arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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My travel scope uses a 1/4" threaded rod as a single-vane spider. Unfortunately, this flexes resulting in a laser spot from a collimator moving by about 1/4" on the primary as the scope moves from horizon to zenith. I am pretty sure that I am right about the source of the collimation shift. The shift decreases by about 30% when I use a 5/16 rod, but I don't want to add weight or thicken the already thick obstruction.
Can one buy threaded rods--or rods with threaded ends--that are less flexible than the standard threaded rods from Lowes or Home Depot?
A friend suggests tempering and offers the use of his blowtorch. Would that help?
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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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polaraligned
sage
Reged: 12/26/08
Posts: 229
Loc: P. R. of New Jersey
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Can't appreciably harden low carbon steel that makes up all commercially available rod.
If you made a rod out of say 4130, then hardened it, the ultimate strength and the yield strength will be much greater but your Young's modulus won't go up significantly. The blow torch will be a big waste of time.
You need to change the cross sectional dimensions of this rod, ie go to say an I-beam or rectangle, or better yet go to a material like carbon fiber rod.
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Moki
super member
Reged: 11/06/05
Posts: 428
Loc: Under the Clouds
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How is the post mounted? Does it hang straight down from the vertical? Assuming you're using a Dob-style mount, this would limit the flexure to one plane. If it is mounted this way then I would experiment with possibly adding a triangular brace to it from behind the post (in its "shadow") to stiffen it in the vertical plane. If the post is offset then you might use a short carbon fiber tube that has a 1/4" or slightly larger internal dimension and epoxy in a short piece of 1/4-20 (or smaller) threaded rod in either end. this would be a direct replacement for the all steel rod you're using now. It'll be stiffer and lighter.
Here's a source for carbon fiber tubing that's pretty reasonable:
Carbon fiber tubing
You could probably safely drop down in thread size a bit (like 10-32) to keep the tubing OD reasonable.
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"In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual." Galileo
12.5" F5.3 Dob on EQ platform
8" F7 planetary Dob / EQ Platform
Current ATM projects:
Xtal-Controlled EQ platform motor drive - finished... It works!
Motorized focus for planetary (moved to the bottom of the list for now)
10" F5.5 Verylight PushTo
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John Jarosz
Astro Gearhead
Reged: 04/25/04
Posts: 2287
Loc: Chicago area, IL
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What Scott said.
Don't know if the carbon fiber would help. This sounds like a design issue. Maybe if you posted a pic we could suggest improvements.......
John
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6" F4.6(w/Paracorr) GEM reflector, 8" F11 Dall Relay Scope
6" F5 RFT Refractor, Garrett Gemini 20x80 LW
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Here is a photo.
What if I epoxied a hacksaw blade, edge-on, to the top or bottom side of the rod? Or maybe a square 3/8" rod?
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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
Edited by arpruss (10/13/09 10:09 PM)
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bbasiaga
professor emeritus
Reged: 05/10/06
Posts: 724
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Is there a second side to that assembly? can you make the rod span it? Supporting the rod at two ends would help.
Also, chaning out your rod for a hollow tube would increase the strength. You might also experiment with an angle iron or C channel (steel, paint it) also available at home depot. You won't find a 1/4" angle, but you might find a 1/4" x 1/2" channel, which you could put on its side so it only presented a 1/4" obstruction. Bot of those shapes should sag less than your rod.
-Brian
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Vixen 80EDSF
Stellarvue 102ED2
Orion Atlas 11 EQ-G
15" Astrosystems Telekit w/ Discovery Optics
Lust for something Larger
Lust for something Larger than that
Past Lovelies:
Oberwerk 20x100 binocs
Meade AR5
Meade LX10 8" SCT
All sold to a good home
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Quote:
Is there a second side to that assembly?
Nope. The light shield hangs off a coathanger wire loop that attaches to the focuser shield (the loop surrounds the light path).
By the way, I checked that the collimation shift is not due to sag in the strut (1" square poplar)--however hard I push on the strut, I can't get much of a shift of the laser.
Quote:
Also, chaning out your rod for a hollow tube would increase the strength.
Could I find a tube that's threaded on both ends?
Quote:
You might also experiment with an angle iron or C channel (steel, paint it) also available at home depot. You won't find a 1/4" angle, but you might find a 1/4" x 1/2" channel, which you could put on its side so it only presented a 1/4" obstruction. Bot of those shapes should sag less than your rod.
Thanks for the idea! It would, however, be a bit of a challenge figuring out how to mount this to the focuser plate and the secondary mount. The current design just unscrews for airline travel.
Also, currently the secondary collimation works Coulter-style--one axis is adjusted by rotating the rod, and the secondary mount bends along the other axis.
Maybe I could use the channel and then JB Weld an inch of threaded rod at each ends of the channel? It would be weaker there, but perhaps the distance would be small enough that it wouldn't matter?
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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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grendel
sage
Reged: 04/12/09
Posts: 243
Loc: Canterbury, Kent, UK
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is the flex in the rod or at the fixing point? I see you have a large washer there, but feel that would need to be pretty tight to stop all flex at that point, if you dont need adjustment at that end you might be able to spot weld the nut to the rod and the washer to the nut, then you could weld a small triangular fillet between the washer and the rod, this would eliminate any flex at this point. you could also weld the channel or a box section to the underside of the threaded rod, which would stiffen it up as well.
Grendel
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scopemankit
super member
Reged: 01/27/09
Posts: 136
Loc: south africa
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Replace the rod with a fiberglass arch.
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Chris
Cape Town
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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I'd scour the local supermarket for a 'tin' around the right diameter to do just the same as the fiberglass....
Just a thought
Owen
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4938
Loc: MA
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Heat hardening steel or other metal doesn't increases the modulus. It only increases the yield point. The threaded rod will be just as flexible if hardened. The only thing that changes is you will be able to deflect if further before it took a set (bent).
