91 replies to this topic

[Biff]

I want to tinker around with a wire spider and have been looking through all the threads on them here and cant find any more detailed information (unless I completely missed it!) other than some people are using guitar strings and some are using music wire. I'm curious about what size of wire people are using. The trial will only be with a 1.5" secondary so not too heavy.

 

Thanks

[Oberon]

In this thread here I show that I use #2 guitar string to support a 3.5 inch mirror, and many other details.

 

http://www.cloudynig...-spider-design/

 

and this is the set of strings I bought.

 

you may get away with the #1 string.

Edited by Oberon, 12 June 2015 - 02:29 PM.

[Pierre Lemay]

Any music store will sell you individual guitar strings.  You can go down to .008" or .009" that way. 

I experimented with wire spiders last winter, to support the 4 inch diagonal of my 20 inch. The wire diameter I chose was .020" stainless steel. My experiments were not very successful and I ended up going back to a solid vane made of 0.030" thick aluminium.

 

To mount the wire spider, I made a jig, supported the holder and attached the wires. It was solid and I could lift the UTA by the diagonal holder, like others have demonstrated. However, I found that if I pinched one of the wires hard enough (something that can happen when handling and transporting the UTA in a car), the whole diagonal/string spider assembly would move and I would have to realign. I was obviously doing something wrong since I've been reading with interest the excellent and successful designs described in this forum. Among other things I was using only two wires for the four vanes and I suspect the successful wire spiders may be using four wires.

 

Before I try again (which I will), a few questions for you experts out there. By the way I'm assuming offset spider arrangements:

1. How do you string the wires for each "vane" (do you use one wire per "vane")?
2. How do you attach the wires so they no longer move and can't slide at the attachement points?
3. How do you stretch the wires to get tension (I know some of you are using guitar tuners. I tried feeding the wires in a hole in a bolt head connected to the UTA and tighten the bolt)?
4. Do you find the exposed wires to be individually fragile if you pinch or stretch one of them like I described above (do you take precautions to protect the wire spider while transporting the telescope in a car? do you frequently move your telescope in a car?)?
5. What diameter of string would you recommend for a 4 inch diagonal taking into account the real life manipulation that occurs when taking down and re-assembling a truss tube reflector?

Thanks.

[GShaffer]

Not done one yet but I have been researching it heavily with the plan being to convert my 12.5" f/5 1st followed by my 20". f/5 after I validate it on the 12.5" for sure.

 

1.  Four "Vanes", two wires per "Vane" vertically stacked (each pair will crisscross each other top to bottom) with the outside ends passing thru the kydex covering inside the UTA  to lockable guitar tensioners mounted on the lower side of the upper ring around the UTA and the upper side of the lower UTA ring. Resulting in the tension coming from both above and below the actual 2ndary. as well as outward tension. Pretty much just as seen here except for the holes in the kydex will be much smaller and metal grommeted as described in #2. This design gives it stability in all directions.

 

2. Don't see sliding at the 2ndary attachment points as an issue since it will be passing thru a small hole only. Installing a small metal "grommet" in the kydex for it to pass thru on the other end so no sliding there either.

 

3. As said, guitar tuners

 

4. Dunno as it isn't done yet .......but I don't intend to use them as handles. Both of my dobs have UTA's that pack away inside the mirror box so no transport issues there.....fully protected.

 

5. #2 guitar strings......

 

This thread here should be of interest to you also......

[Biff]

@ Jonathan, I've seen your work and I see in your write up that you mention using #2 guitar strings... I don't know anything about guitars and such so I don't know if that is an accepted guage thing or what but I haven't found what #2 relates to in wire diameter in my google searches. 

 

@ Mark, I read you tried a few wire sizes in your write up (I've looked at it more than a few times) but I hadn't heard what you ended up using in the field. That's a good tip though about music stores selling individual strings as I haven't found that on line. Buying packs of strings at $15 per and only using one in the pack when I need 8 makes for >$120 just for the wire which seems absurd for what it is. The hobby shop has "music wire" in the finer gages for well under a dollar for the thin stuff for 36" though I haven't brought my calipers down there to see just what size it is.

