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ebfoxbat
member
Reged: 01/21/08
Posts: 23
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I'm just wondering... why aren't strut-tube refractors common.? I realize ATM refractors are not really common as is. But it occurred to me, couldn't one just use a strut design and a drank cloth to lighten the tube?
How about drilling holes in the tube? (Clearly I'm no ATM, I'm just curious.) For example, couldn't one take a store-bought refractor, remove the focusser, mount it lens up, drill out some holes from the aluminum tube and cover the tube in a dark cloth? One would then have a lighter tube tube. Assuming the drilling was done non-violently the lens should stay in culminated. Also, if drilled with some thought, the tube could certainly retain it's structural stability. I would think having an "open" tube would allow shorter thermal cooling time. Also, any weight removed from the tube would be added in the form of an accessory.
Since I've only ever owned catadioptic scopes, I may be way off. Is the weight of a 4" frac tube negligible as is and thus the entire thought not worth while?
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Art Bianconi
Post Laureate
Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
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The questions you've asked are appropriate. As with all "features", the cost of creating them has to be balanced against the benefits they provide.
I know an aircraft builder who built a lovely fabric covered biplane, a Steen Sky-bolt. He spent almost 20 years on that project.
What made his project notable was his fanatical devotion to lightening holes everywhere. It took him almost six months to build the horizontal stabilizer which was all sheet metal ribs and a steel tubing frame.
His devotion to drilling lightening holes in everything was apparent in the stabilizer. The drilling process turned it into a work of art that was only appreciated while it was bare. When covered with fabric, doped and painted, no one would see it. Drilling took him an additional 5 weeks. The weight savings? 14 ounces on a plane that weighs in excess of 1200 pounds!
The difference in weight provided by a drilled tube in an OTA is not likely going to be noticed unless of course, it is to be lifted into a planetary orbit around Saturn by NASA.
While the concept has some merit, it's likely that the idea would be abandoned in favor of an open truss which would have the additional advantage of being lighter, more compact for travel and offer collimation alternatives.
Don't let this response discourage you from thinking about such things. A curious mind that is looking for alternative solutions is a precious thing.
I urge you to keep doing it. Look at Thomas Edison!
Art
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“Everything is on its way to somewhere. . . . . everything!"
____________________ George Malley (John Travolta)
________________________ "Phenomenon"
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1825
Loc: Central Texas
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Quote:
Is the weight of a 4" frac tube negligible as is and thus the entire thought not worth while?
That, and the fact that even a 4 element 4" refractor just doesn't take very long to cool down. A 4" doublet's cooling time is virtually nil .
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David
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ebfoxbat
member
Reged: 01/21/08
Posts: 23
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Honestly, my motivation is this:
(I'm pretty sure) I plan on ordering a Celestron 102 SLT. I'm going to try to heavily load it. Second to adding a second fork arm (which would be complicated and perhaps not too bad of a cloudy night project) I had been thinking about lightening the tube itself since money is rather short (hence the 102 SLT in the first place.
I don't know what that tube is made of. I'm told it heavy so maybe it's not aluminum.
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Rob Willett
Vendor (Degree Circles)
Reged: 02/07/05
Posts: 653
Loc: London, UK.
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Here's a homemade strut refractor. Total cost, very little.
80mm, achro lense from Surplus Shed. Works pretty well and has to be easiest refractor to collimate. I use a 13mm spanner.
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Thanks,
Rob
Edited by Rob Willett (01/23/08 03:45 PM)
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mattyfatz
Pooh-Bah
Reged: 12/27/06
Posts: 1296
Loc: Boise Idaho
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Wow Rob, I've never seen that before. How do you mount it? Does it focus along those struts or is that a helical focuser. There are a bunch od SS junkies around here ( like me  that are going to want to know MORE 
Nice Work
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**Matty**
Stimulating the economy, one piece of equipment at a time.   
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mattyfatz
Pooh-Bah
Reged: 12/27/06
Posts: 1296
Loc: Boise Idaho
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PS...ROB
are those PVC caps, or drain plugs that you used for the cells?
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**Matty**
Stimulating the economy, one piece of equipment at a time.
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jdownie
scholastic sledgehammer
Reged: 02/24/06
Posts: 759
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Old zoom lens focuser?
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ATM project - a terrible waste of good Pyrex.
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Pedestal
Post Laureate
Reged: 03/11/06
Posts: 3857
Loc: Smoggy Bottom, Baytown,Texas
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The tube (it's sheet metal) is the lightest part of the SLT102. The lense assembly on one end/the focuser on the other, either far outweigh the tube. The SLT mount is limited it what it will carry. If you really want to "load it up", it would make more sense to go to a CPC, or GEM mount. The SLT is a great little mount, I love mine for a G&G rig. But I think trying to make it do something for which it is not designed would be an exercise in frustration. And possibly expensive.
Hubert
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Hubert
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www.smoggybottom.org
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jcjr
professor emeritus
Reged: 01/06/08
Posts: 563
Loc: TN, USA
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This link, a google preview section of the book "Unusual Telescopes", page 183--
Much shorter link that doesn't people scroll left and right
Description of a Godzilla-scale Strut Refractor constructed by Hevelius in the 17th century.
