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My very late 14 inch f/5 project...

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#1 Olivier Biot

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Posted 01 August 2010 - 05:53 AM

Time to post some photos of the project as it stands today.

I have the optics, the secondary holder, and a low profile helical focuser.

I have lots of wood that has been cut long time ago, I hope I can reuse part of it.

First photo: UTA (heavy, bulky) with melamine light shield standing on top of a high mirror box:

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#2 Olivier Biot

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Posted 01 August 2010 - 05:53 AM

View from where the focuser should go:

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#3 Olivier Biot

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Posted 01 August 2010 - 05:55 AM

Without the light shield you can see the structure of the UTA. It's made of 18mm plywood (Lauan?) and held together with 20mm x 20mm square aluminium struts and angle profiles, everything bolted together (no gluing or welding):

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#4 Olivier Biot

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Posted 01 August 2010 - 05:57 AM

The four bolts on top of the pink foam pads are the location of the focuser. The foam pads allow for collimating (squaring) the focuser to a certain extent.

Another view of the same, with the secondary mirror holder in place. Manufactured by Gary Wolanski (Canada) at the end of the 1990s (shame on me!):

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#5 Olivier Biot

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Posted 01 August 2010 - 05:58 AM

A view through the UTA:

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#6 Olivier Biot

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Posted 01 August 2010 - 06:02 AM

The secondery holder has a built-in offset so the diagonal is a bit farther from the focuser. The light cone is indeed intercepted at different aperture of the converging cone sent by the primary mirror.

Here's a close-up where you can also see how the spider vanes are attached to the UTA:

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#7 Olivier Biot

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Posted 01 August 2010 - 06:05 AM

A close-up of the offset secondary holder and secondary collimation screws:

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#8 andypod

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Posted 01 August 2010 - 06:36 AM

Thats a very nice looking UTA Olivier. How heavy is it exactly??? It looks quiet heavy.

Andy

#9 Olivier Biot

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Posted 01 August 2010 - 06:47 AM

It not only looks heavy, it is. Without anything more it's 3kg: 2.5kg for the cage, 0.5kg for the shield.

Focuser, mirror, telrad, ... will add at least 1.5kg to the weight. ence the weight MUST go down :grin:

#10 a__l

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Posted 01 August 2010 - 06:53 AM

The plywood 18 mm is superfluous for such diameter. There is enough 12 mm. It is additional weight.

#11 Mirzam

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Posted 01 August 2010 - 06:57 AM

Using single rather than paired struts coupled with a focuser board made of wood may help cut some weight. Also, a lightweight focuser like a KineOptics would help. Or you could go with a single ring design.

http://www.kineoptics.com/HC-2.html

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#12 Sky Captain

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Posted 01 August 2010 - 10:19 PM

Construction looks great Oliver!

#13 rboe

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Posted 02 August 2010 - 10:50 AM

I used four cherry dowels as struts on one upper cage. Then I routered a groove in the plywood ring to house the laminage (royal pain to put together but once done it became a structural member) then morticed in slots to hold the focuser board made from aircraft grade thin plywood. Very strong and light.

Turned out it was too light for the scope so it has never been used as I have not started rebuilding the rest of the scope. :bawling:

Looks like I'll be going in a different direction so it will probably never get used. sigh.

#14 Olivier Biot

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Posted 02 August 2010 - 12:41 PM

The struts are aluminium, hence they don't really add a lot to the weight.

I definitely need to make the UTA lighter and slimmer. What's more challenging, are the 2 plywood rings. Not sure if opting for bike rims is a good option.

Regarding truss pole attachment I really like what Kerry did, but I don't think this is feasible for me right now.

Tonight I'll be finalizing the overall CAD drawings of the scope. This way I can start checking my inventory of plywood for availability.

#15 rboe

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Posted 02 August 2010 - 12:59 PM

Aluminium is very light but you have eight of them, I think you could safely purge four of them. After all a sheet of paper is pretty light but you wouldn't want to be hit by a ream of them!

I think I used 1/2" (~13mm) baltic birch for my rings. At least as strong as your choice due to the thinner plies - and more of them.

I would have you review the way you fasten the al. struts, perhaps go to a single through fastner instead of the braces. You could drop four parts and six fasteners that way.

With four Al. struts you can fasten the spider to them and same the parts that you are using now (changing the number of rings or dimensions as suggested will negate that depending on how you go about it).

You could look at a single ring or a dua ring but reduce the distance between them to the size of your focuser. Opposite the focuser you'll have to have an over sized light shield like Kerry did.

Push come to shove, I think you could find a lighter secondary holder too but that one seems pretty good so I would not be in a hurry to ditch it unless you have to.

#16 Jack Day

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Posted 02 August 2010 - 02:18 PM

Looks great, but I am curious on the design of your spider. Won't you end up with eight sets of diffraction spikes? The vanes of the spider are not lined up...

#17 a__l

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Posted 02 August 2010 - 02:42 PM

Rboe writes about a correct direction for modernization...
In practice it is the basic ideas from the book K&B.

#18 Olivier Biot

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Posted 02 August 2010 - 03:59 PM

Looks great, but I am curious on the design of your spider. Won't you end up with eight sets of diffraction spikes? The vanes of the spider are not lined up...


If the vanes are parallel and perpendicular, then there will only be 4 spikes. Otherwise there will be a multiple of 4 :)

Eventually I'd go for a curved spider (no visible spikes), but that won't happen for Mk I :grin:

#19 Olivier Biot

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Posted 02 August 2010 - 04:01 PM

Thanks for your input Ron! I'll dramatically reduce the height of the UTA. I just keyed in again my numbers in NEWT (I previously computed all this by hand over 10 years ago), and the numbers are right :jump:

The UTA will have an inner diameter of 406mm (16"). That will be perfect for the 14" primary :).

#20 Olivier Biot

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Posted 02 August 2010 - 04:09 PM

Okay, time to see how the puzzle fits together piecewise now.

Here's an image showing the dimensional constraints for my cell. The green zone is the safe zone, the red zone is the "danger zone" where my mirror cell protrudes.

All dimensions are given in millimeters, rounded to 0.1mm.

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#21 Olivier Biot

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Posted 02 August 2010 - 04:16 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?

#22 Olivier Biot

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Posted 02 August 2010 - 04:19 PM

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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#23 Mirzam

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Posted 02 August 2010 - 05:03 PM

Hello Olivier,

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

JimC

#24 Olivier Biot

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Posted 02 August 2010 - 05:53 PM

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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#25 Olivier Biot

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Posted 02 August 2010 - 06:04 PM

Here's another rendering:

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