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White Lightning - the Fastest Hexapod on the Planet

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#1 Jeff Morgan

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Posted 04 May 2021 - 10:34 PM

If you are like me, you spend a lot of time browsing the internet for telescope making ideas. The ATM community is one of incremental refinement, so there are many good ideas to pick and choose from to solve the issues you face.

 

Every so often, progress is punctuated by a big leap of innovation. When I came across Jon Pogson’s Merope thread and his Hexapod truss system, it struck me as one of the big leaps. Not because it used six tubes, but because of the implications for construction and collimation.

 

At the time I became aware of the Hexapod, I was well into the planning stages of a rebuild on a 16” f/7 Dob. The scope worked well, but there was a new driving force: Gen 3 Night Vision astronomy. In the f/7 scope, the NV eyepiece was the most profound leap in visual astronomy I had seen in 45+ years. So, I wanted to build a scope specifically for NV.

 

I was proceeding down the path of Reiner Vogel’s Low Rider concept as it would create more back-focus for focal reduction and also allow the telescope height to be lowered (since no one ever asked for a taller ladder).

 

After seeing Merope I attempted to create a fusion of the Low Rider and Hexapod concepts. It was proving to be a handful. Both upper end prototypes I built were unsatisfactory and the project languished.

 

It was time for a reset. It occurred to me that for NV performance, it would be easier to make a faster mirror slow (when needed) rather than make a slower mirror fast (which is what the image intensifier wants most of the time). Then it just became a question of “how fast”.

 

Conversations ensued with Mike Lockwood of Lockwood Custom Optics, and I made a deposit for a 16” f/2.8 primary mirror and 5” elliptical secondary mirror. Mike was a pleasure to deal with, one of the Good Guys in this little cottage hobby. I am glad to see him back from on CN. In addition to being very available for my design questions, Mike delivered my optics nearly a month ahead of schedule! When he releases me to share the test results on that glass, let’s just say that a lot of LZOS refractor owners will be envious. But that is the subject for another thread.

 

Which such a short focal length (44.7” actual ), the need for the Low Rider concept went by the wayside, allowing for a pure hexapod build. While my pace was slow (outside commitments), the build itself was actually straightforward. Perhaps even easier than the standard Obsession template.

 

Unfortunately I botched the last layer of a 5 color multi-cam pattern last fall. (Yes, I was the kid that flunked Art Class in the 3rd grade.) My paint shop is an unheated garage, and cold weather came early. Was I done until April?

 

My solution to move forward (it seemed like such a good idea at the time) was to select a paint I could apply by brush indoors - exterior latex. Fine for house, but for a telescope not so much. If you get nothing else out of this thread: never - not ever - paint a scope in latex.

 

The good news - such as it is - I am now positioned to capitalize on the spring/early summer observing window. This will provide operational experience and uncover any needed adjustments. When summer monsoon season hits the latex will be stripped and sanded off, and modifications implemented. The repaint will be done in airbrush-applied Durcoat, a very thin and tough paint used to camouflage firearms. Probably the same theme you see here (since I already named the scope), but with a deeper midnight blue as I had originally intended.

 

Enough of the Background material! Photos and feature details to follow ...

 

If I did not mention it, 16" @ f/2.8. Pointing at the zenith the overall height is 52". And ServoCAT of course wink.gif

 

 

 

 

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#2 Jeff Morgan

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Posted 04 May 2021 - 10:42 PM

A view down the tube. The UTA liner is pulled taught between carpet tacks. When I take it all down for painting in July, it will be re-installed to "relax" more against the inner circumference of the ring.

 

If you are wondering, it is ProtoStar black flocking. While it was available, I bought a large roll. That was ten years ago. Still have enough for one more project ...

 

Second picture is the tail-end of the OTA. I use a Silenx 11db computer fan. Ultra-quiet means ultra-low vibration. Still rated for 19 cfm. Note the rear baffle. Also, the termination of the truss tubes is exactly the same point as the connection of the mirror cell.

 

And of course, collimation knobs Need Not Apply - it's a Hexapod! Had the hardest time convincing John Pratte to make the cell that way ...

 

 

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#3 phonehome

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Posted 04 May 2021 - 10:49 PM

It's ALIVE!  The son of "Stubby" lives!!!! 

 

Great job Jeff!  More pics please.

