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Re: [atm_free] thin mirror cell

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  • Nils Olof Carlin
    Benoit wrote: .... Dobs obviously do have the advantage of having a single axis for elevation, so sling type design are easy, but I do not know if there any
    Message 1 of 17 , Feb 1, 2010
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      Benoit wrote:

      ....>>>Dobs obviously do have the advantage of having a single axis for
      elevation, so sling type design are easy, but I do not know if there any any
      sling design which would work for an equatorial, anybody has ideas?

      Finally, how critical is edge support really, anybody with scopes with this
      kind mirror thickness ratio using an RTV glued mirror ?

      There are some known very good schemes for support of thin mirrors - PLOP
      for support on the back, and Robert Houdart's edge support calculator - the
      latter will calculate some selected configurations.

      There are some schemes that are good for avoiding primary astigmatism, the
      single most important deformation. They do so by balancing the vertical
      support forces by horizontal forces, ideally pushing the mirror edges in a
      horizontal direction equal to the vertical forces by direct gravity.

      The 180 deg sling is the theoretically best (of the "simple" solutions!),
      and the 2 point (at +-45 deg to the vertical) support is as good (though it
      will give more higher order deformations) - a 4 point whiffletree
      arrangement seems to perform midways or rather closer to the sling, and
      possibly is easier to implement well. But one fundamental requirement is a
      correct balance between vertical and horizontal forces - merely increasing
      the number of supports won't change this, and neither would nitrogen
      pressure around the perimeter unless you can use graded pressure (and have
      some way of modeling the pressure distribution).

      For an optimum support, it seems essential that the back and edge supports
      are both optimized - the edge supports cannot readily be moved to the back
      of the mirror without loss of performance (see Robert's calculator) - and
      besides, a multi-point (e.g. 18) support of the back is not in general
      balanced for "sideways" forces and may not have a predictable performance at
      all. Even so, you may find the performance good enough for your personal
      taste.

      The fundamental problem here is of course that with an EQ mount, the edge
      forces cannot be distributed in a way that is independent of the orientation
      of the mirror cell (possibly excepting the RTV-ed multipoint cell with its
      own problems). Of course, rotating the tube (or just the cell!) would solve
      this by introducing other perhaps more profound mechanical design problems.

      I don't believe (but would really like to be falsified ;-) that there is a
      simple, passive arrangement for an eq mount that can come close to the ideal
      situation of an optimized alt-az type support. It can no doubt be done with
      computers, or perhaps with purely mechanical means that I can't visualise.
      Else, you need some less than ideal but hopefully good enough solution.
      Like, perhaps, a multi-point RTV-ed cell with a greater number of support
      points than required by PLOP for merely back support, and if possible
      designed for balanced supports also within the plane of the mirror. Or
      perhaps four 90 deg slings, adjusted for a bare minimum of play, giving
      ideal support at 4 orientations and perhaps acceptable performance in
      between.

      More ideas to investigate?

      Nils Olof
    • jacques savard
      a bit off topic but do you make observation or photo whit this kind of big mirror and do you compared the before and after said 8 point or 18 point cell jack
      Message 2 of 17 , Feb 1, 2010
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        a bit off topic

        but do you make observation or photo whit this kind of big mirror
        and do you compared the before and after said 8 point or 18 point cell

        jack 47'n 71'W


        Benoit,

        I Had my first 20" mirror, 2" thickness, glued to the 18 points cell and
        also had lots of astigmatism. I have no astimatism with this mirror, using
        piano wires or rollerskate bearings, plaxced at 90° from each other and
        touching the mirror in the COG plane. For my 1.5" thickness 20" mirror, I
        use the same 18 pointscell and rollerbearings for edge support, without any
        problem. Maybe for an equatorial mount, for bearings at 90° from each other
        and at the COG plane might work?

