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

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#101 mark cowan

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Posted 21 February 2019 - 05:57 PM

:gotpopcorn:



#102 jtsenghas

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Posted 21 February 2019 - 06:18 PM

Sorry this show is taking so long. I'll have to work Saturday, so I might not get a lot done this weekend on the scope. I'll try to get a fire going in the garage woodstove early Sunday and get a lot of the busy work done on small bits early in the day.


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#103 Augustus

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Posted 21 February 2019 - 07:34 PM

Sorry this show is taking so long. I'll have to work Saturday, so I might not get a lot done this weekend on the scope. I'll try to get a fire going in the garage woodstove early Sunday and get a lot of the busy work done on small bits early in the day.

You have heat in your shop? Lucky.



#104 jtsenghas

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Posted 21 February 2019 - 07:42 PM

You have heat in your shop? Lucky.

It's not luck. I built the chimney myself. See post 5 from this same thread. 


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#105 jtsenghas

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Posted 24 February 2019 - 08:23 PM

More Spider Work:

 

Today I did some fiddly work with hardware and played around a bit with the secondary holder and wire spider.  I'm now very encouraged with how these are progressing.

 

First, I drilled some anchoring holes for the RTV adhesive on the back of the secondary holder:

 

20190224_124314_resized_1.jpg

 

For this I angled my drill press table 20 degrees and drilled each of the three holes 1/4" deep from two directions, as shown and with the piece turned 180 degrees. The result is that each hole is about 3/16" in diameter at the surface but quite a bit wider at the bottom.  This will give the adhesive a good anchor even if it doesn't bond well to the wood.  I know from experience this adhesive will bond very well to the glass.

 

By the way, RTV isn't a brand name.  It stands for "room temperature vulcanization" and can be any number of rubbery flexible adhesives that are usually silicone based.  I have a tube labeled "Aquarium Glass Adhesive" that I've used in the past with great success  I tested a blob of it yesterday and, despite it being a couple of years old, it cured overnight very well. I plan on putting a blob of it into each of the three holes and then placing the mirror on top of them with flat, approximately 1 mm thick toothpicks as spacers.


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#106 jtsenghas

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Posted 24 February 2019 - 08:41 PM

I then spent a bit of time reworking each of eight of the aluminum post nuts shown before. I chucked them into a hand drill and spun them against a file and then sandpaper to make the heads just under 3/8" in diameter (they were 0.410" as received)  drilled and tapped the holes in them deeper (8-32 almost the entire length), and bandsaw cut slots in the heads using an oak block as a jig:

 

20190224_153807_resized.jpg

 

That block had a #6 drilled hole in it, which was a slip for for the post nut and a 3/8" counterbore for the head. A larger 1/2" diameter hole in the back of the block gave me the means to push the nut out after I cut a very shallow slot. The miter gage served as a fence to help keep the slots centered.

 

Next  I assembled everything for a preliminary test. I temporarily mounted the secondary mirror with two-sided tape and assembled the spider with cheap painted 26 gage steel craft wire.

 

I liked the clean look of the post nuts and cap screws:

 

20190224_200306_resized_1.jpg

 

I'm glad I gave myself plenty of adjustment for the 1" long cap screws by nearly doubling the depths of the threaded holes, although on this initial stringing most of the wires were a bit on the short side and the screws didn't engage quite as deep as I expected.  I could measure them carefully and make the final wires just a little longer. Ideally each cap screw will extend out of the post nut by 1/4"-1/2" to keep the heads outside of the wall of the UTA to be added later.  There's no harm in the screws slightly entering the UTA, as I've allowed 3/4" inch clearance between the primary mirror and the UTA inside wall. The primary is 12.5" diameter and the UTA interior will be 14" when that 2 mm plywood wall is added later.  I plan to drill the clearance holes in that thin plywood  by the way, by first piloting holes through the post nut holes of the dowels. 

 

I used the jig shown previously to stage the mirror and connect six of the wires. I then removed the jig and attached the remaining two.


Edited by jtsenghas, 24 February 2019 - 11:01 PM.

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#107 jtsenghas

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Posted 24 February 2019 - 09:06 PM

I couldn't believe how incredibly rigid this setup was once the wires were tensioned to slight "plink plink" tension.

The two lower screws on the back of the secondary holder needed to be moved slightly, however. Their heads were unfortunately barely entering the light path outside of the mirror and, more importantly, the wires were just touching the edges of the glass and being bent around it. This may have resulted in slight pinching of the optics. I had tried to set the screw locations for as wide a footprint as possible for stability, but it appeared that I had pushed that just a wee bit too far.

