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Newton wire spider & holder, will it work

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#1 ZielkeNightsky

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Posted 27 December 2012 - 08:42 PM

Merry Christmas

Now my OO 10” mirror finally gave up, I’m in the process of build the final 10” Newton for my observatory. So a 10” F/6 Royce mirror is on the way and my dream scope will be build around this fine mirror.

I’ve thinking and reading a lot about wire and curved spiders and right now I’m hooked on a wire spider. Is it worth going for a wire spider compared to a curved spider. (I think a curved spider I think is somewhat easier to make an install)

Some ideas are from Mark Cowan’s nice design and a couple are from Royce’s secondary holder design.

Secondary holder:
As the renderings shows I use a hinge and therefore only the angel can be adjusted, no tilt/3 axes as we normally see. I can’t fully figure out if it is enough with that together with the rotation of the secondary. Any thoughts?

A link to Royce’s secondary spider/holder design

I’ve also used his suggestion with only one 1/2" square silicone in the centre to clue the secondary mirror and holder together. I’m going for a nice Antares secondary.

The secondary holder will be made in aluminium 4mm. thick, powder coated black.

Wire setup:
The wire is crossed in to dimensions. Will this be strong enough to keep if vibrating?
The tube is made of carbon with a 5mm. hard foam core, a total of 8mm. So it should be strong enough to hold the tension of the wire.

Comments are most welcome.

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#2 glennnnnnn

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Posted 28 December 2012 - 12:27 AM

Just as an added feature you could put a metal insert in the tube at each of the points of attachment. The wire or cable will have a lot of tension and might tend to cut into the tube unless there is a strong anchor. The small part I'm thinking of is known as a threaded insert. You might even be able to use the thread and have the ability to increase or decrease the tension for that segment, so you could more easily "tune it."
What you show in your drawings is a fine design!

EDIT: A tension on each wire can be adjusted by turning the bolt. This is a hollow bolt. I have seen these used in many different applications, but what comes to mind is some sort of bicycle part, like the tensioning for hand-brake cables. There's probably no shortage of bicycle parts in Denmark, or anywhere else in Europe which is bicycle friendly!

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#3 mark cowan

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Posted 28 December 2012 - 12:52 AM

The wire is crossed in to dimensions. Will this be strong enough to keep if vibrating?


Hi, vibration shouldn't be a problem for any reasonable wire thickness. At least a couple people (that I know of) have built my spider design. The limits on wire diameter are going to be set more by how tightly you want it to hold collimation over the range of motion, and because the COG of the overall assembly gets close to where the support converges the moment arm is small which helps a lot.

I understand the hinge but I like my version better. :lol: Of course it doesn't rotate very far and requires more careful setup alignment for that reason. About which I'd have to ask, how often will you need to rotate the secondary and how far? If the hinge were a single pivot and there were two angle adjustments outboard on the front you'd be able to accommodate small amounts of rotation...

You've incorporated the essential elements for vibration stability and I'll be really interested to find out how well it works. :waytogo:

Best,
Mark

#4 ZielkeNightsky

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Posted 28 December 2012 - 06:20 AM

Hi

Thanks for the comments.

About which I'd have to ask, how often will you need to rotate the secondary and how far?
Best, Mark


Mark, you're right. I've been looking a lot on your design, but in the end I was to nervous about the initial setup in your design.


About the wires.
A metal insert (threaded) in the tube will, is in the design now.

How do you set up such a system? How do I find and set the correct tension of the wire?

What about the numbers of wires. It can be 4 in a "loop" or 8 one for each connection.

:question:

#5 KenScharf

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Posted 28 December 2012 - 10:53 AM

I've seen wire spiders before, they work. One fellow used guitar parts to adjust the wire tension for centering the mirror.

#6 bremms

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Posted 28 December 2012 - 03:16 PM

Played with one years ago on my RV6, broke the tube with fender washers. Really needed a metal ring where the wires were attached. I took apart a Novak spider and wrapped the wire through the pins. Worked well until I tightened it up a little too much

#7 mark cowan

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Posted 28 December 2012 - 03:58 PM

I've been looking a lot on your design, but in the end I was to nervous about the initial setup in your design.


You just have to construct a simple jig to hold it in place while you attach the wires and tighten them minimally - since it works by opposing forces (for anti vibration) you don't need much tension at all. 8 or 4 wires it probably doesn't matter much, but it's easier if you lock down one end (of each of 4) on the tube and tighten the other (also on the tube). In my version the lockscrews on the assembly are essential once the position and tension is set...otherwise it will try to flip.

Best,
Mark

#8 Jeff Morgan

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Posted 28 December 2012 - 04:17 PM

Interesting design. I was interested in doing this myself, but get-done-itis caused me to just order a standard 4 vane spider. But from what I learned from those that have built them, lots of wire tension is not required, but should actually be avoided.

I see Mark Cowan has already commented, so I'm sure you've seen his design. Another very interesting design was on one of the German sites - Reiner Vogel I believe. His design minimizes overhang distance by building the diagonal holder as two concentric tubes. The wire supports attach to the outer tube. The inner tube is held and collimated by six bolts, much like a finderscope. There was also a safety mechanism so that the inner tube/mirror could not slide free and fall.