        Jan

        http://members.ziggo.nl/jhm.vangastel/Astronomy/



        ----- Origineel bericht -----
        Van: Benoit Schillings <benoit.schillings@...>
        Datum: maandag, februari 1, 2010 2:45
        Onderwerp: [atm_free] thin mirror cell
        Aan: atm_free@yahoogroups.com


        > Hello Group, I did not post very often, but would like to get some advices
        > from the group.
        >
        >
        > I do have a 20 inch folded newtonian, which has a 20 inch f/4 nova
        > primary,
        > which is 1.5 inches thick. The scope is used on an equatorial mount
        > (paramount) therefore can have any angle from zenith for the primary.
        >
        > The mirror has some issues, and will be re figured in the near
        > future, but I
        > am still worried by the requirements for the mirror mount for this
        > kind of
        > mirror thickness ratio.
        >
        > I currently built an 18 points cell using PLOP and had the mirror
        > RTVed to
        > the cell with 100 mil thick RTV pads, but after more work it looks like
        > below 45 degrees elevation, I might start to have significant
        > astigmatism.
        >
        > I do plan (after re figuring) to keep the 18 point cell, but instead
        > of rtv
        > to use some ball nose plungers (
        > http://www.mcmaster.com/#ball-plungers/=5mcw5p) both for the mirror
        > cell 18
        > points and for edge support (4 ball noise within a few mil from the
        > edge of
        > the mirror).
        >
        >
        > Dobs obviously do have the advantage of having a single axis for
        > elevation,
        > so sling type design are easy, but I do not know if there any any sling
        > design which would work for an equatorial, anybody has ideas?
        >
        >
        > Finally, how critical is edge support really, anybody with scopes
        > with this
        > kind mirror thickness ratio using an RTV glued mirror ?
        >
        >
        > thank you
        >
        >
        > -- benoit
        >


        ------------------------------------

        Yahoo! Groups Links
      • Benoit Schillings
        Jack, this is mostly an imaging telescope, but I do pretty often do get seeing good enough to get stars below 1.5 arcsec FWHM on long exposure, and astigmatism
        Message 3 of 17 , Feb 1, 2010
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          Jack, this is mostly an imaging telescope, but I do pretty often do get seeing good enough to get stars below 1.5 arcsec FWHM on long exposure, and astigmatism can quickly grow to that range. So, yes, I will see the difference

          -- benoit

          On Mon, Feb 1, 2010 at 6:04 AM, jacques savard <jacquessavard@...> wrote:
           

          a bit off topic

          but do you make observation or photo whit this kind of big mirror
          and do you compared the before and after said 8 point or 18 point cell

          jack 47'n 71'W



          Benoit,

          I Had my first 20" mirror, 2" thickness, glued to the 18 points cell and
          also had lots of astigmatism. I have no astimatism with this mirror, using
          piano wires or rollerskate bearings, plaxced at 90° from each other and
          touching the mirror in the COG plane. For my 1.5" thickness 20" mirror, I
          use the same 18 pointscell and rollerbearings for edge support, without any
          problem. Maybe for an equatorial mount, for bearings at 90° from each other
          and at the COG plane might work?

          Jan

          http://members.ziggo.nl/jhm.vangastel/Astronomy/

          ----- Origineel bericht -----
          Van: Benoit Schillings <benoit.schillings@...>
          Datum: maandag, februari 1, 2010 2:45
          Onderwerp: [atm_free] thin mirror cell
          Aan: atm_free@yahoogroups.com

