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Benach
Pooh-Bah


Reged: 01/24/08

Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3989974 - 08/18/10 04:55 AM

Olivier: what about rotating the secondary with the whole uppercage? If you look into your eyepiece at 45degr. you can still place the spider between the trusses.

Dunno what kind of profile pieces you intend to use for the UTA, but most of them should be stiff enough as soon as the ring is fully closed.


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Olivier Biot
Amused
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Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... new [Re: Benach]
      #3990050 - 08/18/10 07:10 AM

I'm not so keen of rotating the focuser 45 since it will be trickier to observe close to zenith then (the rocker and the back of the UTA will be in the way).

Due to the lack of stiffness of my current set of UTA rings (circular cutout in the inside, octagonal at the outside) I'll probably have to make new UTA rings.

I don't know if birch plywood will be stiffer than the lauan I currently have.


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jayscheuerle
Post Laureate
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Reged: 01/16/06

Loc: S. Philadelphia, PA
Re: My very late 14 inch f/5 project... new [Re: Benach]
      #3990221 - 08/18/10 09:36 AM

Quote:

Olivier: I'd go for the 8-truss design. Is stiffer and apart from that: for optimal stiffness, the spider vanes should be mounted between the trusses, seen from above. In your posting of 08/16/10 07:15 they're halfway the trusses.




Interesting. I'd always heard that the 6-trusses are stiffer because they're all working. With 8, you only have 4 supporting the weight of the UTA and 4 controlling side to side motion... - j


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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: jayscheuerle]
      #3990297 - 08/18/10 10:20 AM

I bet the stiffness also depends on how the trusses are connected. If you leave a couple inches between the truss ends, then you remove part of the stiffness.

The hardware that joins 2 trusses often acts like a truss. Its motion is however constrained by means of tightening nuts and bolts.


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Benach
Pooh-Bah


Reged: 01/24/08

Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3990541 - 08/18/10 12:11 PM

Olivier: I don't mean rotating the focuser, I meant rotating the entire upper cage.

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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... new [Re: Benach]
      #3990557 - 08/18/10 12:23 PM

If you test the stiffness of the upper cage "rings" by them selves you will get an unrealistic estimate of their stiffness. You want the upper cage as a whole to be stiff and able to support the things attached to it (spiders, focusers, pictures of Lady Gaga etc.).

To maximized the strength without adding more material and parts you'll want to have struts right where the spider attaches so you can bolt to the strut. The focuser may need its' own board to attach to (I used aircraft grade plywood - very very stiff) the rings. This board will also make the cage stiffer.

If you can make the material between the rings part of the structure (e.g. use laminate that is pinched between the rings so you end up with it being the webbing in an I beam type structure) you can make a very light but strong upper cage.

And light!


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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... new [Re: rboe]
      #3990562 - 08/18/10 12:25 PM

Oh, add a small mounting board or truss for a telerad, rigel or finder scope (rigel is smaller and lighter of the three). I added holes for mounting eye pieces in the focuser board. That way I can stay at the scope to swap out eye pieces.

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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: Benach]
      #3990574 - 08/18/10 12:31 PM

Quote:

Olivier: I don't mean rotating the focuser, I meant rotating the entire upper cage.




That's what I understood as well

If I do so, then the focuser will also be rotated 45 around the mechanical axis of the OTA. That's not a thing I want to do with this scope since I want to avoid requiring a step ladder while stargazing with it (and falling off it in a moment of less attention)


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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: rboe]
      #3990620 - 08/18/10 12:47 PM

Good point(s) Ron.

The original UTA was bolted together, but did not really act as a whole, since there was quite some play.

I have now added a piece of plywood for the focuser board, and I plan on moving the struts where the spider vanes connect to the UTA.

I still have the 1mm thick sheet of formica (melamin resin) that I plan on using as a light shield. If I attach it to the inside of the UTA rings, it will probably add to the stiffness of the UTA.

FWIW this formica sheet has a density of about 1.5kg/dm. With a height of 300mm it weighs around ~0.6kg.

Hmmm... maybe I should use wooden blocks like the focuser board instead of the struts?


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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3990722 - 08/18/10 01:29 PM

Metal will have a higher bending limit than wood for that spider so I would steer you towards a single metal part to attach the spiders to.

I inserted my laminate into grooves I routed into the rings. Huge PITA to assemble, take about four hands it seems like, but the cage had this nice "one part" feel to it once done. You can pinch it a fair amount, unlike kydex.

Look for ways to bolt through the ring instead of using the angle brackets you had before. Even gluing in blocks of wood into the ends of the channel for a wood screw to dig into. Or, cut the channel so that the ribs are shorter and top surface is longer (like a tab). This tab can be screwed into the edge of the ring but that is not ideal.

Make the tab longer so it goes around the ring and folds flat on top. Screw through it into the top of the ring and the screw will have superior holding power.

You could also bend it so it was on the bottom of the ring but that makes attaching a bit more difficult.


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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: rboe]
      #3990768 - 08/18/10 01:44 PM

Quote:

Metal will have a higher bending limit than wood for that spider so I would steer you towards a single metal part to attach the spiders to.