I moved those two screws up a bit on the holder and restrung the wires after supper. It looks like a little sanding of the plywood alongside the screws will keep the wires off of the secondary holder and still avoid touching the glass.

20190224_194925_resized_1.jpg

I couldn't believe how solid this setup turned out to be and couldn't resist a test. I brought the UTA into the house, put a laser collimator into the focuser, turned the UTA upside down, and projected the dot onto the ceiling about two focal lengths away. The UTA was sitting on the floor.

When I rapped hard on the UTA with my hand, really slapping the UTA hard, the dot barely quivered and settled down immediately. By immediately, I mean it had a damping time of only about A TENTH OF A SECOND! When I pushed on or twisted the secondary holder and quickly released it, the result was the same.

I like that the wires are attached only to the dowels of the UTA, and near their ends. This reduces bending forces on the UTA to a bare minimum. That the lines of tension are along the screw axes is nice too.

I can tell this spider will be a little fiddly to collimate the first time, and am glad I'll be using hexapod adjustment for collimation. This design doesn't lend itself well to the addition of collimation screws, but I won't be needing those either. With a bit of an offset and two plates on the holder rather than one, as I've done, one could add collimation to this design. I love that I can make it so flat and light. I'm confident it will hold collimation well with changes of scope altitude angle.

I wouldn't want the UTA to have flatter proportions with this design, either, since this design requires significant height differences of the wire attach points. The diameter of the circle passing through the dowels is just under twice the UTA height of 8 1/2".

Yay! Thank you, Mark for the suggested spider arrangement!

choo-choo.gif

Edited by jtsenghas, 25 February 2019 - 10:16 AM.

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#108 jtsenghas

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Posted 24 February 2019 - 10:30 PM

Note that there is a SLIGHT difference on my setup from the one Mark shared on the previous page. He has purer triangles at the terminations on the UTA, whereas i have eight terminations with a slight distance between each pair (about 1/2"). I don't anticipate a problem with this due to the alignment of the screws and the rigidity of the dowels near their attachment to the UTA rings. This allows me to knot the wires at the secondary and to avoid additional loads on the UTA rings.  It also simplifies adjustment for the centering and rounding of the secondary to the focuser. 


Edited by jtsenghas, 25 February 2019 - 08:18 AM.


#109 mark cowan

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Posted 24 February 2019 - 10:55 PM

I think that J.T.'s version should work really well, and it solves the stringing elegantly as well - something I hadn't really gotten to yet.

 

Whether the strings coincide exactly at the cage or feed into terminations that come to a close approximation isn't likely to make much difference.

 

In favor of making 4 ganged tensioners are the four pure triangle that results.  Against it is that the wires have to be free to move in loops across the hub until snugged down, at which point the extra bits aren't needed. But the wires can run across the back of the hub to accomplish this, and then be clipped off if wanted.  However they can't be looped around the screws, as they won't slide that way, so the fixation by the screws if the wire crossing pieces are clipped will be inferior.

 

But I pretty much like this method better.  waytogo.gif

 

 

 

 


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#110 Oberon

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Posted 25 February 2019 - 04:06 AM

I really like the low mass aspect of this design (which I’m now convinced is extremely beneficial), but am not persuaded that the geometry is optimised. Adequate, probably, perhaps even perfectly adequate with such a low mass, but still, a crossed wire design will place less stress on the wires per gram of load, and that means less flex.

(Excuse me for a moment while I get my flameproof suit and helmet) tonofbricks.gif

 

Meanwhile, may I suggest JT that you reinforce your UTA to resist the spider’s tension no matter how light you think you’ve tensioned it. Those dowel joints just aren’t optimised to resist a diagnonal force and in time you may find your UTA circles misaligned.


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#111 jtsenghas

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Posted 25 February 2019 - 08:41 AM

Does anyone have suggestions for the best permanent wires? 

 

I used some mild steel craft wire that was painted dark green for this initial setup. I actually expect it could suffice permanently, but would rather go for something slightly better.

 

This wire measures about 0.019" diameter painted, which is about 0.003" more than the nominal diameter of 26 gage wire, which it is labeled as.

 

I'm thinking of something along the lines of 28 gauge stainless steel, if it is available.  This would be about 2/3 the diameter, or 0.013".

 

I know Mark has pushed things as small as 0.008", but I'd rather not go so small. That would have only about one quarter the material of my current setup.

 

Guitar strings? If so, what size? 


Edited by jtsenghas, 25 February 2019 - 08:47 AM.


