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Cheap torsionally-stabilized four-vane secondary mirror spider

reflector dob collimation
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#1 Gavin R. Putland

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Posted 25 April 2015 - 05:40 AM

The secondary mirror spider of my cheap 8-inch Dobsonian is constructed as in Fig.1 (front and side views, omitting the mirror and its holder and mounting bolts). It offers little resistance to twisting about the optical axis.

 

non-stabilized.png

 

If the spider were constructed as in Fig.2, it would be even cheaper, due to fewer parts and simpler shapes. But it would be torsionally stabilized. And it would still produce only four diffraction spikes at right angles (provided that the bolt heads are within the central obstruction).

 

stabilized.png

 

This idea is hardly new. It is almost stated in Bely (ed.), The Design and Construction of Large Optical Telescopes, Fig. 6.32 (https://t.co/JlhUyVpyE1), and is an obvious vaned equivalent of (e.g.) Steve Houlihan's wired spider (http://t.co/Y3GrMtEHLt).

 

I merely express my surprise that the simpler superior structure is not seen more often than the more complex inferior one.



#2 Pierre Lemay

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Posted 25 April 2015 - 05:49 AM

I merely express my surprise that the simpler superior structure is not seen more often than the more complex inferior one.

Oh, but the simpler support you describe in fig. 2 is used by many telescope makers, including ATMs. You will find many examples here in CN. As far as I know it is now the preferred way of attaching the spider vanes. 

 

Oh, I should add, welcome to Cloudy Nights. 


Edited by Pierre Lemay, 25 April 2015 - 05:55 AM.


#3 Pinbout

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Posted 25 April 2015 - 07:04 AM

 

the simpler support you describe in fig. 2 is used by many telescope makers, including ATMs.

 

but the states many people aren't use to that config. and when they see it worry about diffraction.



#4 Oberon

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Posted 25 April 2015 - 09:49 AM

I merely express my surprise that the simpler superior structure is not seen more often than the more complex inferior one.

 

Agreed.

Its amazing how even the most precious manufacturers claiming quality and performance seem unaware of this.



#5 Jeff Morgan

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Posted 25 April 2015 - 11:46 AM

Sidgwick presented an off-set vane configuration 50 years ago. He states that excessive tension is not required for stability.

 

I would think this would be a major selling point for those using Sonotube and am also surprised this has not been offered in a commercial product.



#6 Relativist

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Posted 25 April 2015 - 11:51 AM

Id put one on my scopes if there were a retrofit that used the same bolts. (Minus my 10" with a curved vein)

#7 AlphaGJohn

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Posted 25 April 2015 - 12:17 PM

I'm curious about the significance of resistance to axial motion in a spider. It would seem that the main issue would be the tendency of the weight of the secondary to twist the spider's hub's lower and inner edge toward the ground; a force that would be in play except when the scope is pointed exactly at the zenith. Rotation of the tube would be intermittent if the scope was on a GEM and absent entirely with an alt-az mount--unless the tube was mounted in rotating rings (again an interrmitent force). So is there any difference between the OP's illustrated designs in regards to the torsional force in the vertical plane through the center of the spider's hub (imparted by the unbalanced weight of the secondary)? In particular, when the 2nd, simpler design is oriented with the bolts or rivets are in the vertical plane versus at some orientation between that and more horizontal?

I am not familiar enough with the relevant issues to know, but concern about a spider's resistance to rotational forces imparted by rotating the tube seems like a minor consideration. Am I missing something?

John


Edited by AlphaGJohn, 25 April 2015 - 12:23 PM.


#8 starman345

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Posted 25 April 2015 - 12:23 PM

Id put one on my scopes if there were a retrofit that used the same bolts. (Minus my 10" with a curved vein)

 

Drop Gary an e-mail, he'll make whatever you need. His spiders require very little tension and are rock solid.



#9 Mike Lockwood

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Posted 25 April 2015 - 12:38 PM

It would seem that the main issue would be the tendency of the weight of the secondary to twist the spider's hub's lower and inner edge toward the ground; a force that would be in play except when the scope is pointed exactly at the zenith.....

I am not familiar enough with the relevant issues to know, but concern about a spider's resistance to rotational forces imparted by rotating the tube seems like a minor consideration. Am I missing something?

