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Spiders on trial - what gives?

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#51 Oberon

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Posted 27 January 2019 - 05:37 PM

Are you taking bets against it?smile.gif

Not because it isn't well executed, but because the bending stiffness of that design is also finite.

 

Jonathan, a long time ago, on another thread, I asked you how stiff was your wire design. The question arose because at the time I had done my own tests and concluded the spider was the weak link in my scope. At the time it was harder for you to pinpoint the source of flexure but now it's obvious the UTA is the culprit. As you pointed out earlier, it may be difficult to say whether the shift is caused by flexure in the UTA structure or flexure in the wire spider (I bet it's both). But the test with the solid vane spider may help address this issue.

 

In any case, this is very interesting work and I'm looking forward to the following.waytogo.gif  

Sure, the stiffness of my solid vane spider is finite, but I’ll take bets that its at least as stiff as the test structure!


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#52 Jeff Morgan

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Posted 27 January 2019 - 06:54 PM

This study should also be able to show how counterweights for secondary mirrors improve such behavior. When the center of gravity of the secondary support (including counterweights) is not cantilevered, no doubt deflection will decrease. I'm certain most of this deflection is due to angular tilt rather than radial displacement. 

 

I don't think counterweights for the secondary have been used in half a century. The only place I have seen them is in a few (very) old Sky & Telescope illustrations.

 

I looked back through Sidgwick and Texereau to see if they mentioned it, they did not. Both however were on to the offset spider vanes as superior to the "classic" design of vanes converging at the hub.

 

Glad Oberon is taking another look at secondary counterweights!


Edited by Jeff Morgan, 27 January 2019 - 06:56 PM.


#53 MitchAlsup

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Posted 27 January 2019 - 07:01 PM

Then we’ll do curved vanes...

I suspect this will not end well...........curved vanes will underperform for any fixed amount of diffraction.



#54 jtsenghas

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Posted 27 January 2019 - 07:03 PM

I looked back through Sidgwick and Texereau to see if they mentioned it, they did not. Both however were on to the offset spider vanes as superior to the "classic" design of vanes converging at the hub.

Yes, but in their time very large and fast scopes were pretty much unknown. These scopes have MUCH heavier secondary mirrors and MUCH tighter collimation tolerances.  In some cases counterweights may be an advantage where they were relatively unnecessary in the past.


Edited by jtsenghas, 27 January 2019 - 07:04 PM.


#55 mark cowan

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Posted 27 January 2019 - 08:37 PM

 And, after the spider, the next weakest link is the Moonlite focuser. Just sayin’ to help keep things in perspective.

That was what I didn't say.  Plus the rotational change could possibly come from the laser collimator + focus.

 

Counterweights improve flexing designs (I know because I've seen it work about 10 years back) but of course add dead weight to the UTA so aren't all that useful in practice.


Edited by mark cowan, 27 January 2019 - 08:39 PM.


#56 Oberon

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Posted 28 January 2019 - 05:48 AM

I tried the counterweight, taking a punt at weight and moment...

 

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And gave it a spin. Usual start with laser/focuser entering on right...

 

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Massive improvement.

 

But there’s a catch...


Edited by Oberon, 28 January 2019 - 05:51 AM.


#57 Oberon

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Posted 28 January 2019 - 06:31 AM

And here's the catch...

 

med_gallery_217007_10583_233874.jpeg

 

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#58 earlyriser

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Posted 28 January 2019 - 06:51 AM

How so? The moment doesn't change while ever the UTA remains horizontal.

On second thought, I think you are correct. I had it in my head that the longer axis would be cantilevered out further from the point of support, but as long as the secondary is attached at its center of gravity, the magnitude of the torque moment should remain the same as it rotates around the OTA's axis.



#59 ckh

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Posted 28 January 2019 - 10:22 AM

Are you saying it got into vibration, just because of the counterbalance?  You could make the counterbalance lighter by using a longer lever arm for the weight.



