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Merope - engineering a low compromise compact 16" dob

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#201 Trithemius

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Posted 22 April 2017 - 11:05 PM

I wanted to thank you for this build thread.

 

I've been designing and redesigning a truss collimated 12.5" on paper for months now, and I've probably gone through some 30 iterations. I've gone from a ring mirror box, to square, to hexagonal and now back to a ring. I've gone from traditional dob bearings, to a tri-lateral design (which I thought for sure was the one), to a split-bearing design. I've gone from a traditional ground board to a ground ring with flex rocker.

 

Of all the unique configurations I thought for sure I was going to do, my current design (hopefully final), which is one of the most compact so far, nearly resembles Merope, just with a flex rocker. This is just another testament to how much thought you put into your scope. I tried improving on it, but their doesn't seem to be much room for improvement. 

 

I really need to get started on my own scope and make a build thread. 

 

I'm planning on trying truss collimation with eyebolts (poor man's method). Any advice on that front would be welcome.


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#202 jtsenghas

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Posted 23 April 2017 - 08:41 AM

....I'm planning on trying truss collimation with eyebolts (poor man's method). Any advice on that front would be welcome.

Welcome to Cloudy Nights! welcome.gif 

 

The topic of truss tube collimation setups has indeed been discussed on this forum, although the thread has digressed a few times and the moderators have had to perform minor surgery on the last few pages of the thread to keep it on topic.  You can find the discussion that Jonathan started here



#203 Oberon

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Posted 25 April 2017 - 07:01 AM

Merope has been camping...

 

med_gallery_217007_5817_896862.jpg

 

med_gallery_217007_5817_129145.jpg

 

med_gallery_217007_5817_1325007.jpg


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#204 Starman1

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Posted 25 April 2017 - 10:00 AM

Whoa.  Most people would use a ramp, maybe, with wheelbarrow handles.

That's not good enough for the creator of Merope.

No, he has to use a crane!

waytogo.gif


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#205 Pierre Lemay

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Posted 26 April 2017 - 11:25 AM

Jonathan,

I remember suggesting that you get a real hatchback car to move Merope around instead of using the "boot" ("trunk" in real English) of that sedan you showed in pictures. You answered back something like "What fun would that be?".

 

Well, look who's having fun now!


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#206 Oberon

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Posted 03 June 2017 - 12:59 AM

Below are some old shots illustrating how I sanded the altitude trunions. See the discussion on this thread.

First the trunions are screwed together and mounted on the jig (flat sheet) to ensure accurate rotation around the trunion centre.

 

gallery_217007_5817_58448.jpg

 

Then the job is spun by hand against an orbital sander (hidden) that - in this example - is fixed to a piece of wood hinged at one end, and able to be pressed against the job with my fat gut...

gallery_217007_5817_34181.jpg

 

Here is a close up...

gallery_217007_5817_13125.jpg

Watch the movie here...

 


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#207 Pierre Lemay

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Posted 03 June 2017 - 08:05 AM

I knew there was a good reason to keep my tummy!


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#208 Bob4BVM

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Posted 04 June 2017 - 09:43 PM

I wanted to thank you for this build thread.

 

I've been designing and redesigning a truss collimated 12.5" on paper for months now, and I've probably gone through some 30 iterations. I've gone from a ring mirror box, to square, to hexagonal and now back to a ring. I've gone from traditional dob bearings, to a tri-lateral design (which I thought for sure was the one), to a split-bearing design. I've gone from a traditional ground board to a ground ring with flex rocker.

 

Of all the unique configurations I thought for sure I was going to do, my current design (hopefully final), which is one of the most compact so far, nearly resembles Merope, just with a flex rocker. This is just another testament to how much thought you put into your scope. I tried improving on it, but their doesn't seem to be much room for improvement. 

 

I really need to get started on my own scope and make a build thread. 

 

I'm planning on trying truss collimation with eyebolts (poor man's method). Any advice on that front would be welcome.

Oh. please don't use eyebolts !

Rod ends (people here refer to them as heim joints, in the racing world they are rod ends) are available  for a couple bucks a pop on the 'net.  I just ordered a batch of them, 5/16 male & female, LH & RH thread for a race bike build. There were all under $3 a pop. Eyebolts will cost you half that and a lot more in frustration...

 

Welcome to CN !

Bob



#209 stubeeef

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Posted 16 June 2017 - 12:15 AM

I love your design, I wish it was commercially available, congrats you should be very proud.


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#210 Oberon

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Posted 05 August 2017 - 10:59 AM

Here is a self indulgent half finished video I started to make in order to illustrate hexapod collimation, but never got around to finishing. 

 


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#211 Jeff Morgan

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Posted 05 August 2017 - 08:10 PM

A little short on the narrative, but nice soundtrack.



#212 Raiz

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Posted 11 August 2017 - 06:50 PM

Looking at your mirror cell, I am assuming that your mirror edge support is that you glued your mirror on to your mirror cell?

 

Please correct me if I am wrong.



#213 Oberon

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Posted 11 August 2017 - 08:22 PM

No, the mirror is loose. The edge supports consist of a protuding pair of 20mm phenolic balls embedded in the ply ring surrounding the mirror 90 degrees apart. Here is one of them shown close up...

 

med_gallery_217007_4913_159943.jpg

 

If you look carefully you will see it below immediately below the lower left mirror clip.

 

gallery_217007_5817_90156.jpg



#214 Raiz

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Posted 12 August 2017 - 05:25 PM

Interesting, thanks for the quick reply.



#215 VNA

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Posted 03 January 2018 - 03:29 PM

Beautiful and impressive project, but how high will you have to climb to observe?



