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J.T.'s 12.5" F/4.3 Hexapod Dob

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#226 tommm

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Posted 08 July 2019 - 10:10 AM

Beautiful workmanship jtsenghas!


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#227 jtsenghas

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Posted 08 July 2019 - 10:21 AM

The new project, was wondering if you were going to put the push on for 2019.

Alas, I won't be either there or at the Oregon Star Party this year. If I could be in two places at one time those would be my choices. I've made it to Stellafane only once so far and really enjoyed the experience, especially seeing Al Nagler feigning a heart attack at seeing a MEADE eyepiece in my Paracorr! 



#228 jtsenghas

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Posted 15 January 2020 - 09:46 PM

I'm back working on this project again. 

 

When I visited Ed Jones a couple of weeks ago regarding our mutual Chiefspiegler projects he kindly put a number of my flat mirrors in his Zygo interferometer. That day I hadn't the secondary with me for this build because I hadn't found it quickly that morning. 

 

This morning Ed received from me by mail a few more printed lens cells from me as candidates for his 6" f/12 Chief, along with my 2.6" minor axis GSO secondary for testing.  Since his Zygo can handle up to 4" diameter,  it can see the entire surface of this flat.  Ed promptly tested this flat today. 

 

The images were a bit disappointing to me. It appears that along the short axis there is very little power,  but along the long axis there is at least a third of a wave of power as shown below: 

 

IMG_1189_compress45.jpg IMG_1188_compress20.jpg IMG_1187_compress81.jpg

 

So,  how much astigmatism will be created approximately 1/3 wave surface error (fairly regular power) at 45 degrees degrade my views? The 12.5" f/4.3 primary is quite good.  Can anyone help me quantify this? 

 

It's beginning to look to me that this secondary will merely be a placeholder in this build...


Edited by jtsenghas, 16 January 2020 - 06:44 AM.


#229 jtsenghas

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Posted 15 January 2020 - 10:23 PM

By the way,  as I was pushing last year to try to get this scope done for the Cherry Springs Star Party a few minor items weren't coming out exactly to my satisfaction.  The neodymium magnets I had purchased locally were too small and weak for my satisfaction to be effective setup aids.  The 1/16" x 1/2" angle aluminum I'm using as V- block locators for the screw eyes are a little shallower than I'd like for the size of the eyes,  both for height and contact. 

 

When it became clear that at least part of the Star Party last June would be wet and muddy and that this scope would be unsealed if I brought it in a functional if not finished state,  I knew this scope would suffer. 

 

Now I'll take my time to redo a couple of little bits and finish this scope to my satisfaction.  I have ordered some slightly larger aluminum angle from Online Metals (local pickup for me in Toledo) and I have larger magnets to install. Although the existing pieces are epoxied in,  I know I could heat them to remove them,  chop the recesses a little deeper and replace them. 

 

With a few minor refinements I think this will be an effective hexapod.  I'm on the fence about possibly playing with tapering the tube ends on this scope. I'm playing with some ideas to that end.  If I don't use those here I will elsewhere,  and I'll certainly share those ideas and methods in another thread I started a few months ago. 

 

Too many irons in the fire...

 

Stay tuned. 


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#230 jtsenghas

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Posted 07 March 2020 - 10:12 PM

So, I decided to switch this project over to a parallel strut design:

20200307_194854_compress45.jpg



This decision should simplify the design and reduce the number of parts.



JUST KIDDING!

I'm actually using some scrap lumber to perform a focus test on the nearly full moon. (Now,  how many of you furrowed your brows at that photograph? wink.gif )

I'll be using this scope with a Paracorr 2, which needs the shoulder of the focuser about 14 mm inside of prime focus. Since the upper tube assembly has a low profile focuser I want to be careful and have the focuser at least 1/4" away from bottomed out when the Paracorr 2 is used.

Although a hexapod has some additional adjustment for focus with overall truss lengths, the screw eyes I'm using aren't really long and I want to start in the middle of their ranges.

This arrangement with temporary struts turned the upper tube assembly 60 degrees, and put the focuser on top, but that worked out fine for tipping elevation in my backyard.

I started with the lengths of the timbers set at what I estimated would be 5/8" longer than my design calculation. I used the actual 5/16" stainless bolts and anchors the final scope will use. In the end I had to chop the boards down 11/16" from that starting point.

Not bad. My calculations had been right within two millimeters. That small error may be entirely in how centered my roughly collimated secondary is located.

If I had been brave and had instead cut my aluminum tubes and epoxied inserts into them tonight, I would have been fine. I have also verified the straight line distances from the lower to upper attach points 60 degrees apart and can more confidently assemble my tubes.