 

Ed



#4 Jeff Morgan

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Posted 04 May 2021 - 10:50 PM

The key components are:

 

  • ServoCAT with Argo Navis, SkyFi wireless;
  • John Ashcroft primary cell;
  • Lockwood 16” f/2.8 primary mirror, 1.3” thickness, 18.5 pounds, Zambuto coating;
  • Rob Teeter 13” radius aluminum altitude bearings;
  • 1.25” aluminum truss tubes;
  • Lockwood 5” minor axis secondary, 1” thickness, 2.5 pounds, Zambuto coating;
  • FeatherTouch prototype 2” Parallizer focuser;
  • DIY offset four vane spider using the Gary Wolanski design; and
  • Shrouds by Heather Skirt.

 

A six tube design needs some help to use a shroud - the shroud is not adequately supported at the 10 o’clock and 2 o’clock positions and wants to sag into the light path. A problem (hopefully) solved with a few cup hooks placed along the periphery of the mirror box. 

 

And now the part you really wanted to see - a look beneath the skirt!

 

Component details ... manyana.

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#5 Oberon

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Posted 04 May 2021 - 11:09 PM

waytogo.gif waytogo.gif waytogo.gif 

A highly enviable and super neat package. Very flattered to find myself mentioned. 


Edited by Oberon, 04 May 2021 - 11:18 PM.


#6 Jeff Morgan

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Posted 04 May 2021 - 11:27 PM

waytogo.gif waytogo.gif waytogo.gif 

A highly enviable and super neat package. Very flattered to find myself mentioned. 

 

I would be remiss if I did not thank you for your help getting started!

 

And of course, JT Senghas (engineering and wood shop tips) and Mitch Alsup (PLOP analysis).

 

Hopefully I did not miss anyone. A distinct possibility for a project that drags out for five years.



#7 TOMDEY

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Posted 04 May 2021 - 11:44 PM

Very Nice! I especially like the White Lightning Motif. And indeed hexapods rule. Although I never incorporated that into any of my telescopes, it was common on the giant scopes we built at work. We also incorporated those little commercial hexapods into many of our test sets. They would execute x y z roll pitch yaw flawlessly over an envelope about the size of an orange. And then when you hit "home" it would return from whence it came to within an arc-sec and a micron... quite amazing.   Tom

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#8 rob1986

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Posted 05 May 2021 - 01:48 AM

what is the correction like with the elliptical secondary? and is the primary parabolic?

 

I've been thinking about this for years, just haven't had the money (or time!) to invest in actually researching it, and hadn't realized that its an actual known design. but whats the correction on the primary? it has to be very high to avoid the kind of resolution problems SCTs have with their very fast primaries.

 

misread to imply the optics were folded in a Newtonian using surface curvature in the secondary. my bad.


Edited by rob1986, 05 May 2021 - 01:50 AM.


#9 Adam Long

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Posted 05 May 2021 - 06:32 AM

Looks great. waytogo.gif  I'd be interested in some details on your truss terminations, they look very neat.



#10 Jeff Morgan

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Posted 05 May 2021 - 12:07 PM

Very Nice! I especially like the White Lightning Motif. And indeed hexapods rule. Although I never incorporated that into any of my telescopes, it was common on the giant scopes we built at work. We also incorporated those little commercial hexapods into many of our test sets. They would execute x y z roll pitch yaw flawlessly over an envelope about the size of an orange. And then when you hit "home" it would return from whence it came to within an arc-sec and a micron... quite amazing.   Tom

 

Last summer when I did focus testing to verify the mirror spacing, I also did a test collimation.

 

The truss stability and ease of collimation were highly encouraging. The first Newtonian I ever collimated was 1977, a RV-6 on loan from my high school science department. I have seen many arrangements over the years. Nothing was as easy or accurate a this hexapod! The first time you do it, without every taking your eye way from the focuser, it is a revelation. Perhaps I will do you a YouTube video on this?

 

Finished ServoCAT set-up this morning. I think I will run an AUTOCAL test before lunch, but I am ready for first light tonight as-is.


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#11 Jeff Morgan

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Posted 05 May 2021 - 12:31 PM

Looks great. waytogo.gif  I'd be interested in some details on your truss terminations, they look very neat.

 

For the terminations, I used 1.25" Delrin inserts machined by a local gunsmith, 1/4-28 threads left and right handed. The Delrin rod and metal parts are all from McMaster-Carr.

 

From the plugs, threaded rod and female spherical rod ends (aka Heim Joints). A crossbar is spot welded into the eye of a machine hanger screw. The nuts on the cross bar are only present to keep everything attached during storage, transport, and handling. A 1" hand knob with a threaded thru hole is used to tighten everything down.

 

The only tool used for assembly of the scope is a Phillips head screwdriver used to remove the altitude encoder. And I will go to the hardware store this afternoon for a thumb nut to replace that screw.