 

Yes, the weight of the secondary, depending how the offset is implemented, can cause collimation shift, if I am understanding you correctly.  Other flexure with gravity is also common, expecially the flexure of the central stud itself if the mirror is heavy.

 

Also, when the secondary's tilt adjustment screws require significant torque to turn, simply turning those screws will rotate the secondary, and then it will spring back after the torque is removed.  However, this can also cause a shift in where the rotation of the secondary comes to "rest", or to say it another way, it may not spring back to where it was!  This springiness and instability makes secondary collimation quite frustrating, and is a major problem, in my opinion.

 

However, to me the most blatant optical effect/display of the tendency of the secondary to rotate is when it is WINDY - with the unstable (non-offset) spiders, a strong breeze blowing across the spider vanes will cause them to oscillate, which will cause the mirror to oscillate rotationally.  This turns stars into lines!

 

So, I strongly recommend the offset design for fast telescopes.  It has been used by amateurs for nearly 80 years, and is shown in Texereau's "How to Make a Telescope".



#10 Oberon

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Posted 25 April 2015 - 06:54 PM

Also, when the secondary's tilt adjustment screws require significant torque to turn, simply turning those screws will rotate the secondary, and then it will spring back after the torque is removed.  However, this can also cause a shift in where the rotation of the secondary comes to "rest", or to say it another way, it may not spring back to where it was!  This springiness and instability makes secondary collimation quite frustrating, and is a major problem, in my opinion.

 

However, to me the most blatant optical effect/display of the tendency of the secondary to rotate is when it is WINDY - with the unstable (non-offset) spiders, a strong breeze blowing across the spider vanes will cause them to oscillate, which will cause the mirror to oscillate rotationally.  This turns stars into lines!

 

So, I strongly recommend the offset design for fast telescopes.  It has been used by amateurs for nearly 80 years, and is shown in Texereau's "How to Make a Telescope".

 

 

My 8" has a 25+ year old Novak spider which were widely regarded as "the best" when I bought it. But it has proved to be the most signifiant and frustrating cause of miscollimation, and difficult to cure due to its inability to hold steady whilst being adjusted. As Mike says, this springiness and instability make collimation frustrating and is a major problem.



#11 AlphaGJohn

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Posted 28 April 2015 - 11:21 PM

Good point about the torque applied by twisting the collimation adjustment screws. I'm disappointed that didn't occur to me--I have definitely experienced it!

John



#12 JohnH

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Posted 28 April 2015 - 11:43 PM

 

Id put one on my scopes if there were a retrofit that used the same bolts. (Minus my 10" with a curved vein)

 

Drop Gary an e-mail, he'll make whatever you need. His spiders require very little tension and are rock solid.

 

I concur. Have spoken with him a few times and have a spider of his for a 12" I am making.



#13 mconnelley

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Posted 29 April 2015 - 01:29 AM

Hello:
 

    Here's a picture of the telescope I'm currently working on.  It's a 1m f/3.2 dob.  I chose to use the offset spider design precisely for the extra torsional stability.  I noticed that Subaru Telescope's spiders are like this.  The spiders for the telescope I work for are of the conventional design, and when you whack a spider vane you can see the whole assembly (spiders and secondary) do the twist.

 

 I also set the secondary in the middle of the spider rather than hanging down below the spider, to further improve stiffness.  Putting the secondary in the middle of the spider minimizes the change in the load on the spider as the telescope changes in elevation.  A scope this big will never be diffraction limited, so the larger obstruction is irrelevant.  The secondary is 8"x11", or roughly the weight of a 10" primary mirror hanging up-side-down, so it's fairly heavy.  

Cheers

Mike

Attached Thumbnails

  • test_truss.jpg


#14 Pinbout

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Posted 29 April 2015 - 06:58 AM

 

I also set the secondary in the middle of the spider rather than hanging down below the spider, to further improve stiffness.

 

:waytogo:



#15 jtsenghas

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Posted 29 April 2015 - 10:31 AM

There are many ways to make and position vanes to better withstand bending and twisting moments from the mirror and secondary holder,  but the best results I've seen also minimize those loads rather than simply withstand them. Getting the center of gravity of the secondary and holder close the center of the spider is one of those methods, and the above example of putting the secondary in the center of a spider is an excellent one, in my opinion. 

