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Merope - engineering a low compromise compact 16" dob

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#51 Alex Parker

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Posted 14 February 2015 - 09:03 PM

Azimuth Section #1

The azimuth section is a simple ring manufactured from 3 layers of 25mm ply laminated together provides all the support necessary for the OTA. Essentially it is a 70x75mm (3"x3") solid circular beam with a Laminpanel plate on the bottom (which in theory should make it stiffer, but not really necessary). Originally my design was just a ring with no center pin, but as it came together there was really no reason not to put a thin plate on the bottom, so I did, mainly because the plate adds another barrier to creepy crawlies.

 

gallery_217007_4913_64486.jpg

 

The most difficult bit to do was cutting the altitude bearing recesses. Several reasons. First there was no straight edges to work from, and at the time no bottom plate with a center point either. Second I didn't have a tool that could make the cut in a single set up. And third I didn't take enough care building a jig and relied too much on measuring, clamping, unclamping, marking and realigning everything to marks. Consequently the cuts didn't align as well as I expected and needed some rework.

 

gallery_217007_4913_59019.jpg

Mounted in jig from behind

gallery_217007_4913_86930.jpg

 

Jig from front showing angle grinder with 4" saw blade mounted in jig

 

gallery_217007_4913_25025.jpg

 

This is about as dodgy and vicious as it looks. I had to take the guard off to use it and was terrified it would bite and grab and do something really evil. In practice it was as smooth and easy to use, gave no hint of grabbing, and I just shaved the job a few millimeters at a time. This involved lifting the entire cutting assembly to slide up its defining point and then slowly let gravity work its way down slice after slice. Unfortunately with only a 4" cutting blade it soon interfered, hence the step cut to clear space. This meant I had to locate the job 4 times and the saw 4 times, which is not a recipe for repeatability and precision.

So the lessons learned for next time were:-

1. take the trouble to make a very good jig - much better than the photos show

 

2. buy a 6" angle grinder and saw blade

 

3. build a better safer jig for the saw

I would also think about  hogging most of the material out on the tablesaw with a dado set beforehand, and just using the swinging angle grinder jig to fair the final shape.



#52 Oberon

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Posted 14 February 2015 - 10:06 PM

Thank you for the suggestion, that might make it quicker. I don't have a table saw at the moment, so didn't consider it as an option. Initially I imagined I would make the cuts with a router but there was no way to get the cutter in place without fouling the plate.

To be honest there is no need to cut a curve here, except to minimise the material removed in order to maintain maximum stiffness of the ring. Straight angular cuts would do, particular if each piece was cut prior to laminating.

But the engineered look of it appealed to me, and the improved stiffness seemed sufficient justification. And besides, I had to do it for the cutouts in the altitude bearings anyway, so why not be consistent.

 

Would I do it again?

 

Yes.



#53 Pinbout

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Posted 15 February 2015 - 08:35 AM

 

Oberon,

 

do you have a plan view of your mirror box and truss layout to show how much clearance there is - mirror edge to tubes?

 

Just for you Danny....

 

gallery_217007_4913_241690.png

 

 

thanks



#54 woodscavenger

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Posted 16 February 2015 - 01:30 AM

I just keep coming back to look at this amazing structure.  I have a lot of DIY skills but this is just plain beautiful and looks to be a great machine.


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#55 Alex Parker

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Posted 16 February 2015 - 11:36 AM

And another question - given that you can collimate using the truss poles, why bother with the guitar machine heads on the OTA?  They seem like overkill if you just want to tension the wires and then leave them alone (not that I can instantly think of a lighter, simpler tensioning device, of course).  They would obviously be ideal if you wished to use adjustment of the wires to collimate the secondary...

 

And thanks again for posting such a thought-provoking build. 



