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Secondary spider designs?

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#1 tag1260

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Posted 18 September 2013 - 02:33 PM

Looking for input on spider designs. I'm building a 12" truss dob and am going to order a new spider for it. What are the pros and cons of the different designs. 3 or 4 vein, straight versus curved ?

Help me decide what is optimum.

Thanks
Tag

#2 Pinbout

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Posted 18 September 2013 - 03:04 PM

http://www.astrosyst...biz/spiders.htm

$44 for a spider. save a headache, buy it.

#3 hbanich

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Posted 18 September 2013 - 03:14 PM

Hi Tag,

A three vane spider will create six diffraction spikes and is slightly more difficult to physically center in your upper tube assembly.

A four vane spider creates four diffraction spikes and is slightly easier to physically center in the uta.

A curved spider doesn't create any diffraction spikes but spreads diffracted light through the field of view and can be a bugger to physically center in the uta.

So it depends on how many, if any, diffraction spikes you prefer and ease of installation. Four vane spiders seem to be the most popular but all three versions are commercially available and not all that difficult to make yourself. I've made all three on different scopes through the years and prefer the four vane spider.

#4 dpwoos

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Posted 18 September 2013 - 10:26 PM

I have made several of these:

http://woosfamily.net/dennis/Spider/

#5 MitchAlsup

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Posted 19 September 2013 - 11:03 AM

In any event, the thinner the vanes the less light gets diffracted. My 20" DOB gets away with 0.010" thick vanes. I used 4 vanes with a pair meating at 90 degrees on both sides of the secondary fixture. This prevents the fixture from rotating about its center (unlike common 4 vanes at 90 degrees centered on the axis designs).

#6 Starman1

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Posted 19 September 2013 - 03:32 PM

Want curved and thin and relatively stiff?
If your secondary is up to 3", this design should work:
http://www.obsession...elescopes/12.5/
go down the page until you see a pic of the spider on the right and click on the pic for a larger image.
If you install a spider like this, keep it vertically symmetrical, i.e. like )( and not tilted.

#7 Ed Jones

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Posted 19 September 2013 - 06:25 PM

Curved spiders are never as rigid as straight vanes and can be a source of vibration or sag unless they are rather thick. With 3 thin vanes you'll only see spikes on the brightest stars, that's my preference.

#8 GeneT

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Posted 19 September 2013 - 09:35 PM

A lot of people like the curved secondary holders. Call me old fashioned, but I prefer the four vein spiders. I know how to deal with the various issues they present; not so sure about the three vein or curved.

#9 dpwoos

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Posted 19 September 2013 - 10:25 PM

Curved spiders are never as rigid as straight vanes and can be a source of vibration or sag unless they are rather thick. With 3 thin vanes you'll only see spikes on the brightest stars, that's my preference.


When we first rolled out our homemade 10" f/6 dob, the curved spider was made from a single length of stainless steel ruler - a single curve. It was not good, as the vibration was awful. However, we added a second curve on the other side (like Don discussed above) and now it is rock solid - zero vibration - and it provides excellent performance. Our 5" and 6" dobs use a single curve, but made with wider stainless steel (as seen in my link posted above). They too are rock solid, and are excellent scopes. If I was to do the 10" again, I would try the wide single curve. When done carefully, the extra width is parallel to the incoming light and so adds rigidity without adding thickness.

On the other hand, I am totally in agreement that so far as rigidity and simplicity goes, straight vanes are the way to go. In my case, I was tired of having the spikes interfere with my observing double/multiple stars, and so went down the dark and dangerous curved spider road!

#10 tag1260

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Posted 20 September 2013 - 04:53 AM

Thanks everyone . I now am more confused on what to get then ever!!!!! :crazy:

Anyways.....

Don

Where can you find a spider like the one from the Obsession page? Plus, what are the advantages to that design?

Thanks
Tag

#11 jpcannavo

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Posted 20 September 2013 - 06:54 AM

All the above input is right on. I would only add this: if you don't have particularly strong feelings about the presence of diffraction spikes then go with the most straight forward arrangement: 4 straight veins. You can always experiment with alternatives down the road. also, just to be clear, and all things being equal, 3 vein spiders yield 6 spikes, yet each spike is only half as bright as in a 4 vein [3/4 the amount of total diffraction distributed over 3/2 the number spikes, I.e. 2/3 times 3/4=1/2].

#12 Starman1

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Posted 20 September 2013 - 09:24 AM

Thanks everyone . I now am more confused on what to get then ever!!!!! :crazy:

Anyways.....

Don

Where can you find a spider like the one from the Obsession page? Plus, what are the advantages to that design?

Thanks
Tag

Well, the stiffest and strongest will be the 4-straight-vane variety.
The )( design on the Obsession is probably proprietary. It could be made easily enough by using an Astrosystems secondary holder and finding some sheet stainless steel for the vanes. You'd need a vise to bend the metal at the ends and a drill. The center could be wood or a metal block or whatever is convenient.

