[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: [ATM] Ball bearing for triangles

Hi Mark

I know understand what you mean by extra boundary area. I remember I had wondered about this during the construction but then forgot it...
David Vernet told me that it was not the thickness of the mirror that cause the thermal difference between the 2 faces of the miror, but the combinaison of a fin glass and a fast miror. It leads effectively to something like your 5/8 rule. Anyway, I'm not that far from this 5/8 rule (5/8*40mm=25 mm, I have 28 in the center...)
I'm going to have a fan behind the mirror, but this will only help to reach thermal equilibrium faster. The problem of thermal difference between the 2 faces of the miror due to the temperature of space could occur even if the "ground", rear face is in equilibrium.
Maybe a front fan could help to "re-heat" the front face if it its temperature is below the rear face. I don't know...

I think I will need also nearly 1/2 kg of weigh on outside edge to balance my triangle because the weigh you see on the picture is 0.45 kg....


  ----- Original Message ----- 
  From: Mark Suchting 
  To: Raphaël GUINAMARD ; ATM Group 
  Sent: Thursday, September 02, 2004 12:55 PM
  Subject: Re: [ATM] Ball bearing for triangles


  At 07:46 PM 9/2/2004, Raphaël GUINAMARD wrote:



    What do you mean by "the extra triangle boundary area  for a larger support "


  Hi Raphael

  On a triangle that is not equilateral but scalene ,the balance point changes from the theoretical computed point very rapidly  when you add extra even width around w the three support points ( as is normally done ) 

  On the 36" I was involved in , the large bottom triangles needed about 1/2 kilo of weight on outside  edge to balance them otherwise the inner three triangles on the upper layer will be pressing   more or less than the edge triangles.

  I would be much more suspicious of the cell problems than temperature effects. 40mm is really not that thin for a 24" . I have worked 45mm  thick and found no special problems.  I am guessing that David Vernet has done your mirror ?  He has made quite a reputation in making high quality fast mirrors. 

   The mirrors you give example of effects due to different edge/center thickness were probably plate glass.  I remember someone very experienced with experimenting with large thin plate glass mirrors told me of the 5/8 rule. Once the center thickness is less than 5/8 the edge thickness, thermal problems will make the mirror mostly impossible to use.    I don't think you will have so much problem with a Pyrex mirror. Do you have a fan fitted to help cool the mirror more evenly .

  Mark Suchting




_______________________________________________
ATM mailing list http://www.atmlist.net/