mark cowan
Vendor (Obsidian Optics)
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This design is made for a new 14" trilateral string dob. I've taken that out of the title because this thread is just about the wire spider design. 
First make these parts...
Edited by mark cowan (08/05/08 02:37 PM)
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Assemble as shown:
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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2nd view:
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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3rd view:
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Clean gluing surfaces with acetone:
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Blobs away:
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Makes its own spacers! Let RTV cure at least 24 hours:
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Stringing not shown - you'll want to make an alignment jig for sure. These are .014" plain steel electric guitar strings, and have a tensile strength of at least 30 lbs. As shown later, .010" strings work perfectly well, albeit with 1/2 the strength per string.
What you should note here is how the two closest vanes (in this picture) anchor at the far side of the hub assembly, crossing paths to do so. This is the key to what makes this spider so rigid and resistant to vibration. It completely removes the tendency of wire spiders to rotate in vibration along the optical axis of the OTA.
Edited by mark cowan (08/05/08 02:35 PM)
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mark cowan
Vendor (Obsidian Optics)
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The wires could align along 90 degrees, or as 60 degrees, for a kind of fan effect on the spikes... The view from above:
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mark cowan
Vendor (Obsidian Optics)
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Posts: 2383
Loc: salem, OR
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Shown in the mockup secondary cage (ring). Improvised tensioning hardware at this point.  Note the spreaders on the near wires are not symmetrical above/below the ring:
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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View from the focuser position:
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mark cowan
Vendor (Obsidian Optics)
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Posts: 2383
Loc: salem, OR
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Can you say "ultralight"? 
Best,
Mark
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rboe
Reged: 03/16/02
Posts: 51103
Loc: Phx, AZ
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So you have some tall towers under the ring for some wires. Why did you do it that way? 
Very interesting. 
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
Best of ATM
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Thanks!
The "spreaders" are offset below the ring but they're the same height as the inside span on the central hub. It's necessary to have the spreaders to use the short anchor leg on the front of the hub. That allows the secondary to be mounted as far up the hub as possible, which minimizes the moment arm for its torque.
Note all string paths describe triangles (think trusses). If the wires are then fixed at each attachment point (note snubbing screws), either way they locate fixed points in one plane. Consider a string cat's cradle. As built the spider hub can't move in any axis. And by the unequal loading of the crossing pair on the back side from the focuser, the COG is no longer aligned with any major vibration mode.
As built it's insanely rigid and vibration resistant.
[speculation]Thanks to this improved geometry the wires may work well at only .010" in the real 'scope, the tensile strength would still be about 120 lbs for the 8 strands... This hasn't been possible before SFAIK. Wire spiders typically end up needing both high tension and/or thicker wires just to become stable in collimation and resist vibration.[/speculation]
Kinematically its closest cousin - and the direct inspiration for this design - is the triangle based system Mel Bartels used on his 20" trilateral. Easiest to see in real life, but perhaps sufficient to look at the pictures and schematic here, or better yet the article in Aug '04 S&T.
BTW, this was originally intended to be a 3-vane spider with a single leg coming out just above the focuser. Although that worked fine for vibration alone it wasn't proof against collimation changes at different altitudes. So I doubled the wires there, and that did the trick in spades.
Best,
Mark
Edited by mark cowan (08/03/08 10:04 PM)
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2428
Loc: Courtice, Ontario
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Very nice design. How did you actually attach the strings into the hub?
I love the idea of the thin stainless that the secondary is RTV'd to, I'll have to borrow that one. It works out nicely that it's closer to glass in thermal expansion than Al, was that the intent? Just noticed while looking that up that Tungsten is really close, I wonder how that would work. Can't be too pricey as my darts are 90% Tungsten and they're not horribly expensive (maybe a little food for thought). Back OT... it's funny I use the same sort of idea on my homebrew Beismeyer clone tablesaw fence to square it and never thought of using it for this purpose.
One question about that... Why didn't you tack weld a couple nuts onto the back side (to go inbetween the eliptical Al plate and the thin stainless) so that it could be bolted from the back and thus easily removable. I'm not knocking your design in any way. I'm just curious if you thought about that and deemed it a bad idea.
