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Spiders on trial - what gives?

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#26 Oberon

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Posted 26 January 2019 - 06:14 PM

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... 

attachicon.gif PMO 24-F13 Cass.jpg

 

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

smile.gif

Bob

A 24” f/5 would go nicely...



#27 Oberon

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Posted 26 January 2019 - 06:35 PM

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.

The mirror you speak of - I call it the primary - is firmly fixed to the other side of this piece of 4 x 2. It is a 50 x 70mm optical flat. As both lasers look at it any motion is cancelled out.

 

med_gallery_217007_10583_175061.jpg


Edited by Oberon, 26 January 2019 - 06:37 PM.


#28 ckh

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Posted 26 January 2019 - 07:11 PM

Since the green laser is solidly attached to the bottom ring of UTA, the only other things that might move are the top ring and connecting tube. I suppose you could mount the diagonal on something really solid like multiple steel rods attached to upper ring to see how much that part deflects.

 

Could you have done it without the tube?  I'm thinking mount each of the two UTA rings on two ball bearings at +/- 45 degrees. Put the flat on a separate solid stand. Adjust the flat until the green laser stays on target as you rotate the UTA (flat normal to rotation axis).  Of course what you have works.

 

So what spider was in there (apparently there is some deflection in your video)?



#29 jtsenghas

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Posted 26 January 2019 - 10:09 PM

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.

Hmmm... 

By that logic a focuser 30 degrees above the horizontal would show far less deflection of the beam than one 45 degrees from the altitude axis (for a spider equally stiff at all angles).  Very large scopes tend to have focusers that approach parallel to the altitude axis, but a moderate angle up from that tends to be more ergonomic on more moderate sizes,  especially those scopes typically used from observing chairs.  This is a good argument for minimizing that angle, within reason and structural design. 



#30 Bob4BVM

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Posted 26 January 2019 - 10:36 PM

Hmmm... 

By that logic a focuser 30 degrees above the horizontal would show far less deflection of the beam than one 45 degrees from the altitude axis (for a spider equally stiff at all angles).  Very large scopes tend to have focusers that approach parallel to the altitude axis, but a moderate angle up from that tends to be more ergonomic on more moderate sizes,  especially those scopes typically used from observing chairs.  This is a good argument for minimizing that angle, within reason and structurao design. 

Well then what about LR designs , where the focser sits at a large downward angle ?

Seems to shoot down the LR concept, but my experiments with a LR remake of my old Dob , sec mirror and focuser tilted,  did not show any issue with images seen. 

CS

Bob



#31 Oberon

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Posted 27 January 2019 - 02:23 AM

Since the green laser is solidly attached to the bottom ring of UTA, the only other things that might move are the top ring and connecting tube. I suppose you could mount the diagonal on something really solid like multiple steel rods attached to upper ring to see how much that part deflects.

 

Could you have done it without the tube?  I'm thinking mount each of the two UTA rings on two ball bearings at +/- 45 degrees. Put the flat on a separate solid stand. Adjust the flat until the green laser stays on target as you rotate the UTA (flat normal to rotation axis).  Of course what you have works.

 

So what spider was in there (apparently there is some deflection in your video)?

One of my tests will be with my old solid vane spider. I suspect it won’t budge, and that that will confirm that the UTA is solidly attached.

 

The spider currently in test is Merope’s wire spider.



#32 Oberon

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Posted 27 January 2019 - 02:26 AM

Since the green laser is solidly attached to the bottom ring of UTA, the only other things that might move are the top ring and connecting tube. I suppose you could mount the diagonal on something really solid like multiple steel rods attached to upper ring to see how much that part deflects.

 

Could you have done it without the tube?  I'm thinking mount each of the two UTA rings on two ball bearings at +/- 45 degrees. Put the flat on a separate solid stand. Adjust the flat until the green laser stays on target as you rotate the UTA (flat normal to rotation axis).  Of course what you have works.

 

So what spider was in there (apparently there is some deflection in your video)?

Yes, I agree that an effective rotational test could be done without the tube. But without the tube I wouldn’t have known that that was, in fact, a sufficiently effective test.

 

At the end of the day, as I was nutting out ways to perform the test using trusses I realised I had a great big CF tube in the garden...problem solved.


Edited by Oberon, 27 January 2019 - 02:28 AM.

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#33 Oberon

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Posted 27 January 2019 - 07:07 AM

OK another test this time with a true scale in arc minutes. Well sort of...due to accidentally printing at 50% scale the way to read the target is that the outer ring is 60 arc minutes (1 degree) in diameter, not radius. 

 

I printed white on black to absorb the intensity of the lasers, it seems to work.

The following 4 images show a photo taken at each quadrant. 

 

med_gallery_217007_10583_96266.jpg

 

med_gallery_217007_10583_60484.jpg

 

med_gallery_217007_10583_335423.jpg

 

med_gallery_217007_10583_97241.jpg


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#34 earlyriser

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Posted 27 January 2019 - 07:57 AM

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.

