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34" F2.89 Large Fork Mount BHMO Project Build

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#276 elstargazer12

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Posted 14 May 2020 - 12:50 PM

Hi ctcables,

 

I'm wondering... how are you fixing the poles to the wooden braces?  I'm doing something similar with a much smaller strut set and I'd like any ideas.

 

Thanks!

Roger



#277 MitchAlsup

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Posted 14 May 2020 - 03:32 PM

One of the disadvantages of the ">o<" design when mounted on a thin UTA cross section is that it doesn't take much radial force on the UTA to cause a rotation of the diagonal. 

 

When the secondary vanes pick up the forces due to gravity of the secondary intersect at CoM of the secondary, there are no gravitational rotational forces! only bending moments from the center of the vanes not being coincident with the CoM.

 

secondaryCoG.jpg



#278 Pierre Lemay

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Posted 14 May 2020 - 10:09 PM

When the secondary vanes pick up the forces due to gravity of the secondary intersect at CoM of the secondary, there are no gravitational rotational forces! only bending moments from the center of the vanes not being coincident with the CoM.

 

attachicon.gifsecondaryCoG.jpg

Mitch, that’s not what I’m referring to. Hold a UTA equipped with an offset diagonal holder in your hands with each hand placed 180 degrees apart, where the vanes attach to the UTA ring. Now squeeze that upper ring so that both hands are pushing towards the center (towards the diagonal). What happens is the UTA ring is deforming from a circular to an oval shape. Because of the offset, the diagonal support and the mirror attached to it rotate. 

 

It doesn’t take much squeezing to get the 4 inch diagonal of my 20 inch to rotate 2 or 3 degrees, or more, because the UTA rings are very flimsy (bicycle rims). I’ve tried this on many telescopes and they all do it to a certain extent. I’ve even tried it on 24 inch size instruments with much beefier rings than mine, made of 1/2 inch plywood 2 inches wide and I could also see rotation although it took more force. 

 

A flimsy UTA like mine as well as any other thin cross-section UTA ring will not deform if the spider is of the « + » design because the vanes perpendicular to the sqeezing direction will prevent the ring from « ovalizing », something the offset spider cannot do. 

 

Why is this important? Because as the scope moves across the sky the UTA rings are being squeezed at the point where they attach to the trusses. I suspect that some of the laser movement seen when a scope moves from horizon to zenith may be caused in part by this effect, especially for ball scopes like my 20 inch. I hope to experiment different vane configurations later this year to improve my scope. I have several ideas I want to try. 


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#279 MitchAlsup

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Posted 15 May 2020 - 03:38 PM

Mitch, that’s not what I’m referring to. <snip>

 

It doesn’t take much squeezing to get the 4 inch diagonal of my 20 inch to rotate 2 or 3 degrees, or more, because the UTA rings are very flimsy (bicycle rims).<snip>

This is why I quit using an upper ring and built a circular I-beam for the UTA.

 

13Secondary01.JPG


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#280 ctcables

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Posted 16 May 2020 - 08:35 PM

Hi ctcables,

 

I'm wondering... how are you fixing the poles to the wooden braces?  I'm doing something similar with a much smaller strut set and I'd like any ideas.

 

Thanks!

Roger

I use the 2 part epoxy putty and press it in.  First sand the location the brace will sit then mix your putty and wrap small amount around tube then slide brace over the epoxy and finish by pressing around the tube to fill in any gaps and clean up. I found this to work very well and has held up to the test of time on other builds. Hope this helps out.



#281 ctcables

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Posted 16 May 2020 - 08:43 PM

Mitch, that’s not what I’m referring to. Hold a UTA equipped with an offset diagonal holder in your hands with each hand placed 180 degrees apart, where the vanes attach to the UTA ring. Now squeeze that upper ring so that both hands are pushing towards the center (towards the diagonal). What happens is the UTA ring is deforming from a circular to an oval shape. Because of the offset, the diagonal support and the mirror attached to it rotate. 

 

It doesn’t take much squeezing to get the 4 inch diagonal of my 20 inch to rotate 2 or 3 degrees, or more, because the UTA rings are very flimsy (bicycle rims). I’ve tried this on many telescopes and they all do it to a certain extent. I’ve even tried it on 24 inch size instruments with much beefier rings than mine, made of 1/2 inch plywood 2 inches wide and I could also see rotation although it took more force. 

 

A flimsy UTA like mine as well as any other thin cross-section UTA ring will not deform if the spider is of the « + » design because the vanes perpendicular to the sqeezing direction will prevent the ring from « ovalizing », something the offset spider cannot do. 

 

Why is this important? Because as the scope moves across the sky the UTA rings are being squeezed at the point where they attach to the trusses. I suspect that some of the laser movement seen when a scope moves from horizon to zenith may be caused in part by this effect, especially for ball scopes like my 20 inch. I hope to experiment different vane configurations later this year to improve my scope. I have several ideas I want to try. 