 

AFAIK the "music wire" in the hobby shops are a high tensile steel which I think is a high carbon steel, the guitar strings are phosphor bronze.  Any idea of the strength differences?

[Jeff Morgan]

One can get a variety of wire from McMaster-Carr online. Steel, stainless steel of various alloys, etc. Would any of these be better than guitar wire?

[Oberon]

@ Jonathan, I've seen your work and I see in your write up that you mention using #2 guitar strings... I don't know anything about guitars and such so I don't know if that is an accepted guage thing or what but I haven't found what #2 relates to in wire diameter in my google searches. 

 

Biff, the link I gave you here provides the size of each of the strings, and #2 is defined as "B-2nd Stainless Steel  0.016 inch or 0.41mm in this table:-

Specifications:
A206-L                                                  inch          mm
E-1st Stainless Steel                            .012         0.30
B-2nd Stainless Steel                           .016         0.41
G-3rd Coated Copper Alloy Wound       .024         0.61
D-4th Coated Copper Alloy Wound       .032         0.81
A-5th Coated Copper Alloy Wound       .042         1.07
E-6th Coated Copper Alloy Wound       .053         1.35

Also, the link I provided is for 10 packets of 6 strings at a total cost of $17:99 total so you get 2 spares of the size you choose (in case you break one), plus get to experiment with different sizes. If you shopped around hard enough you could probably do even better, but I figured that less than $20 was cheap enough for a spider, and stopped looking further.

[Oberon]

 

Any music store will sell you individual guitar strings.  You can go down to .008" or .009" that way. 

I experimented with wire spiders last winter, to support the 4 inch diagonal of my 20 inch. The wire diameter I chose was .020" stainless steel. My experiments were not very successful and I ended up going back to a solid vane made of 0.030" thick aluminium.

 

To mount the wire spider, I made a jig, supported the holder and attached the wires. It was solid and I could lift the UTA by the diagonal holder, like others have demonstrated. However, I found that if I pinched one of the wires hard enough (something that can happen when handling and transporting the UTA in a car), the whole diagonal/string spider assembly would move and I would have to realign. I was obviously doing something wrong since I've been reading with interest the excellent and successful designs described in this forum. Among other things I was using only two wires for the four vanes and I suspect the successful wire spiders may be using four wires.

 

Before I try again (which I will), a few questions for you experts out there. By the way I'm assuming offset spider arrangements:

1. How do you string the wires for each "vane" (do you use one wire per "vane")?
2. How do you attach the wires so they no longer move and can't slide at the attachement points?
3. How do you stretch the wires to get tension (I know some of you are using guitar tuners. I tried feeding the wires in a hole in a bolt head connected to the UTA and tighten the bolt)?
4. Do you find the exposed wires to be individually fragile if you pinch or stretch one of them like I described above (do you take precautions to protect the wire spider while transporting the telescope in a car? do you frequently move your telescope in a car?)?
5. What diameter of string would you recommend for a 4 inch diagonal taking into account the real life manipulation that occurs when taking down and re-assembling a truss tube reflector?

Thanks.

 

Pierre,

seeing as guitar strings are so cheap I decided to use one string per wire.

1. so thats two wires per vane. Additional benefit...if one breaks...well, so what? Just replace it. The mirror stays where it belongs, nothing collapses.

2. guiter strings come with a ring fitted on one end. Feed the wire through a small hole on the secondary support and tension it against the ring, as per the image below.
3. guitar string tuners or "machine heads" are very cheap and very convenient. I used "locking" machine heads which grab the wire. Only ultra-light UTA's might benefit from avoiding machine heads to minimise weight.
4. My wires and mirrors are protected by the UTA structure. No further protection seems necessary.
5. I'm supporting a 88mm diagonal with #2 strings, and could probably push it to 100mm or 4", but only just. My UTA is compact which means the angles of the wires are at a minimum, separated only by 120mm at their machine heads, which is way less than ideal. A deeper more standard UTA would perform better, and with, say, 250mm span to work with then a 4" secondary would not be an issue, and would be very rigid.

Edited by Oberon, 13 June 2015 - 06:17 AM.

[Pierre Lemay]

 

 

Any music store will sell you individual guitar strings.  You can go down to .008" or .009" that way. 