#60 CrazyPanda

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Posted 28 January 2019 - 10:25 AM

Are you saying it got into vibration, just because of the counterbalance?  You could make the counterbalance lighter by using a longer lever arm for the weight.

Won't that have the same moment of inertia though? 



#61 Jeff Morgan

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Posted 28 January 2019 - 10:28 AM

Won't that have the same moment of inertia though? 

 

Vibration frequency will be higher with a lower weight, damping faster.



#62 TonyStar

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Posted 28 January 2019 - 01:09 PM

Vibration frequency will be higher with a lower weight, damping faster.

????

Adding weight far from the pivot point inevitably increases the moment of inertia, thereby lowering the resonance frequency (bad)

But the issue here is also that the wire spider has a very LOW damping coefficient.

I don't remember noticing wild vibrations when experimenting with counterweights, but I wasn't kicking and bouncing the scope either, just tilting it back and forth.


Edited by TonyStar, 28 January 2019 - 01:14 PM.


#63 ckh

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Posted 28 January 2019 - 01:40 PM

As the length from the CG is increased the weight goes down by the same proportion, but the moment of inertia increases with the square of the length and decreases linearly with smaller mass.  So if you double the length and halve the mass, you get twice the moment of inertia.  You can compensate for that by moving the back wires to the counterbalance weight. Then I think the effect of the moment of inertia is a wash and the assemble is still lighter. 

 

If you mount the diagonal at the middle of a thin cylinder with a big hole in the side, you can balance the diagonal and minimize both the moment of inertial and overall weight.

 

gallery_240847_5047_20720.jpg


Edited by ckh, 28 January 2019 - 01:42 PM.

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#64 Oberon

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Posted 28 January 2019 - 01:54 PM

The real way to improve deflection is improve geometry. Do not increase inertia, do not add mass, don’t even consider using a counterweight.

 

The answer is to increase the distance between the upper and lower support points of the secondary stalk, that is, to have a longer taller stalk. For Merope that would also mean a taller UTA, and risk losing the benefit of packing flat into the boot of my car. Classic compromise of a compact scope.

 

In my case I could also reduce the mass of the stalk by replacing the Delrin with something lighter.



#65 ckh

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Posted 28 January 2019 - 03:32 PM

By making the tube above longer you get more leverage from the wires (as you say).  I'd use a piece of carbon tube for the above diagonal holder (if only one could buy short pieces) and make the back part somewhat longer.

 

The more the diagonal overhangs from the front wires, the larger the tilting moment of inertia (goes up as the square of the distance).  The moment of inertia is inversely related to the vibration frequency for tilting modes.  If the diagonal is centered between the front and back wires, the tilting moment of inertia is minimized.

 

I have an 0.04" sheet of Sorbothane the might be useful for damping spider vibrations.  I'm going to try that in a wire spider.

 

How about just putting a backplate on the diagonal and suspending it only from the plate with wires? I think someone here fooled with that.


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#66 TonyStar

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Posted 28 January 2019 - 03:33 PM

 

If you mount the diagonal at the middle of a thin cylinder with a big hole in the side, you can balance the diagonal and minimize both the moment of inertial and overall weight.

That's a good solution which has been implemented by CN users before (and certainly by someone else) but the cylinder's diameter has to be significantly larger than the secondary to avoid vignetting. Those obsessed with CO will object. Although obstruction can be minimized with a clever cutout. It's definitely worth refining... 


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#67 ckh

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Posted 28 January 2019 - 04:48 PM

Cool example, thanks.  The tube only vignettes the edge of the diagonal which is often not as flat anyway. Carbon tubes can be quite thin and still stiff.  Besides which carbon doesn't add much weight or moment of inertia.

 

This mounting also limits the cantilever of the focuser when you use a single UTA ring because the focuser can be closer to the ring (as in your linked example).



#68 mark cowan

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Posted 28 January 2019 - 05:58 PM

Vibration frequency will be higher with a lower weight, damping faster.

And you'll need to increase the string size if the overall suspended weight increase, as now you have a harmonic oscillator with less damping.  Its a catch22.
 

