Telescope Reviews: Mirror side support - captive ball bearing

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bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Mirror side support - captive ball bearing - LB16"
#5342917 - 07/30/12 07:51 AM

I will tell you this story backwards

Here is what I did after I got rid of the silicons Meade used to fix the mirror in the cell:

I have placed 2x 3/8" torlon balls into the cavities I prepared on the mirror cell. The cavities are at 45 degrees from vertical at the bottom of the the mirror cell and in line with COG of the mirror.

Here is a short clip showing how the torlon balls are freely rotating.

Short movie

I did the same test when the mirror was at 45 and 60 degrees and the system worked without a problem. However, since I was alone and two handed only, couldn't film those trials.

If the experts on this forum approve this solution and you want to know more, I will continue.

Thanks for your time.

Sedat

Post Extras:

rockethead26
Carpal Tunnel

Reged: 10/21/09

Loc: Indiana, USA

Re: Mirror side support - captive ball bearing - LB16" new

[Re: bilgebay]
#5343148 - 07/30/12 10:48 AM

I'm certainly no expert, but wouldn't the ball bearings create two stress points by not evenly supporting the edge of the mirror like a sling or cable would?

Post Extras:

Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Mirror side support - captive ball bearing - LB16" new

[Re: bilgebay]
#5343164 - 07/30/12 11:00 AM

that is great. love the movie.

Post Extras:

Howie Glatter
Vendor

Reged: 07/04/06

Re: Mirror side support - captive ball bearing - LB16" new

[Re: bilgebay]
#5343211 - 07/30/12 11:27 AM

Hi Sedat,

Looks good! I think the cavities are a little bit wider than the balls because you can see a tiny bit of side motion of the ball when you reverse the mirror rotation, but I don't think that's a problem. Does the ball bear against a countersink in the aluminum cell casting?

Post Extras:

bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Re: Mirror side support - captive ball bearing - LB16" new

[Re: Howie Glatter]
#5343687 - 07/30/12 03:52 PM

Hi Howie,

I am glad you liked the idea. You are absolutely right, there is some lateral play caused by the imperfections of my hand held drilling. I will do this again ( by rotating the aluminium mirror cell by 120 degrees) but this time in my machine shop and with great precision. This was just a test run and in fact will function as is if I leave it alone.

You can see where the ball sits in the photo below:

Hi res photo is here

Initially, I planned to use plastic screws but they didn't look strong enough to me, especially with the angle I drilled the holes. There simply was too much side load on these 4mm plastic screws.

Why did I drill the holes at such a stupid angle ? To leave enough material at the edge of the mirror cell while keeping the tip of the screw on the COG of the mirror. It wasn't a good idea though.

Post Extras:

bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Re: Mirror side support - captive ball bearing - LB16" new

[Re: Pinbout]
#5343695 - 07/30/12 03:55 PM

Hi Danny, thank you.

Post Extras:

bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Re: Mirror side support - captive ball bearing - LB16" new

[Re: rockethead26]
#5343727 - 07/30/12 04:07 PM

Hi Jim,

I reckon the sling cable is the best solution. However, it would be difficult for me to implement that solution to the existing mirror cell. A while ago, there was a thread by Project Galileo where 2-point edge support was discussed.

Quote:

Once I marked the COG on the metal edge I set and drilled a hole in the side so I could tap new threads. A tap and some lubricating oil had me in business soon. Be careful. It is an aluminum cell and is very soft and easy to work with. I opted for a 10/24 screw size so I had many threads in the thinner metal cell side.

In the close up picture you can see the hole I drilled is 2.5 mm below the COG mark above it. I chose to back up the support to allow for more metal at the lip of the hole and cell. I felt I would be drilling and tapping a hole too close to the edge using the true COG.

The Edge Support Calculator showed that by changing the support position relative to the COG 1/20th of the mirror thickness (2.5 mm in my case) it would change the RMS Surface Error to 3.3 nm. Still within the 5 nm goal and undetectable with the eye. I opted to move the screw in trade for more strength.

The new 10/24 nylon screws fit perfectly. I will round and smooth the tips of the screws to lower their friction on the mirror's edge too. Finally, I will use left over UHMW tape pads stuck to the side of the mirror where it contacts the nylon screws to lower friction even more.

Ta da! Mirror cell improved and done for now!

You can read about M42's evolution here.

I will be following Doc's footsteps for a while

Thank you for this beautiful thread once again Doc.

Post Extras:

bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Re: Mirror side support - captive ball bearing - LB16" new

[Re: bilgebay]
#5343767 - 07/30/12 04:30 PM

Howie, I forgot to give you the crucial dimension, here it is:

You should be somewhere between 11.00 and 11.40 mm to keep the mirror at a distance from the cork material around the periphery of the cell.

And for those who are wondering how I was able to rotate the mirror so easily, here is the magical chemical I have used for that: McLube Sailkote.

We are using this stuff a lot on our sailboats to help everything move much easier so I said why not here

I have sprayed the back of the mirror with this and it made a huge difference. Of course, I have removed the original support pads and the mirror is on bare floating supports. I have also used Sailkote at the fulcrum of the floating supports both on the mirror cell and the support side. They are really floating now. Needless to say, the seats where the 2x 3/8" Torlon balls are seated were also sprayed with Sailkote. It takes most of the friction out of the equation.

Post Extras:

rockethead26
Carpal Tunnel

Reged: 10/21/09

Loc: Indiana, USA

Re: Mirror side support - captive ball bearing - LB16" new

[Re: bilgebay]
#5343854 - 07/30/12 05:28 PM

Quote:

Hi Jim,

I reckon the sling cable is the best solution. However, it would be difficult for me to implement that solution to the existing mirror cell. A while ago, there was a thread by Project Galileo where 2-point edge support was discussed.