 

I tried leaving the encoder attached during assembly and disassembly (it seemed like such a good idea), but the dangling encoder arm was an accident waiting to happen. After a few mishaps of bumping the arm into things (the last collision bending the aluminum arm!), I counted myself lucky the optical encoder was not damaged.

 

Attached are photos of the terminations. The UTA is pretty straightforward. A hole in the ring, the spherical rod ends bear against a large washer attached to the underside. Tighten the hand knob, and it draws the rod ends up into the washer.

 

The LTA works in a similar fashion, the hole going thru the center of the primary mirror cell mounting bolts.

 

If you have every assembled a Moonlight or similar ball & socket system, you know how that can be at first - like herding cats! The truss tubes want to flip and flop every which way. After a few assembly attempts, one discovers the best way to manage that.

 

So far, what I do is the lower connections first, then tighten the crossbar nuts enough to hold the crossbar vertical. Then I lower the UTA getting one of the hanger bolts up thru a UTA hole. At that point the weight of the UTA is supported by a truss pair and the other two pairs can be lined up.

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#12 MitchAlsup

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Posted 05 May 2021 - 12:37 PM

When I first built my 13" F/3 it stood low to the ground

 

FtDavis01.JPG


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#13 MitchAlsup

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Posted 05 May 2021 - 12:39 PM

After this first use, I raised it up to eyepiece height (about 62 inches):

 

13newbase02.JPG

 

I like it a lot better at its taller height.


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#14 Mike Lockwood

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Posted 05 May 2021 - 08:44 PM

Great looking scope Jeff, since you're a pilot I'm going to have trouble not calling it "Blue Thunder" given the color and graphics!

 

I look forward to many Sharpless photos taken through it, among other things.

 

By the way, Ed mentioned my 14.5" f/2.55 above, which I have started calling "Stubby".



#15 Jeff Morgan

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Posted 05 May 2021 - 11:48 PM

After this first use, I raised it up to eyepiece height (about 62 inches):

 

attachicon.gif13newbase02.JPG

 

I like it a lot better at its taller height.

 

That is a good (and smart) looking tripod!

 

I added a couple of inches to the rocker box. More about that later.

 

Too early to be sure, but I think I will need some type of additional "lift kit".



#16 Jeff Morgan

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Posted 05 May 2021 - 11:49 PM

Great looking scope Jeff, since you're a pilot I'm going to have trouble not calling it "Blue Thunder" given the color and graphics!

 

I look forward to many Sharpless photos taken through it, among other things.

 

By the way, Ed mentioned my 14.5" f/2.55 above, which I have started calling "Stubby".

 

I wondered about the Son of Stubby reference ...



#17 peleuba

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Posted 06 May 2021 - 09:52 AM

 

If I did not mention it, 16" @ f/2.8. Pointing at the zenith the overall height is 52". And ServoCAT of course wink.gif

 

Wow.  That's nice!   

 

Mike Lockwood is making me a 16" F/3.7 for a JP Astrocraft, and can't wait to see it later this year.

 

I really like the Spider/Secondary Holder design you've implemented.



#18 Augustus

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Posted 06 May 2021 - 10:18 AM

Looks fantastic!



#19 Jeff Morgan

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Posted 06 May 2021 - 12:36 PM

Wow.  That's nice!   

 

Mike Lockwood is making me a 16" F/3.7 for a JP Astrocraft, and can't wait to see it later this year.

 

I really like the Spider/Secondary Holder design you've implemented.

 

Thanks, the design is essentially a scaled up version of the unit Canadian amateur Gary Wolanski used to sell. Minus the collimation screws of course, not needed on a hexapod. Unfortunately, Gary retired.

 

Mike Lockwood gave me some input as to material thickness and lugs. IIRC, 0.022 thickness mild steel. All told, parts and materials ran about $8. Except for the shroud of course. That is AstroSystems. I ordered his secondary holder, but only used the shroud! 

 

The holder only requires two adjustments: rotational and longitudinal, both used for centering the diagonal under the focuser. It is secured by the brass thumb knobs. Note the hitch pin to prevent the secondary from doing a swan dive into the primary. The offset of 7/16" away from the focuser is built-in. After painting I put the front plate on the drill press and made a small dimple (the shiny spot) used to mechanically center during installation.

 

One aspect of this design: If desired, I could quickly remove the secondary holder for storage and transport, and install for use. During my initial testing I was able to install the secondary and finish collimation thru the autocollimator in under ten minutes, easily.

