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An offset wire spider design

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#26 careysub

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Posted 22 January 2015 - 02:50 PM

I'm also wondering if a nylon swivel bearing could be incorporated to replace the lower collimation screws, something like McMaster-Carr part number 1071K12. In this arrangement, the safety device could be a simple cotter pin in the central bolt.

I was thinking exactly the same thing. The 6 screws seem to make an over-determined system.

 

One draw-back, this tends toward a long secondary assembly which is inconvenient with single ring or short cage designs.


Edited by careysub, 23 January 2015 - 01:09 AM.

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#27 mark cowan

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Posted 22 January 2015 - 06:47 PM

The first set of screws could be the security blanket, just some oval head screws inside the tube (or through holes in the rod) locked to the outside, and the second top set would adjust.  Then you've got a built-in battery holder if it's a tube. :waytogo:



#28 Pinbout

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Posted 22 January 2015 - 09:42 PM

 

Then you've got a built-in battery holder if it's a tube. :waytogo:

 

:grin:



#29 FHarry

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Posted 23 January 2015 - 02:52 AM

Ihhhh,ahhh,

no don't copy my design. It's awful:

 

>>this one out. it has a similar method to chriske's but on a wire support and uses thumbscrews

>>http://www.astrotref...Fangspiegel.jpg

 

The idea of having two tubes sliding inside each other is brilliant. You automatically have the possibility of adjustment in height. But i find the collimation awful. You have to loosen the opposite screws to tighten the other. And perhaps i used the wrong tubes? Maybe it would work better if the gap between inner and outer tube is not that big? Collimation is not really predictable. The tube does not really move smooth and into the direction you would think.

 

My next build went back to the more classic design. See attached image.

 

Harry

Attached Thumbnails

  • SecondaryHolder.JPG

Edited by FHarry, 23 January 2015 - 02:53 AM.

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#30 careysub

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Posted 23 January 2015 - 03:26 PM

How about using two orthogonal adjustment screws in the outer tube, with opposed springs? 

 

Take a tube and drill straight through it twice at 90 degree angles, tap all four holes identically, then insert adjustment set screws on two adjacent sides, and use these spring set screws:

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

to insert and load springs to push against the adjustment screws.

 

Use this arrangement with the McMaster-Carr spherical bearing for the pivot/holder. The tubes could the normal round, or square, or hexagonal:

http://www.mcmaster....-tubing/=vlfe2g

Round tubes/rods would probably do fine, but perhaps a flat bearing surface might have some advantage (of course in inner round tube/rod could just have its surface filed flat where the adjustment occurs).

 

A set screw shaft collar:

http://www.mcmaster....collars/=vlff7i

cemented to the bottom part of the spherical bearing could allow height adjustment.

 



#31 reiner

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Posted 24 January 2015 - 03:58 AM

Oberon's spider and secondary holder are truly excellent. The short lever between secondary and spider allow for only small forces exerted by the secondary.

 

I have built a number of wire spiders which all follow this design, which is in some aspects very similar to Oberon's, yet the secondary holder is different

 

hut_5.jpg

 

Hut2.jpg

 

Yes, this design has six setting screws and one to prevent the secondary holder to slide out. Yes, this is an overdetermined system. But does this matter? I don't think so. Such a secondary holder is rock solid (my biggest one holds a 150mm secondary). It takes a bit more time to adjust the secondary holder as compared to any three screws system, but not very much. And keep in mind: How often do you adjust your secondary? I check mine and make minor ajustments about once a year.  When you made the step and use a Cheshire instead of a laser toy to adjust your primary, this is all it takes :-)


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#32 Pinbout

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Posted 24 January 2015 - 02:05 PM

 

Oberon's spider and secondary holder are truly excellent. The short lever between secondary and spider allow for only small forces exerted by the secondary.

 

:waytogo:



#33 Chriske

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Posted 24 January 2015 - 02:18 PM

Nice projects...! :waytogo:



#34 mark cowan

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Posted 24 January 2015 - 03:31 PM

Agreed. 



#35 Oberon

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Posted 01 February 2015 - 08:35 PM

 

 

I adjust my trusses for collimation. Both mirrors.

 

I've heard about people like you.... :lol:

 

thanks for clarification on collimation.

 

 

For yet more clarification...check out the pinned "Post your home made scope" thread and specifically this post here...