#112 CrazyPanda

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Posted 25 February 2019 - 10:29 AM

More Spider Work:

 

Today I did some fiddly work with hardware and played around a bit with the secondary holder and wire spider.  I'm now very encouraged with how these are progressing.

 

First, I drilled some anchoring holes for the RTV adhesive on the back of the secondary holder:

 

attachicon.gif 20190224_124314_resized_1.jpg

 

For this I angled my drill press table 20 degrees and drilled each of the three holes 1/4" deep from two directions, as shown and with the piece turned 180 degrees. The result is that each hole is about 3/16" in diameter at the surface but quite a bit wider at the bottom.  This will give the adhesive a good anchor even if it doesn't bond well to the wood.  I know from experience this adhesive will bond very well to the glass.

 

By the way, RTV isn't a brand name.  It stands for "room temperature vulcanization" and can be any number of rubbery flexible adhesives that are usually silicone based.  I have a tube labeled "Aquarium Glass Adhesive" that I've used in the past with great success  I tested a blob of it yesterday and, despite it being a couple of years old, it cured overnight very well. I plan on putting a blob of it into each of the three holes and then placing the mirror on top of them with flat, approximately 1 mm thick toothpicks as spacers.

Is there any concern of an air pocket being created by the mechanical "slot" for the RTV? When applying blobs of RTV to flat surfaces in the past, I've noticed there can be a propensity to form air pockets if done improperly, so I'd think you'd either want to be careful and "prefill" the mechanical slot with silicone first (e.g. start at the bottom of the hole, and pull up as you apply the RTV blob), or drill a small through hole in the bottom of the slot so that air can escape and you don't get an air pocket.



#113 Jeff Morgan

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Posted 25 February 2019 - 10:36 AM

I know Mark has pushed things as small as 0.008", but I'd rather not go so small. That would have only about one quarter the material of my current setup.

 

Guitar strings? If so, what size? 

I ordered 8 gauge guitar strings:

 

https://www.amazon.c...0?ie=UTF8&psc=1

 

My 4" secondary mirror was much thicker than anticipated - 1". This should be interesting ....


Edited by Jeff Morgan, 25 February 2019 - 10:36 AM.

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#114 jtsenghas

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Posted 25 February 2019 - 10:44 AM

Is there any concern of an air pocket being created by the mechanical "slot" for the RTV? When applying blobs of RTV to flat surfaces in the past, I've noticed there can be a propensity to form air pockets if done improperly, so I'd think you'd either want to be careful and "prefill" the mechanical slot with silicone first (e.g. start at the bottom of the hole, and pull up as you apply the RTV blob), or drill a small through hole in the bottom of the slot so that air can escape and you don't get an air pocket.

My tube has an orifice slightly smaller than the hole it will fill. The method I use is to insert the tip into the hole, squeeze until the adhesive fills the hole and emerges around the tip, and then to back off, applying a little more if necessary.  If any air remains in the hole it should be trivial and the adhesive will still certainly be wider beneath the surface than at the top of the hole.

 

I don't expect this Baltic Birch to have significant amounts of oil that the adhesive will resist adhering to anyway.  As we all know when it comes to suspending one expensive mirror above another, better safe than despondent! 


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#115 jtsenghas

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Posted 25 February 2019 - 11:01 AM

I ordered 8 gauge guitar strings:

 

https://www.amazon.c...0?ie=UTF8&psc=1

 

My 4" secondary mirror was much thicker than anticipated - 1". This should be interesting ....

Can you tell me the diameter of these? Clearly this is not the same as the scale of American Wire Gauge (AWG) in which all candidates would be in the 24-28 range .

 

Are guitar wires sometimes chosen smaller for the higher pitches? I'm not sure I'd want my scope wearing a "G-string"! 

 

wink.gif



#116 jtsenghas

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Posted 25 February 2019 - 11:18 AM

I ordered 8 gauge guitar strings:

Apparently  for guitar strings the gauge is the thickness in thousandths. So 8 gauge is 0.008".

 

See this


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#117 ckh

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Posted 25 February 2019 - 11:44 AM

J.T.,

 

Nice minimalist spider concept. A few questions:

 

How are the wires attached to cap screws? Did you drill holes crosswise through the heads?

 

Do you adjust using the screw slots?

 

What prevents the cap screw from turning with the slotted adjustment socket? Do you hold the cap screw as you adjust the slotted end with a screwdriver?

 

Do you have a link to Mark's original idea?

 

Thanks,

Carl

 

Edit: Sorry I got behind in following the thread. Some of these questions are answered on the previous page.


Edited by ckh, 25 February 2019 - 11:48 AM.