The real way to improve deflection is improve geometry. Do not increase inertia, do not add mass, don’t even consider using a counterweight.

All the above was part of the motivation that led to the experimentation I did to find a better geometry that was stable against vibrations and didn't require wire tension for that. There are ways to do this that don't involve increasing the angles on the legs excessively and that work in practice. It's explained here with ample pics and paragraphs and diagrams, and the CN thread (it's ten years old now!) is here.  I've posted it often and several people have built their own with reportedly good results.

 

Although I tested that design for both deflection and vibration (with laser setup) and found it worked very well with light wires I didn't particularly test it for heavy diagonals since that wasn't in the original application although I used c-clamps on the prototype on the test jig to tease out the salient improvements before ever building the actual one.

 
However picking up on the need for somewhat more massive diagonals holding tighter collimation the basic principle of opposing tensioning is used in a next generation design for a 20" ultralight/fast mensicus scope.  I believe J.T. might be implementing the geometry and he'll likely beat me to the first finished one, but one way or another it'll get thrashed out. wink.gif


Edited by mark cowan, 28 January 2019 - 06:26 PM.


#69 ckh

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Posted 28 January 2019 - 06:37 PM

Stiffness of the wire goes up by the square of the diameter so you don't have to increase wire diameter by much to get the same frequency. It's not like you can't win. 

 

Tension has little to do with stiffness of a wire along its axis which is how it's used to limit motion of a diagonal.  Increased tension will increase the traverse vibration frequency of the wire, but the wire has so little mass that doesn't have much effect anyway.

 

Once the wires are taut, they have essentially the same amount of give regardless of tension.



#70 earlyriser

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Posted 28 January 2019 - 11:08 PM

Stiffness of the wire goes up by the square of the diameter so you don't have to increase wire diameter by much to get the same frequency. It's not like you can't win. 

 

Tension has little to do with stiffness of a wire along its axis which is how it's used to limit motion of a diagonal.  Increased tension will increase the traverse vibration frequency of the wire, but the wire has so little mass that doesn't have much effect anyway.

 

Once the wires are taut, they have essentially the same amount of give regardless of tension.

Does the stiffness of the wire matter? It seems the tensile strength would be all that matters in this application, but I think that also goes up with the square of the area. I would think increased tension equates to less deflection. 



#71 earlyriser

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Posted 28 January 2019 - 11:15 PM

Vibration frequency will be higher with a lower weight, damping faster.

Has anyone tried adding some type of damper to lower the Q of the system? Something that works like like a muffler on a piano wire would kill vibrations, I would think.



#72 Oberon

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Posted 28 January 2019 - 11:26 PM

Does the stiffness of the wire matter? It seems the tensile strength would be all that matters in this application, but I think that also goes up with the square of the area. I would think increased tension equates to less deflection. 

The effect of increased tension is not great, perhaps so miniscule you won’t notice. Increased tension in a typical scenario eliminates loose components, thus increasing stiffness. But a thin wire already in tension is not loose, it is elastic, and will simply stretch further as you increase the tension until it fails.

Consequently, as is so often the case, mass is the enemy.


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#73 Oberon

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Posted 28 January 2019 - 11:37 PM

Has anyone tried adding some type of damper to lower the Q of the system? Something that works like like a muffler on a piano wire would kill vibrations, I would think.

The vibrations on a wire spider are not much different to those on a stringed instrument. The natural frequency is high, they dampen quickly and are not a concern. Pling pling!

The oscillations however that a wire spider permits when supporting a mass is at a much lower frequency, and not the sort of thing a muffler would manage with much practical effect.



#74 Oberon

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Posted 29 January 2019 - 03:47 AM

This post here pretty sums up my findings on the counterweight...



#75 Oberon

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Posted 29 January 2019 - 07:05 AM

I found a more convenient way to lift my monster tube from horizontal through to vertical when she-who-must-be-cherished reminded me that I have an electric crane fitted onto the back of my ute...

 

med_gallery_217007_10583_363280.jpg

 

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Edited by Oberon, 29 January 2019 - 07:06 AM.

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