicely if the tube expands significantly.
Also, the PVC can deform a little to let eyepieces in and out, since the eyepieces are supported only along three fairly narrow lines. This could well be a problem for a helical Crayford focuser that requires a very round tube, but it won't be a problem for a standard Crayford which has no such tolerances to speak of, or for thepush-pull Crayford that I will use it in (the Teflon in the focuser can deform to compensate).
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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: 854
Loc: Waco, TX
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After the drawtube spent about an hour in the freezer, my laser collimator went in with a bit more difficulty, but indeed it did go in and it did come out. This was a worst-case scenario because (a) the laser was room-temperature and the PVC was cold while in real life both would have been cold and the aluminum housing on the laser would have been slightly contracted, and (b) the laser is, I think, slightly thicker than all my eyepieces. After the laser, I put my Hyperion 13mm in, and it went in just fine, but that was less fair as a test since the PVC had warmed up a bit.
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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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