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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bbasiaga
professor emeritus
Reged: 05/10/06
Posts: 724
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You may not be able to find hollow tube that is threaded, but you could epoxy a threaded rod small enough to fit the inner diameter in place to get your connection points.
-Brian
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Vixen 80EDSF
Stellarvue 102ED2
Orion Atlas 11 EQ-G
15" Astrosystems Telekit w/ Discovery Optics
Lust for something Larger
Lust for something Larger than that
Past Lovelies:
Oberwerk 20x100 binocs
Meade AR5
Meade LX10 8" SCT
All sold to a good home
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Lots of suggestions for me to think about. Nobody has commented yet on my idea of epoxying a parallel rod or a hacksaw blade. Are those ideas bad?
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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: 855
Loc: Waco, TX
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Inspired by the I-beam idea, I JBWelded a 3/16" square rod to the two inner (lock)nuts on the threaded rod, so there are now two rods about 1/4" apart. I am thinking that vertical bending will now be rather difficult, as it will require the square rod to compress or stretch (depending on whether I put it under the threaded rod or above--I'll experiment), assuming the epoxy holds well. I'll be able to test the theory tomorrow once the JB Weld cures.
How much collimation shift is too much for F/3.93? The current setup had the laser beam shifting about 1/4" on the primary and about 1 mm, or less, on the return.
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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
Edited by arpruss (10/14/09 10:26 AM)
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John Jarosz
Astro Gearhead
Reged: 04/25/04
Posts: 2287
Loc: Chicago area, IL
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Adding the hacksaw blade will increase stiffness in only one direction. I think you need more than that. I agree with the idea of the arched thin blade support that Chris suggested. It could be a metal arch .030" or .045: thick. I have used these and they are quite stable.
Here's a photo of mine.
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6" F4.6(w/Paracorr) GEM reflector, 8" F11 Dall Relay Scope
6" F5 RFT Refractor, Garrett Gemini 20x80 LW
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Quote:
Adding the hacksaw blade will increase stiffness in only one direction.
Only one direction is needed because the secondary support on this scope is perpendicular to the alt axis, so the load is always in one plane.
Quote:
I think you need more than that. I agree with the idea of the arched thin blade support that Chris suggested. It could be a metal arch .030" or .045: thick. I have used these and they are quite stable.
Thanks for the photo--looks very nice. However, I would have to redo the secondary mount as a standard three screw collimation mount, rather than the super-simple two-axis Coulter-style mount that I am using. So, currently I need the secondary to be able to rotate around the support axis. I may eventually redo with a standard secondary mount, and then the arched thin blade support will be attractive.
By the way, one reason I feel free to use something as thick as a 1/4" rod is because a large chunk of the primary is covered up (the primary has a chip in it, apparently due to having been dropped at some point--I got it really cheap), and almost half of the rod is over the covered up portion.
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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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John Jarosz
Astro Gearhead
Reged: 04/25/04
Posts: 2287
Loc: Chicago area, IL
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An F4 scope is a challenge to collimate even if the the hardware is perfect and unbelievably robust.
Vic Menard's Collimation book
The tolerances for component placement are very small at F4. I recommend Vic's book to really understand what you are up against. Even F6 is easy compared to F4.
Sorry if you've already been thru this, but I had a very steep learning curve to be able to collimate mine and this seems to be an area where the requirements are not too well understood.
John
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6" F4.6(w/Paracorr) GEM reflector, 8" F11 Dall Relay Scope
6" F5 RFT Refractor, Garrett Gemini 20x80 LW
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Quote:
An F4 scope is a challenge to collimate even if the the hardware is perfect and unbelievably robust.
I'd qualify it: Only if one is demanding. If one just wants to have fun, see some faint fuzzies, have a look at the moon, show Jupiter to friends and family, all the while sticking to magnifications that don't exceed 150-200X, one can do just fine with a quick and very imperfect collimation. And that's me with this scope. If I can easily do something to improve the situation, as I am hoping my modification to the secondary support will, of course I'm very happy to do that. There are many kinds of ATMers out there, and I am on the newbie and low end of the quick-and-dirty side. :-)
Even with things as they were, after a quick laser collimation, stars were sufficiently sharp for my taste with my Hyperion 13mm (62X--a nice magnification for open clusters and the like). They were acceptable, though less sharp, with my TMB/BO 6mm (133X). I don't remember what stars looked like at 200X, but the moon looked fine at 200X--rather less sharp than in my 13" F/4.5 at the same magnification, but what more can I expect of a smaller aperture scope with $39 worth of optics, including a broken primary?
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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
Edited by arpruss (10/14/09 05:01 PM)
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Well, with the double rod, things are indeed better, at the expense of about 1.5oz. (I could get it back if I shifted to a double aluminum rod arrangement, I guess.) Vibration settles an order of magnitude faster. Collimation shift is half of what it used to be.
I just took it outside. At 133X, Jupiter has two belts, with no detail, but Callisto's shadow is beautifully clear. (There is a problem with a bloom of light emanating from Jupiter. This is not, I think, collimation related, but has something to do with the asymmetrical covering up of a large chunk of the mirror. When I symmetrically cover up the opposite side, the bloom disappears.) I'm pretty satisfied. It's only a travel scope, plus something light (17 lbs) for me to take outside quickly (I can hold the scope with one hand, and with the other I can hold the scope's base and the eyepiece case).
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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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StarStuff1
scholastic sledgehammer
Reged: 04/01/07
Posts: 977
Loc: South of the Mason-Dixon Line
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Sounds like you have solved your problem to the point you are satisfied and that is what is important. Enjoy!
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Tools that make objects very far away appear much closer than they actually are.
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