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Equipment Discussions >> ATM, Optics and DIY Forum

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Newton wire spider & holder, will it work new [Re: careysub]
      #5601292 - 01/02/13 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?


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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Newton wire spider & holder, will it work new [Re: mark cowan]
      #5601600 - 01/02/13 05:06 PM

Quote:

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.


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bratislav
sage


Reged: 09/07/06

Re: Newton wire spider & holder, will it work new [Re: GlennLeDrew]
      #5601667 - 01/02/13 05:57 PM

Quote:

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.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Newton wire spider & holder, will it work new [Re: bratislav]
      #5601703 - 01/02/13 06:19 PM

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

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bratislav
sage


Reged: 09/07/06

Re: Newton wire spider & holder, will it work new [Re: GlennLeDrew]
      #5601773 - 01/02/13 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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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: Newton wire spider & holder, will it work new [Re: GlennLeDrew]
      #5601884 - 01/02/13 08:20 PM

Quote:

Has anyone made a text rig having differing obstructor widths?




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

Best,
Mark


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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Newton wire spider & holder, will it work new [Re: bratislav]
      #5602090 - 01/02/13 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.


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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Newton wire spider & holder, will it work new [Re: ZielkeNightsky]
      #5602551 - 01/03/13 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.


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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Newton wire spider & holder, will it work new [Re: Dick Jacobson]
      #5603582 - 01/03/13 07:35 PM

Quote:

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.


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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: Newton wire spider & holder, will it work new [Re: Dick Jacobson]
      #5603799 - 01/03/13 10:18 PM

Quote:

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. [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...

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


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Lightning
sage


Reged: 07/04/10

Loc: Canberra, Australia
Re: Newton wire spider & holder, will it work new [Re: mark cowan]
      #5604034 - 01/04/13 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


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ZielkeNightsky
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Reged: 10/01/06

Loc: Denmark
Re: Newton wire spider & holder, will it work new [Re: Lightning]
      #5604427 - 01/04/13 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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NHRob
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Reged: 08/27/04

Loc: New Hampshire
Re: Newton wire spider & holder, will it work new [Re: ZielkeNightsky]
      #5604449 - 01/04/13 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?

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Ed Jones
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Reged: 04/06/04

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Re: Newton wire spider & holder, will it work new [Re: ZielkeNightsky]
      #5604490 - 01/04/13 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.

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ZielkeNightsky
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Reged: 10/01/06

Loc: Denmark
Re: Newton wire spider & holder, will it work new [Re: Ed Jones]
      #5604620 - 01/04/13 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.

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Newton wire spider & holder, will it work new [Re: ZielkeNightsky]
      #5605301 - 01/04/13 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.


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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: Newton wire spider & holder, will it work new [Re: GlennLeDrew]
      #5605996 - 01/05/13 04:02 AM

That's a start.

You might want to look at http://home.digitalexp.com/~suiterhr/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


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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Newton wire spider & holder, will it work new [Re: mark cowan]
      #5606660 - 01/05/13 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.


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careysub
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Reged: 02/18/11

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Re: Newton wire spider & holder, will it work new [Re: mark cowan]
      #5606707 - 01/05/13 02:10 PM

Quote:

... 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.


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bratislav
sage


Reged: 09/07/06

Re: Newton wire spider & holder, will it work new [Re: Ed Jones]
      #5607527 - 01/05/13 11:29 PM Attachment (13 downloads)

Quote:

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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