#56 Oberon

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Posted 16 February 2015 - 05:22 PM

As I account for every gram in my UTA I've been asking myself the same question. I could lose 300 - 400 grams if I got rid of them, and that translates into losing 2-3kg at the LTA. The answer is that this was my first attempt at strings and most other examples I had seen looked kind of lashed up and unfinished, experimental and messy. Guitar machine heads give a wire spider the look of a properly engineered and finished assembly, and they make it very very easy to get everything right the first time, and being a commercial product are very low cost for the level of engineering they bring to the job. However, as you point out, with my truss collimation system in place they are also somewhat overkill.

 

What would I do in future?

 

For all the above reasons I would still rather have them than not and compensate for the weight. But if ultra-light weight is my objective then I would revert to something more boffinish. Drill a hole through a small thread and tighten with a nut, or something of the sort.



#57 starman345

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Posted 17 February 2015 - 05:59 PM

 

gallery_217007_4913_86930.jpg

 

 

 

 

 

 I see the pivot bolt behind the angle grinder  allowing the blade to move into the cut at the proper radius but can't see how you are raising and lowering it? Is the whole assembly raised and lowered on the pivot bolt?



#58 Oberon

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Posted 17 February 2015 - 07:20 PM

Yes. Its crude. I just lifted the assembly by hand; its sits pretty tight on the pivot even when lifted about 3" off the surface. When rotating it back and forth carefully gravity assists and pulls it down, all I had to do was provide enough support to control the rate of descent. It was pretty easy to do a thin slice at a time.

But I stress...aside from the successful use of a saw blade cutting tool I am not recommending this method, that is just what I did. Next time I will engineer something better.



#59 starman345

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Posted 18 February 2015 - 08:48 AM

Thanks, you are a brave man! That tool looks like a prop from a horror movie.  But, I think it is the only way to get a matching curve, I could do it with a router but it cannot take a full depth cut in plywood, which means it would have to be moved little by little further into the cut after each pass. It would still require a holding jig similar to yours, a bit bigger as the router is bigger around than my angle grinder. Better I think just to use a method similar to yours with a saw blade.

Rigging up to cut these angles seems the hardest part to building a scope like this.



#60 Oberon

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Posted 18 February 2015 - 07:47 PM

Mirror Ring #1

Having concluded that a triangular steel frame directly connected to a hexapod Stewart Platform would provide the ideal structure for an 18 point mirror support system and for maintaining optical alignment I was left with a few loose ends to tidy up with respect to the mirror cell.

 

1. mount the altitude bearings

2. support the mirror edge

3. protect the mirror when in storage

4. protect the mirror from falling out if tipped too far

5. permit good ventilation

6. must pack flat to fit telescope into car

It didn't take long to realise that all these objectives could easily be achieved with yet another simple laminated plywood ring. Better still, the ring could be cut from the inside of the Azimuth rings as shown in the cutting plans @ post #32. This solution had the added benefit of naturally packing flat and may be considered a defining feature of the Merope design. In short, a Merope style telescope operates and packs flat as a compact series of circular sections working within circular sections rather than stacked on top. Relatively speaking, the cell is suspended *within* the azimuth section which is suspended *within* the ground plate, best illustrated with this photo.

gallery_217007_4913_108684.jpg

 

This early stack give a rough indication of just how flat the telescope could potentially be packed. My constraint was >400mm to fit into my car boot. I knew I had up to 1000mm square in width and depth so could afford to spread out. If portability is the driver, height is the primary constraint.

 

gallery_217007_4913_3915.jpg

 

At this point I hadn't worked out what to do with the altitude bearings and was thinking that a pair of semicircles on top would complete the stack. Eventually I decided to size them so they could pack neatly around the mirror cell like so...

gallery_217007_4913_11958.jpg

 

...at which point I had achieved a pack 350mm (14") flat, well within my requirements for >400mm, and with the final build very easily packed into the trunk of an ordinary sedan with loads of space to spare for luggage. In a final triumph even the wheels pack flat around the UTA.