Edited by jtsenghas, 08 March 2020 - 05:18 PM.

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#231 jtsenghas

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Posted 13 March 2020 - 09:26 PM

Small steps,  but getting there. 

 

Tonight I scuffed the insides of the tube ends,  mixed up some JB Weld, and epoxied the inserts into the truss tubes. 

 

Then,  for some "belts and suspenders" insurance,  I also riveted each insert with three 1/8" aluminum rivets left over from some gutter and downspout work. 

 

20200313_214302_compress72.jpg

 

I discovered by luck that after installing one rivet at each end,  the 3/4" guide groove in my Shopsmith allowed me to rotate the tube almost exactly 120 degrees each way when the first rivet head contacted the table.  This made it easy to equally space the rivets at each end.  I quickly got all 36 rivets popped into place. 

 


Edited by jtsenghas, 14 March 2020 - 05:34 AM.

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#232 SteveV

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Posted 13 March 2020 - 10:00 PM

The patience and thought in this is wonderful.  


Edited by SteveV, 13 March 2020 - 10:03 PM.


#233 Oberon

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Posted 13 March 2020 - 11:33 PM

Small steps,  but getting there. 

 

Tonight I scuffed the insides of the tube ends,  mixed up some JB Weld, and epoxied the inserts into the truss tubes. 

 

Then,  for some "belts and suspenders" insurance,  I also riveted each insert with three 1/8" aluminum rivets left over from some gutter and downspout work. 

 

attachicon.gif20200313_214302_compress72.jpg

 

I discovered by luck that after installing one rivet at each end,  the 3/4" guide groove in my Shopsmith allowed me to rotate the tube almost exactly 120 degrees each way when the first rivet head contacted the table.  This made it easy to equally space the rivets ast each end.  I quickly got all 36 rivets popped into place. 

I must be a tightwad. After installing one single rivet per end I decided that the plug couldn’t go anywhere so that was that!  grin.gif



#234 jtsenghas

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Posted 14 March 2020 - 05:46 AM

I just realized some work I did on February 23 got reported in "What did you work on Today? ", but not here. 

 

Well,  I received a load of firewood,  got a nice fire going in my workshop woodstove, and finally resumed work on my 12.5" dob.  I had failed to stock up on firewood this year,  and time playing in my detached garage shop nearly stopped because of this. 

 

On that cold day I honed a nice edge on a couple of chisels and reworked the V block nests for my hexapod eyebolts on both the UTA and base.

 

20200222_165557_compress13.jpg

 

I had used that jig to chisel deep enough recesses for the 1/16" x 1/2" x 1/2" aluminum angle I had picked up locally last year.  I decided later that the eyebolts were a little large for that and nested practically at the edges.

 

I live within driving distance of an Online Metals distribution location, so I didn't have to pay exorbitant shipping costs for a $3 piece of 1/16" x 3/4" x 3/4" aluminum 24" long. This belt sanded neatly to about 5/8" at the right angle.  I moved a few screws on my jig to widen it and quickly chopped recesses the perfect depth almost with hand pressure alone. I love using sharp tools.

 

20200222_163749_compress74.jpg

 

These pieces of aluminum were roughed up with 80 grit paper on the outside and epoxied into place.

 

Shown at left in the last photo in the vise is a piece of leather I glued to a block and rub with green compound.  Thirty quick strokes of a chisel against that strop makes it "wicked pissa' sharp"!

 

 

 


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#235 jtsenghas

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Posted 14 March 2020 - 05:52 AM

I must be a tightwad....

... says the man using carbon fiber on a binoscope including on experimental corrugated secondary mirror spider vanes! 


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#236 jtsenghas

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Posted 15 March 2020 - 03:47 PM

This afternoon I sharpened some of my chisels and lathe tools and tried sweeping the wings back on my small detail gouge a bit and making the bevel sharper more like the "fingernail gouges" I see used by some expert turners on YouTube.  This shape makes it easier to roll the gouge while making curves on changing grain. 

 

While doing that I looked over at the Shaker knob on the mirror cover for this scope.  Funnily enough,  that "Shaker" knob left over from a kitchen project was probably CNC turned in China.

 

Meh.

20200315_162408_compress62.jpg

 

I made a small screw chuck next,  which is something I've been meaning to do for some time and made a replacement. 

 

20200315_162437_compress60.jpg

 

There!  That should work MUCH better at covering my mirror! wink.gif

 


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#237 a__l

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Posted 15 March 2020 - 05:42 PM

jtsenghas, 

Did you use a router for all of your circles or did you do some with a blade?



