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Equipment Discussions >> ATM, Optics and DIY Forum

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Benach
Pooh-Bah


Reged: 01/24/08

Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3985804 - 08/16/10 08:01 AM

Olivier: I'd recommend you to redesign your collimation system. I don't see how it's working. I highly recommend you to use a kinematic collimation system. Then you have to align your primary only once and can do the rest of the collimation on the secondary mirror.
A wall thickness of 2mm has a far better stiffness, so I'd go for that 30mm as well.
Threaded inserts (helicoils) are excellent! However, if you intend to use them, insert them after anodising the construction. Most helicoils I know, are made of stainless steel which will ruin the anodising solution.


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Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: My very late 14 inch f/5 project... new [Re: Benach]
      #3985818 - 08/16/10 08:23 AM

Align your primary only once? We all wish! Bumpy roads to dark sky sites are not so forgiving.

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Benach
Pooh-Bah


Reged: 01/24/08

Re: My very late 14 inch f/5 project... new [Re: Mike I. Jones]
      #3985857 - 08/16/10 09:03 AM

Mike: a Kelvin-clamp (cone-vee-flat) is highly shockresistant as you know. Stiffness then only depends on the springs used.

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m. allan noah
scholastic sledgehammer


Reged: 08/14/09

Loc: Virginia, USA
Re: My very late 14 inch f/5 project... new [Re: Benach]
      #3985880 - 08/16/10 09:25 AM

Benach- How about a drawing of the collimation setup you envision?

allan


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Benach
Pooh-Bah


Reged: 01/24/08

Re: My very late 14 inch f/5 project... new [Re: m. allan noah]
      #3985935 - 08/16/10 09:58 AM

Allan: I don't have a drawing over here, but on this site you can see another kinematic mount.

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Gary Fuchs
Pooh-Bah
*****

Reged: 05/22/06

Loc: Easton, PA, USA
Re: My very late 14 inch f/5 project... new [Re: Olivier Biot]
      #3985944 - 08/16/10 10:03 AM

Quote:

the weight MUST go down




Hi Olivier,

I think you have at least 48 bolts and nuts that could be replaced with nylon and maybe save about 1/3 kg assuming the bolts are around 30-32mm x 6 or so and my crude calculations are anywhere near correct.

(Thanks for your note on the other forum.)

Gary


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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: Gary Fuchs]
      #3985989 - 08/16/10 10:35 AM

Quote:

Quote:

the weight MUST go down




Hi Olivier,

I think you have at least 48 bolts and nuts that could be replaced with nylon and maybe save about 1/3 kg assuming the bolts are around 30-32mm x 6 or so and my crude calculations are anywhere near correct.

(Thanks for your note on the other forum.)

Gary




Thanks Gary

For what it's worth, I entirely disassembled the UTA today, and here are the weight figures:

  • UTA rings: 2 x 400g = 800g
  • Nuts and bolts (56 of each): 320g
  • Aluminum trusses and corners: 850g
  • Spider and diagonal holder: 400g
  • Formica light shield: 500g


Without any focuser or any other extras I already had a bulky 3kg UTA


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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]
      #3986856 - 08/16/10 07:11 PM Attachment (61 downloads)

Here's a significant update.

First of all, here's a rendition (in perspective) of the OTA on the rocker box:


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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]
      #3986863 - 08/16/10 07:15 PM Attachment (55 downloads)

As you can see I changed my mind regarding the trusses. I used rectangular section trusses here with a section of 40mm by 20mm, and I have the choice between 1.5mm wall thickness (0.462kg/m) or 2mm wall thickness (0.605kg/m). Given the section I believe 1.5mm wall thickness should be fine.

What is still missing is the collimation system and the ground board. I didn't draw the focuser either .

Here's another view, from the business end (again 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]
      #3986876 - 08/16/10 07:23 PM Attachment (54 downloads)

As you can see from the image above, I use an "offset spider" (made by Gary Wolanski over 10 years ago). This spider will produce 4 diffraction spikes like a regular spider, as long as I keep the vanes parallel and perpendicular.