#9 ZielkeNightsky

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Posted 28 December 2012 - 06:02 PM

Thanks for the new comments.

I'll have look at Reiners work on his website.

So good support at the tube, a jig to get everything in place, some lock-screws, guitar parts for adjustment of tension.

I'll alter the design an upload it here.

So one last question before I go ahead with building it. Is it worth doing compared to the curved spider?

#10 mark cowan

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Posted 28 December 2012 - 06:08 PM

Ha ha ha ha ah ha ha ha. Seriously?

Best,
Mark

#11 ZielkeNightsky

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Posted 28 December 2012 - 06:49 PM

Not really. Then it should have been the first question before spending time designing it.

Nevertheless, always nice if others confirm what I think is the right path to go

BTW: Here is the design I first was going after, pretty much your design Mark. For the reasons about rotation and adjustment of position I ended in the current design.

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#12 mark cowan

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Posted 28 December 2012 - 08:13 PM

A curved spider works fine at doing what it does - replacing wider dispersion of smaller diffracted energy with closer dispersion of larger diffracted energy. I've never compared them directly, though. I do kind of like not having to make the vanes themselves for the wire spiders.

Best,
Mark

#13 ZielkeNightsky

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Posted 28 December 2012 - 09:09 PM

Thanks for all the advice so far.

This sketch, I think is close to what I'll start building now.

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And there is a plan B - buy a curved spider if I find it to difficult to make.

#14 NHRob

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Posted 29 December 2012 - 11:32 AM

are the diffraction spikes from such a wire spider nearly invisible?
Are they significantly less intrusive than a thin, solid spider?

#15 ianm

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Posted 29 December 2012 - 12:18 PM

I built a wire spider for my 12.5 inch dob using Mack Cowan's design. It works beautifully and is not as hard to make as one might think. I never see any diffraction spikes. Click here

#16 allardster

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Posted 29 December 2012 - 01:16 PM

Here is an interesting, recent thread comparing different secondary holders and their impact on diffraction:
http://www.cloudynig...4137547/page...

Both wirespiders and curved cone out very well.

#17 ZielkeNightsky

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Posted 01 January 2013 - 08:03 AM

Thanks for comments and links.

I've started building one now.

#18 NHRob

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Posted 01 January 2013 - 09:56 AM

how about a curved wire spider ??
:john: :john: :john: :john:

#19 careysub

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Posted 02 January 2013 - 11:21 AM

...

Secondary holder:
As the renderings shows I use a hinge and therefore only the angel can be adjusted, no tilt/3 axes as we normally see. I can’t fully figure out if it is enough with that together with the rotation of the secondary. Any thoughts?
...


After seeing Royce's secondary holder design I was puzzling this out myself.

This is what I conclude.

The design has two axes of rotation adjustment - rotating around the center bolt, and adjusting the mirror angle tilt. These are axes are both perpendicular to the focuser sight line, vertically, and horizontally.

There is one additional missing degree of freedom - and that axis is the focuser sight line.

It is helpful to try this with a model - say a piece of corrugated cardboard stuck on a pencil at a 45 degree angle (the corrugations keep the card flat, the hex cross section of the pencil keeps it from turning on the pencil).

Look at the card from the side so that you see the 45 degree angle. Now spin the pencil 90 degrees (this is an extreme "adjustment" to make the change involved clear).

What you see is the card is still at 45 degree angle, but that angle is no longer inclined to the vertical, but to the horizontal. And if you made the card long like the elliptical secondary you will notice that the long axis is now pointed horizontally as well.

Now here is the thing. Look at the rotated card from the front (along the pencil axis). It is inclined to your point of view, and this can be fixed by rotation along the support bolt axis. The card is now flat to your view line, but not tilted at 45 degrees. Using the tilt angle adjustment fixes that.

The final problem that hasn't been fixed is that the mirror's long axis is no longer pointing up. If you can rotate the mirror in or with its mount to adjust its axis then this problem can be fixed.

But remember that in this design the spider is actually perfectly rigid along the focuser sight-line axis. It is impossible for new mis-alignment to arise here unless the hinge axis is actually inclined with respect to the optical axis in which case tilt adjustment will create a very small mirror ellipse axis rotation as well. This doesn't affect collimation though and with an appropriately sized secondary shouldn't create any issues. In a well made and installed spider assembly (we are not assuming perfection here, just no obvious manufacturing and assembly errors) this should not be a significant problem.

#20 mark cowan

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Posted 02 January 2013 - 01:08 PM

Yes. You only need to be able to make the last few tiny tweaks with the adjustments on the spider, the rest is taken out during installation with a jig.

