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Nathan F
sage
Reged: 10/10/08
Posts: 223
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After reading a recent post concerning the use of four dob feet versus three, it made me recall a question I had after reading Kriege/Berry's "The Dobsonian Telescope." On page 112, they state the following:
"At this juncture, it is tempting to ask why not simply rest the mirror on a solid steel plate. Wouldn't this provide plenty of support? Sadly, the answer is no. If you place a rigid object on another rigid object, they contact each other at exactly three points. You can support an uncooked pizza on a flat steel plate because the pizza sags until it rests on the steel, but that's no way to support a telescope mirror."
I have no doubt that they are correct about the support of telescope mirrors, but I don't quite understand the statement that any two rigid objects will only contact on 3 points unless one sags.
Can someone explain this?
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12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
After reading a recent post concerning the use of four dob feet versus three, it made me recall a question I had after reading Kriege/Berry's "The Dobsonian Telescope." On page 112, they state the following:
"At this juncture, it is tempting to ask why not simply rest the mirror on a solid steel plate. Wouldn't this provide plenty of support? Sadly, the answer is no. If you place a rigid object on another rigid object, they contact each other at exactly three points. You can support an uncooked pizza on a flat steel plate because the pizza sags until it rests on the steel, but that's no way to support a telescope mirror."
I have no doubt that they are correct about the support of telescope mirrors, but I don't quite understand the statement that any two rigid objects will only contact on 3 points unless one sags.
Can someone explain this?
Well I'm not too smart but I bet its because rigid surfaces generally don't need more then a tripodal contact point. The sagging results when one part is not rigid and thustly it sags and contacts in more then 3 points.
But 3 contact points are all two surfaces need to be stable against each other. Look at it this way...when you set up your telescope on its tripod...all that scope and all its doodads are touching the earth by 3 points...and it's stable.
However with a mirror against a steel plate...the mirror will sag and then become ground/polished incorrectly. Placing it on a *floatation* system like carpet liners does indeed suport the mirror evenly as it is being worked.
See...... 
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Gene Baraff
sage
Reged: 03/22/09
Posts: 246
Loc: Berkeley Heights, N.J.
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Quote:
I have no doubt that they are correct about the support of telescope mirrors, but I don't quite understand the statement that any two rigid objects will only contact on 3 points unless one sags.
Can someone explain this?
They exaggerate just a bit to make a point.
If you have two surfaces, both absolutely plane, they will contact each other over the entire surface. (Don't quibble about the exact meaning of "absolutely".)
But Krieg/Berry were talking about the real world. Dob construction is supposed to be possible in Joe Sixpack's garage.
The planar surface Joe and I could make would (unless Joe is much better than me) contact at only three points.
Gene Baraff
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jpcannavo
sage
Reged: 02/21/05
Posts: 347
Loc: Long Island New York
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The kernal here is basically that any three distinct points (in euclidian space) that are not colinear, define a single plane.
Joe
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Joseph Cannavo
16" F5 Zambuto, front collimating mirror cell. Dob Driver being installed
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Mr Keeyoots (My Cat)
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Howie Glatter
Vendor
Reged: 07/04/06
Posts: 196
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"If you place a rigid object on another rigid object, they contact each other at exactly three points."
This statement should not have been made without having stated the qualifications of the particular case under consideration. If the objects are close to perfectly rigid, it would be true most of the time ("perfectly rigid" does not exist in the material universe).
In most real situations, very flat and rigid surfaces will contact at many points due to deformation under gravity.
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 669
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IF you put a penny on a railroad track the track will go down a little /i think
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jayscheuerle
Post Laureate
Reged: 01/16/06
Posts: 4076
Loc: S. Philadelphia, PA
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Same works for ball-scopes supported in a ring (there is no perfect sphere or perfect ring), but the reality is that there's flex to most materials, so while there may be three primary points of contact that bear the weight, other parts may brush along close enough touch the surface and keep the dust off. - j
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Fight indignorance!
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rboe
Reged: 03/16/02
Posts: 45411
Loc: Phx, AZ
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Would have been better if they said a "minimum" of three points. Being an imperfect world it could be more than three, but you would need at least three for the reasons stated above.
It's not the best example they could have run with.
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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It is the principle of kinematics, any flat surface contacting another flat surface will always contact on three of the highest points and so stabilize, the exception is if the flats are perfect or nearly so. IE optical flat testing.
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