"One approach used two boards, attached along their lengths to form a trough with an L cross-section. This worked well enough on dark nights but during twilight observations or on nights with strong Moonlight, stray light entering the eyepiece decreased contrast... One of the later solutions was to place a series of black aperture stops along an open-frame tube assembly. From the eyepiece, no portion of the sky could be seen except thru the objective."
If one constructed a strut refractor frame of arbitrary length, and 'healthy' diameter (6 inches?), it could be like an optical bench on a tripod? One could swap out objectives and make a few adjustments, and be good to go.
Got thinking thataway from the sled focuser thread. Add an objective, adjust the basic optical length within the range of a helical focuser on the diagonal, and there would be no obstructions for wide low power views?
If one finds a promising objective lens that seems to work well, one could easy/cheap make a set of aperture stops from construction paper or other light material to insert within the struts? Rather than laboriously ray-tracing the minimum number of stops, just trace the light cone with lightweight baffles spaced about an inch apart?
Another alternative would be to construct a moderate-sized scope with model-airplane design strategy, though perhaps composites 'better' than balsa wood could be found?
I had been wondering about weight vs rigidity. A small scope's tube is thin but pretty rigid. Maybe you couldn't save weight with rods, since there would be fewer rods, but they would need to be thicker-heavier?
That said, Rob's strut refractor LOOKS very rigid.
Edited by Don W (01/24/08 10:31 AM)
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Rob Willett
Vendor (Degree Circles)
Reged: 02/07/05
Posts: 653
Loc: London, UK.
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This refractor was done as a winter project in the UK as we have had basically 100% cloud cover and the Chief Financial Officer refused to let me buy another telescope. The woman can buy as many pairs of boots as she wants, but a small new Tak, no way 
The picture is older, it now has a simple plywood base fixed between two of the struts. A vixen dovetail connects to that.
The plastic ends are very thick drain pipe ends. The rod is 8mm rod I had lying around. I actually do collimate it with a 13mm spanner.
The focusor is an old M42 telephoto lens with all the glass removed. This works well and has the advantage that a camera will fit straight on.
The scope is pretty rigid. The more I tighten it the better it gets.
The reason for the truss's are that I couldn't get any tube of the right size at all. Plastic tubing in the UK is very standardised and nobody stocks intermediate sizes at all. It's all 110mm (4.5in) drain pipes so I had to come up with a better way of doing it and the truss idea just popped up.
My intention was to make a pair of these as binoculars but I forgot my eyes aren't six inches apart so have given up on that. This version will be for web camera and DSLR camera usage and I will build a second version with far shorter trusses that will allow a diagonal to be used. I have an old focusor off a Tasco or something similar that I was given.
I made this in a few evenings but the interest it's generated is quite funny. I took it down to my astronomy club and a large crowd gathered around it, my nice 100mm TMB was wholly ignored in place of a $30 home made scope.
I'll post more pictures once it's mounted and the clouds clear away and I can actually use it.
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Thanks,
Rob
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mattyfatz
Pooh-Bah
Reged: 12/27/06
Posts: 1296
Loc: Boise Idaho
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I want one
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**Matty**
Stimulating the economy, one piece of equipment at a time.
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Rob Willett
Vendor (Degree Circles)
Reged: 02/07/05
Posts: 653
Loc: London, UK.
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matt,
I am happy to sell it to you or better still swap it for your 13.1 Coulter. Let me know where to pick up from?
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Thanks,
Rob
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TheDarkOne
super member
Reged: 01/16/08
Posts: 121
Loc: Bishop
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I am very interested in this scope design. I am planning a trip to south america, and I need something that will travel well. The ability to break this scope down for a backpack, then re-assemble it in the field makes it very appealing. I have posted several threads here about travel scopes, but this seems to be my solution. you are a true geniouse. Do you have any ideas for a portable, or modular mount and tripod?
thank you for posting.
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Things are not as they appear...
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jcjr
professor emeritus
Reged: 01/06/08
Posts: 563
Loc: TN, USA
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The three-rod idea looks great!
Haven't had much experience, but recently reworked and baffled a small cheap 70mm refractor as an initial experiment. Made the baffles out of worn-out sanding belts, which are stiffer than cardboard, have a rough surface to randomly-scatter light, and of course worn-out sanding belts are not really good for much else (grin).
I cutout little rings and cylinder sections painted flat black. Stuffed them one by one down into the scope tube.
It wasn't hitech or difficult, but OTOH a disc or cylinder sized to closely fit the scope tube, can be pretty annoying to shove down to the correct place in the tube. It would seem a lot easier to mount such baffles on an open frame.
One possible light, stiff, inexpensive material, could be that thin white waterproof plastic/fiberglass wallboard. It is pretty rigid in small pieces.