I experimented with wire spiders last winter, to support the 4 inch diagonal of my 20 inch. The wire diameter I chose was .020" stainless steel. My experiments were not very successful and I ended up going back to a solid vane made of 0.030" thick aluminium.

 

To mount the wire spider, I made a jig, supported the holder and attached the wires. It was solid and I could lift the UTA by the diagonal holder, like others have demonstrated. However, I found that if I pinched one of the wires hard enough (something that can happen when handling and transporting the UTA in a car), the whole diagonal/string spider assembly would move and I would have to realign. I was obviously doing something wrong since I've been reading with interest the excellent and successful designs described in this forum. Among other things I was using only two wires for the four vanes and I suspect the successful wire spiders may be using four wires.

 

Before I try again (which I will), a few questions for you experts out there. By the way I'm assuming offset spider arrangements:

1. How do you string the wires for each "vane" (do you use one wire per "vane")?
2. How do you attach the wires so they no longer move and can't slide at the attachement points?
3. How do you stretch the wires to get tension (I know some of you are using guitar tuners. I tried feeding the wires in a hole in a bolt head connected to the UTA and tighten the bolt)?
4. Do you find the exposed wires to be individually fragile if you pinch or stretch one of them like I described above (do you take precautions to protect the wire spider while transporting the telescope in a car? do you frequently move your telescope in a car?)?
5. What diameter of string would you recommend for a 4 inch diagonal taking into account the real life manipulation that occurs when taking down and re-assembling a truss tube reflector?

Thanks.

 

Pierre,

seeing as guitar strings are so cheap I decided to use one string per wire.

1. so thats two wires per vane. Additional benefit...if one breaks...well, so what? Just replace it. The mirror stays where it belongs, nothing collapses.

2. guiter strings come with a ring fitted on one end. Feed the wire through a small hole on the secondary support and tension it against the ring, as per the image below.
3. guitar string tuners or "machine heads" are very cheap and very convenient. I used "locking" machine heads which grab the wire. Only ultra-light UTA's might benefit from avoiding machine heads to minimise weight.
4. My wires and mirrors are protected by the UTA structure. No further protection seems necessary.
5. I'm supporting a 88mm diagonal with #2 strings, and could probably push it to 100mm or 4", but only just. My UTA is compact which means the angles of the wires are at a minimum, separated only by 120mm at their machine heads, which is way less than ideal. A deeper more standard UTA would perform better, and with, say, 250mm span to work with then a 4" secondary would not be an issue, and would be very rigid.

Jonathan, thanks for those very useful details. One often sees wire spider examples but details such as number of cables used and how they are attached are rarely provided.

 

From my experiments last winter, I concur that using two wires per vane is the most robust solution since accidentaly breaking one wire will not spell disaster for either the diagonal or primary mirrors. On the other hand using only two wires for the four vanes, like I did, could damage the diagonal after an accidental wire failure. I suppose using one wire per vane is an acceptable compromise, which is probably what most people do. However I prefer your 8 wire approach and that's how I will try it the next time.

 

As for wire thickness, I purchased a coil of 0.025" stainless steel wire from McMaster Carr: http://www.mcmaster.com/#8860K13 . With a tensile strength of 75,000 PSI, it's yield strength is approximately 37 pounds which is more than good enough. I will order a coil of 0.015" stainless steel wire and try that (13 pounds yield strength). I can space my triangle base on the UTA by at least 12 inches (300mm) which should provide ample strength with minimal wire pull force.

 

Finally, I will have to do something about protecting the UTA during travel. At the moment the UTA nests inside the hemisphere and anyone could throw a jacket on top of it, which would land  and rest on the spider and diagonal holder. The diagonal is protected by a cover but the wire spider is not and could accidentaly be pinched or pulled on.

[ckh]

Pierre,

 

Music (piano) wire can have a tensile strength over 300,000 PSI. It's more expensive because it's high purity and processed specially but, for the amount you need, it's cheap.

 

To exploit the full strength, you need to do the end connections properly.

 

Carl

Edited by ckh, 13 June 2015 - 02:24 PM.

[Pierre Lemay]

Music (piano) wire can have a tensile strength over 300,000 PSI. It's more expensive because it's high purity and processed specially but, for the amount you need, it's cheap.