 

gallery_217007_4913_87064.jpg

 

How many other 16" telescopes are there that take up so little room inside an ordinary car? For comparison, check out SDM's Obsession style 14.5" telescope being touted for its portability. It takes up the trunk and the entire back seat.

 


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#61 Oberon

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Posted 18 February 2015 - 08:43 PM

Mirror Ring #2

 

At this point the ring has been cut, laminated, sanded and varnished and is now being prepared for its glossy surface. The masking tape makes it easier to remove excess glue.

 

gallery_217007_4913_11410.jpg

 

I thought it might be a good idea to embed magnets in the ring to help keep the lid in place. I haven't used them yet but there are there...

gallery_217007_4913_37199.jpg

 

After laminating with some scrap high gloss Alupanel (or a similar product) and trimming with a router. The aluminium is extremely thin and routes beautifully (although you'll want to vacuum up the mess asap before children walk into the workshop). Note the short sections of plastic water pipe temporarily attached inside as router guides. These prevent the mirror from falling out and are curved gently to prevent additional refraction spikes. I attached some sticky sided felt to their underneath and they are so effective you can safely tip the cell over and allow the mirror to rest on them while working on the cell (like I had to do to add more weights at first light).

gallery_217007_4913_34948.jpg

 

The holes were then re-drilled through from underneath (undersized) and enlarged from surface. On that note I can't recommend a set of these countersink deburring tools highly enough. They are essential for thin metals and invaluable for countersinking all materials from steel through to wood and plastic.

 

NEW-font-b-Countersink-b-font--font-b-De

 

Then the truss assembly could be tested. Just one bolt to tighten and the Heim joints were extremely well defined against the surface, yet, due to their ball-shaped head could be manually adjusted to point to any angle without becoming loose or floppy. However as suspected I needed to do something to prevent rotation (yaw).

 

gallery_217007_4913_42611.jpg

 

After considering and rejecting washers glued to the surface, I routed a series of V slots in the surface. This also helps keep everything flush. The V groove follows the curve to keep it all neat. I also fitted 10mm thick teflon strips at the 45 degree points to support the mirror, and another at 12' o'clock (not touching) to keep the mirror roughly centered with respect to the mirror clips when the cell is tipped over. I could have made a whiffle tree *inside* the timber ring by cutting out a section of the middle laminate, but my mirror maker Mark Suchting assured me that would not be necessary, that two supports 90 degrees apart was adequate. Since building this I have learned more about centering the edge supports and next time would set a 20mm teflon ball into the sides at the correct point. This would be easier to get right than the teflon strips. I'll probably do it anyway as nothing has been compromised.

 

gallery_217007_4913_69912.jpg

 

Finally I drilled small holes and inserted magnets into the grooves to hold the truss assembly in place prior to fixing.

 

gallery_217007_4913_51419.jpg


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#62 abberation

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Posted 20 February 2015 - 12:11 PM

I have been reading up on mirror cell design, did you design your 18 point with plop?

 

After reading a few web pages, "refocus" gets mentioned a lot but never defined, do they just mean turning the focuser knob or something different?

 

If it is just turning the knob, I wonder if I could design a Serrurier truss that changes the length to match the refocus needed...

 

I do a lot of borrowing in my design, thanks for using details that are tempting to steal.



#63 mark cowan

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Posted 20 February 2015 - 01:34 PM

"Refocus" is just adjusting the focus to reach the best image, it's a very small adjustment usually.  The reason is the minimal aberration position isn't always the same as the best focus, SFAIK.

 

Oberon, did I miss it or have you described the azimuth bearing details somewhere?  Liking the six-pole collimation. :waytogo:

Best,

Mark


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#64 Oberon

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Posted 22 February 2015 - 08:12 PM

Bearings #1: Azimuth

 

The Azimuth section sits on a flat teflon pad like so...

gallery_217007_4913_21269.jpg

 

...most of which is exposed just for looks. It tidies up the hole around the 3/4 threaded leveling screw nicely.