#216 KJL

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Posted 03 January 2018 - 04:05 PM

Beautiful and impressive project, but how high will you have to climb to observe?

I'm guessing given its 1800mm focal length that the resulting zenith EP height is probably around 65-70 in. There is a good chance he doesn't have to climb anything at all (I wouldn't).


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#217 Olle Eriksson

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Posted 03 January 2018 - 05:53 PM

gallery_217007_4913_33700.jpg

 

Note my original plan when this photo was taken was for the teflon pads on the feet to run against the side of the azimuth bearing and constrain it, so no need for a center pin. But something didn't work as intended so I went to plan B and fitted a bottom plate, and those feet with their lovely slabs of 10mm teflon were shelved.

An absolutely beautiful telescope! I've looked at your thread many times in the past, but never commented. But yeah, it's been said by others so many times as well, great work!

 

On this picture you said that something didn't work as intended in regards to having the side of the azimuth bearing ride against teflon making a center pin unnecessary. May I ask what that was?


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#218 Oberon

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Posted 04 January 2018 - 06:58 PM

 

Beautiful and impressive project, but how high will you have to climb to observe?

I'm guessing given its 1800mm focal length that the resulting zenith EP height is probably around 65-70 in. There is a good chance he doesn't have to climb anything at all (I wouldn't).

 

Thats right. Due to the nested design the mirror sits low and I stand flat on the ground for all but very close to zenith, which for me requires tippy toes.



#219 Oberon

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Posted 04 January 2018 - 07:03 PM

 

gallery_217007_4913_33700.jpg

 

Note my original plan when this photo was taken was for the teflon pads on the feet to run against the side of the azimuth bearing and constrain it, so no need for a center pin. But something didn't work as intended so I went to plan B and fitted a bottom plate, and those feet with their lovely slabs of 10mm teflon were shelved.

An absolutely beautiful telescope! I've looked at your thread many times in the past, but never commented. But yeah, it's been said by others so many times as well, great work!

 

On this picture you said that something didn't work as intended in regards to having the side of the azimuth bearing ride against teflon making a center pin unnecessary. May I ask what that was?

 

I was experimenting with a thin floppy base. A protoype worked well (was rock solid) on my 8” but failed on Merope. I’ve yet to really determine why the difference and at the time figured that the benefit (20mm lower eyepiece) wasn’t worth the time and effort to resolve, so went with a more standard stiff baseplate.



#220 Bob4BVM

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Posted 04 January 2018 - 11:08 PM

Jonathan

Can you comment on how much the tubes actually move up/down when you do a collimation touch-up ?

I was considering mounting my focuser in the triangle formed at the top of 2 of the truss tubes, looking upward at the sec mirror ( LR design), but was wondering how much shift i'd see of the focuser position when twisting the tubes... Maybe I could have one of the 3 pairs fixed ?  Just kicking this around in my head

CS

Bob

 



#221 Oberon

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Posted 05 January 2018 - 08:01 AM

Hi Bob, adjustment of the tubes is typically less than a thread, less than a single rotation of the tube. However I would strongly recommend against constraining the tubes in any way with the focuser; truss tubes should only experience tension and compression forces down their centerline. Also you cannot leave a pair truss tubes fixed, in a hexapod it is only possible to fix a single truss tube without losing an axis of movement.

 

To my mind you are better off with shorter trusses (stiffer) and a short but classic UTA that holds the focuser between two rings. If you want to go LR then shorten the trusses further, or build a stiff UTA ring and a sold focuser support hanging off that ring. Keep the trusses free to do their job.


Edited by Oberon, 05 January 2018 - 08:05 AM.


#222 Bob4BVM

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Posted 05 January 2018 - 01:54 PM

Hi Bob, adjustment of the tubes is typically less than a thread, less than a single rotation of the tube. However I would strongly recommend against constraining the tubes in any way with the focuser; truss tubes should only experience tension and compression forces down their centerline. Also you cannot leave a pair truss tubes fixed, in a hexapod it is only possible to fix a single truss tube without losing an axis of movement.

 

To my mind you are better off with shorter trusses (stiffer) and a short but classic UTA that holds the focuser between two rings. If you want to go LR then shorten the trusses further, or build a stiff UTA ring and a sold focuser support hanging off that ring. Keep the trusses free to do their job.

Yeah I figured that would be the case

Everything gets easier if I go with a classic UTA anyway, wirespider, focuser mount, finder mount etc.

Interesting that it takes so little truss movement to collimate.

Thanks for the feedback, it puts a lot of questions & what-ifs behind me !

CS

Bob



#223 kb58

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Posted 25 December 2018 - 11:52 PM

If you had gone with a vane-type spyder, I'm curious if - carrying on with the three-point truss - would you have used a 3-vane part?



#224 Oberon

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Posted 25 December 2018 - 11:59 PM

Definitely not.

 

A 3 vane spider cannot be made sufficiently stiff and rigid. If you look earlier in the thread I started with a 4 vane-type spider, and later changed for wire as part of a weight loss exercise. The critical design element for rigidity is a pair of triangles in tension with a straight section between them, as in >-<.


Edited by Oberon, 26 December 2018 - 12:00 AM.


#225 kb58

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Posted 08 January 2019 - 04:59 PM

Should I then assume that your answer would be the same regarding a 3-vane wire spider?

 

The reason I keep asking about it is that the three-truss theme is tempting to run through the entire assembly. That is, triangular mirror cell, bolted straight to the 3-point truss bases, with the truss tops bolting directly to the secondary housing spacers, with a 3-wire spider coming off of them. I very much defer to your experience on this though and will go with a 4-wire set spider if  you say so.


Edited by kb58, 08 January 2019 - 05:00 PM.



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