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Newton wire spider & holder, will it work

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#26 mark cowan

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Posted 02 January 2013 - 08:20 PM

Has anyone made a text rig having differing obstructor widths?


Yes. We should discuss wire spiders over a couple of beers sometime. :lol:

Best,
Mark

#27 careysub

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Posted 02 January 2013 - 10:43 PM

Very interesting analysis.


I am familiar with all of the physics of all of these physical phenomenon, but this had never occurred to me.

#28 Dick Jacobson

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Posted 03 January 2013 - 08:55 AM

Has anyone tried making a spider out of thin, tape-like material, for example videotape? The advantage would be ultra-thin profile along the optical axis. For example, you could use a long central bolt and four loops of tape. Each loop would make a 90-degree bend around the bolt. There would be one pair of loops at the top, oriented to make a "+" pattern. A few inches down would be a second pair of loops to keep the bolt parallel to the tube. Outside the tube, each end of the tape would be wrapped around a shaft which could be tightened as needed.

Personally, I agree with the previous post that questioned whether it is really worth all the effort to build an ultra-thin spider. But for purists who want to reduce diffraction to an absolute minimum, it would be interesting to see how well a tape spider could perform.

#29 careysub

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Posted 03 January 2013 - 07:35 PM

Has anyone tried making a spider out of thin, tape-like material, for example videotape?...


Looking up the specs on videotape, it is thin I grant you that: 1.5 mils for old, old tapes, to 0.5 mils for recent ones (0.0015" - 0.0005"). Maybe too thin.

It is made of PET, and is 1/2" wide for VHS family tapes. From PET tech data I calculate a tensile strength of no more than 2.7 lb for recent tapes, and 8 lb for old ones. I read about people with very thin tensioned spiders using something like 20 lb force to hold them rigid. And at a near breaking-strength load (as would be necessary unless doubled up a few of times) it would have something like a 2% stretch.

OTOH you can get very high modulus carbon fiber unidirectional tapes that are 0.006" (6 mils) thick in widths as narrow as 1/2". Using a minimal epoxy binder (otherwise the tape would start fuzzing) you would get a highly flexible tape with a tensile strength of 860 lb (1/2" wide).

Carbon fiber tow (thread) would seem to be a promising material for wire spiders, you would probably wipe it with resin and twist it before it sets to bind the fibers together. Unfortunately it is only available in spools used for weaving, and a spool is a lot more than you (or you and 99 other ATMers) would ever use for this purpose.

#30 mark cowan

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Posted 03 January 2013 - 10:18 PM

Personally, I agree with the previous post that questioned whether it is really worth all the effort to build an ultra-thin spider.


This is what I hear regularly, but rarely if ever from anybody who's actually built one, because they know it's not difficult at all. :lol: [EDIT: I'm only referring to a wire spider here, for good reason.]

And tensile strength is not an issue if the geometry is solved correctly. Which is why I did what I did. And they're CHEAP to build. Exotic materials are not required, though I did experiment with Vectran yarn (too fluffy, like all polyamarids).

Even Mel Bartels (though a latecomer to wire spiders) now raves about the ease of construction, durability, and performance. As well as how CHEAP THEY ARE TO BUILD. He used a spool of wire from Lowes... :grin:

I was hoping someone else would refute Glenn's argument, but since they're no takers I'll guess I'll have to do it - when I've got a little more time and tact on hand.

Best,
Mark

#31 Lightning

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Posted 04 January 2013 - 03:12 AM

Beta Pictoris by Rolf Olsen

The extra contrast gained from the small diagonal and thin vanes are a good part of what let Rolf capture the protoplanetary disc of Beta Pict. I think that says it's worth it right there.

Cheers,
Cam

#32 ZielkeNightsky

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Posted 04 January 2013 - 10:03 AM

Great posts.

I'm still working on the new spider, although I've made a few changes. I'm still convinced that there's a little performance gain with the thin spider wanes (wire). I haven't found a single ATM that's complaining over the performance. (Maybe I haven't looked hard enough?)

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#33 NHRob

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Posted 04 January 2013 - 10:15 AM

There has been some discussion in the other forum over cooling fans for the primary. It seems that if one used a rear tube fan, gently pushing air up the tube, this would help reduce the thermal layer on spider vanes, and reduce spider diffraction. Has anyone experimented with this?