Maybe use 0.25" - 20 tpi threaded steel rod for the three struts. Cut a bunch of round fiberglass wallboard baffles. Paint em black. A bunch of 1" plastic or aluminum spacers slightly bigger than the threaded rod. Each baffle gets slipped down the rods, held against the spacers with nuts.
With a stack of ring baffles spaced about every inch down the struts, locked down on the threaded rod, perhaps the assembly would be quite stiff? And pretty good contrast?
It probably would not save much weight. Perhaps it would be heavier than an equivalently-sized tube. But construction would be very straighforward and 'modular'? One could perhaps modular-design the back-end with diagonal and focuser so that the back piece could be easily swapped between different frames?
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CPC 1100, C102SLT, SV F80, Meade 70 & 60 AZT
Q70 38mm, Pan24, Meade 5K 18mm UW, Axiom LX 15mm, 10mm, 7mm, Nagler 13T6, Expanse 20mm, 9mm, 6mm, BO/TMB 5mm, 2.5mm
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Rob Willett
Vendor (Degree Circles)
Reged: 02/07/05
Posts: 653
Loc: London, UK.
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I thought about how to make it more modular and easy to break down. The only thing holding it in place are the nuts either side of each of the ends. If you epoxied the internal nuts and marked which rods go in what hole then it should stay reasonably collimated. The other solution is to use lock nuts on the inside and allow yourself a little amount of collimation.
No real thoughts on mounts as I haven't needed to yet. I use a small photo tripod and it works for me. I don't think I can get it smaller than a decent tripod.
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Thanks,
Rob
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Rob Willett
Vendor (Degree Circles)
Reged: 02/07/05
Posts: 653
Loc: London, UK.
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Quote:
The three-rod idea looks great!
Haven't had much experience, but recently reworked and baffled a small cheap 70mm refractor as an initial experiment. Made the baffles out of worn-out sanding belts, which are stiffer than cardboard, have a rough surface to randomly-scatter light, and of course worn-out sanding belts are not really good for much else (grin).
I cutout little rings and cylinder sections painted flat black. Stuffed them one by one down into the scope tube.
It wasn't hitech or difficult, but OTOH a disc or cylinder sized to closely fit the scope tube, can be pretty annoying to shove down to the correct place in the tube. It would seem a lot easier to mount such baffles on an open frame.
One possible light, stiff, inexpensive material, could be that thin white waterproof plastic/fiberglass wallboard. It is pretty rigid in small pieces.
Maybe use 0.25" - 20 tpi threaded steel rod for the three struts. Cut a bunch of round fiberglass wallboard baffles. Paint em black. A bunch of 1" plastic or aluminum spacers slightly bigger than the threaded rod. Each baffle gets slipped down the rods, held against the spacers with nuts.
With a stack of ring baffles spaced about every inch down the struts, locked down on the threaded rod, perhaps the assembly would be quite stiff? And pretty good contrast?
It probably would not save much weight. Perhaps it would be heavier than an equivalently-sized tube. But construction would be very straighforward and 'modular'? One could perhaps modular-design the back-end with diagonal and focuser so that the back piece could be easily swapped between different frames?
I thought about doing this and haven't got around to it due to time. I agree that using baffles that mount on the rods will increase stiffness. I was going to use that astronomers composite favourite, plywood. 3mm should be OK and stiff enough. Paint it with blackboard black and job done.
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Thanks,
Rob
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mattyfatz
Pooh-Bah
Reged: 12/27/06
Posts: 1296
Loc: Boise Idaho
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Why not just wrap in in a black sheet like a big old truss dob?
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**Matty**
Stimulating the economy, one piece of equipment at a time.
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mattyfatz
Pooh-Bah
Reged: 12/27/06
Posts: 1296
Loc: Boise Idaho
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Quote:
matt,
I am happy to sell it to you or better still swap it for your 13.1 Coulter. Let me know where to pick up from?
I'll tell you this... that DOB rarely makes it out of my garage. Its clumsy and it weighs over 100 pounds. Its always out of collimation and it takes forever to get it right. I'd sell it if it was worth it, but for what I put into it so far it isnt. I think I'm keeping it for spite. I thought about converting it to a truss Dob but its too much work for me. I do enjoy the views when all the BS is done though.
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**Matty**
Stimulating the economy, one piece of equipment at a time.
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jcjr
professor emeritus
Reged: 01/06/08
Posts: 563
Loc: TN, USA
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Quote:
Why not just wrap in in a black sheet like a big old truss dob?
Matty, that might be the way to go. Maybe black velvet, with velcro along the seam for attachment?
Had been thinking maybe aperture masks could give a cleaner image, but if there are no reflections, maybe the baffles would be overkill?
Or perhaps one could do both-- A few baffle rings might enhance rigidity, and also provide a well-defined profile for the black velvet sleeve to 'hug'?
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CPC 1100, C102SLT, SV F80, Meade 70 & 60 AZT
Q70 38mm, Pan24, Meade 5K 18mm UW, Axiom LX 15mm, 10mm, 7mm, Nagler 13T6, Expanse 20mm, 9mm, 6mm, BO/TMB 5mm, 2.5mm
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