 

For those just making minor modifications to existing secondaries that may be offset significantly from the center of the vanes an added counterweight may be a reasonable solution. As long as it doesn't interfere with collimation adjustments or enter the light path its presence would hardly be noticed. It may not affect the response to vibration much, except to reduce the frequency, but a counterweight to some secondary mirrors would certainly reduce shifts from static loading at different altitudes.


Edited by jtsenghas, 29 April 2015 - 10:33 AM.


#16 Oberon

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Posted 29 April 2015 - 12:38 PM

Mmmm...I hear this a lot about bending and twisting moments, and agree in principle, but I wonder how much of that is due to other inherent weaknesses either of the design or implementation. Let me explain using Mike's ginormous telescope pictured above which he has obviously put a lot of thought into...

1. The telescope has offset spider vanes (excellent) and

2. the secondary is placed in the vertical center to minimise moment, and thus minimise flexure from elevation changes.

 

This sounds great.

What could go wrong?

Look closer...because the mirror is centered, the vanes cannot continue straight through (there is a mirror in the way).

OK, no problem, that's been done before, just make sure the mirror tube is stiff enough.

Is it stiff enough?

Judging from the photo, the upper vanes appear to pulling at a reinforced section of tube which will adequately resist the likely tension. Unfortunately the lower vanes are pulling at a thin metal tube that is not reinforced. This seems like a weak link which will easily distort under tension, and must greatly limit the force that may be applied compared to a design where the vanes continue straight through (see photo below). And that leaves the tube is highly vulnerable to flexure. Bummer. Exactly what Mike wishes to avoid.

It may get worse that this. Depending on the tolerances and the tension applied, the tube may even distort enough to put pressure on the mirror. I'm sure Mike doesn't want that. So to apply enough force to prevent flexure somehow the secondary support tube will need to be reinforced down the bottom end to ensure it is more or less as stiff as the top end. And that won't be easy without increasing the obscuration. (Not saying it can't be done, and perhaps Mike is already aware of this weakness and has compensated for it; obviously I can't tell from the photo, but its worth pointing out that this is an issue just in case he hasn't).

In short, I'm predicting that the unreinforced section of tube holding the secondary mirror limits the tension that may be effectively applied through the vanes to resist flexure, and the difference in rigidity between the reinforced top and non-reinforced bottom of the tube will permit a rotational effect on the mirror under the force of gravity as the telescope is tilted; exactly what we thought would not happen.

In contrast I recall testing the stiffness of Merope's first spider and UTA (no longer in use). I was blown away how stiff, rigid and strong it was. I could have stood and jumped on it with little effect. It was unbelievably stiff even under very light tension, and would not have experienced any noticable flexure even with a much greater moment. Yes it was over engineered, it just didn't need to be so strong...but it wasn't the amount of metal in it that made it so strong and rigid, it was the way the components worked together. Look at the photo below and you can see why; the weakest link is the outer ring. The spider is as strong and rigid as the outer ring permits it to be, and stoutly resists flexure in any direction.

Of course, being too heavy defeats a design constraint, so I'm not holding this up as a great UTA. In that regard it was a fail. Its obvious Mike has put a lot of thought into his UTA, especially with regard to minimising weight, and it looks like he is almost ready to fit the secondary and test for UTA flexure. I'll be very interested to your report Mike. Best wishes...

 

gallery_217007_4913_246150.jpg
 



#17 Oberon

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Posted 29 April 2015 - 12:43 PM

Hello:
 

    Here's a picture of the telescope I'm currently working on.  It's a 1m f/3.2 dob.  I chose to use the offset spider design precisely for the extra torsional stability.  I noticed that Subaru Telescope's spiders are like this.  The spiders for the telescope I work for are of the conventional design, and when you whack a spider vane you can see the whole assembly (spiders and secondary) do the twist.

 

Exactly!

So which telescope do you work for?



#18 Jeff Morgan

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Posted 30 April 2015 - 12:50 AM

 

Id put one on my scopes if there were a retrofit that used the same bolts. (Minus my 10" with a curved vein)

 

Drop Gary an e-mail, he'll make whatever you need. His spiders require very little tension and are rock solid.