#13 hottr6

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Posted 20 September 2013 - 10:33 AM

Well, the stiffest and strongest will be the 4-straight-vane variety.

Don,

Are there any studies done on this? Intuitively, I sense that a 3-vane system will be as rigid as a 4-vane given the short beam-lengths we are using. A 3-vane spider accurately defines a plane, but a 4-point spider defines 2 planes, suggesting that a 4-vane spider will have out-of-plane forces acting on the secondary support. An engineer is needed here to describe the forces at work.

I'll also contend that a 3-vane system is much easier to adjust, and achieve equal tension on all vanes. This has implications for people using very-thin walled tubes.

3-vane spiders are likely lighter than a 4-vane.... an important consideration for the ultra-light weight contingent.

For the rest of us, I would say that the choice between a 3- and 4-vane system is a matter of personal preference..... 6 faint diffraction spikes vs 4 relatively bright spikes. I have both in my Newts, but I prefer 3-vane spiders.

#14 garret

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Posted 20 September 2013 - 11:00 AM

The ASA newtons has double (very thin) vanes:

Attached Files



#15 MitchAlsup

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Posted 20 September 2013 - 11:41 AM


Well, the stiffest and strongest will be the 4-straight-vane variety.

Don,

Are there any studies done on this?


Yes, the 4 vane of >-< architecture is the only one that completely eliminates rocking couples of the secondary mirror on the optical axis. Both sides of the secondary holder attach to vanes at 90 degrees so all rocking forces are in pure tension.

Its 3 vane varient >-- has a rocking couple where the other side of the secondary holder attaches to the vane. The rocking couple on this end is in quadradic tension (i.e. not good).

The 4 vane varrient >< has rocking couples because the vanes prevent rotation around the optical axis only in quadradic tension.

#16 vct123

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Posted 20 September 2013 - 12:24 PM

If viewing will be mostly deep sky, then go with 4.
If you plan on using the scope for planetary use, then go curved 3 or 4 vanes.

#17 csrlice12

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Posted 20 September 2013 - 12:39 PM

I've often wondered, why can't the secondary be built into the focuser....you'd only need one vane and possibly some tilt adjustment screws. The secondary would be permanently aligned with the focuser tube that way except for the tilt....Of course this could be a mess if you ever decide to replace the focuser....

#18 Starman1

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Posted 20 September 2013 - 02:26 PM


Well, the stiffest and strongest will be the 4-straight-vane variety.

Don,

Are there any studies done on this? Intuitively, I sense that a 3-vane system will be as rigid as a 4-vane given the short beam-lengths we are using. A 3-vane spider accurately defines a plane, but a 4-point spider defines 2 planes, suggesting that a 4-vane spider will have out-of-plane forces acting on the secondary support. An engineer is needed here to describe the forces at work.

I'll also contend that a 3-vane system is much easier to adjust, and achieve equal tension on all vanes. This has implications for people using very-thin walled tubes.

3-vane spiders are likely lighter than a 4-vane.... an important consideration for the ultra-light weight contingent.

For the rest of us, I would say that the choice between a 3- and 4-vane system is a matter of personal preference..... 6 faint diffraction spikes vs 4 relatively bright spikes. I have both in my Newts, but I prefer 3-vane spiders.


In a 4-vane assembly, each vane tension increase also increases the tension of the blade opposite. This actually lessens the necessary forces on the vanes because the forces are directly in line with the vanes. A higher tension down the length of the blade (necessary to hold the vane in place) results from less force applied to the bolts holding the vanes[than with vanes that are not opposite one another].

In a 3-vane assembly, each vane tension increase increases the tension on both of the other blades but not all of the tightening force goes into tightening the other blades because a substantial portion of it is lateral rather than direct in line with the blades.

Since thin flat blades have little resistance to lateral forces, the blades in a 3-vane arrangement have to be thicker than those in a 4-vane arrangement or the entire arrangement has to have a higher tension than a 4-vane arrangement. In other words, because tightening one blade tends to bend the other two vanes laterally as well as applying tension along their lengths, the tension has to be brought up to a higher level in a 3-vane arrangement to hold the center as tightly as in a 4-vane arrangement.
And since holding the center tightly is essential to preserving collimation as the scope moves up and down, the 3-vane arrangement is at a disadvantage.

Also, it seems, from a couple studies, that rigidity of the spider to resisting gravitational forces on the secondary as the scope points lower requires a vertical symmetry in the attachment of the spider. I.e. a 4 vane arrangement has to be attached as either an X or a + while a 3-vane arrangement should be attached as a Y or and upside down Y.