#238 jtsenghas

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Posted 15 March 2020 - 05:47 PM

jtsenghas, 

Did you use a router for all of your circles or did you do some with a blade?

I had nearly as many inside arcs as outside arcs to make,  and the altitude bearing pieces nested,  so I used a router with a straight fluted bit.  The homemade circle jig can be found on page 2 of this thread in post 31.


Edited by jtsenghas, 15 March 2020 - 06:11 PM.


#239 jtsenghas

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Posted 15 March 2020 - 10:18 PM

Whew! 

 

The hexapod mechanisms appear to work perfectly now that the V blocks are a little wider.  I did have to shorten by about 2 mm the opposite end of the "toe clamp" as I'm calling this miniature hold down in order to adjust for the deeper V block. I haven't yet embedded screws, washers and magnets at that fulcrum location. You can see a temporary washer standing about a millimeter proud of the surface under the button head screw pivot. 

 

20200315_214337_compress40.jpg

 

Shown above is one of six identical truss termination points. This one is on the inverted UTA.

 

 

Here's a view of it from the side. 

 

20200315_214820_compress8.jpg

 

Since the 5/16" stainless cap screw is at the middle of the clamp, only half its force is directed onto the eye bolts. That's still more than enough to hold it really securely though with very low torque.  I might make just a ball end 1/4" Allen wrench on a screwdriver handle for assembly of the six attach points to allow for a tilted tool between inserts. 

 

The underside of the homemade "cross nut" has been ground to a taper so that the eyebolts are pulled against the tapered sides of the clamp at exactly the correct angle for the trusses.  I showed this last July on the previous page of this thread here

 

I put two jam nuts on the right handed one and it took a LOT of torque before it tried to swivel at all.  This makes me confident that my eye bolts will not tend to shift or rotate during collimation.  They should still tip the  negligible angles needed with modest length adjustments. 

 

On my first attempt with slightly smaller angle aluminum the eyes tended to catch on the edges of the V blocks and had to be coaxed into position with a squeeze during tightening. With these 1/8" wider ones they lock right down radially in the V and tangentially against the clamp with mere inch pounds of torque. 

 

The cross nut rods are long enough to allow the truss assembly to fold up parallel, and the end washers and screws should keep everything together. 

 

All I need is to color code one such attach point for a consistent assembly direction to minimize collimation adjustments at set up.  The plastic tie wrap on one of the screw eyes is just to remind me which ones are left handed until I'm done.  I was afraid I might make at least two trusses with same handed bolts,  which would make adjustment impossible. I think I'll put all left handed ones at the bottom. Adjustment will be more intuitive for me if the typical right handed threads are at the top where I will grasp the poles. 

 

I'm STOKED! choo-choo.gif

 

 

 

 

 

 


Edited by jtsenghas, 15 March 2020 - 11:53 PM.

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#240 jtsenghas

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Posted 21 March 2020 - 09:06 PM

Well,  I'd better take advantage of what short sessions I can on this project.  I'm actually still quite busy at work programming water jet robots for a new truck hood launch that is a little behind schedule. Fortunately almost all my interaction at work is with yellow robots in the midst of this COVID-19 Virus situation. 

 

Tonight I bored three 1/2" diameter holes 0.300" deep in each my UTA and base exactly 5/8" from my truss attachment holes.  I slightly countersink some #8 washers for 3/4" long flathead #4 wood screws which just fit through my holes in the 1/2" diameter neodymium magnets. The mostly hollow UTA rings are nearly 7/8" thick and solid at these locations. 

 

20200321_191319_compress91.jpg

 

I decided to stack two magnets at each location for extra holding force as setup aids.  The steel screw and washer help to transmit the magnetic force quite well and I'm confident that my steel button head screws on my clamps will grab well enough to allow the entire scope to stand before any of the six stainless steel bolts are installed.  Assembled,  the screw and washer are just below flush to the surface so that the stacked scope shouldn't get scratched by them. This should be a bit of fun to set up. 

 

Thanks for the suggestion,  Jonathan.  I'm using it at both ends of the scope,  but with Mark's suggestion to  protect the magnets from stress or impact. The clamping pressure will be primarily on the screw.


Edited by jtsenghas, 22 March 2020 - 01:54 PM.

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#241 jtsenghas

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Posted 21 March 2020 - 09:10 PM

I also cut,  ground and filed a 1/4" Allen wrench. 

 

20200321_210213.jpg

 

Does anybody want to venture a guess as to why on earth I did that


Edited by jtsenghas, 21 March 2020 - 09:10 PM.


#242 Oberon

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Posted 22 March 2020 - 01:29 AM

I also cut,  ground and filed a 1/4" Allen wrench. 

