Does anyone have experience or factual information regarding the Plettstone or Dobstuff truss designs? I can see how these designs would be lighter than a regular six pole truss-tube with a UTA, but I'm wondering if there are any major drawbacks. For one, it appears that your optics are much more exposed. I'm wondering about flexure, ease of setup/breakdown, or any other issues you may know about.
Three Pole Lightweight Truss Designs
Started by pwp818, 05/31/2007 07:42PM
Posted 05/31/2007 07:42PM
Opening Post
Posted 05/31/2007 09:54PM
#1
Light shrouds must be specially made to hold their shape, if you need one of those.
Tear down and set up looks like it is a bit more work with the DobStuff design, Plettstone too, but I guess that is not a big deal maybe a couple of minutes. But I have not actually put one together, so I am going by what I see.
The optics are more exposed and Dew is a bigger issue. Stability is good, the tubes are large diameter. Lightweight is an advantage they have. Portability is good.
I tried to get Plettstone to build a scope around my 10" f/6 Zambuto mirror, but the design was not applicable they said and could not justify modifying it for a one off scope. To bad.
Plettstone is a very well made scope of excellent design. Scopestuff looks like a nice version of this type too.
Tear down and set up looks like it is a bit more work with the DobStuff design, Plettstone too, but I guess that is not a big deal maybe a couple of minutes. But I have not actually put one together, so I am going by what I see.
The optics are more exposed and Dew is a bigger issue. Stability is good, the tubes are large diameter. Lightweight is an advantage they have. Portability is good.
I tried to get Plettstone to build a scope around my 10" f/6 Zambuto mirror, but the design was not applicable they said and could not justify modifying it for a one off scope. To bad.
Plettstone is a very well made scope of excellent design. Scopestuff looks like a nice version of this type too.
[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue
Amateur Imager
Posted 06/01/2007 12:53AM
| Edited 06/01/2007 01:08AM
#2
Hi:
I recently completed a Dobstuff conversion of a 16 inch Meade Starfinder. I have been very happy with the scope in general and with working with Dennis Steele, he's a prince.
-----
Apparently there is some issue with flexure of the poles. Initially there was quite a bit of movement a laser collimator beam as the scope was moved from horizontal to vertical, it was on the order of a quarter of an inch.
According to Dennis, Albert Hinge who is responsible for the strut scope design, all experience some flexure, but not on this level.
I spent a good deal of time tracking the collimator "dot shift" down. I made a solid top plate out of a 1.125 inch 24 inch laminated pine disk to investigate whether the thin Meade spider with the 4 inch secondary was part of the problem. This seemed very likely. I clamped the laser to the poles to look at their bending, moved the focuser around. I spent about 20 hours looking at various possibilities, trying another spider, installing the secondary upside down, installing the secondary with long rod with a heavy weight hanging from it....
Anyway, what I discovered was that there were two main problems. The less than ideal Orion Crayford I had waylaid for this project actually was the source of some of the flex but most was caused by the top ring, it was not stiff enough. I finally just cut an 18 inch hole in the the test plate, installed the spider and the "dot shift" of the collimator was reduced down to about 0.040"-0.060".
Dennis at Dobstuff is going to double the thickness of the top plate by laminating a second plate with the original top plate. He will do this at no cost if I ever get the gumption to send him the original.
After I made this major improvement, I still worked some more on reducing the "dot shift" of the collimator beam.
One issue to be aware of is that many collimators do not put out a nice round dot surrounded by diffraction rings but my 7 year old Howie Glatter puts out these perfect little stars. With the open design, I can get within inches of the dot so I can see minute shifts that would difficult to see without a top notch laser like a Howie. It would probably be difficult to see if I were not so close to it but it could be seen.
OK: Back to the story. The Dobstuff design uses 3 long poles between the top and bottom plates and 3 short poles between the bottom and middle plates.
Since my measurements showed there was still some flex, ie "dot shift", I figured out that I could use some tensioning rods between the middle plate and the top plate, from the short poles to the long poles to stiffen things up.
These were easy to make an only required some 1/4 threaded rod, some 1/4-20 screw eyes and some turn buckles. I had to drill one hole for each rod and I was done.
I tried two and I tried six, I decided two was enough, I will post a photo later. It reduced the "dot shift" even further and it is at a level I find acceptable.
-----
Enough of the flex: My evaluation of the scope... I haven't used a lot of truss scopes...
I like it, the OTA weighs 55lbs, the base 32 lbs, pretty light for a scope with a 26lb mirror. The action is nice with those big altitude bearings. The scope is not as stiff as my 12.5 inch Tube DOB but it settles down quickly. From a stiffness standpoint, it is very much like a friends Astrosystems 16inch.