Ryan
[edit]
NM I figured out how you attached the strings after looking at the pics closer.
[/edit]
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
Edited by Biff (08/03/08 11:16 PM)
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mark cowan
Vendor (Obsidian Optics)
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Posts: 2383
Loc: salem, OR
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Thanks, I used the stainless for its springiness, but I'll take thermal expansion as a bonus. It works with minimal hardware and was the last part of the design to come together, this is the second shaped plate I made after finding that a simple elliptical version makes a nice "cricket toy" but a poor collimation plate.
I suppose you could tack-weld nuts on the collimation plate as you suggest, if you wanted to be able to swap secondaries, or you could just glue the screws on and hold them with nuts instead. But I wanted it to be as sturdy and failure-proof as possible - with the tapped holes I can torque all the parts together and know it's not coming apart. It's easy enough to cut the secondary free if needed.
Alternatively you could fix rare-earth magnets to the plate and to the secondary and I did consider that briefly - until Murphy's law cleared its throat. The worst case scenario occurred to me where the magnet system has had a demo, and somebody tries it out, and drops the secondary...
Best,
Mark
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2223
Loc: Boechout, Belgium
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Mark,
Maybe I'm paranoid here, but once you start collimating that secondary mirror you will introduce tension in that mirror ending up with some sort of warping..
Did you already tested this unit in field, startest..?
But again, maybe it's just me, just asking stupid questions ...
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Chris
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mark cowan
Vendor (Obsidian Optics)
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Posts: 2383
Loc: salem, OR
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Tension how? From the RTV? You could be right, I'll have to wait and see once the rest of the scope is built. No, it hasn't been field tested, it's just me taking a guess about the forces involved. Since the spider can be positioned very accurately during stringing up, the collimation plate is only for small motions, tweaking that is. In those motions my intuition (and the law of small angles) said it would work but a rough calculation might help.
Let X be the span of the diagonal from the center of the RTV pad to a line orthogonal to the point Z where the diagonal plate is fixed. Let Y be the span of the diagonal plate from the the center of the RTV pad to the same point. Let E be the elevation of the plate lobe by the collimation screw, then if only one pad is adjusted the movement of the diagonal center relative to Z is roughly E/2. See the crude illustration for slightly more clarity.
You then have that the lateral displacement D absorbed by the RTV pad (which applies twisting force to the diagonal) is approximately D=X-sqrt(Y^2-E^2/4).
For the span of the longest lobe on the collimation plate (29mm) and the maximum expected collimation screw travel (2mm?) this works out to .02mm or 20 microns. At 1mm travel it would be .004mm or 4 microns. I don't think it'll matter as these are very small displacements and RTV isn't that strong in shear, but it'd probably be good practice to advance the collimation screws to the center of expected travel before the RTV sets, which I'll do with the quartz diagonal that I don't have yet.
In any case I'll let you know if I see any effect. Good point, BTW!
Best,
Mark
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2223
Loc: Boechout, Belgium
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Yes I was thinking of the blobs of siliconen. If they're not 'flexible' enough or to large you could end up with warping. I would dare to do this myself with very small blobs. In case of large blobs I would get suspicious.
Keep us informed...
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Chris
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mark cowan
Vendor (Obsidian Optics)
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Loc: salem, OR
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Okay, here's a couple new things. After working with the secondary a bit I thought that if the wires that converged on the focuser side of the hub went from top-bottom and bottom-top instead of describing a trapezoid the resistance to bending along that side would improve. It does. A trapezoid is less stable than a triangle, which in turn is less stable than a crossed-over double triangle, even for a very small amount of cross-over. Try it with a loop of string if you have doubts. It's the same principle as having the two wire vanes on the far side cross over to the opposite side, this is what makes it super stable.
After that I thought I'd just go ahead and try stringing it with .010" wires and see how it worked. The results are most impressive - I can't measure any deflection from vertical to horizontal with a dial gauge, and even at medium tension the settling time from rapping on it is unaffected (less than a second).
Best,
Mark
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reiner
super member
Reged: 09/28/05
Posts: 127
Loc: Freiburg, Germany
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Hi Mark,
for rigidity it is definitely better to cross the wires and to make the lengths of the bases on the outer ring and the secondary holder as long as reasonably possible. I have done a wire spider for my 22" Lowrider Dobson and it holds a 6" secondary without any problems.