#36 abberation

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Posted 16 March 2015 - 09:42 PM

 

This explains it all. One spider is removed to make it clear.

spider3.jpg

 

Black screws to attach the vanes to the secondary holder, red set screws to position/collimate the mirror in its holder.

The setscrews closest to the mirror are fastened first. But before you do that the secondary mirror is positioned exactly in front of the focuser in both directions, rotating that mirror in it's holder and also adjust it in height. You also correct for secondary offset at the same time. At this point you never try to collimate the optical axis..!

After that is all done you start collimating the mirrors optical axis using only the other 3 setscrews.

 

spider4.jpg

 

 

I'm really liking this design. It will give me a lot to think about this morning.

 

A couple of things I might change:

 

1) For a three vane system, I would use a section of hexagonal bar stock and drill out the middle. Might be a bit easier than cutting the flats into the outer collar.

 

2) I would incorporate some type of wire loop safety connecting the center bolt to the outer collar so that if an upper and lower set/collimation screw came loose the secondary mirror could not fall free and turn your Newtonian into a Cassegrain.

 

I'm also wondering if a nylon swivel bearing could be incorporated to replace the lower collimation screws, something like McMaster-Carr part number 1071K12. In this arrangement, the safety device could be a simple cotter pin in the central bolt.

 

For the center bolt, how about a bolt?

 

You could put a washer under the head to allow a wider hole in the hex for more adjustment.

 

You could make an O-ring groove at the bottom of the hex to center the bolt at the bottom of the hex and stack O-rings between the piece that holds the secondary and the hex.

 

That would reduce the number of setscrews, prevent the secondary from falling and construction is pretty simple.


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#37 abberation

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Posted 16 March 2015 - 10:11 PM

Honestly, I like the guitar string spider for its mechanical elegance but can anyone quantify its diffraction performance?

 

I know the obstruction is very small but the wires are straight and thin, wouldn't that cause diffraction?

 

I like Chriske's half circle but I would try the changes I suggested and maybe make it helical.

 

The dark horse is the carbide single stalk.  I would need to get tricky with the mirror mounting and adjustment to minimize mass, I could put three adjustments out on the ring so the stalk doesn't have to handle so much weight.  If its light enough I could make the portion of stalk in the light path an elipse to reduce the obstruction/diffraction.



#38 dag55

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Posted 17 March 2015 - 08:31 AM

Honestly, I like the guitar string spider for its mechanical elegance but can anyone quantify its diffraction performance?

 

I know the obstruction is very small but the wires are straight and thin, wouldn't that cause diffraction?

 

I like Chriske's half circle but I would try the changes I suggested and maybe make it helical.

 

The dark horse is the carbide single stalk.  I would need to get tricky with the mirror mounting and adjustment to minimize mass, I could put three adjustments out on the ring so the stalk doesn't have to handle so much weight.  If its light enough I could make the portion of stalk in the light path an elipse to reduce the obstruction/diffraction.

I would think that 8 No. .008" wires totally up to .064" would be far less defraction than a .187" stalk, which I would think minimal for a 7.25" to 8" radius of a 12.5" scope,  and that is the least I would imagine hanging a 1.83"-2/14" sec. on with collimation screws and such. I am building a 8" planetary scope, I know smaller than the 12.5" that I would like but the seeing in the midwest doesn't allow for that but maybe three times a year and invariably I am at work those nights. I located some .007" extra super light E string high carbon wire ( Just Stings) for my 1.52" sec. should perform very well, 19% CO. Note I could have went with 1.3" sec. 16.25% but with the fast fall off of illumination do to long focal length I chose the 1.52" for good globular viewing ,I have read numerous reports that once under 20% CO a smaller CO be the 2.75% that the 1.3" would have been would be all but impossible to detect the difference. This scope is a poor mans refractor so to speak,1500mm focal length, I would love to have a Tec. or AP in the 180mm size but can't justify the expense.

I borrowed Oberon's design with one mod. I used angle aluminum instead of the plate he mounted the delrin to,  this allows the three collimation screws on top for standard collimation. I also used a hardwood 1.25" dowel to fabricate the sec. mount, lighter than delrin and easy to work.

Dane


Edited by dag55, 17 March 2015 - 11:43 AM.


