115 replies to this topic

[TonyStar]

Jon,

 

thanks for your detailed report, I also doubt the source of flexure is with the trusses, first of all because there is no slop in your trusses by design and second of all because your UTA doesn't look so heavy (maybe around 2.5 Kg without finder and eyepiece?).

 

The simplest way to rule out the spider would be to remove the secondary mirror (and as much weight as possible) from the hub and replace it with a tiny reflecting surface. If the laser spot moves the same amount as with the heavy secondary in place, then the issue is definitely with the UTA...

Edited by TonyStar, 16 February 2017 - 06:09 PM.

[Bob4BVM]

Not likely because it isn't a simple matter of the UTA load sitting on 3 or 4 points. The UTA is an integral part of the truss, forming the 3rd leg of each triangle, and so a weak UTA makes for a weak truss, and adding more trusses won't fix that.

 

But lets keep it in context; my problem arose because I was on a weight cutting exercise and I abandoned my usual approach and went with methods that seemed "tried and true" because everybody else was doing it. Merope, remember, was always an experiment, a test bed for construction methods and design. So I looked at the methods other atm's used for UTA and decided that I was being too conservative and could afford to go thinner and to not treat the tubular section as structural. In hindsight I should have stuck with my intuition. Well OK you can and in practice Merope performs perfectly well at all the altitudes that matter...but it could be better, and I know why.

 

A perfectly engineered Merope would use carbon truss tubes and a carbon monocoque UTA. Anything less is a compromise, and life is full of compromises. Stick with your hexapod, you will love it. And remember that it eliminates the need for adjustable components in your mirror cells.

OK Jonathan, thx again for the reassurance on the hex.

Once I get my new test UTA/Spider assembly together I will post a pic or two.

I am looking at 2 rings of foam laminated between layers of CF or maybe even fiberglass since I have that around for my canoe-building pastime.  I'm hoping for a UTA that is as rigid but a whole lot lighter than my 1991 version which has 2 rings of 3/4" ply joined by 1/4" all-thread & 1" aluminum tubes.

Has anyone here tried UTA rings of foam/CF sandwich ?

 

CS

Bob

[Oberon]

I think foam + CF is the way to go. I'm planning something like that for Sterope, unfortunately delayed till funds are available to fully get into what I need for vacuum infusion CF. 





 

[astroman33]

 

Honestly, I like the guitar string spider for its mechanical elegance but can anyone quantify its diffraction performance?

 

I know the obstruction is very small but the wires are straight and thin, wouldn't that cause diffraction?

 

I like Chriske's half circle but I would try the changes I suggested and maybe make it helical.

 

The dark horse is the carbide single stalk.  I would need to get tricky with the mirror mounting and adjustment to minimize mass, I could put three adjustments out on the ring so the stalk doesn't have to handle so much weight.  If its light enough I could make the portion of stalk in the light path an elipse to reduce the obstruction/diffraction.

I would think that 8 No. .008" wires totally up to .064" would be far less defraction than a .187" stalk, which I would think minimal for a 7.25" to 8" radius of a 12.5" scope,  and that is the least I would imagine hanging a 1.83"-2/14" sec. on with collimation screws and such. I am building a 8" planetary scope, I know smaller than the 12.5" that I would like but the seeing in the midwest doesn't allow for that but maybe three times a year and invariably I am at work those nights. I located some .007" extra super light E string high carbon wire ( Just Stings) for my 1.52" sec. should perform very well, 19% CO. Note I could have went with 1.3" sec. 16.25% but with the fast fall off of illumination do to long focal length I chose the 1.52" for good globular viewing ,I have read numerous reports that once under 20% CO a smaller CO be the 2.75% that the 1.3" would have been would be all but impossible to detect the difference. This scope is a poor mans refractor so to speak,1500mm focal length, I would love to have a Tec. or AP in the 180mm size but can't justify the expense.

I borrowed Oberon's design with one mod. I used angle aluminum instead of the plate he mounted the delrin to,  this allows the three collimation screws on top for standard collimation. I also used a hardwood 1.25" dowel to fabricate the sec. mount, lighter than delrin and easy to work.

Dane

 

Do you have a picture of this?

[PrestonE]

Hi Bob,

 

Yes, in the 20" RC build that I did, we did a CF VARTM (vacuum assisted resin transfer molding)

using blown PVC and layers of CF...

 

It was Very Very Stiff...

 

The problem at the time was making and locating all of the connecting

hardware into the process of doing the molding in order to NOT 

comprimise the integrety of the final CF skin...

 

At the time, we ran an FEA analysis and determined that the aluminum

rings were about as strong and within the design figures for stiffness.

 

Also, the problem of galvanic corrosion with aluminum and CF was something

that I did not want to have to worry about, as the scope would not me in

my control for repairing in the future.

 

You could likely use aluminum inserts or titanium if your worried and cast them

into the structure...

 

The blown PVC was much better in compression strength and only weighed double

the 2 pound polystyrene...thus, still very light and not the problem of damaging the

fragile light weight polystyrene during the infusion process.