More images of the spider are here
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Reiner
22" and 14" Dobs on EQ platforms and Deep Sky Observing
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Reiner,
Thanks for the comments, that's a nice design.
Best,
Mark
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2223
Loc: Boechout, Belgium
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Hi Mark,
Maybe a connection (soldering?) on the crossing would improve stiffness of the wire-spider even more. By connecting the wires at the marked spot you get a triangulation.
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Chris
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reiner
super member
Reged: 09/28/05
Posts: 127
Loc: Freiburg, Germany
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Hi Chris,
this is a good point. I stabilized the crossing points by winding some thin wire around it and soldering it. It sure helps (I think the indoor race bikers did this with their spokes to increase the stiffness of the wheel).
The stiffness would be even more increased if the second triangle becomes larger. This can be achieved by increasing the length of the base at the secondary holder.
Cheers
Reiner
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Reiner
22" and 14" Dobs on EQ platforms and Deep Sky Observing
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jg3
sage
Reged: 05/27/07
Posts: 332
Loc: near Auburn, CA
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One thing I liked in Mark's original wire layout is that the trapezoidal pairs can be aligned in the same vertical plane, so each lower wire is in the shadow of an upper wire, minimizing diffraction from on-axis targets. The cross-wire alternative precludes such alignment. (I realize that some, especially photographers, might prefer the more consistent spike intensities from crossing or otherwise off-plane wire pairs.)
The other thing it shows is that not all spans need to be strung alike. Too much symmetry can leave some modes of motion poorly resisted.
 One idea I'm getting from this is ringside collimation. This is by vertical shifting of either of the two ring attachment points at which two wires meet. The top lobe of the mirror would be stationary thanks to the opposite hub-vertex wire pairs. The only problem (for me at least) is the tilting of the plane of the other ring-vertex wire pair.
 I'm skeptical of the "good-better-best" diagram's implication that crossing the strings is what gains rigidity. It's the increased slope of the wires. (Of course, crossing them achieves a given slope in a smaller space.)
There's nothing like a mock-up, or a test in a real scope. Modes of motion are so tricky to visuallize, that I've experimented with string layouts using stuff in the garage only to discover motions not directly resisted at all. 
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Kobayashi
scholastic sledgehammer
Reged: 07/10/08
Posts: 977
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I'm not sure I understand the mechanism. Is the thin stainless piece designed to act as a flexure? But one of the RTV pads is on the part that doesn't flex, so wouldn't the adjustment put stress on the mirror?
Also you say it "makes its own spacers" but you want the mirror to not touch those bolts underneath, do you?
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Good comments. I can test the suggestion about soldering the crossing points by just clamping them temporarily. Well - it doesn't seem to make any difference. If the crossing were in the center of the spans, I expect it might, though.
JG3 says
Quote:
I'm skeptical of the "good-better-best" diagram's implication that crossing the strings is what gains rigidity. It's the increased slope of the wires. (Of course, crossing them achieves a given slope in a smaller space.)
The increased slope is a factor, but it's far outweighed by putting the strings in tension against the remaining freedom of motion (the arcs I depicted). Even over a very small "positive" leg it makes a noticeable difference. Try the geometry out with a loop of string if you like.
I take your point about the crossing increasing the shadow of the wires. This spider is designed for absolute minimum diffraction spikes. Crossing 2 wires will increase the total vane area by 25%, but I've already reduced the wire size nearly 50% from the design width due to the far better performance of this geometry! The small crossing reduces the vibration settling time for motion about 4 times to around 1/4 second. Seems like a good trade.
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
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Quote:
But one of the RTV pads is on the part that doesn't flex
All the pads are on the flexing plate. There are 3 collimation screws but the one at the bottom is too close to the light path to put a knob on top of it.
"Makes its own spacers" just means you don't have to put nickels or matches under the secondary while the glue sets...its a play on an old TV commercial. In use the plate needs to keep some tension on the collimation screws so that lifts the mirror off the bolt heads.
Best,
Mark
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Kobayashi
scholastic sledgehammer
Reged: 07/10/08
Posts: 977
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Quote:
All the pads are on the flexing plate. There are 3 collimation screws but the one at the bottom is too close to the light path to put a knob on top of it.