The Azimuth bearing itself is located in a step on the side of the Azimuth ring rather than the bottom, like so...

gallery_217007_4913_63264.jpg

 

The bearing material is the same Lamipanel (now Laminex Aquapanel) product referred to in this post here and here.

 

"Aquapanel sheets are 2.7mm thick compact laminate. Phenolic resin saturated kraft papers are bonded under heat and pressure to create a high moisture and impact resistant material. The surface is smooth, providing a tough, durable, non porous surface that easily resists marks, stains, steam or moisture. The sheets can be used in dry, wet or hygiene areas."

 

gallery_217007_4913_20417.jpg
 

Unlike the newer glossy Aquapanel, my sheets have a very mildly textured surface (Melamine) and work well against teflon despite the size of my bearing being 760mm diameter. Applying a little wipe of beeswax and citrus oil furniture polish makes it spin very freely and I wouldn't hesitate to use a larger diameter bearing even on a short telescope if the design required it; at one point I was planning a 1 meter bearing for my binoscope version of Merope. I like large bearings; enough resistance for the breeze and very stable.

Design Note:
As Merope is built now there is no real requirement for the stepped bearing, and it would have been easier and just as effective to use the bottom surface. But I didn't know that when designing it and was concerned the bearings would have too much resistance. The step was put there to permit use of 32mm ball bearings without adding height. My misgivings arose from another telescope which has a gloss surface that had so much resistance despite a smaller diameter that I changed the teflon pads to ball bearings.

However, having done it, having gone to the additional work, there are no disadvantages, and the thicker feet required to support the teflon serves to make the leveling screws so stable they don't need locknuts.


Edited by Oberon, 22 February 2015 - 09:08 PM.


#65 Alex Parker

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Posted 22 February 2015 - 08:48 PM

Those are some nice countersinks...



#66 Oberon

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Posted 22 February 2015 - 09:06 PM

I have been reading up on mirror cell design, did you design your 18 point with plop?

 

After reading a few web pages, "refocus" gets mentioned a lot but never defined, do they just mean turning the focuser knob or something different?

 

If it is just turning the knob, I wonder if I could design a Serrurier truss that changes the length to match the refocus needed...

 

I do a lot of borrowing in my design, thanks for using details that are tempting to steal.

 

yes I designed the cell with 'Plop' essentially by taking a screen shot of the result and dumping the image into Illustrator, scaling it and drawing over it as in this image. It was one of the first things I did so the design seen here bears little resemblance to the Merope cell we see today.

gallery_217007_4913_129007.png

 

Refocus in this thread refers to adjusting the truss length so that the focuser travel can be optimised. I don't have to cut my tubes down or extend them, I just twist them all a few turns either way. I have also provided additional thread in the bottom end so that I can adjust my truss length somewhere in the order of 100mm (4") if I want to for any reason.

So far as using the tubes for focus, yes it would work but you wouldn't want to do it unless you could adjust them all together in a controlled and repeatable way.

In fact I have a design for stepper motorised linear actuators that sit at the bottom of the trusses, one for each tube. This design is for my binoscope. When designing binoscopes one of the big problems is all the mechanisms required for keeping the eyepieces in the right place for every observer, and that means adjusting IPD, focus and alignment. Because the Stewart Platform has six axis of freedom it can do all of these, making the heavy focusers redundant, making the rotating top end mechanism redundant, making the secondary and tertiary mirror adjustment mechanisms redundant. Such a motorised Stewart Platform should provide enormous potential benefits for portable binoscope design because it makes the UTA's so much lighter. However it does needs 12 linear actuators and their controllers etc, such as the Arduino based Rhumba.