#118 jtsenghas

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Posted 25 February 2019 - 12:02 PM

J.T.,

 

Nice minimalist spider concept. A few questions:

 

How are the wires attached to cap screws? Did you drill holes crosswise through the heads?

 

Do you adjust using the screw slots?

 

What prevents the cap screw from turning with the slotted adjustment socket? Do you hold the cap screw as you adjust the slotted end with a screwdriver?

Yes, a couple of these questions can be answered on the previous page. Mark shared his stereoscopic view on that page. I drilled the head diagonally at about 40 degrees with a hole slightly larger than two wire diameters to wrap the wire around the threads and exit back through the middle of the head.

 

I held the cap screw head between two fingertips while turning the post nuts with a flat head screwdriver. Even when the wires got "plink" tight that was easy enough to do. About a full turn before the first six were tight (they gave a dull "thunk" when plucked) , I removed the board jig and attached the last two upper wires across the diagonals. 


Edited by jtsenghas, 25 February 2019 - 12:11 PM.


#119 ckh

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Posted 25 February 2019 - 12:16 PM

Well I have to say that's a pretty cool diagonal setup. What's especially nice is that the diagonal is supported quite close to it's CG so the moments are about as low as they can get.

 

How did Mark make those stereoscopic images? They are very helpful in grasping the geometry.  Perhaps it's a technique we should use more.



#120 mic1970

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Posted 25 February 2019 - 12:18 PM

That is a really nice spider setup.



#121 jtsenghas

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Posted 25 February 2019 - 12:26 PM

Well I have to say that's a pretty cool diagonal setup. What's especially nice is that the diagonal is supported quite close to it's CG so the moments are about as low as they can get.

 

How did Mark make those stereoscopic images? They are very helpful in grasping the geometry.  Perhaps it's a technique we should use more.

Yes, it is literally a DIAGONAL offset spider.

 

I've made stereoscopic images like that by simply taking two cell phone photos about 60 millimeters apart. When put up on a computer monitor you can focus past the screen and see in 3d like those "magic eye" images. 


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#122 ckh

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Posted 25 February 2019 - 01:12 PM

I cross my eyes to merge the images, but it seems to work. Nice going on your build.



#123 jtsenghas

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Posted 25 February 2019 - 01:48 PM

I cross my eyes to merge the images, but it seems to work. Nice going on your build.

Thanks. I guess if the distance between the centers of the images are less than your interocular distance (62 mm for me) you would have to cross your eyes slightly for the effect to work. I find the effect easiest when the distance between images is scaled close to my interocular (also called interpupilary) distance and I relax my eyes to face towards infinity, yet focus closer than that. This is how I also look at Magic Eye images. 



#124 mark cowan

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Posted 25 February 2019 - 06:24 PM

I really like the low mass aspect of this design (which I’m now convinced is extremely beneficial), but am not persuaded that the geometry is optimised. Adequate, probably, perhaps even perfectly adequate with such a low mass, but still, a crossed wire design will place less stress on the wires per gram of load, and that means less flex.

(Excuse me for a moment while I get my flameproof suit and helmet) tonofbricks.gif

 

Meanwhile, may I suggest JT that you reinforce your UTA to resist the spider’s tension no matter how light you think you’ve tensioned it. Those dowel joints just aren’t optimised to resist a diagnonal force and in time you may find your UTA circles misaligned.

Is as does.  J.T. took this ball and ran with it.  :waytogo:


Edited by mark cowan, 25 February 2019 - 06:25 PM.


#125 mark cowan

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Posted 25 February 2019 - 06:28 PM

Does anyone have suggestions for the best permanent wires? 

 

I used some mild steel craft wire that was painted dark green for this initial setup. I actually expect it could suffice permanently, but would rather go for something slightly better.

 

This wire measures about 0.019" diameter painted, which is about 0.003" more than the nominal diameter of 26 gage wire, which it is labeled as.

 

I'm thinking of something along the lines of 28 gauge stainless steel, if it is available.  This would be about 2/3 the diameter, or 0.013".

 

I know Mark has pushed things as small as 0.008", but I'd rather not go so small. That would have only about one quarter the material of my current setup.

 

Guitar strings? If so, what size? 

Steel guitar strings around 0.0000010, err, 0.010-0.012" will work nicely and not be prone to breakage. 

 

Stainless from McMaster Carr would be a good option at that size or slightly smaller.   

 

In any case, there will be multiple isolated diffraction spikes that will probably prove impossible to see except on something like Vega.  That was part of the design, only overlapping spikes if you laid it out that way.


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