Thanks, I understand now. Initially I thought there were just two adjustment knobs, which would mean the back surface of the mirror stays in contact with (or very very close to) one of the bolts.
By the way, as for tying soldering the crossed wires, nobody does that with bicycle wheels anymore. It's been reported (e.g. The Bicycle Wheel by Jobst Brandt) that tying and soldering the crossed spokes has no measurable effect on stiffness. It's an archaic practice from the days of high-wheelers, and only intended to prevent a broken spoke from thrashing around and breaking other spokes. I doubt it would have any benefit on a telescope either; if it flexes enough that tying/soldering them would make any difference, it would flex too much regardless.
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Kobayashi
scholastic sledgehammer
Reged: 07/10/08
Posts: 977
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Quote:
JG3 says
Quote:
I'm skeptical of the "good-better-best" diagram's implication that crossing the strings is what gains rigidity. It's the increased slope of the wires. (Of course, crossing them achieves a given slope in a smaller space.)
The increased slope is a factor, but it's far outweighed by putting the strings in tension against the remaining freedom of motion (the arcs I depicted). Even over a very small "positive" leg it makes a noticeable difference. Try the geometry out with a loop of string if you like.
Um, I think JG3 is right about this one. The resistance to up/down movement only depends on the angle of the wires, not the relative position. And a pair of wires do not resist against tilt, no matter how they are arranged; the resistance to tilt comes from having three pairs of wires.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Nevertheless the reality of the structure doesn't agree, so in this case we're probably all mistaken. 
I've strung it twice both ways during the course of experimenting, and the effect is definitely there. Also I've had it setup both ways with the snubber screw for the shown pair installed (I'd left that out earlier) and it didn't make any difference - which is what I'd expect given the fairly sharp angles the wires make going into the holes.
If it depends only on the angle of the wires, the change in damping time is far too great to be explained by the small change in angle. Therefore it must depend on something else.
As it turns out the real explanation of the effect is more subtle. Take a look at the picture below. The vibration mode involved (which was the largest remaining of a small set) consists of pivoting across the two back vanes, essentially as a rigid structure. This is shown by the small purple arc (I drew the arcs backwards the first time, just to make if confusing I suppose).
In the top geometry the lower attachment point first moves to the left, then to the right, as it pivots up through the plane. In the bottom geometry the top attachment point always moves to the right as it pivots up. The restraining string thus resists the pivoting much better. This explains what happens in the real system, and it also explains why it works in general in wire spiders such as Reiners.
BTW I originally built the spider with a longer span on the "short arm" but a broken 6-32 tap on the very last hole on the flat plate rendered that one so much scrap.  I suspect but can't prove that lengthening the span in question would have little effect on the stability, given the geometry at work here, but it's something else to try. As it sits I'm extremely pleased with the performance so far.
Thanks for all the comments and suggestions!
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3659
Loc: Northeast
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This looks neat! But I can't grasp one concept about the wires. Does each wire contribute 2 "edges of diffraction"? Or is there something I haven't thought of that improves this defect? Or, maybe that isn't the goal? Does the obstruction of a typical sheet metal vane worsten diffraction in areas off axis?
Mark
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So many projects, so little time!
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jayscheuerle
Post Laureate
Reged: 01/16/06
Posts: 4413
Loc: S. Philadelphia, PA
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Quote:
Yes I was thinking of the blobs of siliconen. If they're not 'flexible' enough or to large you could end up with warping. I would dare to do this myself with very small blobs. In case of large blobs I would get suspicious.
Keep us informed...
I was thinking the same thing. If it does warp the secondary, perhaps a rigid plate between the two (flex plate - silicon - rigid plate - silicon - secondary) would remove the effect.
Essentially, when you flex the plate from flat, the points move closer together... but not by much. I'm looking forward to these results too! - j
--------------------
Fight indignorance!
120ED, 12" f/5 Green Goblin, 6" f/5 Eero2, 4.5" f/8 PortaBowl, 8" f/5 Big Red Ed.
The PortaBowl-a $100 4.5" f/8 ball-scope YOU can build!
