700 replies to this topic

[Oberon]

As many readers know, I’ve long appreciated and been interested in the secondary spider as an opto-mechanical structure. It probably goes back to my days at the AAT when it was my routine responsibility to swap out the Top End (UTA) of the 3.9m telescope, an operation performed with a 7T crane fitted integral within the dome. In control from the crane platform at the inside top of the dome high above the telescope I would lift and exchange the entire UTA from a choice of 4, providing the AAT with a wide range of focal length options and Prime Focus instruments. It was from there that I first noticed that the spider vanes were offset from the center, and curiosity piquied I had to ask why.

Thirty years later its just a hobby, but...I still enjoy playing with the engineering principles that make for an excellent secondary support system. And with some difficult decisions to make for my planned 2 x 16” bino, I decided to move from theory and run a series of performance test on a wide range of spiders, from wire to curved thru to some of the exotic shapes we arrived at in this thread here. 

 

I want to see what gives, what moves, what flexes, and what holds. And take no prisoners.

Hence this monster in my garage...

 

 

Edited by Oberon, 23 January 2019 - 03:57 AM.

[CrazyPanda]

Are you going to be taking any measurements or just going by feel for stiffness? 

[Pierre Lemay]

Neat! It looks like you are going to do with spiders what Albert Highe did for truss tubes. Looking forward to learning along with you. 

[Starman47]

Please keep the CN community informed about your results.

[mark cowan]

[JohnH]

After reading a number of articles on these, one conclusion I have is that collimation/alignment is more critical.

Having a spider design that makes these adjustments both easy to do AND long lasting should over ride the various designs that only improve the image marginally.

[earlyriser]

After reading a number of articles on these, one conclusion I have is that collimation/alignment is more critical.

Having a spider design that makes these adjustments both easy to do AND long lasting should over ride the various designs that only improve the image marginally.

I agree with this 100%. If diffraction is that much of a concern, how about using a sheet of optically flat glass to support the secondary? Of course, I suppose that is only practical up to a certain size. 

[GShaffer]

I agree with this 100%. If diffraction is that much of a concern, how about using a sheet of optically flat glass to support the secondary? Of course, I suppose that is only practical up to a certain size. 

 

Not being much of a glass pusher myself I can tell you that true quality optical windows dont come cheap......I built a 6.7" f/9.3 newt using an optical window and one made to good specs will cost you some coin.....I sent out for quotes to a dozen or more known good sources and the best I could do was about $1300.00 for an IF test verified good 7.3" one that wouldn't significantly impact the performance of an excellent mirror. I dont regret the basically no expense spared build as it is an awesome little newt for planets but I wont be doing it again lol

[TonyStar]

That's a commendable effort, (sound) data from (sound) experiments are worth a billion words!

 

All I can say is I did test the source of flex in my travelscope (see sig.), specifically secondary spider, trusses, and primary cell. There was no need to build a specialized rig, I just unloaded each component separately by removing the mirrors and measured structure deflections using a laser while the scope was tilted from zenith to horizon.

The results showed that the main and most significant source of flex was in the spider. It's not a lot and I could easily improve it by reducing the bending moment on the spider but in practice I always tweak collimation at the target altitude for hi-res work so I'm living with it.

 

My spider vanes are only ~0.2mm thick. I don't use a wire spider because my scope is fully collapsible to fit in a suitcase and a design constraint is quick assembly-disassembly. I would anticipate your wire design is a bit stiffer than mine (assuming same wire gauge) because of the larger angles between crossed wire couples. However, you'll certainly find the same, that is, nothing really holds, in the sense that structures with minimal cross section needed to minimize diffraction, inherently have a finite bending stiffness and therefore flex under the weight of (heavy) secondary mirrors. 

 

If you do a comparison, it would be fair to compare spiders based on cross sectional parameters and weight.

 

I personally don't care about diffraction spikes and want my airy disc as tight of possible. With 0.2mm-thick vanes, the only time diffraction was really bothering me was with Mars at opposition last summer    

 

But I recognize that is just matter of taste.

 

Hope you enjoy the ride, and looking for some interesting results.

Edited by TonyStar, 25 January 2019 - 10:49 AM.

[Starman47]

I discovered with the help of many on CN, that I had flex in my spider at the point where the Astrosystems spider and secondary connect.

 

https://www.cloudyni...shift-on-a-dob/
 

 

And since I hope to build a few scopes, I look forward to learning more about spiders from this discussion. .

[JohnH]

That's a commendable effort, (sound) data from (sound) experiments are worth a billion words!

 

All I can say is I did test the source of flex in my travelscope (see sig.), specifically secondary spider, trusses, and primary cell. There was no need to build a specialized rig, I just unloaded each component separately by removing the mirrors and measured structure deflections using a laser while the scope was tilted from zenith to horizon.

The results showed that the main and most significant source of flex was in the spider. It's not a lot and I could easily improve it by reducing the bending moment on the spider but in practice I always tweak collimation at the target altitude for hi-res work so I'm living with it.