Do you mean I should go into the direction of a single metal UTA ring?

Quote:

I inserted my laminate into grooves I routed into the rings. Huge PITA to assemble, take about four hands it seems like, but the cage had this nice "one part" feel to it once done. You can pinch it a fair amount, unlike kydex.




In my case that'd require a very thin router bit (1-2mm). Not sure that will be feasible. I'd have to do it with the Dremel.

Quote:

Look for ways to bolt through the ring instead of using the angle brackets you had before. Even gluing in blocks of wood into the ends of the channel for a wood screw to dig into. Or, cut the channel so that the ribs are shorter and top surface is longer (like a tab). This tab can be screwed into the edge of the ring but that is not ideal.

Make the tab longer so it goes around the ring and folds flat on top. Screw through it into the top of the ring and the screw will have superior holding power.

You could also bend it so it was on the bottom of the ring but that makes attaching a bit more difficult.




Do you mean that I should use corner irons with a longer "tab" on which I attach the spider vane? And that this corner iron makes contact on the flat side of the ring as well as on the edge (outer cylindrical surface) of the ring?


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gaidash
super member


Reged: 12/19/09

Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3991359 - 08/18/10 06:18 PM

Quote:

Russian babushka dolls




Matreshka


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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... new [Re: gaidash]
      #3991530 - 08/18/10 07:28 PM

NO, toss those corner irons entirely. Machine the metal trusses you have now to build in the tabs.

I used a 1/8th inch router bit I think. Larger than the thickness but good enough. Used a regular router, no problem. Well, it was a Trend router, they are a bit smaller than your typical woodworking router like your Porter-Cables and DeWalts. Best done before you cut them out.

Still use the double wood rings.


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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: rboe]
      #3991595 - 08/18/10 08:03 PM

What do you mean with 'tabs' here? I'm a bit lost

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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3991705 - 08/18/10 08:56 PM

Your original channels or struts are C shaped. If you mill off the top and bottom legs of the C you have an I where you milled. That is your tab. I'm thinking you can mill off an 1" and bend that tab in to close off the end of the channel and you now have a tab parallel to the ring that can be used as a mount point.

Mill off more, place the ring on the C part then bend the surplus part of the tab OVER the ring so it captures the ring and use that to fasten to the ring.

This tab can be under or over the ring, just has to be longer if you go over.

Since it is part of the original channel there are no parts to buy, no extra weight to add. Save the screw. Maybe a washer.

I may have to get a napkin and draw you a picture like a real engineer.


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Olivier Biot
Amused
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Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: rboe]
      #3991729 - 08/18/10 09:08 PM

In the original UTA cage the struts were square section aluminum, 20mm x 20mm and 1.5mm thick. They're not "C" shaped.

If I understand it right I'd have to trim off 3 sides of the square section struts at the top and bottom, so that there's a 'tab' protruding. This tab then should go in a routed recess.

Is that what you meant?


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rboeAdministrator

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Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3991958 - 08/18/10 11:23 PM

Shakey memory. Could have sworn they were C channels.

You could make a recess for them, a mortise like you would for a hinge. It would weaken the ring somewhat so I would not do it myself unless I made the ring thicker.


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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3991968 - 08/18/10 11:26 PM

Quote:

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):




Well looky there, square tubes. I did notice that the plies were darn thick on that plywood. It would help a lot if you found some plywood with much thinner plies (which will up your ply count which is always good).

If you can't and you still need to make it that small to nest it then do what Sky Captain did and go with metal.

But not cast iron or bronze.


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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: My very late 14 inch f/5 project... [Re: rboe]
      #3991974 - 08/18/10 11:29 PM

Well poo, the picture didn't come through on the quote.

They make spring disc threaded inserts for round tubes, it would save you some trouble if they made them for square tubes too. Have not looked into that. Otherwise, consider going with round tubes.

Square would be stiffer mounts for the spider but a round one should be stiff enough. Lighter too.


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Olivier Biot
Amused
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Reged: 04/25/05

Loc: 51°N (Belgium)
Re: My very late 14 inch f/5 project... [Re: rboe]
      #3992310 - 08/19/10 06:16 AM

Thanks for your recommendations Ron.

I'll get some other plywood for the scope. I'll try to get birch as it is know for its many plies.

I am still considering 18mm (3/4") thick plywood.

Since I'm going for new wood I have no reason to question the UTA ring diameter anymore. Here are the properties of 4 ring configurations:

  1. Original, 406mm I.D., 446 O.D. octagon (20mm ring width at narrowest point): volume = 0.636dm (1dm = 1 liter) (reference)
  2. Round, 406mm I.D., 446 O.D. (20mm ring width): volume = 0.482dm (-25%)
  3. Round, 406mm I.D., 456 O.D. (25mm ring width): volume = 0.609dm (-5%)
  4. Round, 406mm I.D., 446 O.D. (30mm ring width): volume = 0.740dm (+16%)


This means that I'm having a 5% lighter and stronger UTA ring if I take a 25mm wide round ring section than with my original octagonal section.

Going from 25mm to 30mm will add about an ounce or two.

I think I'll stick to 25mm width for now.


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