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Equipment Discussions >> ATM, Optics and DIY Forum

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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3959693 - 08/02/10 05:16 PM

If you look closely you'll see the dimensional constraint space is rectangular. It's narrower left-right, and wider top-down.

The "vertical" clearance I need is at least 107.6 + 107.6 + 215.2 = 430.4mm. It is determined by the triangular cell support. I was thinking to use 440mm. That still gives me 9mm of play, which I believe is safe.

The "horizontal" clearance I need is at least 372.7mm. It is determined by the lateral support system of the mirror. This is almost 6 cm less than the "vertical" clearance.

Should I go for a "square" section of the mirror box, or should I go for the slimmer rectangular shape?


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3959702 - 08/02/10 05:19 PM Attachment (77 downloads)

Here's an idea of what a mirror box with a "square" section and an inner space of 440mm would look like:

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Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3959862 - 08/02/10 06:03 PM

Hello Olivier,

Although the rectangular box may be more compact, the square allows more space for internal counterweights and batteries if needed.

JimC


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Mirzam]
      #3959956 - 08/02/10 06:53 PM Attachment (59 downloads)

Quote:

Hello Olivier,

Although the rectangular box may be more compact, the square allows more space for internal counterweights and batteries if needed.

JimC




That's an interesting thought!

So square it remains for now.

Here is a drawing with 400mm altitude trunnions in place:


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3959975 - 08/02/10 07:04 PM Attachment (59 downloads)

Here's another rendering:

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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3959978 - 08/02/10 07:06 PM

Of course I have no clue where the altitude trunnions should be now, as I don't know the center of gravity of the scope yet.

I opted for 400mm trunnions since I have 2 400mm disks already. I believe they are large enough for a 14".


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3959987 - 08/02/10 07:09 PM

I want to have a low profile rocker since I don't want to have to use a stool when I observe. That may be a bit challenging though if my COG is too high.

Also, the cell as I designed it today has this protruding triangle point to the bottom. I could reduce the size of the mirror box and trim part of the mirror box at the bottom if I don't make a full triangle but only use sections of it, attached to a metal frame. Maybe this is even easier to weld than my triangular cell.

The rocker I envision looks like Kerry's: a steel frame on which a low profile wooden trunnion support is attached.

I'll draw that frame tomorrow

Edited by Olivier Biot (08/02/10 07:32 PM)


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3961475 - 08/03/10 01:09 PM Attachment (62 downloads)

One option is to eliminate protruding elements as much as possible. Here is a front view of the cell in the same square section mirror box, without the triangular U-profile unit that keeps the 3 pivots (visible in this image) in place:

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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3961536 - 08/03/10 01:34 PM Attachment (60 downloads)

A simplified option, which takes less real estate, and will allow me to have a smaller mirror box, is sketched below. The transparent yellow beams replace the triangular support and the black T-shaped collimation frame:

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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3961570 - 08/03/10 01:53 PM Attachment (50 downloads)

When compared to the option with the full triangular support, there's some gain:

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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3962006 - 08/03/10 05:47 PM Attachment (48 downloads)

Still struggling with a couple constraints for my cell.

Here's a simpler cell, where I removed the 45 whiffletree lateral support. This approach reduces the height of the cell of more than 2cm, and the height at the focuser will be impacted more since there are no more obnoxious "corners" in the way while moving in altitude:


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3962010 - 08/03/10 05:47 PM Attachment (49 downloads)

Here's another view of the same:

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bertb
member


Reged: 10/10/08

Loc: Achterveld, the Netherlands
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3962990 - 08/04/10 04:50 AM

Look how Roel with his 12 inch F4 travel dobson designed his mirror cell. Very effective and easy to make!

Edited by bertb (08/04/10 04:51 AM)


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: bertb]
      #3963610 - 08/04/10 01:16 PM

Thanks for that pointer Bert!

Anyway, I just realized I misinterpreted the lateral support performance figures for my mirror. The good news is that I can replace the bulkier and more complex 4 points whiffle tree support with a lateral support consisting of 2 bearings at 90 (45 from vertical at both ends).

Stay tuned for yet another picture of my slow progress


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3963977 - 08/04/10 04:14 PM Attachment (54 downloads)

Here is tonight's iteration on the cell. I used angle L irons for the lateral supports. They can slide a bit sideways to adjust the manufacturing tolerances of the cell (slots). At the top there is a large washer that plays the double role of securing the lateral support in place and preventing the mirror from falling off the cell.

I will add 2 more identical clips to the top (where you can see the slots).

Image:


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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3963988 - 08/04/10 04:20 PM Attachment (52 downloads)

Here's a close-up of the lateral support and mirror retaining clip in place. The white 'blob' represents the contact point. I was thinking about using some PTFE (Teflon) here:

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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3964003 - 08/04/10 04:25 PM Attachment (48 downloads)

Here's a view of option 3 in perspective:

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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3964064 - 08/04/10 04:52 PM

I have one question though: can this cell be made? The remaining issue I see now, is drilling the holes for the 3 pivots. Indeed, the 2 oblique U profiles cannot be drilled once they are in place.

Any thoughts?


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Benach
Pooh-Bah


Reged: 01/24/08

Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3964083 - 08/04/10 05:01 PM

Olivier: jups, can be made rather easy. You simply have to drill the holes before you weld the assembly. However, I have serious doubts about the edge supports. I strongly suspect it will wobble a lot. BTW, you can bond metal to glass without stress with RTV (Siliconenkit).

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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... [Re: Benach]
      #3964124 - 08/04/10 05:18 PM

Pre-drilling will cure any difficulties. But I don't see any play to allow the glass to expand (is it even needed? )

Are you going to weld the big U channels together?


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