 

 

Thanks for the tip. I emailed Gary about a spider for my 8", if it is as good as advertised I'll retrofit one to my 16".



#19 RossSackett

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Posted 30 April 2015 - 10:19 AM

I think many of us learned of the offset spider from Texereau's 1951 book.

Here's my implementation on an 18" dob.

Attached Thumbnails

  • Spider.JPG

Edited by RossSackett, 30 April 2015 - 05:35 PM.


#20 Oberon

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Posted 30 April 2015 - 05:19 PM

Nice spider. The shame is how few manufacturers have learnt. 


Edited by Oberon, 30 April 2015 - 06:54 PM.


#21 mconnelley

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Posted 01 May 2015 - 04:33 AM

Hello:

 

   That's a keen eye to notice that the second reinforcing ring hadn't been installed yet when I took that photo.  I had always planned to have a reinforcing ring at each end of the secondary tube, and both are currently installed.  The design has the spiders bolting into tapped holes in the reinforcing rings, trapping the tube in between.  For the photo, I just bolted the lower spiders to the tube itself.  This photo is fairly old, but I haven't recently take a photo of the hole thing put together.  

 

   I may have to trim the lower reinforcing ring for about 1/4 of it's length to allow for more clearance to the secondary mirror.  This won't interfere with where the spider's attach, so overall stiffness shouldn't be hurt too much.  

 

   So far I'm fairly satisfied with the stiffness of the top end ring.  I can pick it up by the secondary mirror tube, twist it around, and the outer ring follows along without any noticeable torsional oscillation.  I have yet to have first light with this scope, so we'll only know if it's stiff enough when I can test it on sky.  

 

   One other detail is that my spiders are not tensioned.  They're made of an aluminum-plastic composite panel, with thin face sheets of aluminum with corrugated plastic in between.  It's 1/4" thick, light weight, and very strong (and the plastic is a good vibration damper).  Given that the spiders are mounted above the upper ring, tensioning them would twist the ring.  I think that they're inherently stiff enough that tensioning them wouldn't help much.  I figure that with a 1m aperture, 1/4" thick spiders aren't a problem.  Heck, Subaru telescope (8m aperture) has 8" thick spiders.  

 

   My 'day job' is as a support astronomer for the NASA Infrared Telescope.  

Cheers

Mike



#22 davidpitre

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Posted 02 May 2015 - 04:36 PM

Oberon.

I really like that UTA and think it has great potential for a large aperture dob.

How are the ends of the spiders held to what looks like plywood outside the UTA tubes?



#23 Oberon

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Posted 04 May 2015 - 03:17 PM

Oberon.

I really like that UTA and think it has great potential for a large aperture dob.

How are the ends of the spiders held to what looks like plywood outside the UTA tubes?

A picture tells a thousand words. I can follow up later with more explanation and dimensions etc, but its the concept that matters and you can probably work it out from there.

gallery_217007_4746_9880.png

 

Basically the spider is held in tension by a single M6 screw for each vane threaded into a short section of 20mm Delrin rod. The end of the vane is supported by two 3mm aluminium adaptors with 2 3mm aluminium spacers to provide room for the screw and ensure tension is centered through the vane with no risk of twisting. The spacers, adaptors and vanes are riveted to ensure that the vanes can be very thin and not distort due to stress.

What you saw as a raw plywood panel was temporary, my intention was to reinforce it to make the ply stronger and stiffer; that proved not necessary as very little tension was required.

The plywood panel is supported in turn by a pair of 25mm aluminium tubes with Moonlite inserts recessed into the UTA rings. The tubes are not penetrated or weakened and sit only 15mm apart, giving only 2mm clearance.

As I said...unbelievably rigid and strong for my 16" (I replaced it later with a wire spider to reduce weight) but as a method I expect it should work very well for much larger sizes.

gallery_217007_4913_10308.jpg

gallery_217007_4746_112572.jpg
 



#24 Oberon

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Posted 04 May 2015 - 08:49 PM

I think many of us learned of the offset spider from Texereau's 1951 book.

Here's my implementation on an 18" dob.

 

Ross, surely your spider deserves more exposure than one dodgy photograph in the dark!  :)  More construction info please...




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