 

Here is a draft concept sketch showing a single linear stepper actuator attached to a single truss pole with Heim joint. The truss pole does not rotate and the threaded rod from the stepper motor is free to slide up and down inside the truss tube. The linear actuator pushes and pulls on the Heim joint to lift the truss tube. Not shown is a releasable mechanism for connecting and disconnecting the actuator from the truss tube, desirable to avoid connecting and disconnecting wires when removing the trusses.

gallery_217007_4913_17940.png

However I have abandoned that approach now for two reasons.

1. I hate electrical wiring and motors and all that crap over my telescope

 

2. Dual Stewart Platforms mounted on a common base have the disadvantage that the UTA's must be decoupled for it to work. Decoupling the UTA's makes the design more vulnerable to flexure and misalignment in actual use. So the design is high gain with high risk, and high risk isn't something I want to invest a lot of time money and energy into for my first binoscope.
 



#67 Oberon

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Posted 22 February 2015 - 10:33 PM

Bearings #2: Altitude

The altitude bearings are designed to fit snugly around the mirror cell when packed flat for traveling, and have the same diameter as the azimuth bearing. They are cut from two layers of 25mm ply laminated together and then a section cut out for mating them to the mirror cell. The bearings are mirrored identical twins. So having made two identical arcs I then proceeded to cut out the mating section with my wonder tool...

gallery_217007_4913_44919.jpg

 

Unfortunately this proved to be the wrong way to do it. Here we see I have managed to get the cutouts identical...

 

gallery_217007_4913_42203.jpg

 

...but at the expense of the bearing itself, which is now misaligned like so...

 

gallery_217007_4913_99940.jpg

 

Bummer! I had to screw them together, mount them on a circular plate and run them past my sander for an hour to bring their bearing surface back to true.

The right way to do it would have been to rough them out, cut the mating surface, and use the mating surface as the mounting point reference from which to complete cutting trimming and sanding the bearings. Lesson learned!
 

Wow it fits!

 

gallery_217007_4913_51590.jpg

 

The sections were all aligned and clamped etc and made to "work" before drilling through the cell into the bearings.

gallery_217007_4913_19620.jpg

 

2 x M8 inserts were fitted after drilling through from the cell. Note also the small section carved out in the foreground with a Dremel to prevent the steel section of the mirror cell triangle and its bolts from interfering.

 

gallery_217007_4913_30200.jpg

 

Bolting up for the first time...

 

gallery_217007_4913_69281.jpg


Edited by Oberon, 22 February 2015 - 10:39 PM.

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#68 Oberon

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Posted 22 February 2015 - 10:55 PM

Bearings #3: Altitude

The bearings were then finished as follows:-

1. black felt on the inside to protect the azimuth setting circle when packed flat

2. laminex bearing surface

3. altitude setting circle

4. stain and varnish on remaining surfaces

 

(not necessarily in that order)

gallery_217007_4913_35570.jpg

 

The Laminex chosen has a textured surface seen best in this close up. I think it is called "Flint" finish although their "Dimensions" finish may perhaps have been even better with its coarser texture. But it works really well as it is.

 

gallery_217007_4913_32781.jpg

 

This next pic shows the black sticky sided felt fitted to the inside surfaces, and the stained and varnished sections

 

gallery_217007_4913_72892.jpg

 

And of course the setting circles was glued in place and trimmed with the router.

 

gallery_217007_4913_47541.jpg

 

IIRC I think the order of finishing went

 

1. sand seal sand and seal

2. setting circle and trim

3. Laminex bearing surface and trim

4. black felt

5. stain and varnish
 


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#69 jonathanCR

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Posted 08 June 2015 - 10:19 PM

I keep visiting this thread over and over again. Amazing build. I hope you get great views with this instrument.



#70 Oberon

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Posted 10 June 2015 - 12:02 AM

Thank you. I should finish it I suppose. It seems I've put more completed photos and info spread elsewhere that should also appear in this thread, if only so I can find them myself!