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jpcannavo
sage
Reged: 02/21/05
Posts: 401
Loc: Long Island New York
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Mark C,
I especially like the design and execution of the mounting collimation mechanism. I also agree that “hypotenuse effect” on the diagonals figure should not amount to much - thickness of the substrate (quartz?) along with the yield in shear of the silicone should handle this. (I do however like something along the lines of Jay's suggestion as a possible fix, if the issue emerges)
Curious to see if significant deflection of the secondary towards/away from the primary (within the plane roughly defined by both the primary optical axis and the reflected optical axis of the secondary) as drawn in your above post becomes an issue with changes in altitude. In my own experiments with stringing – no tested models yet – I have had bad luck with triangles oriented “apex-at/close to-secondary”. But if it holds position here, I may give some version of this a go. Do caliper measurements show anything?
Mark H,
Wire spiders do avoid the increasing off-axis profile of sheet metal. They also have low thermal mass, which some have argued avoids an otherwise significant boundary layer of cooled air sitting along the surface of less than ambient temp metal. Personally I have also been intrigued by the eliminated possibility of light scatter from very low angle of incidence paths, striking an otherwise flat-black sheet metal surface. How contributory this effect is in traditional spiders though…?
Joe
Edited by jpcannavo (08/07/08 07:50 AM)
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3659
Loc: Northeast
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"How contributory this effect is in traditional spiders though…?
Joe "
*************
Exactly what I was wondering. I don't really know any source that would have specific information.
M.
--------------------
So many projects, so little time!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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I also came up with the idea of installing a second plate in between the secondary and the flex plate. Easy enough to do if it turns out to be needed.
In the quest for the limits of the design I restrung the spider with .008" strings last Friday. This works perfectly well, which is not too surprising considering the progress so far - the settling time changes from about 1/4 second to about 1/3 second. All vibrational modes with this geometry have strong damping. Next step will be seeing how that is in practice, as I'm calling it done for now.
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Joe,
I'm going to rig up a laser pointer mounted to the test jig along with a centered target to see if I can see any changes in collimation position with altitude. I haven't been able to measure any yet. I'll have the entire structure available pretty soon to mount it in.
Mark H,
The wires when aligned for overlap in on-axis present only the one pair of edges, that's the intention. This overlap for off-axis eventually fails of course - take the span of the wires at the center of the hub for the calculation - 85mm with a width (now) of .2mm. Sin(0.135 degrees)=0.2/85=0.0024. So over a true FOV of about 1/4 degree it still looks like one vane (worst case). More representative would be the average span of 45mm for which the the FOV is about 1/2 degree with the wires looking like one obstruction, albeit twice as wide. Pretty obvious though that for high power use (well collimated!) the second wire falls in the shadow of the first.
Best,
Mark
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kfrederick
scholastic sledgehammer
Reged: 02/01/08
Posts: 887
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hi mark nice spider are you going to paint it black ? how much better image do you think it will make ? how tight are you keeping the wires / thanks for the info kevin
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calan
sage
Reged: 06/16/07
Posts: 457
Loc: Oklahoma City, OK
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nice design... I take it you are familiar with guitar strings?
Did you pre-stretch them (like play them on an actual guitar for a couple of weeks), or are they new?
I'd also be curious to see how different types of strings work, as they all have different stretch properties and durability. (nickel, steel, stainless, etc.)
I would think the best would be something like GHS or similar that are made for extreme tremelo use without breaking.
--------------------
Orion f/4.7 XT10 (completely rebuilt, DOB or GEM mountable...details here)
Meade f/5 LXD75 6" Newt w/mods
Nikon 10x50 AE Extreme
Hyperion 8-24mm Zoom
Astro-tech 20mm SWA (2)
Siebert Optics BK bino/Power Mag Wheel
Various Plossls
Time flies like an arrow, but fruit flies like a bannana.
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jayscheuerle
Post Laureate
Reged: 01/16/06
Posts: 4413
Loc: S. Philadelphia, PA
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Quote:
hi mark nice spider are you going to paint it black ? how much better image do you think it will make ? how tight are you keeping the wires / thanks for the info kevin
I'd think painting them would kind of defeat the purpose of them being so thin as paint does have a thickness! Perhaps a black Sharpie would suffice?