 

My spider vanes are only ~0.2mm thick. I don't use a wire spider because my scope is fully collapsible to fit in a suitcase and a design constraint is quick assembly-disassembly. I would anticipate your wire design is a bit stiffer than mine (assuming same wire gauge) because of the larger angles between crossed wire couples. However, you'll certainly find the same, that is, nothing really holds, in the sense that structures with minimal cross section needed to minimize diffraction, inherently have a finite bending stiffness and therefore flex under the weight of (heavy) secondary mirrors. 

 

If you do a comparison, it would be fair to compare spiders based on cross sectional parameters and weight.

 

I personally don't care about diffraction spikes and want my airy disc as tight of possible. With 0.2mm-thick vanes, the only time diffraction was really bothering me was with Mars at opposition last summer    

 

But I recognize that is just matter of taste.

 

Hope you enjoy the ride, and looking for some interesting results.

The best version for a spider that satisfied my various needs was a serious ATM one done by Gary Wolanski, near where I live in Vancouver. He got all the important ideas right: Thin vanes under tension, offset built into support, 5 degrees of freedom for various adjustments and a vane design where the vanes DO NOT pass through the center support stalk and help keep vibrations dampened. The method of using ATV to "glue" the secondary on needs some work, like a safety cable to stop the secondary from free falling onto the primary.

[Oberon]

So back to the monster in the garage...

 

The large tube is carbon fibre, a 2500mm long by 750mm diameter air ventilation duct scavenged a few years ago from the dump of a coal mine where I was working at the time. It had been underground so was filthy and had some minor superficial blemishes, but scrubbed up pretty well. I stored it behind a shed for years not knowing what I was going to do with it, just knowing it was begging for a job.

 

[Oberon]

Fitted out with a hexapod it has an optical path 3.15m, and with flat mirror mounted at the other end I have a 6.3m beam length, which I hoped would be sufficiently brutal for analysing deflections. By my calculations a degree of deflection at the spider will be 110mm, and almost 2mm per arc minute. 

Here is Merope's UTA mounted on top of the hexapod with a red Glatter laser fitted in the focuser.

 

A green laser is fitted on to an adjustable platform to provide a reference standard, to ensure I am only measuring deflections of the spider. An O-ring forms the switch.

 

 

 

 

And there are plenty of adjustments available to get the lasers lined up. Above you see the green and red lasers dots together.

 

 

Finally a view from the other end of the tube...

 

[Oberon]

Unfortunately it stands so tall that it is non-trivial measuring deflections at angles coming down from zenith...

 

 

But once horizontal it is trivial to roll it around the lawn...

 

 

...and the deflections were immediately apparent...

 

 

So at least we know it works very nicely. I can be happy about that.

 

[Oberon]

After the lawn test showed that the severest test is rolling the tube through 360 degree whilst horizontal I figured I might as well keep it in the workshop and build a jig to roll it on the bench. And fit some extras, like an old iPhone running Clinometer.

 

This video here shows a full rotation (sped up 4 x). 

At this point I'm happy I have a functional test bench and will now do some refinements before testing more seriously.

[MitchAlsup]

Well done, well done indeed.

[Bob4BVM]

NIce work Jonathon !

You do realize your structure is just begging to be fitted with a 16"-20"F13 mirror... maybe even with a center hole....  I can visualize a big classic Cassegrain on a monster permanent single-fork mounting, in a proper dome of course... 

 

Anyway, carry on, I look fwd to more enlightenment...

Bob

Edited by Bob4BVM, 26 January 2019 - 02:49 PM.

[Starman1]

This is all well and good, and will return excellent information.

However, how will you differentiate tube sag from spider deflection?

Because there kind of has to be SOME tube compression or sag.

[mark cowan]

He can mount the secondary on a solid support for that...

[TonyStar]

However, how will you differentiate tube sag from spider deflection?

That's what the green laser is for. Tube sagging (assuming hexapod is stiff enough) is what causes the green laser to move in a loop as he rotates the tube.

[mark cowan]

Well OK but it doesn't eliminate attitudinal changes in the flat mirror support, whatever that actually is.  Since they're both moving in the video.  Rigidly fixing the secondary would test the full support portion that isn't the spider.

 

Liking the idea though.

[Starman1]

Indeed.  What the market lacks is much in the way of experimental evidence.

[TonyStar]

.

I was puzzled initially by the fact that the red dot draws an ellipse, but it's just due to the 45* mirror support.

Edited by TonyStar, 26 January 2019 - 04:32 PM.

[Oberon]

The ellipse makes perfect sense, but I had to stop and think about it for a bit. I think of it this way....it is possible to tilt (or rotate) the secondary on a plane that has no effect because its a flat. In our case the sideways tilt more closely approximates that effect and so the laser moves nowhere near as much as when the mirror tilts back and forth toward and away from the beam.

 

Therefore I concluded that flex of the spider/secondary is effectively the same in all directions, but effects differ.

 

The ellipse is an important observation, and is an argument for having the focuser parallel with the altitude bearings.

Edited by Oberon, 26 January 2019 - 06:26 PM.

[Oberon]

This is all well and good, and will return excellent information.

However, how will you differentiate tube sag from spider deflection?

Because there kind of has to be SOME tube compression or sag.

Thats what the green laser does. Both lasers follow the same path down the tube and back to the paper sheet, both are mounted on the same hexapod. Only the red laser sees spider deflection.