#39 macdonjh

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Posted 17 March 2015 - 08:49 AM

Question:  with these offset spiders, do you get four pair of diffraction spikes rather than four individual spikes?



#40 jtsenghas

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Posted 17 March 2015 - 10:05 AM

Question:  with these offset spiders, do you get four pair of diffraction spikes rather than four individual spikes?

 

With ANY straight vane or spider you get two spikes perpendicular to the edge of the vane for each bright object anywhere in the field of view, so any vanes or wires that are parallel have coincident spikes and their effects add. It doesn't matter whether the vanes are in line with each other or not.  If you stretched a large number of parallel wires across the aperture (as Glenn once suggested in a similar conversation for explanation of this phenomenon), you would get one pair of bright spikes for each bright object.  Even offset, if two angles are involved, the diffraction pattern of offset vanes would be indistinguishable from straight radial vanes.


Edited by jtsenghas, 17 March 2015 - 10:08 AM.


#41 Oberon

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Posted 17 March 2015 - 12:03 PM

In short, the number of diffraction spikes you see is determined by the number of angles involved with respect to the star. As an offset spider introduces no additional angles, the spikes look the same as a classic centered spider.



#42 macdonjh

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Posted 17 March 2015 - 12:42 PM

 

Question:  with these offset spiders, do you get four pair of diffraction spikes rather than four individual spikes?

 

With ANY straight vane or spider you get two spikes perpendicular to the edge of the vane for each bright object anywhere in the field of view, so any vanes or wires that are parallel have coincident spikes and their effects add. It doesn't matter whether the vanes are in line with each other or not.  If you stretched a large number of parallel wires across the aperture (as Glenn once suggested in a similar conversation for explanation of this phenomenon), you would get one pair of bright spikes for each bright object.  Even offset, if two angles are involved, the diffraction pattern of offset vanes would be indistinguishable from straight radial vanes.

 

 

 

In short, the number of diffraction spikes you see is determined by the number of angles involved with respect to the star. As an offset spider introduces no additional angles, the spikes look the same as a classic centered spider.

 

Interesting, I would have thought that the diffraction pattern in your scopes would have looked like a double cross hair reticle.



#43 Oberon

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Posted 17 March 2015 - 06:22 PM

Think of it this way; diffraction spikes always extend from the star. There is no such thing as an offset diffraction spike.


Edited by Oberon, 17 March 2015 - 06:30 PM.


#44 michaeldurban

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Posted 22 September 2015 - 03:35 PM

I'm going to machine chriske's secondary mounting design, maybe

with some small changes..

 

It does seem like a very simple and stable design, even though collimation

may prove to be finicky..

 

great design..



#45 I forge iron

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Posted 20 October 2016 - 11:01 PM

Some state the diffraction spikes are the result of thermal turbulance and the thin wire having less mass produces less turbulance and less diffraction spike.   Support or discount?



#46 Oberon

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Posted 21 October 2016 - 12:10 AM

Sort of. 

My understanding is that in the 1950's Andre Couder theorized that spider vane differential cooling caused refraction effects making the vanes effectively thicker. They found that polished spider vanes gave noticeably better results. See Sky & Telescope, April 1978 page 354. This is because these effectively thicker vanes would bring the diffractive energy of the spike closer to the star, thus reducing contrast. The refractive effects themselves would also reduce contrast by adding a "seeing" component. 

However it is important to appreciate that diffraction spikes are the result of interference causes by parallel boundaries or edges in an optical path; in short, any spider with straight edges (and any mirror with straight edges) will generate a diffraction spike, including wire spiders. The intensity of the diffraction spike is determined by how its energy is distributed, and that is determined by the distance between the parallel edges (i.e. the thickness of the vane). The thinner the obstruction, the further the energy is spread away from the object of interest, improving contrast. The thicker the obstruction, the worse the contrast. See here for a far more authoritative description.

In any case, here is a translation of a summary of the original paper with thanks to Pierre Lemay.

On a thermal effect observed in reflecting telescopes
by André Couder.

Original Summary of Couder’s article: Olivier Ruau, France 
English translation (December 2013): Pierre Lemay, Canada

In the September-October 1949 issue of « L’Astronomie », (bulletin de la Société Astronomique de France),  André Couder wrote an article on the thermal effects in reflecting telescopes. This extract of what he wrote applies particularly to spider supports and the analysis of the phenomena. The following document is my translation, from French to English, of French Amateur Astronomer Olivier Ruau’s summary of Couder’s original article, which I have unfortunately not been able to get my hands on (next time I’m in Paris…).   Pierre Lemay.