--------------------
Fight indignorance!
120ED, 12" f/5 Green Goblin, 6" f/5 Eero2, 4.5" f/8 PortaBowl, 8" f/5 Big Red Ed.
The PortaBowl-a $100 4.5" f/8 ball-scope YOU can build!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Kevin,
"I see a spider and I want to paint it black" 
With apologies to the RS, well, no. Except for the parts of the hub itself, if any, that are visible through the focuser - which if I baffle correctly won't be any. I have tried using Sharpies to make blackened wires. But I eventually just cleaned 'em off again, as it makes no difference for scattered light. Why? Any light that hits the wires goes off in all directions, and none of it has any kind of focused effect.
How will it work? Considering the extra aperture, it should work better than the .008" wire spider on my 8", which works better than any other spider I've seen. The total obstructed area is about the same %, but the relationship between the mirror diameter and vane diameter is a lot higher. We'll see.
Calan,
Thanks! Yeah, I've broken more than a few thin strings. So I know how strong they are in practice. No, I didn't prestretch them, the tuning on the spider is all microtonal, and it doesn't matter very much. They're sitting around F above middle C (that's about 700Hz).
Different strings - these are plain steel, the store I frequent had heard of stainless strings but didn't have any. OTOH you can get stainless wire in any thickness by the roll from www.smallparts.com , though if you're only doing one it'd be spendy - $37 for 100 feet of .008", about the same price/foot as strings from the store.
Best,
Mark
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calan
sage
Reged: 06/16/07
Posts: 457
Loc: Oklahoma City, OK
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Quote:
No, I didn't prestretch them, the tuning on the spider is all microtonal, and it doesn't matter very much. They're sitting around F above middle C (that's about 700Hz).
I was thinking more along the lines of them maybe stretching and potentially letting your mirror sag after some temperature changes and use, unless they are pre-stretched before tightening down the mirror assembly.
Cool idea and nice machine work!
--------------------
Orion f/4.7 XT10 (completely rebuilt, DOB or GEM mountable...details here)
Meade f/5 LXD75 6" Newt w/mods
Nikon 10x50 AE Extreme
Hyperion 8-24mm Zoom
Astro-tech 20mm SWA (2)
Siebert Optics BK bino/Power Mag Wheel
Various Plossls
Time flies like an arrow, but fruit flies like a bannana.
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Kobayashi
scholastic sledgehammer
Reged: 07/10/08
Posts: 977
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Quote:
I was thinking more along the lines of them maybe stretching and potentially letting your mirror sag after some temperature changes and use, unless they are pre-stretched before tightening down the mirror assembly.
If the wires are tensioned during assembly, that means they are "pre-stretched."
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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He's referring to the settling-in period that new strings go through. You put them on a guitar and play them for about an hour before they'll hold pitch reliably. Or just bend the heck out of them, which is what I generally do anyway.
On the spider the tension doesn't matter very much so long as they all have some, so it shouldn't make any difference.
Best,
Mark
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2428
Loc: Courtice, Ontario
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Mark,
Since you're actively testing new wire spider ideas I thought I'd throw out an idea. What if you twisted the wires where they crossed to maybe reduce some vibration? I got the idea when I saw a picture of a ridiculously laced wheel on my cycling forum (probably just some wheel builder showing off), I included a picture of that wheel for a visual of what I mean. Worth a try?
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
My house.
DRAACO
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CoreDump
member
Reged: 11/06/06
Posts: 56
Loc: France
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Hi,
Have you tried or think about braid dyneema fishing line?
I was thinking of using this since it exhibit high young modulus (about 100GPa), quite lower than high strength steel but it could be usable.
Edited by CoreDump (09/05/08 08:37 AM)
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Gene Hunter
sage
Reged: 12/29/07
Posts: 292
Loc: SC
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Very cool stuff, I would think though braiding the wires would cause their thickness to bulge at the point of the braid. It certainly looks like it would be much stronger though.
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Clear Skies
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2428
Loc: Courtice, Ontario
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It can only be 2x the thickness, same as if they just cross each other. Though that 2x width would be a little longer if they were twisted together.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
My house.