#71 Oberon

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Posted 10 June 2015 - 12:16 AM

After a few observing runs I convinced myself I was suffering from some astigmatism most likely brought about by mirror support. The only thing I could blame was the edge supports which were flat sections of teflon rather crudely embedded into the sides of the primary mirror cell timber ring. Basically I had cut a shallow flat trench with a Dremel and hoped for the best, which wasn't good enough. Last month I replaced the teflon with 2 x 20mm diameter Phenolic ball knobs (virtually identical to the balls used by Moonlite) carefully recessed into the ring at the right depth and location, and fixed from behind with M6 screws.

Here is the original cell ring showing the flat teflon supports...

gallery_217007_4913_25306.jpg

And this photo shows the new Phenolic balls recessed in their place...

gallery_217007_4913_347795.jpg

Arguably a Delrin ball might be even better, but they don't come with a brass insert thread. Still they could be drilled and tapped. In any case, the Phenolic ball works well and the astigmatism has gone.


Edited by Oberon, 10 June 2015 - 12:18 AM.


#72 Starman1

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Posted 10 June 2015 - 12:22 AM

I presume the balls contact the edge of the mirror at the COG of the mirror?

And how do you effect cooling?  It looks like your mirror has no way for the internal heat to escape except through the face of the mirror.



#73 Oberon

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Posted 10 June 2015 - 12:55 AM

I presume the balls contact the edge of the mirror at the COG of the mirror?

And how do you effect cooling?  It looks like your mirror has no way for the internal heat to escape except through the face of the mirror.

Yes, at COG.

Fan forced cooling was only ever intended for the rear surface, which is wide open except near zenith, and the front surface is so exposed that rising thermals are flushed away by any breeze. At least that's the theory.

gallery_217007_4913_12659.jpg

In practice I haven't fitted a fan yet, and won't until I redesign or remove the weights from the rear of the cell. So without a fan it is pretty pointless trying to observe with a hot mirror, and I don't.

OTOH where I live many good observing nights are mild, dry and steady and coming out of an air-conditioned home the mirror doesn't require ventilation. Although this is rarely true for autumn (fall) and winter, it is commonly true in spring and summer, so I observe then and ventilation is not high on my list of priorities.

But I will get around to fitting a fan on the rear of the cell one day so that I can observe in the freezing cold too like everyone else. If tests prove that I need another fan to sweep away rising thermals from the surface, then I'll either fit another for that, or just use a floor mounted fan sat nearby, which I recently experimented with to see if it made a difference. It didn't on that occasion, meaning I was dominated by atmospheric seeing.


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#74 Oberon

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Posted 10 June 2015 - 03:04 AM

Some random pictures of the finished Merope that haven't appeared in this thread...

gallery_217007_4913_13946.jpg

gallery_217007_4913_101140.jpg

 

gallery_217007_4913_54043.jpg

Note the long focuser tube. This enables me to fit my DSLR without requiring extension pieces for eyepieces. The tube only protrudes into the beam when taking photos, which being a Dob is rather whimsical, such as during a lunar eclipse, and so isn't a bother. When using eyepieces the tube does not protrude into the beam.

 

gallery_217007_4913_6541.jpg

My light-shield was a quick and dirty lash-up made from some Coreflute I had laying around with sticky-backed black felt on one side. Slits cut with a knife parallel to the cores enable it to curve in one direction, while remaining stiff enough to serve as a protective UTA cover when not in use.

 

gallery_217007_4913_4247.jpg

I rarely use the 10x50 finder since fitting a green laser.

 

gallery_217007_4913_95482.jpg


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#75 BinoGuy

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Posted 15 January 2016 - 10:23 PM

What a wonderful thread sir.  I bow to your design and layout skills.  I am especially interested in the wheels.  I watched your video and read through both threads.  I believe they are simple bicycle wheels with the hubs, spokes and nipples removed, as opposed to a 'spokeless wheel' product that you purchased from somewhere.  Can you confirm or point me to the thread where you discuss them please?

 

Again, excellent work.




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