#34 Ed Jones

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Posted 04 January 2013 - 10:34 AM

I believe the wires will probably not be line up on top of each other and thus will double their small diffraction. I prefer scope with no spiders but when I do I think the best solution is a 3 vane spider with vanes as thin as your wires. I built one wire spider on a 6 inch and it never was as solid as a vane spider, but then it wasn't this design either. I have a 3 vane spider with .010 inch vanes and you don't see spikes except on the brightest stars.

#35 ZielkeNightsky

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Posted 04 January 2013 - 11:42 AM

Good point with the line up on top of each other. Maybe it's better to not line them up, but instead increase the distance so I end up with 8 wires.

#36 GlennLeDrew

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Posted 04 January 2013 - 06:00 PM

In the 'vein' of try it your self and see...

I just performed a test, with a 60mm f/7 Carton refractor and 4mm long eye relief eyepiece (0.57mm exit pupil.) I placed a white LED flashlight down at the far end of my apartment building's hall. A piece of foil had numerous pinholes, producing 'stars' of at least Sirius brightness down to near invisibility.

At the front end of the shade/dew cap I taped two straight pieces of wire diametrically, and mutually perpendicular. The thicknesses are 0.47mm (0.0185") and 1.7mm (0.067"), the latter being 3.62X thicker than the other.

In the main, the *overall* brightness and extent of the diffraction spikes is fairly similar. The most outstanding difference is the frequency of the sinusoidal variation in intensity along the spikes (visible on only the brightest couple of 'stars'.) Not having first measured the wire thickness, my estimate had the thicker wire's spike varying some 3.5-4 times more rapidly (compare to the 3.6X thickness ratio.) So we see that this angular frequency apparently scales linearly with obstructor width, at least over a limited range.

The thicker wire's spike immediately beside the 'star' was certainly brighter than that produced by the thinner wire. But it dimmed more rapidly with distance, the first minimum in the sinusoidal variation occurring closer to the 'star.'

When I threw the image well out of focus, on those 'stars' sufficiently bright to show it the superimposed linear component of diffraction was certainly more prominent, too.

But the surprising thing I found is that for the dimmer 'stars' (for which spikes were too faint to see) it was most difficult to detect at all that there were obstructors present. Even while spinning the lens shade. The first and second diffraction rings surrounding the Airy disk appeared hardly altered from uniform circularity and azimuthal uniformity of intensity.

In this experiment, using a 60mm aperture, the 0.47mm wire presented an areal obstruction of 1%, and the 1.7mm wire, 3.6%.

My early conclusions are that over the range explored here, while the differences in the diffraction spikes differ much in detail, in the main they are not so different. Total extent and overall brightness are not obvious at first glance. The worst aspect of a thicker obstruction is the spike's higher brightness adjacent to the source.

Yes, a thinner vane is better, but things do not change so very greatly over any reasonable range, particularly in the Fresnel pattern of the image point. I should try a thinner wire yet to see if the trend continues linearly, or if the realm of diminishing returns is soon (or has been ) reached.

This was a quickly performed test using stuff ready to hand. I should drag out one of the 120mm refractors so that something closer to a Newt's aperture is used.

#37 mark cowan

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Posted 05 January 2013 - 04:02 AM

That's a start. :)

You might want to look at http://home.digitale.../TM/Spiders.htm if you haven't. Try thinner wires and larger apertures. You can lay a thin wire at angles over any spidered dob and compare easily.

Best,
Mark

#38 careysub

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Posted 05 January 2013 - 01:46 PM

The Suiter page gives some very good insights that I have not seen presented elsewhere (certainly not so clearly).

Note that in his final "apples-to-apples" comparison of the 1%, 2% and 4% vanes (thickness fraction of aperture) that the ratios are very close to the simple ratio of vane thickness.

#39 careysub

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Posted 05 January 2013 - 02:10 PM

... Exotic materials are not required, though I did experiment with Vectran yarn (too fluffy, like all polyamarids).


Most (probably all) fibrous filamentary materials (Vectran, Kevlar, CF, Spectra and fiberglass) would need to be wiped with resin to bond the fibers together to prevent eventual (or immediate) fuzzing. But it wouldn't take much.

#40 bratislav

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Posted 05 January 2013 - 11:29 PM

I believe the wires will probably not be line up on top of each other and thus will double their small diffraction.