I think the torque moment the elipitical mirror applies to the support varies as you rotate it, with the moment being greatest when the long axis of the ecliptical mirror is vertical, and least when the short axis is vertical. This would cause the deflection to vary as the tube is rolled along the ground.


Edited by earlyriser, 27 January 2019 - 07:58 AM.

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#35 Oberon

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Posted 27 January 2019 - 08:05 AM

How so? The moment doesn't change while ever the UTA remains horizontal.



#36 ckh

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Posted 27 January 2019 - 09:32 AM

Looks like about 1 moon's worth of variation.  Have you thought about the fact that the UTA does not rotate on this axis in actual use.  It would be interesting to see how the diagonal tilt varies with inclination along the scope's altitude axis.  For that you do need the tube. wink.gif

 

The orientation of the spider has an effect too, which you can assess with the rotation you are doing.


Edited by ckh, 27 January 2019 - 09:34 AM.


#37 Pierre Lemay

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Posted 27 January 2019 - 09:59 AM

Have you thought about the fact that the UTA does not rotate on this axis in actual use.

It does with a ball scope! And with equatorials. Jonathan's study is fascinating for me because I have noticed these issues with my 20 inch ball scope (the oval movement of the red dot). I'm rebuilding the UTA this year, including the diagonal holder, so this study could not come at a better time.


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#38 jtsenghas

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Posted 27 January 2019 - 10:16 AM

This study should also be able to show how counterweights for secondary mirrors improve such behavior. When the center of gravity of the secondary support (including counterweights) is not cantilevered, no doubt deflection will decrease. I'm certain most of this deflection is due to angular tilt rather than radial displacement. 


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#39 jtsenghas

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Posted 27 January 2019 - 10:37 AM

 Have you thought about the fact that the UTA does not rotate on this axis in actual use.

Even if you were interested only in deflection seen by alt-azimuth scopes such as dobs, for relatively symmetrical spiders the difference in readings between normal orientation and 180 degrees from that should be quite close to twice the range one would see in actual use from the horizon to zenith.

 

I like this setup! With about 2.5 meters of tube and perhaps nearly another half meter to the mirror, the total laser distance must be close to six meters. Arc minutes are quite large indeed!

 

Edit -- Hmmmm ... Is the angular scale on your black reference chart off by a factor of two, Jonathan? I don't know the size of your phone in the image but it looks to me that you might not be using twice the distance to your "primary" for your calculations. 


Edited by jtsenghas, 28 January 2019 - 09:16 AM.


#40 TonyStar

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Posted 27 January 2019 - 11:04 AM

This study should also be able to show how counterweights for secondary mirrors improve such behavior. When the center of gravity of the secondary support (including counterweights) is not cantilevered, no doubt deflection will decrease. I'm certain most of this deflection is due to angular tilt rather than radial displacement. 

Good point, counterweights work well. I experimented with them when I built my scope and I could compensate for most of the flex using a counterweight. Indeed I tapped some holes in my spider hub to attach a cantilever arm. 

I'm not sure what you mean by "radial displacement", but the largest laser shift occurs when the focuser axis is oriented along g and the mirror gets slightly shifted towards/away the focuser. 


Edited by TonyStar, 27 January 2019 - 11:05 AM.


#41 ckh

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Posted 27 January 2019 - 11:33 AM

Angular displacements will show up much better than radial with Jonathan's setup. As long as the radial displacements are limited to a few hundredths of an inch, they don't matter too much - slight refocus is all.

 

I agree that getting the CG centered in the spider will reduce the tilting that we are seeing. If the spider is symmetrical, that should only leave radial displacement.


Edited by ckh, 27 January 2019 - 11:35 AM.


#42 tommm

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Posted 27 January 2019 - 11:41 AM

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.

...and movement of the laser/laser fixturing itself. But quibbling aside, it clearly shows the secondary is moving much more than these other sources.  Would be nice if (any) movement of the hexapod could be separated from the secondary fixturing movement. Might be useful to bolt or clamp the UTA directly to the end of the tube to compare to the present setup. Wouldn't expect significant tube flex with it rolling with both ends supported.

 

Nice work!



#43 TonyStar

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Posted 27 January 2019 - 01:22 PM

I suspect it won’t budge

Are you taking bets against it?smile.gif

Not because it isn't well executed, but because the bending stiffness of that design is also finite.

 

Jonathan, a long time ago, on another thread, I asked you how stiff was your wire design. The question arose because at the time I had done my own tests and concluded the spider was the weak link in my scope. At the time it was harder for you to pinpoint the source of flexure but now it's obvious the UTA is the culprit. As you pointed out earlier, it may be difficult to say whether the shift is caused by flexure in the UTA structure or flexure in the wire spider (I bet it's both). But the test with the solid vane spider may help address this issue.

 

In any case, this is very interesting work and I'm looking forward to the following.waytogo.gif  


Edited by TonyStar, 27 January 2019 - 01:55 PM.