All of this matters little to the users of 8" and smaller scopes or 2" secondaries and smaller. But when the weight of the secondary and holder get higher (2.6" and heavier mirrors), resisting gravitational torque on the vanes with altitude change in the scope matters quite a bit if preserving collimation is desired. This puts curved vanes at a disadvantage as well as the 3-vane configuration.

#19 obin robinson

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Posted 20 September 2013 - 05:00 PM

This discussion has me thinking of if I should use a 4-vane spider in my 4.5" astrograph I am rebuilding. This will be used primarily for imaging satellites especially geostationaries. Should I use my 3-vane spider or have a custom 4-vane one built? The money doesn't matter as I want to make this thing as perfect as possible on my first attempt.

obin :question:

#20 Starman1

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Posted 20 September 2013 - 05:10 PM

This discussion has me thinking of if I should use a 4-vane spider in my 4.5" astrograph I am rebuilding. This will be used primarily for imaging satellites especially geostationaries. Should I use my 3-vane spider or have a custom 4-vane one built? The money doesn't matter as I want to make this thing as perfect as possible on my first attempt.

obin :question:

Well, since your secondary will be small, it matters more how you want the images to come out. With a 4-vane spider, you could have exceptionally thin vanes (0.01" or less) and reduce the diffraction, while getting those lovely crosses on the bright stars (which some refractor users ADD to the images). With 3 vanes, you'll get six spikes on every bright star.

It will matter a lot how long the focal length is and how large a chip you use.
If the scope is f/5 or faster, you will need some form of coma correction--especially on a large chip camera.
If the focal length is shorter than 1000mm, you might need a field flattener if the chip in your camera is a decent size.
Those factors will probably matter more than the number of vanes in your spider.

#21 obin robinson

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Posted 20 September 2013 - 07:20 PM

Don,

The focal length is a f/7.9 and I have tested both my DSLR and CCD camera in the tester configuration. The images look fine. I do notice the multiple diffraction spikes though. I might just consider a thinner 4 vane as long as it will make a positive difference.

obin :question:

#22 Jon Isaacs

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Posted 21 September 2013 - 05:46 AM

Don,

Are there any studies done on this? Intuitively, I sense that a 3-vane system will be as rigid as a 4-vane given the short beam-lengths we are using. A 3-vane spider accurately defines a plane, but a 4-point spider defines 2 planes, suggesting that a 4-vane spider will have out-of-plane forces acting on the secondary support. An engineer is needed here to describe the forces at work.



I am not exactly sure what you are concerned about here. A 4 vane spider has two pairs in direct tension. The pairs are at 90 degrees and have essentially stiffness at 90 degrees so for small deflections, adjusting one pair does not affect the other pair.

As far as easy to adjust, I think 4 are easier to center simply because can be done independently in X and Y. With 3, when you adjust 1 you then have to adjust the other two. They are also easier to tension for the same reason.

I think that scope design issues can guide the number of vanes. In a single upper ring three strut design ala Albert Hinge, three spiders that are aligned with the struts reduces the bending forces on the upper ring.

Diffraction spikes don't bother me, the secondary shifting with altitude bothers me. The job of the spider is to accurately and robustly position the secondary mirror. A 3 or 4 vane straight vane spider is the best way to do this..

In the spoked wheel designs, one never sees a bicycle wheel with curved spokes unless they are very wide and made of composite.. there is a lesson there.

Jon

#23 Sarkikos

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Posted 22 September 2013 - 11:23 AM

If viewing will be mostly deep sky, then go with 4.
If you plan on using the scope for planetary use, then go curved 3 or 4 vanes.


Why? Is it really better to spread the diffraction across the image rather than have it concentrated in four spikes? I don't think so. Also, four vanes tend to have better stability and hold collimation better than curved. Close, stable collimation is important for planet viewing.

I think the only advantage to curved vanes is for double stars and for observers who have a personal aesthetic preference for no spikes in the image.

Mike

#24 dpwoos

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Posted 22 September 2013 - 12:52 PM

My (homemade) curved spiders don't exhibit any collimation issues. Maybe for really large/heavy secondaries it is an issue, but then I would expect a good design to account for this.

To me, the issue boils down to diffraction, and how one sees the trade-offs. I think the facts are well known, and what is left is all subjective. Really, not such a big deal and if the views are great I don't care that much whether I see diffraction spikes or not, nor how many.

#25 Jon Isaacs

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Posted 22 September 2013 - 02:07 PM

My (homemade) curved spiders don't exhibit any collimation issues. Maybe for really large/heavy secondaries it is an issue, but then I would expect a good design to account for this.



As a mechanical engineer, I agree. Good mechanical design for a spider is simple, straight vanes, pure tension, no bending. Look at a spoked bicycle wheel...

If minimizing diffraction is a concern, thinner vanes are better and straight vanes, because they are in tension and not out of plane bending, can be thinner.

Jon






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