It is not that hard to make wires line up perfectly. This wire contraption needed rather basic tools (drill press and a router to cut big circles were most advanced).
And it is not wobbly either! (those are 2.5kg weights each - 11 lb in total)

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#41 mark cowan

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Posted 05 January 2013 - 11:33 PM

... Exotic materials are not required, though I did experiment with Vectran yarn (too fluffy, like all polyamarids).


Most (probably all) fibrous filamentary materials (Vectran, Kevlar, CF, Spectra and fiberglass) would need to be wiped with resin to bond the fibers together to prevent eventual (or immediate) fuzzing. But it wouldn't take much.


I suppose you could do that. But the strength advantage is mostly by weight, not diameter, and even if you wind them tight you still get more thickness than thinner wires (SS) that can be plenty strong.

Best,
Mark

PS if anybody wants some Vectran yarn just ask - I've got a hundred yards or so.

#42 telfish

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Posted 06 January 2013 - 08:17 AM

There is a spectra fishing line called Power Pro which has a waxed coating. The 50 lb strength is 0,014"

I have used it for fishing for years and it's plenty strong and does not fuzz. It's woven braid.Spectra lines have almost zero stretch.

The 20Lb which may be enough is 0.009"

http://www.powerpro....ro_v2/produc...

Terry

#43 careysub

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Posted 06 January 2013 - 11:56 AM

There is a spectra fishing line called Power Pro which has a waxed coating. The 50 lb strength is 0,014"

I have used it for fishing for years and it's plenty strong and does not fuzz. It's woven braid.Spectra lines have almost zero stretch.

The 20Lb which may be enough is 0.009"

http://www.powerpro....ro_v2/produc...

Terry


Looks like Spectra line holds the edge in minimum diameter for a given strength, well above trolling leader wire, and somewhat above music (aka "piano") wire.

The Proline specs for the 0.009" line give it a strength of 470 KSI, music wire in this diameter range has a minimum strength of 400 KSI or so (this is extremely high for steel, essentially maraging steel). It is an edge, but not much of one.

#44 telfish

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Posted 06 January 2013 - 05:07 PM

Being a Guitarist and a Fisherman I can tell you that you are far more likely to get an unexpected failure in a guitar string that is under far less pressure than a power pro fishing line with a 40 lb fish on the end!

The high E sting or the high G string on a 12 string guitar is apt to break if it is over tightened or can cut itself if crossed over during winding.

No sure what would happen with a 40 Lb fish on the end of a G string!

#45 mark cowan

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Posted 06 January 2013 - 10:59 PM

I've broken many an E string as well. The tensions used in a properly designed spider (i.e. relies on geometry not tension for holding position) are far less than in that application. So if you're doing it right it's really a nonissue. Can't emphasize this enough, as people ALWAYS go to the "must have lots of tension in a wire spider" mindset.

Best,
Mark

#46 telfish

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Posted 07 January 2013 - 07:55 AM

I am guessing the wire used in this application would need to be stainless. Guitar wire tends to rust.

#47 mark cowan

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Posted 07 January 2013 - 12:20 PM

Not really a problem in application, but stainless is preferable.


Best,
Mark

#48 ianm

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Posted 07 January 2013 - 05:32 PM

Can't emphasize this enough, as people ALWAYS go to the "must have lots of tension in a wire spider" mindset.


Very true. I was surprised by how little tension was needed to make my spider rock solid. I used .01 in ss fishing wire and if I make another I'll use .005.

A lot of emphasis has been put on the size of diffraction spikes and while I enjoy that aspect weight was just as important. My scope is a single pole hybrid with one 3/4 inch square x 1/16 inch aluminum ring as UTA and had to be as light as possible. An aluminum holder/ss wire design is hard to beat.

Another positive is the wow factor. I've had a chance to show my scope to large groups and at first glance the wire spider seems like magic. From a few feet away the wires are invisible.

#49 mark cowan

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Posted 07 January 2013 - 06:02 PM

Hey, if you have info online I'd like to link it on the page I put up...

Best,
Mark

#50 ianm

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Posted 07 January 2013 - 07:30 PM

Hi Mark,

The only info I have on line is the post I made here last year. If you want to use some of those photos that would be fine. There is a link above on my first comment. Also, I could email you what photos I have of the spider. I really think more people should consider it. I'm in the early planning stage of an 8 in (haven't got a mirror yet) and would definitely use a wire spider on it.

Ian






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