…Only a support made up of flat blades under tension (secondary mirror support) can provide the necessary rigidity in a large instrument.  In principal it is advantageous to make the blades as thin as the resistance of the materials allows.

Without revisiting the diffraction theory of thin screens inserted in the entrance pupil of an instrument, this reminder: the diffracted energy is smallest and is spread over the largest angle when the screen is narrowest, a double reason to diminish the visible spikes.  Take a telescope having a diameter “D”, in which the support blades (vanes) have a thickness “e”; Assume a photographic exposure that records stars down to magnitude “M1”; it is possible to evaluate by calculation from which point “M2”, inferior to “M1”, the stars will appear in the photograph showing diffraction spikes.  The M1-M2 difference measures the success with which the secondary mirror support is mounted.  Observing in the 80cm telescope (at the Haute Provence observatory in southern France)… I found a M1-M2 difference inferior to what I was expecting, of 3 or 4 magnitudes.  The spikes formed an unexpectedly bright excess.  Second remark: the alternating maxima and minima brightness, characteristic of the diffraction figure of a slot having a width “e”, is not visually perceptible.  Thirdly, the brightness of the spikes observed visually in the vicinity of the star Vega, for example, varies widely from night to night, and even during the same night.  Finally, an electric wire which happened to cut the light path one day produced a spike less bright than the spikes generated by the vanes even though this wire, round and smooth surfaced, had a diameter almost twice the thickness of the spider vanes.  “We are obviously in the presence of the same basic phenomena that produces dew:  The spider vanes are loosing heat by radiation; the surrounding air cools when in contact with them… the vanes of the 80 cm telescope being approximately 10 cm in height… correspond to an air temperature drop of 2.7°.  A round wire, substituted for the spider blade also surrounds itself with cold air, but the wire’s insulator is cylindrical: the light’s trajectory through one of these chords is very short; this explains one of the remarks made earlier” (Couder follows with a demonstration of the lowering of temperatures).  “… The thermal emissivity of the black paint that covers the spider vane is 0.9; explaining the temperature drop of 4.4°, in calm air.  When recalculating with a doubling of the convection coefficient, we find -3.1°.  These numbers match well with the experimental results of -2.7° obtained earlier: the vane’s surrounding air is obviously warmer than the vane.  The regularity of the observed appearances is a consequence of the speed of air circulation being much too small to be accompanied by turbulence: the airflow is laminar.”

The spikes being mainly caused by refraction through a layer of cold air, many remedies can immediately be called upon:
• Reducing the length of the optical trajectory in this cold air by reducing the height of the vanes;
• Removing material in the vanes, where possible, while maintaining sufficient strength to properly hold the secondary mirror;
• Substituting steel wires instead of flat vanes;
• Increasing the coefficient of convection using forced air;
• Adding a tube extension as long as the observatorie’s dome will allow, to reduce the solid angle of sky seen by the instrument;
• Finally, reduce the thermal emissivity of the spider vanes.  Here, a few explanations are necessary.

The radiating energy exchanged with objects at ordinary temperatures has its maximum intensity, for a wavelength of λm, near 10m.  In this spectral domain paints and varnishes are very absorbent: their power of emissivity is only slightly inferior to that of a black body.  Note in passing that this property is almost independent of the nature of the pigment and is a consequence, mainly, of the absorption of the coating product… On the other hand most polished metals have an infrared emissive power inferior to 0.12.  Introducing this number in the preceding formula… we find the temperature is reduced by 1.2° or 0.65° (depending on convection rate), almost a quarter or a fifth.

To check these predictions on the 80 cm telescope, I simply covered (glued) a sheet of thin polished aluminum to the vanes as they are currently setup.  The operation having been done on only one of the two crossed vanes, we first observed the image with a low power eyepiece.  The favorable effect is immediately obvious, although the eye is a poor judge of two linear sources: the spike corresponding to the treated vane appeared (the wind speed being variable) weaker and 2 to 6 times shorter, than the untreated one… Of course the glued aluminum foil was but a crude, temporary setup.  On an instrument under construction we will use polished steel vanes, nickel or chrome plated.