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#20 peleuba

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Posted 07 May 2021 - 08:30 AM

Thanks, the design is essentially a scaled up version of the unit Canadian amateur Gary Wolanski used to sell. Minus the collimation screws of course, not needed on a hexapod. Unfortunately, Gary retired.

 

 

Thanks for the additional details and extra photos.  It looks terrific!  I did notice the built-in offset.  And, I remember Gary Wolanski.  Also like the hitch pin to prevent a disaster...  the elegance of this is in its simplicity.  bow.gif

 

Funny - I have a box of secondary parts - mostly Astrosystems - as I harvest the shroud from these holders, too.   On my Portaballs, I use some of the Astrosystems parts, then have some other pieces made to my specifications by a machine shop.  In the past I have used PreciseParts, but he seems to not want to do anything except adapters for astrophotography.  So I was able to find several guys on CN who do high quality machining.



#21 Aperturefever

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Posted 07 May 2021 - 06:19 PM

Wow. Now that is what I call a light bucket.
Sit that next to a blue tube Coulter to get an idea of the evolution of a dobsonian!

#22 Jeff Morgan

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Posted 08 May 2021 - 09:27 AM

A better look at the components, Let’s start from the ground up. The rocker box is of standard design. What else can one say about a rocker box? It is strictly speaking perhaps 2” longer on each dimension than would be expected for this aperture (24" per side), but I will touch on that in a future post (hold those thoughts).

 

It looks more complicated (and crowded) than it is primarily due to virtually all of of the electronics residing inside the rocker box. The only externalities are the ones that absolutely had to be outside. And most of the cabling is far longer than needed. I’ll address that during weather or full moon down-time. If for no other reason, the cables were migrated from my last Dob and have seen heavy usage over the last 10 years.

 

One cable seems to have developed a problem: the altitude motor DSC cable that the ServoCAT box uses to read motor position. It appears to be responsible for erratic motor movement. So first light did not happen this week, Gary Myers of StellarCAT is already sending the replacement hopeful arriving Monday.

 

A ServoCAT powered ground board is utilized. 12VDC comes in from a plug on one of the feet and feeds through the 1” diameter azimuth pivot. A nice system to avoid cord wrap.

 

Bronze sleeve receptacles for wheelbarrow handles are incorporated, but I don’t anticipate using them. With a new hatchback sedan on the way, the scope had to fit that vehicle also. The scope components are sized to go in the hatchback as separate pieces of manageable weight. ServoCAT complicates that slightly, more on that in the Mirror Box post.

 

The lifting weight of the rocker box is 38.6 pounds. That includes the ground board you don't see in the photos plus the electronics and motors. If you're wondering, that is with a 3/4" thickness bottom board and 1" thickness altitude bearing boards. I could do a few lightening cuts here and there, and holes for handles would be nice. Weight savings would be marginal though while providing more dust ingress as the holes would necessarily be close to the ground. (How I miss those grass observing fields of the midwest.)

 

I made the rocker box a few inches taller than it needed to be to make viewing the southern Milky Way a bit more ergonomic. This also provides easy reach under the OTA for securing of the truss connectors to the LTA, since my arrangement has the securing mechanism bottom-mounted. The ServoCAT box can be mounted at an upward angle such that status lights and switches are easily seen when needed.

 

With the scope being so compact, the hand pad coiled cable is long enough to be available from any observing position. No more wireless hand pad! For storage is rides in the mirror box, in use it is attached with velcro to the altitude bearing just below the focuser.

 

In fact, all of the electronic components now reside on the rocker box floor, even Argo Navis. The cables have enough slack that I can withdraw ARGO for star alignment, then put it back in the rocker box for the rest of the night. DSC computers necessarily have two serious user interface limitations: small controls and a very small alpha-numeric data display.

 

YMMV, I find using a touch screen tablet with a GUI as the “front end” of the DSC computer is a vastly better user experience. My Modus Operandi is to hand off control from Argo to SkySafari via WiFi once 2-star alignment is complete. So while Argo has a key role in system operation, my nightly interaction with it is generally done in the first five minutes. That did not warrant dedicated stalk space for it. Out of sight, out of mind.

 

For for those of you keeping score, so far I have been able to eliminate:

 

1) The ladder;
2) The wheelbarrow handles;
3) The car ramps;
4) The ServoCAT wireless hand pad; and
5) The wired CATStalk.

 

There is a stalk a 1” aluminum tube and Markless Astronomics shelf for the iPad. Strictly speaking I could ditch that too, but it is more convenient than leaving the iPad on a nearby folding table with the eyepiece case.