 

To exploit the full strength, you need to do the end connections properly.

Carl, yes I'm aware of that. McMaster-Carr offer other type of Stainless Steel wires that have 200,000 PSI yield strength. The reason I chose the wire I did is because it is more ductile. Ductility makes it easier to bend and stay bent. I wanted to try something that was easier to work with. I have afraid a harder material might not like being twisted and bent, but I could be wrong.

[MitchAlsup]

I experimented with wire spiders last winter, to support the 4 inch diagonal of my 20 inch. The wire diameter I chose was .020" stainless steel. My experiments were not very successful and I ended up going back to a solid vane made of 0.030" thick aluminium.

 

About a decade ago I experimented with wire spiders for my 20" DOB with 4" secondary. I tried a variety of geometries and wire thicknesses (all high end guitar stuff) from 0.008 through 0.020.

 

I wounded up making a conventional vane spider with 0.008 thick vanes that works way better (in the stiffness and diffraction departments.)

[Oberon]

How was your spider configured? Centered as in X or offset like >-< ?

[ckh]

Carl, yes I'm aware of that. McMaster-Carr offer other type of Stainless Steel wires that have 200,000 PSI yield strength. The reason I chose the wire I did is because it is more ductile. Ductility makes it easier to bend and stay bent. I wanted to try something that was easier to work with. I have afraid a harder material might not like being twisted and bent, but I could be wrong.

 

 

Yes you are right about that. There are even special jigs for winding loops in the ends of music wire. If you get a kink, it's difficult to remove. Thin wires are particularly prone to kinking if you're not careful.

 

 

I've been looking for information about the diffraction patterns for very thin wires, but I haven't seen it in this thread or any of the ongoing related threads.

 

Jonathan has some sort of diffraction simulator, but the examples he's presented do not specify the width of the spider vanes. So, I'm still wondering how much very thin wires reduce the problem.

[Pierre Lemay]

These things are a lot stronger than people think.  I break high strings on my guitar occasionally but it takes a lot of work.  Like I said elsewhere when I built the simple wire spider for my 8" sonotube scope, with .008" wires, I could pick up the entire tube by the secondary with no problem.

Mark, thank you for that testimony. So at .008" you can barely see the crossed difraction pattern with your 8 inch?

[raal]

Mark, about the spikes on Jupiter, how close they start from the planet limb?

[raal]

I'm not sure about it, but I was hoping their intensity would be peaking a bit away from the limb.

Regardless, I'll go for wire spider instead of curved vanes.

 

I also wanted to ask you, with the wire arrangement in your 8", are you happy with oscillation damping?

 

I'll also build a tube scope, so anchor points that are far apart on the tube is not a problem and I'd like to get away without crossing the wires (and virtually thicken the shadow).

 

BTW,that is a very good illusion of disappearing ring, thanks!

Edited by raal, 14 June 2015 - 04:31 PM.

[GShaffer]

Hah! The disappearing ring works really well......until I look away and see the ring imposed on the view

[Oberon]

Oscillation damping is all about design and geometry. Get the geometry right and the wire spider is fine. Most vane spiders have poor geometry and are unable to resist even small rotational forces, using wires in the same way without offsetting from the center exposes the weakness of that design.

[ZeroID]

Wire works for me. .023mm 'E'strings and guitar machine heads for tensioning.

Once you get it close to centred it's very easy to tweak into place and very stable.

Optically not tested yet as it has just been completed but the clouds are being uncooperative.

Keep it simple is the mantra ...

Some more pix of the scope in  http://www.cloudynig...on-today/page-8

Attached Thumbnails

• 

Edited by ZeroID, 14 June 2015 - 07:23 PM.

[jtsenghas]

Wire works for me. .023mm 'E'strings...

Would that be 0.23 mm?

[raal]

I actually meant the longitudinal component, but Mark answered it even if I wasn't clear about it.

And the answer doesn't surprise me, given the the lack of steel-to-steel rubbing friction as the wires aren't crossing over one another. I believe that this friction is large enough to help a lot in damping.

 

Strictly speaking, oscillation damping is about friction alone. Geometry only determines the system stiffness (compliance), or resonant frequency (given the same hung mass). Applied force will determine the amplitude.