Quote:

Once I marked the COG on the metal edge I set and drilled a hole in the side so I could tap new threads. A tap and some lubricating oil had me in business soon. Be careful. It is an aluminum cell and is very soft and easy to work with. I opted for a 10/24 screw size so I had many threads in the thinner metal cell side.

In the close up picture you can see the hole I drilled is 2.5 mm below the COG mark above it. I chose to back up the support to allow for more metal at the lip of the hole and cell. I felt I would be drilling and tapping a hole too close to the edge using the true COG.

The Edge Support Calculator showed that by changing the support position relative to the COG 1/20th of the mirror thickness (2.5 mm in my case) it would change the RMS Surface Error to 3.3 nm. Still within the 5 nm goal and undetectable with the eye. I opted to move the screw in trade for more strength.

The new 10/24 nylon screws fit perfectly. I will round and smooth the tips of the screws to lower their friction on the mirror's edge too. Finally, I will use left over UHMW tape pads stuck to the side of the mirror where it contacts the nylon screws to lower friction even more.

Ta da! Mirror cell improved and done for now!

You can read about M42's evolution here.

I will be following Doc's footsteps for a while Thank you for this beautiful thread once again Doc.

Thanks for clarifying Sedat. Looks like the stress change to the mirror's figure is insignificant. Looks like a good solution for you.

Post Extras:

GeneT
Ely Kid

Reged: 11/07/08

Loc: South Texas

Re: Mirror side support - captive ball bearing - LB16" new

[Re: bilgebay]
#5343885 - 07/30/12 05:52 PM

Very interesting. Please post how it works in the field.

Post Extras:

bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Re: Mirror side support - captive ball bearing - LB16" new

[Re: rockethead26]
#5344601 - 07/31/12 02:31 AM

You are welcome Jim.

Post Extras:

bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Re: Mirror side support - captive ball bearing - LB16" new

[Re: GeneT]
#5344604 - 07/31/12 02:33 AM

Hi Gene,

I bet it will be much better than the original but I will certainly report my findings.

Post Extras:

bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Re: Mirror side support - captive ball bearing - LB16" new

[Re: bilgebay]
#5344606 - 07/31/12 02:39 AM

Another implementation can be ball point support screws. Just like the ballpoint pens...

This was not applicable to LB16 mirror cell but could be used by others and ATM'ers. In this case you will not be limited to 2 contact points. You could use 4 points and more with careful craftsmanship.

Post Extras:

The bear
Pooh-Bah

Reged: 02/11/08

Loc: rushville, indiana

Re: Mirror side support - captive ball bearing - LB16" new

[Re: bilgebay]
#5345068 - 07/31/12 11:24 AM

do they make structures like the ball on a stalk like you show in your diagram i have some ideas i could use such a thing thanks
doc

Post Extras:

bilgebay
Post Laureate

Reged: 11/06/08

Loc: Türkiye - Istanbul and Marmari...

Re: Mirror side support - captive ball bearing - LB16" new

[Re: The bear]
#5346340 - 08/01/12 02:17 AM

Any machine shop should be able to do it for you.

Post Extras:

Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Mirror side support - captive ball bearing - LB16" new

[Re: The bear]
#5346507 - 08/01/12 07:03 AM

Quote:

do they make structures like the ball on a stalk like you show in your diagram i have some ideas i could use such a thing thanks

mcmasters sells them, but the ball is spring loaded.

http://www.mcmaster.com/#standard-spring-plungers/=inp0r1

Post Extras:

piaras
professor emeritus

Reged: 01/26/09

Loc: Niagara Region

Re: Mirror side support - captive ball bearing - LB16" new

[Re: Pinbout]
#5352838 - 08/05/12 10:18 AM

Some of the "plunger ball screws" have a tension screw on the opposite end. One can remove the spring and replace it with either a pin, like a dowel pin, or even a longer screw to lock the ball in place.

Also one can weld ears on the mirror support ring to allow the screws to be in the correct place and orientation.

Post Extras:

Mark Peterman
super member

Reged: 08/07/12

Loc: Texas

Re: Mirror side support - captive ball bearing - LB16" new

[Re: Pinbout]
#5566328 - 12/11/12 05:21 PM

Quote:

Quote:

do they make structures like the ball on a stalk like you show in your diagram i have some ideas i could use such a thing thanks

mcmasters sells them, but the ball is spring loaded.

http://www.mcmaster.com/#standard-spring-plungers/=inp0r1

They also have these set screws with a ball bearing in the end. (non-spring loaded)

McMaster

Post Extras:

Project Galileo
professor emeritus

Reged: 11/14/07

Loc: Jefferson County, Colorado

Re: Mirror side support - captive ball bearing - LB16" new

[Re: Mark Peterman]
#5566687 - 12/11/12 10:32 PM

I love this idea. The captive ball bearing precision adjustment set screw may be just the thing for me too. I can only see them as decreasing friction and freeing up the mirror to float better.

Post Extras:

d.sireci51
Arachnoid

Reged: 11/19/09

Loc: The Tholian Web [S. Wisconsin]

Re: Mirror side support - captive ball bearing - LB16" [Re: Project Galileo]
#5566758 - 12/11/12 11:18 PM

I like the spring loaded ball bearings. If you cant set the ball bearings exact, the spring would compensate. Also it would hold the mirror snug while moving the tube from horz to vert.

D

Edited by d.sireci51 (12/11/12 11:19 PM)

Post Extras:

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