Best,
Mark

#21 GlennLeDrew

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Posted 02 January 2013 - 01:50 PM

Is it *really* worth the effort to go with wire? Does a thinner material truly amount to a change in diffraction intensity of any note? I ask because even the finest roof prism, with its effectively micron-thin roof line produces an easily visible spike. If going from a 'thick' vane to a thinner wire did result in a sensible diminution, one would naively extrapolate and ascribe to to the *vastly* thinner prism edge a vanishingly small diffraction energy.

My thoughts go as follows. Within reasonable limits of practical vane thickness, their small obstruction areas relative to the unobstructed area of the objective is hardly ever greater than 2%. For example, a 10" with 0.05" thick vanes would have the vane obstruction at about 1.2% The secondary central obstruction whose relative diameter is 0.12 is considered as having a barely detectible impact. Such an obstruction has an area of 1.4% that of the aperture. And so even relatively stout vanes offer obstruction area only of this order. Because of the fact that even a 'wavelength-thin' vane already introduces readily visible diffraction, the damage is already done, and the difference between this and a 'thick' vane is really minimal.

The linked-to thread from a couple years back is all well and good. But the investigator missed an opportunity to do a *proper* comparison with his simulated vane/wire setup. And that is creating a version where one diametrical vane is the thicker and the other is the thinner. In such a way one had a *direct*, unbiased comparison where any difference seen will be real, and not reliant upon fallible memory. I am always leery in the extreme where a test relies upon taking the time to swap out the apparatus and then require for comparison one's memory of the previous appearance.

Has anyone made a text rig having differing obstructor widths? I know I should, just to see for myself.

I should add a final factor which puts me off from wire spiders. And that is that they require precise longitudinal alignment. If to the incoming light they present as not two exactly overlapping obstructors, their effective with is now larger. And even if of the most precise construction, for light coming from any distance off-axis, the wires no longer exactly overlap, presenting as thicker. At some point off axis, depending on wire thickness and their longitudinal separation, they will present as two obstructors, with a concomitant doubling of diffraction!

In the 'cross-braced' wire arrangement as proposed here, it's obvious that the wires cannot mask their counterparts, thus projecting as having greater width; the advantage of thinness so desperately sought is nullified at a stroke! And for each of the four wire pairs, at some small off-axis angle it will project as an 'X'-shaped obstructor, with portions of the 'vane' being doubled, which, again, doubles diffraction.

Where the wires project as separate obstructors, the diffraction will most definitely be worse than that of even a quite thick singleton; you can take that to the bank.

Truly, is all the bother really worth it?

#22 careysub

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Posted 02 January 2013 - 05:06 PM

Yes. You only need to be able to make the last few tiny tweaks with the adjustments on the spider, the rest is taken out during installation with a jig.

Best,
Mark


And I have a suggestion for a collimation tool or jig for tweaking the vertical axis rotation.

The problem with adjusting this is that unlike using a screw to adjust the mirror tilt, there is no mechanical reduction involved that converts a large motion into a small one. And if you are rotating the whole mirror assembly on the support bolt, then tightening the locking nut will tend to create torque that un-collimates what you have carefully adjusted.

The idea is to have a top plate (for example) on the mirror assembly with two holes drilled in it. Make a beam long enough to rest across the top of the UTA with two prongs that fit in the holes.

That's basically it.

You now have long arm to assist in precisely adjusting the mirror assembly rotation, and using the arm you can hold the assembly in place while tightening up a locking nut.

#23 bratislav

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Posted 02 January 2013 - 05:57 PM

Is it *really* worth the effort to go with wire? Does a thinner material truly amount to a change in diffraction intensity of any note?


The main advantage of a wire spider is not that they are thinner. They have less mass. Not for the immediately obvious reason.

Let's remember where spider usually is - at top of the tube, or in an upper cage (e.g. large Dob). Both allow spider assembly to be exposed to large areas of the night sky, which in turn promotes a lot of radiative cooling. In such environment (typical night sky will be around 50 Kelvin or so), anything that is directly exposed will achieve equilibrium below ambient air (sometimes WELL below ambient air. That is why car is often covered in ice even if night temperatures stay well above freezing point).
Now, more mass keeps more (in this case negative) energy. That is, more massive spider will cool more surrounding air. This cooler layer will have different refractive index from the surrounding air, and that will in turn increase the effective spider profile, making it look thicker.
Wire spiders will have less of this 'effective' profile, and show less diffraction most of the time. It will be still there, no question about it, but it will be less noticeable.
Of course, as with anything else, our logarythmic senses make those advantages anything but subtle (as you pointed out, even thinnest obstruction is very obvious); nevertheless the effect, and the advantages, are real.

#24 GlennLeDrew

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Posted 02 January 2013 - 06:19 PM

Then how about a lower emissivity coating on all metal components exposed to the sky? And lightweight tube extensions ('dewcaps').

#25 bratislav

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Posted 02 January 2013 - 07:15 PM

People have reported that insulation (e.g. cork) works well in metal tubes. But considering that there is usually a generous space inbetween the tube and a light path, it is probably not so important (as to remove this boundary layer - it can't be much more than a mm thick. It may be important for different reasons, as in reducing tube currents).
But spider is right in the middle of the tube, and anything that can make it look thinner must help.






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