A system with higher resonant frequency will need greater friction to settle after the same number of periods (same Q factor) and it will last for shorter amount of time because periods are shorter.

 

As for torsional oscillation component, it's compliance is determined by the distance from the axis (the center of mass of sec. and it's holder) to the conjoining point of vanes (or wires).

In case of centered vane spiders, they all virtually conjoin on the center, or zero distance to center of mass, so the stiffness depends only on bending strength of vanes.

in case of offset vane spiders, they conjoin away from the center of mass, so there is some leverage. Rotation of the sec. will require elongation of vanes, which requires far more force for the same angle.

We're dealing with greatly increased stiffness in that case, but only proportionally increased (if at all) damping (internal friction in metal), so probably the duration of vibration is much greater, but the amplitude of oscillation is much smaller (same hung mass and applied displacement force), so it is taken as lasting much shorter as we can't resolve the smear during longer settling time.

 

If by geometry, we also mean of crossing wires that rub each other, then we are introducing damping as well, and that may be the decisive factor in case of low strength-low tension vanes like wires are.

Edited by raal, 14 June 2015 - 08:32 PM.

[raal]

Mark, I use terms here that may seem to mean the same thing in practice, but for theoretical explanation (understanding the design) they must be used in their strict sense. Damping (or dampening) and amplitude reduction may bring to the same practical result of indiscernible movement of the mirror, but they work in very different ways and how we go about designing a spider is how much of either we will use in getting to the goal.

 

Using wire in stretching mode is not really dampening the oscillation, it is reducing the amplitude, while it may not have been damped at all. For example, tightening the guitar string does not dampen it.

The only case when stretching could provide any damping is if the wire would be under so much tension that metal surpasses it's elastic deformation capability and starts entering into plastic deformation zone and this is not the case here. That is the case in guitars (wire sag after some playing), but even then the damping is insignificant.

 

If there really is any damping here, not just amplitude reduction, it comes from other than using wire in stretching mode.

An easy experiment in frictional damping would be if you cross two wires on the guitar, and see how much tone sustain you get then. The amount of movement is almost irrelevant. Pick them as softly as you possibly can (to avoid buzz) and they will still not sustain the tone.

 

I'm not going to calculate the difference now in actual stretching force of the wires in your three examples, but I can say with confidence that their angles are not different enough to provide significant difference in the force that stretch the wire under movement of sec holder.

 

Anyway, even if crossing the wires does not allow for sufficient dampening and even if the angles of "attack" are not significantly greater, then something else there either dampens the vibration, or if not that, it reduces it's amplitude significantly.

 

I think that what you have gradually done in showing the three examples in the drawing is the displacement of leverage point further and further away from the center of mass, very significantly increasing the arm length that opposes the momentum of hung mass (with practically the same stretching force), plus, introduced certain dampening from crossed wires.

 

Whether I'm not fully right on this or not, your design is excellent, nonetheless.

 

The changes that I would do is increasing the damping proper, by passing the wires (at sec. holder) through some hard rubber or soft plastic grommets and possibly rearrange the wire angles and attach points to try to avoid crossing while maintaining the momentum arm length to center of mass...

 

Cheers,

[careysub]

Pierre,
...

3. guitar string tuners or "machine heads" are very cheap and very convenient. I used "locking" machine heads which grab the wire. Only ultra-light UTA's might benefit from avoiding machine heads to minimise weight.

 

 

To scale the wire spider scheme down for ultralight rings, or simply small reflectors perhaps a wire-locking socket cap screws could be used:

http://www.mcmaster....-screws/=xmuepd

 

Normally wires with pairs of wire-locking screws are used to prevent the screws from coming loose, but a slight modification in technique could be used to tension spider wires I suspect. Thread the wire through the first screw and turn until the appropriate tension is achieved, then thread it through the second screw, twist it (perhaps) and pull the wire tight with the second screw.

[jtsenghas]

That is an interesting proposed use of drilled-head cap screws. I've used these screws often in their intended manner. One could also run the wire through one screw to maintain wire position and around a second for adjustable tension. If that second screw is mounted through a clearance hole and retained with lock washers and lock nuts, it could be adjusted for the optimal amount of wrap angle.