Here's a close-up of the secondary holder as seen through the focuser hole:


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Owen
professor emeritus


Reged: 06/21/07

Loc: New Zealand
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3987682 - 08/17/10 04:27 AM

Olivier,

I'd be looking to put the uta on a diet, and reduce the truss count by 2... Both will save weight.

Have fun


Owen


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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: Owen]
      #3987690 - 08/17/10 04:42 AM

Quote:

Olivier,

I'd be looking to put the uta on a diet, and reduce the truss count by 2... Both will save weight.




Hi Owen,

Thanks for your recommendations.

If I reduce the truss count from 8 to 6 I can no longer use square section trusses. In that case I'll have to go for 25mm or even 30mm diameter round trusses.

Will the increased forces on the remaining trusses not require a sturdier and heavier mirror box, and more complex truss clamps?

Cheers,

Olivier


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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]
      #3987711 - 08/17/10 05:41 AM Attachment (55 downloads)

Regarding collimation, Chriske suggested I use push-pull.

Since I plan on collimating the scope from the front, I basically inverted the push-pull setup.

I attached a concept paper drawing to this message, and you'll notice I didn't add a compression spring (yet).

The mirror cell rests on the bottom board, the top board contains the push-pull hardware pointing towards the stars. The top board will be firmly attached to the side of the rocker board (e.g.,with L irons at top and bottom to prevent sagging and play).

The left bolt is the PULL bolt: it regulates the collimation. The bolt to the right is the PUSH bolt: it locks the collimation in place.

The head of the PULL bolt rests on a large washer to avoid imprinting in the top board. The bottom of the PUSH bolt makes contact with the bottom board on a hard surface, e.g. the flat head of a countersunk wood screw.

This setup requires 2 threaded inserts with their top pointing downwards. This way both threaded inserts are pulled into the wood under normal compression load.

What do you think?


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Owen
professor emeritus


Reged: 06/21/07

Loc: New Zealand
Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3987740 - 08/17/10 06:40 AM

Hi Olivier,

You would still be able to use a square section, but the geometry of the clamps does get a little more challenging - most incidences that I've seen take the angle from the mirror box, and transpose that on the uta.

The box sections specified are immensely strong as they are, and moving to 6 rather than 8 trusses will triangulate the structure more effectively - so potential is there to reduce the wall thickness further.

Making the mirror box structure a bit more substantial will help overall balance...

Have fun



Owen


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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: Owen]
      #3987859 - 08/17/10 08:40 AM Attachment (48 downloads)

I'll give the 6 truss concept a try this afternoon.

Here's an illustration of a push-pull system with compression springs (the collimation bolts do not rotate in a threaded insert - they're fixed to the mirror cell board):


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Benach
Pooh-Bah


Reged: 01/24/08

Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3987883 - 08/17/10 08:59 AM

Quote:

What do you think?



Read your PM to know what I think


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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]
      #3987932 - 08/17/10 09:29 AM Attachment (43 downloads)

Here's a rendering with lateral rails in place (4 slanted corner irons). This way I can use smaller diameter setscrews for the push-pull system.

Eventually I may give up entirely the idea of the compression springs in the collimation unit. They're not needed in a true push-pull system. This would further reduce the footprint of my cell


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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]
      #3988288 - 08/17/10 12:21 PM Attachment (50 downloads)

Here's a first rendering of the OTA with 6 round trusses (30mm diameter, 2mm wall thickness):

Edited by Olivier Biot (08/17/10 12:24 PM)


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Benach
Pooh-Bah


Reged: 01/24/08

Re: My very late 14 inch f/5 project... [Re: Olivier Biot]
      #3988304 - 08/17/10 12:33 PM

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.

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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: Benach]
      #3989139 - 08/17/10 07:12 PM

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.




The 8 truss design is simpler for a first scope. Especially since I am building it like Russian babushka dolls (UTA cage fits in mirror box fits in rocker box).

I can't choose where to place the spider vanes due to the type of secondary holder.

What I'm more concerned with, is that the current UTA rings (octagons, in fact) are not sufficiently stiff for the job. I can easily bend them out of shape.

Hence, I'll need to have other wood (birch or beech plywood instead of the current Chinese lauan), and most likely wider UTA rings. This means I won't be able to stack the secondary cage inside the mirror box.

Bummer.


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