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#44 mark cowan

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Posted 27 January 2019 - 02:15 PM

I think the torque moment the elipitical mirror applies to the support varies as you rotate it, with the moment being greatest when the long axis of the ecliptical mirror is vertical, and least when the short axis is vertical. This would cause the deflection to vary as the tube is rolled along the ground.

More likely the support itself has a stronger axis for loading in some orientations.  IIRC Johnathon's spider is a vertical block spawning wires from the edges.  This same effect was noticed elsewhere for that sort of design, and is getting tested, also elsewhere.

 

The way around that is not to have the vertical block approach but that's an entirely different thread most likely. wink.gif

 

That said unless you're building an equatorial or a rotating UTA, for a dobsonian the changes that matter are with vertical to horizontal not in rotation, and the spider can be oriented in its strongest direction for use.


Edited by mark cowan, 27 January 2019 - 02:22 PM.

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#45 Oberon

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Posted 27 January 2019 - 04:24 PM

This study should also be able to show how counterweights for secondary mirrors improve such behavior. When the center of gravity of the secondary support (including counterweights) is not cantilevered, no doubt deflection will decrease. I'm certain most of this deflection is due to angular tilt rather than radial displacement. 

Yes, counterweights are on the plan.


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#46 Oberon

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Posted 27 January 2019 - 04:34 PM

Even if you were interested only in deflection seen by alt-azimuth scopes such as dobs, for relatively symmetrical spiders the difference in readings between normal orientation and 180 degrees from that should be quite close to twice the range one would see in actual use from the horizon to zenith.

 

I like this setup! With about 2.5 meters of tube and perhaps nearly another half meter to the mirror, the total laser distance must be close to five meters. Arc minutes are quite large indeed!

 

Edit -- Hmmmm ... Is the angular scale on your black reference chart off by a factor of two, Jonathan? I don't know the size of your phone in the image but it looks to me that you might not be using twice the distance to your "primary" for your calculations. 

Total laser distance is 6.3 meters (plus focuser to secondary). As the focal plane to secondary is 0.3 meters the amplification of effect is 21. Consequently (and to my amazement) the largest deflections seen here from a 180 degree axial rotation translate to a mere ~3mm at the eyepiece, which bodes very well for any normal movement expected from a f/4.5 Dob where the coma free sweet spot is 2mm. 

 

But more on that later. I’ve figured out a more convenient way of getting altitude measurements...



#47 Oberon

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Posted 27 January 2019 - 04:39 PM

...and movement of the laser/laser fixturing itself. But quibbling aside, it clearly shows the secondary is moving much more than these other sources.  Would be nice if (any) movement of the hexapod could be separated from the secondary fixturing movement. Might be useful to bolt or clamp the UTA directly to the end of the tube to compare to the present setup. Wouldn't expect significant tube flex with it rolling with both ends supported.

 

Nice work!

While I can and do go around pushing and pulling to see what wobbles the laser, the hexapod is not a weak link here. And, after the spider, the next weakest link is the Moonlite focuser. Just sayin’ to help keep things in perspective.



#48 Oberon

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Posted 27 January 2019 - 04:44 PM

Are you taking bets against it?smile.gif

Not because it isn't well executed, but because the bending stiffness of that design is also finite.

 

Jonathan, a long time ago, on another thread, I asked you how stiff was your wire design. The question arose because at the time I had done my own tests and concluded the spider was the weak link in my scope. At the time it was harder for you to pinpoint the source of flexure but now it's obvious the UTA is the culprit. As you pointed out earlier, it may be difficult to say whether the shift is caused by flexure in the UTA structure or flexure in the wire spider (I bet it's both). But the test with the solid vane spider may help address this issue.

 

In any case, this is very interesting work and I'm looking forward to the following.waytogo.gif  

In this series of tests in time I will isolate the following components;

 

1. the UTA structure

2. the wires (v vanes)

3. the angle of the wires

4. the thickness of the wires

5. tension of wires

6. anything else about wires we think may be important

 

So while Merope’s wire spider is my starting point, once it is properly characterised I’ll start making changes.

 

Then we’ll do curved vanes...

 

6. lots and lots of variables to be quantified...


Edited by Oberon, 27 January 2019 - 04:45 PM.


#49 Oberon

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Posted 27 January 2019 - 04:49 PM

Looks like about 1 moon's worth of variation.  

Only if the distance between your focal plane and your secondary is about 6 meters...


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#50 Oberon

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Posted 27 January 2019 - 04:53 PM

Edit -- Hmmmm ... Is the angular scale on your black reference chart off by a factor of two, Jonathan? I don't know the size of your phone in the image but it looks to me that you might not be using twice the distance to your "primary" for your calculations. 

Yes. I accidently printed at 50% scale. However, as noted in the post, the scale is correct so long as one treats the dimensions given as diameters, not radius.


Edited by Oberon, 27 January 2019 - 04:53 PM.

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