8"GSO dob, EQ platform, a case full of EPs
Terry Terry McInturff President, Terry C. McInturff Guitars, Inc. SkySpy 10 prototype...10" f6 dual strut dob, premium glass Various ES, TV,UO,Ultrascopic, etc eyepieces 4.250" f4 RFT Newt doubles as SkySpy "finder"
ATM Nut.6" F15 Jaegers Refractor on Homebuilt EQ mount12.5" Homebuilt NewtProjects:16" F7.2 Reflector
Quote:Most materials are stronger in tension than compression. In general, this means you need more material on the bottom than the top of the tube. When pointing at the horizon, you could replace the top tube with a string. Now, all that theory is a bit wasted as soon as you point upward.allan
Quote:Quote:Most materials are stronger in tension than compression. In general, this means you need more material on the bottom than the top of the tube. When pointing at the horizon, you could replace the top tube with a string. Now, all that theory is a bit wasted as soon as you point upward.
If you do the math, I think one finds that neither the compression strength nor axial stiffness of the tubes are important, it's the bending stiffness that is important and at first guess, it doesn't matter the orientation in terms of whether the two struts are below or above.
Equal spacing of the tubes probably optimizes the stiffness of the upper ring which ties the tubes together and increases the stiffness of the entire structure in bending. A single pole with one fixed end is a cantilever, if the upper end is "guided, which means it can move laterally but it is prevented from rotation at what would be a free end free end, then the pole is 4 times stiffer. This is one purpose of a stiff upper ring.
Quote:Most materials are stronger in tension than compression. In general, this means you need more material on the bottom than the top of the tube. When pointing at the horizon, you could replace the top tube with a string. Now, all that theory is a bit wasted as soon as you point upward.
I have some questions about the string scope mod though.
My plan is to use one length of metal cable with a turnbuckle. My Lower OTA endring with the old Lightbridge truss clamp hardware would be rotated 120º. ie. the 3 old clamps would be equidistant between the 3 struts. The cable would loop under the smooth shaft of the old truss clamp bolts up over a bolt through the top of the next strut, down to the next old truss clamp bolt, up...... etc etc If needed I could fit pulley type bearings to the bolts so that the looping cable doesn't snag and that when I turn the turnbuckle it applies equal pressure on all the struts.
Jon you mentioned previously that I couldn't use moonlite ball and socket clamps for the UTA attachment to the struts and that I should use UTA clamps instead. Can you explain why the moonlite ball and sockets don't work in letting the string tension the struts and prevent flexure?
Celestron 8SE Dobstuff 13.1": Swayze refigured Coulter mirror, 6 pt mirror cell (2 pt edge support) and CF focuser board made by me StarBlast 4.5 ST80/PortaMount II Zhumell 20x80/Oberwerk 15x70 on a Seronik-style tripod boom mount Hubble Optics 18 inch F/4 mirror.
Quote:Jon you mentioned previously that I couldn't use moonlite ball and socket clamps for the UTA attachment to the struts and that I should use UTA clamps instead. Can you explain why the moonlite ball and sockets don't work in letting the string tension the struts and prevent flexure?
Quote:careysub, is Dyneema/Spectra better than the archery string everybody uses?
Quote:\ Ball and SocketsThe ball and socket on the tops of the struts idea was to ease replacement of the UTA when setting up after a full scope breakdown for transport. I accept Jons reasoning for it not necessarily being a good idea in terms of Cantilever etc. However, what about ordinary Moonlite truss clamps for two of the struts and a ball and socket on the third strut. Would I then be able to have my cake and eat it too. ie. Ball and socket on one strut allows me to firmly anchor/take the weight of the UTA on one strut leaving both arms free to guide the UTA clamps onto the other struts. While the Truss clamp attachment to the other two struts prevents the problems assosciated with using all ball and sockets???
Quote:That is, (and I believe true for the original discussion above as well) if the B&S connectors allow rotation once clamped. I think they resist rotation, meaning that while the arrangement won't be as stiff as, say, 1.25" long cylindrical clamps, it will be much stiffer than if you mounted the ring on the B&S connectors and didn't tighten them down (the factor of four Jon mentioned). Most flex may come from the neck of the ball, not from rotation.I'll be interested to hear if Jon has some comment on this last part.
and if you spring-load the poles then none of the strings need a separate tensioner (the poles themselves provide this.
Quote:Joint looks bendy to me.