I have no worry about the rings on my 34" deforming because it is .25" thick flange welded to make a ring the angle makes the ring very stiff and because there is a top and bottom ring the rings are reinforcing creating a very very strong cage. I am confident the secondary is solid in all scope angles.

Attached Thumbnails

  • secondary cage2.jpg
  • spider 2.jpg


#282 Bob4BVM

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Posted 17 May 2020 - 01:30 AM

I have no worry about the rings on my 34" deforming because it is .25" thick flange welded to make a ring the angle makes the ring very stiff and because there is a top and bottom ring the rings are reinforcing creating a very very strong cage. I am confident the secondary is solid in all scope angles.

Everything moves of course, if you measure close enough :)

 

That said, I'd guess that once you tension the vanes, whatever movement you initially induce will stay right where you put it, looks rock solid to me.

I wish I had the option for such a beefy UTA on my b-scope, best sec support so far is a V-vane and it still moves a little with altitude change. Probably need wider aluminum bar stock for my 1.5+# sec's.

 

Lookin good so far, keep churning away on it !

 

CS

Bob


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#283 ctcables

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Posted 15 September 2020 - 10:19 AM

So going with 50mm struts, to much flex in fishing poles. I would need to use many more to make it work. So the new carbon fiber 48mm ID 50mm OD will work great. Also making a base for the true 3" focuser that will hold the 3" comma corrector and work like a sips system.  Some photos of the drawings. 

Attached Thumbnails

  • strut 50mm.jpg
  • strut cap..jpg
  • strut cap1.jpg
  • focuser base FT.jpg
  • base focuser 1.jpg
  • base focuser 2.jpg

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#284 ctcables

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Posted 20 September 2020 - 11:52 PM

Bit of a redesign, instead of a clamp I am going to use a thumb screw to lock in the comma corrector. This part will screw into the focuser mount.  This way it is fixed and will just unscrew just like the 2" SIPS. 

Attached Thumbnails

  • mount screw2.jpg
  • mount screw.jpg
  • cutout mount.jpg


#285 ctcables

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Posted 21 September 2020 - 12:21 AM

finished tube end in PLA, Carbon fiber filament is on the way.  I am going to print everything up first in PLA to test it before using the expensive stuff. 

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  • tube end.jpg

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#286 ctcables

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Posted 27 September 2020 - 02:04 AM

38H print and the test base for the focuser is done. 

Attached Thumbnails

  • FB_IMG_1601189723699.jpg
  • FB_IMG_1601189769749.jpg
  • FB_IMG_1601189743358.jpg

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#287 ctcables

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Posted 27 September 2020 - 08:38 PM

showing assembled. Screw holder for the comma corrector needs the threads cleaned up so it screws in better but it works. Love the printer, 38H for the base part no errors in PLA, will move on to the harder carbon fiber print next week.

 

Attached Thumbnails

  • printed 3d base2.jpg
  • printed 3d base1.jpg

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#288 ctcables

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Posted 21 October 2020 - 11:23 PM

Strut ends complete, will epoxy to struts when length from primary to secondary cage is set in stone. Very happy with how the printed parts turned out, used PC Carbon Fiber Filament.

Attached Thumbnails

  • strut ends1.jpg
  • strut ends2.jpg

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#289 MitchAlsup

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Posted 22 October 2020 - 07:33 PM

showing assembled. Screw holder for the comma corrector needs the threads cleaned up so it screws in better but it works. Love the printer, 38H for the base part no errors in PLA, will move on to the harder carbon fiber print next week.

Looks a lot like what I came up with (I call mine MIPS):: P2 on "other side" of base plate, focuser outside of upper assembly::

 

assembly34.JPG

 

Notice I use the thumbscrew to both prevent the P2 from falling out of the focuser, and to focus the telescope. I have not found the need for a precise focuser in positioning the P2:: it seems to tolerate anything within a couple of mm of where it is supposed to be mathematically.

 

I lightweighted the "base plate" and use it as a stiffener; as it is the height of the upper assembly tube.


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#290 MitchAlsup

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Posted 22 October 2020 - 07:35 PM

Strut ends complete, will epoxy to struts when length from primary to secondary cage is set in stone. Very happy with how the printed parts turned out, used PC Carbon Fiber Filament.

Your ball ends look like mine, too::

 

assembly24.JPG


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#291 calypsob

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Posted 22 October 2020 - 09:53 PM

Strut ends complete, will epoxy to struts when length from primary to secondary cage is set in stone. Very happy with how the printed parts turned out, used PC Carbon Fiber Filament.

These respond well to sanding print lines down with 200-400 grit 


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#292 ctcables

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Posted 23 October 2020 - 09:44 PM

Your ball ends look like mine, too::

 

attachicon.gifassembly24.JPG

Little bit.  I have set it up so the ball slides in from the side for easy set up and no loose parts. 

Attached Thumbnails

  • FB_IMG_1603507244222.jpg
  • FB_IMG_1603507239997.jpg
  • FB_IMG_1603507233933.jpg

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