Will the polished surfaces not create unwanted reflections? To understand that this fear is unfounded, let’s first consider the observation at the Cassegrain focus: since the vanes are parallel to the optical axis, from a view a short distance from the optical axis, we see a reflected image of the sky very oblique on the vanes.  This image has a brightness not much lower than that of the sky itself.  It appears under a very small solid angle as compared to the larger opening of the telescope.  Consequently, the brightening the focal plane undergoes is insignificant. We can also take comfort in the fact that the reflexion properties of the best paints and varnishes is far from insignificant under such high angles of reflexion.  As for Newtonian focus: the eye placed at the focus can see the reflections off the vanes of certain areas at the top of the tube.  These areas must be blackened.

The advantages that procure low emissivity coatings can be extended to every component in the upper part of the tube assembly, including the secondary mirror support.  I have never observed a cold current effect on the secondary mirror supports on the telescopes at the Haute Provence Observatory (French Alps).  The reason, I believe, is this: We can consider constant the air temperature surrounding the secondary mirror vanes, thin and lightweight, which quickly attain their equilibrium temperature.  Such is not the case with the secondary mirror supports of large telescopes, the mass of which is higher than 15kg.  The speed of cooling of these components for a temperature difference of a few degrees is comparable to the drop in speed of air temperature during the night… … It is the heat accumulated in these massive components during the day which supplies the loss by radiation.  The surrounding air around the small mirror appears when it is lightweight.  Its existence was noted by M.P.J. Hargreaves, who blames it for the apparent overcorrection of spherical aberration often observed.  The cold air column having no more than 1 cm of thickness, this explanation for overcorrection seems true, to me, only for small telescopes.  All the same, the remedy I have just described can be applied without difficulty.”

 


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#47 Jeff Morgan

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Posted 21 October 2016 - 12:11 AM

Support.

 

http://www.astrosurf...thermique_e.htm



#48 Oberon

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Posted 21 October 2016 - 12:18 AM

In short, your premise as stated is wrong or badly stated (and so on that score may be discounted), but there is support for a better stated premise that a super-cooled solid vane affects diffraction because it is in effect thickened by the refractive effects of a boundary layer, reducing contrast.



#49 I forge iron

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Posted 16 January 2017 - 10:39 PM

Hi Oberon,

 

  Two years gone by... I hope you have not become weary of questions about your wire spider.  In one of my abandoned shechems I was worried about the guitar brass anchor points and attachment points  flexing... That was before enormously streaching my seconday wires.  I bought enough (12?) guitar strings to use 8.  Somewhere in stringing my secondary I notcied that the wires passing threw the small  holes didn't move. So in my seconday there are only 4 guitar strings being used lacing front to back.  No attachment points passing threw the aluminum secondary stem.

 

You may have been my inspiration to use the locking guitar heads... I, outside of fiddling around with the new guitar machine heads out of the package have never locked them.  Do you lock yours?  In collimating the wire suspended secondary I am amazed how fast and easy it is to move the secondary mirror around.  It's like you can move one of the 8 machine heads and it immediately will work or not, instant feed back!  I am at a compete loss of how to effectively string the words together to convey the ecstatic delight in adjusting the mirror.  Is it because there are no tools involved?  Is it like a 'joystick' on an airplane? 

 

Well!  Somewhere in here there should be another "THANK YOU"!  My first mock up used tensioning sliding  bolts with little holes drilled in the ends for the wire to pass threw...and it was cute... but with a wrench in your hand... it's not like holding onto a joystick! 

 

Fred

 

P.S.  (I should have placed a date on the wooden mock up wire spider.... I fear it was much longer than 2 years ago!) 

 

Drat!  Can't get a photo to upload! 



#50 Oberon

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Posted 16 January 2017 - 11:36 PM

Hi Fred the guitar tuners are just great. They just work so well. Glad to hear you're enjoying yours.

So far as locking is concerned, my locks only lock the wire into the hole to ensure nothing slips when tightening, they don't prevent adjustment (tuning). So yes I locked them prior to adjusting them as described here.

If you are enjoying the machine heads for collimation, wait until you try the Stewart Platform described here and here. Simply twist the truss poles to adjust collimation of both mirrors, no other adjustments necessary, and no tools required. It makes collimation trivial and intuitive, and is perfect for star collimation because you can make your final adjustments while looking through the eyepiece. 




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