Quote:A good presentation of the principles of a string truss telescope is section VII of this document:http://bossanova9.org/astro/ATM/5-Tubes&Trusses.pdfA true string truss does require a minimum of 3 pairs of cables, all six of which are separate tension elements. The key difference between a strut telescope and truss telescope is that the struts bend, one side of each strut is under tension and the other under compression, while in a truss the elements are each either under pure compression or pure tension - no bending. A string truss is thus a true truss - it relies on pure tension with rigid poles under pure compression serving only to maintain the tension.If you adopt a true string truss design then the requirements on the poles are greatly relaxed compared with the strut telescope. You could reduce the number of poles to two, make them much lighter, and if you spring-load the poles then none of the strings need a separate tensioner (the poles themselves provide this).You can also make a hybrid "string assisted" strut telescope. This uses struts to provide the basic structural rigidity, with strings to take up additional load in certain directions. To handle the deflection caused by shifting in elevation, only one string on top is needed, though you could use one on either side instead. You can provide load handling in all directions with 3 strings, you don't need six.
Quote:Thanks - note that they did work in his 8 inch scope though. Some sort of non-springy screw-adjustment pole, a version of which he used for his "Mark 2" model is probably the best all-around approach.
Quote:...When you talk about 3 pairs of strings, do you still mean 6 discrete cables because you have used 'pair' and 'discrete' in the same sentance so I am still confused.
Quote:I had actually realised how I might be able to have 3 physical pairs. A loop on each end and in the middle of the cable. I've seen the metal cable doodads for this. Perhaps a new drawing will show what I mean
Quote:I presume this new idea eliminates the problem of cable creep that was talked about when I asked if I could just loop a single run of cable over and under bolts. While at the same time meaning I don't have to deal with 6 individual cable runs but instead 3 pairs so to speak.
Quote:So if we have arrived at a workable cable/string system and a workable tensioning system given what you said in your last paragraph, can I switch back to using 1" OD poles and clamps instead of the 1.25" versions. In the grand scheme of things given the weight of the Lightbridge UTA, Moonlite focuser, Paracorr, 21mm Ethos, 80mm finder etc etc the weight savings in the poles is not a consideration. What is, however, is that I already have 4 spare 1" Moonlite Truss clamps and would save myself $72 by not having to buy all 10 I would be using on this project. After all the Lightbridge truss poles are 1" OD so with a 'Virtual Truss' with the strings could I get away with my 3 struts also being 1" OD.
Quote:...Now that I have struts that run parallel to the OTA I am wondering can I move my Stellarvue F80 Finderscope (2.5Lbs) from my overcrowded OTA to the uppermost strut. I would mount it to another one of those moonlite clamps on the strut. ie nothing to do with those same types of clamps I use to affix the struts to the OTA but using yet another one of those clamps a few inches down the strut from the TA as a movable mounting along the length of that uppermost strut. Why a Moonlite clamp instead of just bolting a Dovetail shoe to the strut?? Twisting the strut for tension might put the shoe at the wrong angle. With the shoe bolted to the moonlite clamp I just loosen the clamp and rotate it.
Quote:... So the question is would this affect the rigidity of the strut? Final position of the shoe on top of the strut or final position pointing sideways towards me standing at the side of the scope. This would mean I wouldn't have to lean in as far to the Finderscope eyepiece. ie. Striaght up and the weight would be in the direction of the strut, whereas mounting off the side of the strut would put the lever arm alongside the strut. Would this torquing of the strut cause issues?
Quote:If it did cause issues to a 1" OD strut, Could I just make the Finderscope strut a 1.25 incher but leave the other 2 struts as 1 inchers. Thus still being able to use the 1 inch clamps I already own.
Quote:...Its just clicked for me what you were saying about the relationship of the strings and struts. ie. That when you add strings to a strut system the struts cease to be the main structural member so to speak, its the strings doing all the hard work and the strut just becomes the means of keeping the strings under tension as it were.
Quote:So would I be correct in saying that I can have my threaded joint that enables tensioning by means of twisting at any point along the strut. You see I thought I had to have a contiguous length of strut between the Lower OTA clamp and UTA clamp because the tiniest amount of play between telescopic strut pieces would mean collimation shift. However I think I now realise that a tiny amount of lateral play doesn't matter in the strut if the strut is only there to keep tension on the strings which are the real structural elements and maintainers of collimation.
Quote:So I can do away with my Double Lower OTA clamps idea and have the short twistable section of strut up near the top of the struts with an outer sheath of tube over the split threaded sections. Or inner sheath I guess. Which ever I find first that fits outside a 1" OD pole or inside a 1" OD .050" wall thickness pole. Hopefully this inner or outer sheath tube keeps the sections in alignment with little or no play but that a small amount of play is not a big deal anyway.
Quote:I have made a couple of 10", 3 truss scopes. Very successful although I did have two stiffening wires to stop vibrations.