 

attachicon.gif20200321_210213.jpg

 

Does anybody want to venture a guess as to why on earth I did that

“ I might make just a ball end 1/4" Allen wrench on a screwdriver handle for assembly of the six attach points to allow for a tilted tool between inserts.”



#243 jtsenghas

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Posted 22 March 2020 - 07:25 AM

“ I might make just a ball end 1/4" Allen wrench on a screwdriver handle for assembly of the six attach points to allow for a tilted tool between inserts.”

But why that specific step and notch? Do any old timer Yankees know? 



#244 roscoe

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Posted 22 March 2020 - 12:08 PM

You gave that one away!!  Old-time "Yankee".....  I won't go further until someone else guesses.....


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#245 Bill Schneider

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Posted 22 March 2020 - 01:33 PM

I know too.

 

JT - That's so turn-of-the-century. I'm not talking about the early 2000s, but the one before that.


Edited by Bill Schneider, 22 March 2020 - 01:33 PM.

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#246 jtsenghas

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Posted 29 March 2020 - 09:40 PM

Well, three people including Roscoe and Bill Schneider have sent private messages recognizing what the strange rework of the Allen wrench was about.  No one has called it out on this thread,  though. 

 

Jonathan showed us how he considered assembling his six bolts on his 16" hexapod Merope with a long hand tool about five years ago,  but quickly went the route of using a battery operated drill driver. 

 

I'm going to try something in between those options that should work quickly and would also help to avoid applying excessive torque.

 

How many of you are familiar with the Yankee Screwdriver? 

 

20200321_221903_compress30.jpg

 

"Oh yeah!  My Dad had one in his toolbox and I used to play with it as a kid!"

 

This spring loaded tool,  from more than a century ago and actually patented in 1895 can rapidly drive screws or bolts and disassemble them with either a ratcheting or pump action.  The long helix spirals can turn the bit from one and a half to several full turns per stroke in either direction depending on the model. I've got one of the bigger later ones that may date back to the 1940's. It is model 131A. It has a sliding latch that sets the tool in forward,  reverse,  or locked mode. 

 

The bits that fit these tools come in three diameters depending on the size of the model,  but all have the profile seen in my Allen wrench in the previous post. The diameters are just under 7/32", 9/32" or in my case 5/16".  The step at the top of the bit fits into a recess in the tool and is for anti-rotation. The notch about an inch below that is a detent for a retainer.  Pulling back on the knurled bit holder allows the bits to be changed. Yankee screwdriver bits and drill bits can still be purchased,  and I was surprised to learn recently that these screwdrivers were still in production until 2007.

 

A few years ago I realized that a 1/4" hex bit holder for modern bits could be reworked across the long diameter just like I did on that Allen Wrench of the same size, and it would fit my 131A model only a little loosely. With that holder any modern 1/4" bit or drill could be held in it. If I had the next size down,  the 9/32" version,  only the corners of the hex would have to be ground off.  For the 7/32" version it would have to be spun on a grinder just a little more than necessary to get it round before the notches were made. Neither feature is close tolerance,  but that retention notch has to be carefully placed along the length of the bit or bit holder to latch the bit securely. 

 

Bill Schneider pointed out in a PM to me last week that Lee Valley Tools actually sells such bit holders for Yankee Screwdrivers,  complete with Neodymium magnets for the bits! Here's the link he sent me. 

 

I expect this scope to be fun to assemble.  The strong magnets should allow me to stand the strut assembly on the circular mirror box like a teepee, and then place the UTA on the top ends. The long length of my model of Screwdriver should also allow me to steady the scope while driving the lower screws without much stooping.  I might put a rubber tube around the bit to hold the approximately 1/2" diameter head of the cap screw so that screws can be preloaded to the bit and then poked into place. 

 

The  six screws should zip right in,  particularly if I rework the screw tips to copy an automobile assembly screw design feature I've seen used for years. 

 

If a screw is at least several threads longer than it needs to be,  the tip for about a diameter of length can be turned down to just under the minor diameter of the threads.  The resulting fastener becomes virtually impossible to cross thread,  and certainly easier to start. The tip can also be radiused for easier insertion. I've seen this used at my various jobs since the 1980s. If I grind the tips of my stainless bolts this way I should be able to push each screw in with the Yankee Screwdriver and send it home with a few pumps. A final tweak of each in ratchet mode should make each of the six  JUST tight enough. 

 

 


Edited by jtsenghas, 30 March 2020 - 05:37 AM.

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#247 Oberon

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Posted 30 March 2020 - 04:22 AM

Brilliant! Especially for the bottom.


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