 

If you have any questions regarding VARTM, I will be happy to assist where I can.

 

Very Best Regards,

 

Preston

[sixela]

could afford to go thinner and to not treat the tubular section as structural

 

I built something similar to yours and still treated the tubular section as not structural and got the same problems with 9mm plywood and pretty much your arrangement.

 

Then I used 12mm instead of 9mm and between the rings alternated long tangential squares (as you have) and short radial planks in between those squares. That seems to have fixed a lot of issues. You can even drill circles out of the larger squares to make it lighter again (and even lighten the rings by drilling them out, but that's a lot of work). But if I had to do it again, perhaps I would have used some aluminium tubing between the rings.

 

One ATMer in the Netherlands went completely overboard with a _very_ nice design where the rings are with a hollowed out centre portion and then two thin plywood rings glued to the tob and bottom. That, too, gives you very stiff but light rings.

[Giany1967]

Well if i would see any of those pictures without descriptions i would say what the hell is that!

[Jeff Morgan]

Doing some parts shopping, and I came across something that may be of interest. Improved gear ratio (not sure how important that is, but sure sounds good), and more importantly lighter weight:

 

https://www.grotro.c...ines-143-Series

[Trithemius]

Doing some parts shopping, and I came across something that may be of interest. Improved gear ratio (not sure how important that is, but sure sounds good), and more importantly lighter weight:

 

https://www.grotro.c...ines-143-Series

Not as high of a gear ratio, but ukulele tuners are very lightweight.

 

http://www.stewmac.c...g_Machines.html

[Jeff Morgan]

Or these, made from solid billet aluminum:

 

http://sperzel.com/guitar-tuners.php

 

And available in sets of 8, in anodized colors.

[Jeff Morgan]

I picked up the Sperzel tuners, 8 right-handed units. In hindsight I should have gotten left and right units based on the way I want to mount them. But, I can just flip half of them and likely only a guitar player would notice the difference.

 

They appear to be very well made units, and smaller than I realized (a good thing). I weighed the group of them at 3.6 ounces, or 102 grams if Siri has her conversion correct.

 

After noodling on this project for the better part of two years, all I have to do is finish my list of carbon fiber parts and it's time to start making sawdust!

 

 

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[howardcano]

I picked up the Sperzel tuners, 8 right-handed units. In hindsight I should have gotten left and right units based on the way I want to mount them. But, I can just flip half of them and likely only a guitar player would notice the difference.

 

They appear to be very well made units, and smaller than I realized (a good thing). I weighed the group of them at 3.6 ounces, or 102 grams if Siri has her conversion correct.

 

After noodling on this project for the better part of two years, all I have to do is finish my list of carbon fiber parts and it's time to start making sawdust!

The fact that they have the locking feature will bring some peace of mind, as well as making them easier to string.

Edited by howardcano, 02 June 2018 - 09:56 AM.

[Jeff Morgan]

This configuration has the secondary holder somewhat nested within the highly angled wires. As a result the center of gravity of the secondary is not cantilevered very far from the supports, which further reduces the loads and torques you are concerned about. 

 

I'm not too worried about the mirror holder shifting, but it occurred to me that the UTA rings are (generally) thin plywood rings. In typical construction, four aluminum tubes (columns) separate and support the rings. And the weight of the secondary is supported by these columns.

 

In a wire spider, typically the rings support secondary weight.

 

Wouldn't the section (arc) of the UTA ring between the tubes act like a beam, subject to a small bending load if a weight was suspended from it?

 

Could this be material, or am I overthinking it?

[Oberon]

It is material and it does matter. A spider in tension will exert a force on the ring that will tend to warp or twist the ring. The force applied is far greater than the gravitational load. However, so long as the force is sufficiently greater than the gravitational forces involved it will remain stable at all angles and thus from an engineering pov won’t matter. But you may notice it, your UTA ring may look slightly wonky.

 

[Jeff Morgan]

It is material and it does matter. A spider in tension will exert a force on the ring that will tend to warp or twist the ring. The force applied is far greater than the gravitational load. However, so long as the force is sufficiently greater than the gravitational forces involved it will remain stable at all angles and thus from an engineering pov won’t matter. But you may notice it, your UTA ring may look slightly wonky.

 

 

My initial thought was that since the tuner supports are distributed symmetrically on the upper and lower rings to put a strut between each post to prevent the potato chipping.

 

But if it is stable I think I'll pass on the extra cost/weight. It's not like the wires will be under lots of tension and the wonky look might have it's own charm. 

Edited by Jeff Morgan, 13 June 2018 - 06:32 PM.

[David Stevenson]

I am thinking on improving my UTA, I would like to introduce a wire spider in my truss collimation 12 inch hexapod. I am a bit put off by stiffness, do these thing have the same or comparable stiffness to a traditional lamina with offset spider? I would like to go down the Merope route for this, maybe trying with more distance between guitar pegs.
But my concern is stiffness, in other words how it keeps collimation from zenith to horizon.
Thanks