As far as setup time. I timed myself setting it up the first time I took it to the mountains. It took right at 10 minutes. This included the time required to move the parts off the bed in the motor home and out the door and then set the scope up. I would guess if everything were together, it would take about 5 minutes.
I put the scope together myself and I am not a woodworker, I am a researcher/mechanical engineer so I built it like it was metal, lots of 1/4-20 bolts, aluminum corner pieces, lots of tapped holes. I think it took me about 3 solid days.
There is still some stuff to do but I have to say I am one happy camper, it is certainly in a different league than the 100lb+ starfinder OTA....
Yesterday I completed another strut design Newtonian, this one is a 4 pole design and I have included a photo of it alongside the upper ring of my Dobstuff scope. (The top ring in the photo is the crude test ring I made and sawed with a sabersaw.)
They say the Meade Starfinders are a "Scope in Progess." This one has progressed a long way...
Best to all
Jon
I recently completed a Dobstuff conversion of a 16 inch Meade Starfinder. I have been very happy with the scope in general and with working with Dennis Steele, he's a prince.
-----
Apparently there is some issue with flexure of the poles. Initially there was quite a bit of movement a laser collimator beam as the scope was moved from horizontal to vertical, it was on the order of a quarter of an inch.
According to Dennis, Albert Hinge who is responsible for the strut scope design, all experience some flexure, but not on this level.
I spent a good deal of time tracking the collimator "dot shift" down. I made a solid top plate out of a 1.125 inch 24 inch laminated pine disk to investigate whether the thin Meade spider with the 4 inch secondary was part of the problem. This seemed very likely. I clamped the laser to the poles to look at their bending, moved the focuser around. I spent about 20 hours looking at various possibilities, trying another spider, installing the secondary upside down, installing the secondary with long rod with a heavy weight hanging from it....
Anyway, what I discovered was that there were two main problems. The less than ideal Orion Crayford I had waylaid for this project actually was the source of some of the flex but most was caused by the top ring, it was not stiff enough. I finally just cut an 18 inch hole in the the test plate, installed the spider and the "dot shift" of the collimator was reduced down to about 0.040"-0.060".
Dennis at Dobstuff is going to double the thickness of the top plate by laminating a second plate with the original top plate. He will do this at no cost if I ever get the gumption to send him the original.
After I made this major improvement, I still worked some more on reducing the "dot shift" of the collimator beam.
One issue to be aware of is that many collimators do not put out a nice round dot surrounded by diffraction rings but my 7 year old Howie Glatter puts out these perfect little stars. With the open design, I can get within inches of the dot so I can see minute shifts that would difficult to see without a top notch laser like a Howie. It would probably be difficult to see if I were not so close to it but it could be seen.
OK: Back to the story. The Dobstuff design uses 3 long poles between the top and bottom plates and 3 short poles between the bottom and middle plates.
Since my measurements showed there was still some flex, ie "dot shift", I figured out that I could use some tensioning rods between the middle plate and the top plate, from the short poles to the long poles to stiffen things up.
These were easy to make an only required some 1/4 threaded rod, some 1/4-20 screw eyes and some turn buckles. I had to drill one hole for each rod and I was done.
I tried two and I tried six, I decided two was enough, I will post a photo later. It reduced the "dot shift" even further and it is at a level I find acceptable.
-----
Enough of the flex: My evaluation of the scope... I haven't used a lot of truss scopes...
I like it, the OTA weighs 55lbs, the base 32 lbs, pretty light for a scope with a 26lb mirror. The action is nice with those big altitude bearings. The scope is not as stiff as my 12.5 inch Tube DOB but it settles down quickly. From a stiffness standpoint, it is very much like a friends Astrosystems 16inch.
As far as setup time. I timed myself setting it up the first time I took it to the mountains. It took right at 10 minutes. This included the time required to move the parts off the bed in the motor home and out the door and then set the scope up. I would guess if everything were together, it would take about 5 minutes.
I put the scope together myself and I am not a woodworker, I am a researcher/mechanical engineer so I built it like it was metal, lots of 1/4-20 bolts, aluminum corner pieces, lots of tapped holes. I think it took me about 3 solid days.
There is still some stuff to do but I have to say I am one happy camper, it is certainly in a different league than the 100lb+ starfinder OTA....
Yesterday I completed another strut design Newtonian, this one is a 4 pole design and I have included a photo of it alongside the upper ring of my Dobstuff scope. (The top ring in the photo is the crude test ring I made and sawed with a sabersaw.)
They say the Meade Starfinders are a "Scope in Progess." This one has progressed a long way...
Best to all
Jon
Attached Image:
Funding Member