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#23 Jeff Morgan

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Posted 09 May 2021 - 12:02 PM

Next section is the mirror box. Except, of course it is not a box. But after 50 years of Dobsonian nomenclature, probably easiest to call it a "box".

 

Traditional eight tube geometry favors square shapes, while six tube geometry favors round shapes. One could argue that in terms of space efficiency, the square is better.  This 16” aperture sits in a structure whose outer dimensions are 24”.

 

OTOH, is cubage in the back of your vehicle the limiting factor? And how close do you want to crowd your mirror with heat-retaining and air-flow limiting structure? Not to mention more closely spacing the observers body to the optical path. The human body is reported to give off 400 BTU’s at rest (the equivalent of a 100 watt incandescent bulb).

 

Long and the short of it, I find the bulk trade-off acceptable. (And since I went down the hexapod path, it is not like I had much choice, right?)

 

There is a generous 1.5” air gap around the primary mirror and everything still fits easily in the car. The mirror box weighs in 33 pounds, which includes 15 pounds for the steel cell and a few more pounds for the aluminum altitude bearings. Less than half of that 33 pounds is wood.

 

The ring was also much easier to build than a box, no worries about joint strength or whether or not the box is actually square. Setting a dozen clamps while squaring the box before the glue sets can be a hectic wood shop exercise, even with a longer setting glue like TiteBond III.

 

The ring is constructed from four sheets of Baltic Birch plywood laminated together for a total thickness of 2.45”. The resulting slab was trimmed rectangular on a table saw to 24” x 20-5/8”.

 

After router work to round the long sides and remove the center, I was left with 2.45” x 12” flat surfaces for attachment of the vertical altitude bearing support boards. This did not strike me as adequate surface area to glue a simple butt joint, so each support board is reinforced with one 1-1/4” and several 7/8” hardwood dowel pins.

 

The altitude bearings are Teeter 13” radius aluminum with Silver Vein powder coated finish, bolted to the vertical support board. Silver Vein has a wonderful feel on Teflon, and of course does not use contact cement. It should last virtually as long as the underlying metal.

 

If I left the rocker and mirror sections nested for transport and handling (as most ServoCAT owners do) they would be too tall to fit in the back of a hatchback. So I needed a few small mods to make the ServoCAT connections quicker. The first of the three connections is the altitude encoder cable, a RJ11 jack which is as simple as it gets.

 

The second connection is the altitude drive cable. When the cable is unwrapped from the drive roller, it goes slack. During reassembly, aligning the slack cable to the channels in the teflon pads is like trying to herd cats. This was solved with a simple bungee tensioner. The bungee solves that problem neatly.

 

The third connection is the altitude encoder arm. Virtually every ServoCAT installation features the arm at an angle with respect to the vertical (typically 45 degrees). Perhaps to avoid conflict with wheelbarrow handles? Whatever the reason, it makes lowering the mirror box onto the rocker box awkward.

 

What I did was mount it at 90 degrees so it can be lowered straight down. Taking a file to the bottom of the arm to flare the slot opening makes it easy guide the arm over a fixed guide pin on the rocker box. I have used this approach on a smaller Richard Berry style refractor mount where it works well. The piece is small and relatively light, the arm can easily be rotated out of the way by a finger on the lifting hand.

 

However, that idea proved to be awkward when scaled up. The overall piece is heavier, and the long encoder arm is a hazard to bumping into things, which happened several times. The last time was going around a corner in the house with an impact sufficient to bend the aluminum encoder arm! I got lucky on that one, the encoder itself did not appear damaged and read normally in power-up testing.

 

Plan B is removing the entire encoder arm via the 10-32 set screw. After a few trials, removing the encoder arm entirely is the safest and preferred method, at the cost of an extra set-up step. Currently a long cap screw is used for that which requires a hex key for assembly. I always carry a hex key set in my accessory bag, but I'll order a 2" thumb screw to return to no-tool assembly.

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#24 bigbangbaby

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Posted 09 May 2021 - 12:50 PM

^^Jeff, nice design and execution! Please tell us how it performs.



#25 MitchAlsup

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Posted 09 May 2021 - 04:29 PM

Traditional eight tube geometry favors square shapes, while six tube geometry favors round shapes. One could argue that in terms of space efficiency, the square is better.  This 16” aperture sits in a structure whose outer dimensions are 24”.

The most efficient design is one whee the poles deliver/take the loads from the mirror support frame without going through a "mirror box". You design does this properly--the poles connect to a structure that passes the loads to the mirror frame by way of very short and stiff paths.

 

I have been using this philosophy since 1989 when I build my first DOB.

 

Congratulations--well done




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