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  • atmpob
    That does happen with plate glass mirrors. It can take several hours for the mirror temperature to stabilize after a figuring session. It happend on my 400
    Message 1 of 5 , Jul 1, 2004
      That does happen with plate glass mirrors. It can take several hours
      for the mirror temperature to stabilize after a figuring session. It
      happend on my 400 mm 2.5 CM thick mirror. It usually took all night
      for it to cool and stabilize.

      It is not so much of a problem with Pyrex however.

      Dale Eason

      --- In atm_free@yahoogroups.com, "guido gaethofs"
      <guido.gaethofs@p...> wrote:
      > hi folks
      > im guido gaethofs im living in belgium and im a telescopebuilder
      from about 10 years.im made several telescopes in the range of 10 cm
      to 32cm.im also a mirrorgrinder.my telescope is a 32 cm dobson ,now
      im building a equatorial platform for the 32 cm.on this moment im
      grinding a 25 cm mirror for a telescope for handicapt persons.this
      year im going to grind a 50 cm .ive have also a quistion about the 25
      cm mirror.at evening ill test the 25 and he is sferical the day after
      he is not,this is a problem ill never had.this is the sixt time that
      it is happen.
      > greetings guido
    • Richard Schwartz
      Guido, greetings from California. What is the focal length of your 25 cm mirror? What kind of glass are you using? Did you check the annealing of your glass
      Message 2 of 5 , Jul 1, 2004
        Guido, greetings from California.
         
        What is the focal length of your 25 cm mirror?   
         
        What kind of glass are you using?   Did you check the annealing of your glass by viewing it between crossed polaroids?   A bad piece of glass looks like this:
         
         
        (The second polarizer is on my camera lens.)
         
         
         
        For mirror testing, you need to have the mirror isothermal.   After you wash it and put it on the testing stand, you must wait one hour for temperatures to equalize.   If the mirror is warm, the edge will cool faster than the center and change the figure.   After a long time, the mirror will reach equal temperature all over and you will see the correct figure.   This problem is common to all kinds of glass, but it is worse for plate glass than for pyrex, borofloat, or duran low expansion glass.   The expansion problem does not exist for fuzed silica, zerodur, or other zero-expansion materials.
         
        . . . Richard
         
         
        ----- Original Message -----
        Sent: 7/1/2004 3:03:24 AM
        Subject: [atm_free] mail

        hi folks
        im guido gaethofs im living in belgium and im a telescopebuilder from about 10 years.im made several telescopes in the range of 10  cm to 32cm.im also a mirrorgrinder.my telescope is a 32 cm dobson ,now im building a equatorial platform for the 32 cm.on this moment im grinding a 25 cm mirror for a telescope for handicapt persons.this year im going to grind a 50 cm .ive have also a quistion about the 25 cm mirror.at evening ill test the 25 and he is sferical the day after he is not,this is a problem ill never had.this is the sixt time that it is happen.
        greetings guido
         

      • guido gaethofs
        hi richard greetings from belgium the 25 cm have a focal length of F6 it is duran 50 glass the thikness is 6cm about 2.5 inch. im waiting 4 hours to testing.im
        Message 3 of 5 , Jul 1, 2004
          hi richard greetings from belgium
          the 25 cm have a focal length of F6 it is duran 50 glass the thikness is 6cm about 2.5 inch.
          im waiting 4 hours to testing.im grinding in my garage the temperature there is allways between 20 and 25 degrees.i put the mirror in cool water and wait 4 hours.is it posible that duran 50 has stretching.
          guido
          ----- Original Message -----
          Sent: Thursday, July 01, 2004 11:37 PM
          Subject: RE: [atm_free] mail

          Guido, greetings from California.
           
          What is the focal length of your 25 cm mirror?   
           
          What kind of glass are you using?   Did you check the annealing of your glass by viewing it between crossed polaroids?   A bad piece of glass looks like this:
           
           
          (The second polarizer is on my camera lens.)
           
           
           
          For mirror testing, you need to have the mirror isothermal.   After you wash it and put it on the testing stand, you must wait one hour for temperatures to equalize.   If the mirror is warm, the edge will cool faster than the center and change the figure.   After a long time, the mirror will reach equal temperature all over and you will see the correct figure.   This problem is common to all kinds of glass, but it is worse for plate glass than for pyrex, borofloat, or duran low expansion glass.   The expansion problem does not exist for fuzed silica, zerodur, or other zero-expansion materials.
           
          . . . Richard
           
           
          ----- Original Message -----
          Sent: 7/1/2004 3:03:24 AM
          Subject: [atm_free] mail

          hi folks
          im guido gaethofs im living in belgium and im a telescopebuilder from about 10 years.im made several telescopes in the range of 10  cm to 32cm.im also a mirrorgrinder.my telescope is a 32 cm dobson ,now im building a equatorial platform for the 32 cm.on this moment im grinding a 25 cm mirror for a telescope for handicapt persons.this year im going to grind a 50 cm .ive have also a quistion about the 25 cm mirror.at evening ill test the 25 and he is sferical the day after he is not,this is a problem ill never had.this is the sixt time that it is happen.
          greetings guido
           


        • Richard Schwartz
          I think that four hours is enough time for duran to reach thermal equilibrium. In fact, I work with plate glass, and I only wait one hour. If there is a
          Message 4 of 5 , Jul 2, 2004
            I think that four hours is enough time for duran to reach thermal equilibrium.   In fact, I work with plate glass, and I only wait one hour.  If there is a problem, I go out for beer and pizza, then come back and try again.
             
            However, pizza is not pitch.  I have never polished my glass with a pizza lap.
             
            What about the stress testing with polarized light?   That can show problems in the glass.  I never have annealing problems because I anneal the glass myself.    See www.megspace.com/science/sfe if you want to visit my garage and see what I am doing in there.
             
            .. . Richard
            ----- Original Message -----
            Sent: 7/2/2004 3:38:11 AM
            Subject: Re: [atm_free] mail

            hi richard greetings from belgium
            the 25 cm have a focal length of F6 it is duran 50 glass the thikness is 6cm about 2.5 inch.
            im waiting 4 hours to testing.im grinding in my garage the temperature there is allways between 20 and 25 degrees.i put the mirror in cool water and wait 4 hours.is it posible that duran 50 has stretching.
            guido
            -----
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