DRAACO
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Coredump:
Fishing line works - I've used it in a pinch - but it creeps and the strength is not nearly as good. I've experimented with Vectran as well (see Vectran supplier they also sell Kevlar thread). It has less creep than Kevlar and better strength (or at least strength/weight, not sure). It's very strong but is essentially a yarn and without waxing or some treatment it frays a lot.
Re braiding:
At first I'm not sure what value braiding the wires would have. I doubt it adds to the strength as it acts like mild kinks, and I think stringing would become a nightmare - you need to adjust all the wires relative to each other and their anchor points in the hub and tensioners as you position the hub in the final position. Takes some iteration to get it just right and then under tension, but braids would possibly bung this up. Looks cool though.
Then I went and tried it out roughly - though my initial impression is that the braids will add to the cross section viewed along the optical axis over more of the wires than simply crossing, from holding a couple of braided strings up to look at. I'll have to put them under working tension and see what happens to be sure. It may be that if the braid is done right it will reduce the overall cross section, which would be more than cool - that'd be awesome.
Best,
Mark
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4590
Loc: Atlanta, GA
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Actually, the main advantage I can think of for braids is that they may make the edge of the wire curve like a curved-vane spider. In other words, they may spread the diffraction spikes out.
Mark - if you have the scope out, try stringing a single braided wire across at an angle between the other wires, and see if it makes a new diffraction spike at all.
Jarad
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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Well, that scope is still made of raw materials in many places but I see what you mean. Even if a pure sinusoidal curve on both sides (like an idealized screw thread would look in profile) the braid will still produce a good spike - the reason is a bit convoluted if approached from diffraction theory of obstructions, but here goes...
Each segment of the edge scatters in an orthogonal direction. In the upper crude picture (the braid) the predominant direction that results is 90 degrees to the main body (the little red arrows). Without converting it to frequency, in the lower crude picture (the pure sine curve of the stretchy wire) it gets much closer to spreading it out, but still there's necessarily more edge that runs left-right then up-down.
Put another way, if you add up all the little orthogonal arrows, neither case scatters 360 degrees, and the braid case with its added obstruction does considerably worse than the single wire, albeit the latter is longer and thus has more edge. I have accumulated some sample obstructions that illustrate these effects and eventually when I happen to have the right mirror to put them in the tester path will try to get some artificial star examples of the actual diffraction spikes that result.
In any case you don't need a wire spider to test what a braided wire produces - just align it on any newt (or refractor-like-object) and look at a bright star.
Best,
Mark
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4590
Loc: Atlanta, GA
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Quote:
Put another way, if you add up all the little orthogonal arrows, neither case scatters 360 degrees
You don't need a single wire to scatter 360 degrees, only 180 (i.e. you want to spread the two spikes out into two 90-degree fans.) The other wire will spread out its spikes into 2 90 degree fans as well, oriented perpendicular to the first pair. Together they will add up to 360.
Also, getting exactly 360 isn't critical - if they were only spread out into 4 45-degree fans vs. 4 spikes, they would still be greatly dimmed.
Yes, the easiest way to test is to just string a braided wire across any scope - either at a different angle than the existing spider on a newt, or better yet cross a refractor or other spiderless design (so any spike you see is from the braided wire).
Jarad
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2383
Loc: salem, OR
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No, what I mean is there will still be spikes from any single (or braided) wire/vane that has a predominant straight line component, as in the examples. Most of the diffracted light will appear in a slightly blunted spike. Although this requires testing to see what the effect is, it's one of those things where if it did work it would be in common use - but that's not a guarantee. And the second case is useless in practice.
BTW you can easily tell what a braided wire is doing even with existing spikes by just rotating it about the aperture... I did a lot of those kinds of experiments when first looking at diffraction effects.
Best,
Mark
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jg3
sage
Reged: 05/27/07
Posts: 332
Loc: near Auburn, CA
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Twisted or braided wires are a case where the "popular" diffraction theory of edges runs afoul of the well-established but mathematically complex physics theory. The clearest counterexample to the edge misunderstanging is to make an eccentric aperture mask, with *two* circular apertures instead of one. This actually results in a spike! Even more if you string a bunch of circular apertures in line, similar to braided or twisted wire. Diffraction is so full of surprises!
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