          > Hello Group, I did not post very often, but would like to get some advices
          > from the group.
          >
          >
          > I do have a 20 inch folded newtonian, which has a 20 inch f/4 nova
          > primary,
          > which is 1.5 inches thick. The scope is used on an equatorial mount
          > (paramount) therefore can have any angle from zenith for the primary.
          >
          > The mirror has some issues, and will be re figured in the near
          > future, but I
          > am still worried by the requirements for the mirror mount for this
          > kind of
          > mirror thickness ratio.
          >
          > I currently built an 18 points cell using PLOP and had the mirror
          > RTVed to
          > the cell with 100 mil thick RTV pads, but after more work it looks like
          > below 45 degrees elevation, I might start to have significant
          > astigmatism.
          >
          > I do plan (after re figuring) to keep the 18 point cell, but instead
          > of rtv
          > to use some ball nose plungers (
          > http://www.mcmaster.com/#ball-plungers/=5mcw5p) both for the mirror
          > cell 18
          > points and for edge support (4 ball noise within a few mil from the
          > edge of
          > the mirror).
          >
          >
          > Dobs obviously do have the advantage of having a single axis for
          > elevation,
          > so sling type design are easy, but I do not know if there any any sling
          > design which would work for an equatorial, anybody has ideas?
          >
          >
          > Finally, how critical is edge support really, anybody with scopes
          > with this
          > kind mirror thickness ratio using an RTV glued mirror ?
          >
          >
          > thank you
          >
          >
          > -- benoit
          >

          ------------------------------------

          Yahoo! Groups Links


        • Benoit Schillings
          Ok, so there seems to be a few paths which might work: - Astatic levers at the edge of the mirror, not too bad to build but another thing that can fail. -
          Message 4 of 17 , Feb 1, 2010
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            Ok, so there seems to be a few paths which might work:

            - Astatic levers at the edge of the mirror, not too bad to build but another thing that can fail.
            - Double or triple sling so that the mirror will be supported at every angles.
            - 4 rest points around the edge, ball bearings or similar with as little distance possible between the edge and the rest point so that the mirror can settle agains 2 or 3 points.
            - N rest points with RTV filled gap around the edge.
            - Mercury bag (my wife would love this)

            And of course an 18 points cell for the back side.

            The primary is perforated (2 inch center hole), could the edge support be replaced by a central support point combined with the mirror cell? Teflon coated center hub or similar?


            -- benoit


          • Richard
            Another possibility is to fill the tube with a clear fluid or gas that has the same density as the mirror, then clamp the mirror clips down extra tight. Sent
            Message 5 of 17 , Feb 2, 2010
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              Another possibility is to fill the tube with a clear fluid or gas that has the same density as the mirror, then clamp the mirror clips down extra tight.

              Sent from Richard's iPhone

              On Feb 1, 2010, at 1:52 PM, Benoit Schillings <benoit.schillings@...> wrote:

               

              Ok, so there seems to be a few paths which might work:

              - Astatic levers at the edge of the mirror, not too bad to build but another thing that can fail.
              - Double or triple sling so that the mirror will be supported at every angles.
              - 4 rest points around the edge, ball bearings or similar with as little distance possible between the edge and the rest point so that the mirror can settle agains 2 or 3 points.
              - N rest points with RTV filled gap around the edge.
              - Mercury bag (my wife would love this)

              And of course an 18 points cell for the back side.

              The primary is perforated (2 inch center hole), could the edge support be replaced by a central support point combined with the mirror cell? Teflon coated center hub or similar?


              -- benoit


            • Benoit Schillings
              Yes, but I am worried of the top of the tube leaking when I do not look @ zenith, I will now need an optical window in the front too ! Also, how do you prevent
              Message 6 of 17 , Feb 2, 2010
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                Yes, but I am worried of the top of the tube leaking when I do not look @ zenith, I will now need an optical window in the front too ! 

                Also, how do you prevent temperature gradients in the liquid ;-)

                Thank you for the useful idea Richard,


                -- benoit

                On Tue, Feb 2, 2010 at 8:30 AM, Richard <richard1941@...> wrote:
                 

                Another possibility is to fill the tube with a clear fluid or gas that has the same density as the mirror, then clamp the mirror clips down extra tight.

                Sent from Richard's iPhone

                On Feb 1, 2010, at 1:52 PM, Benoit Schillings <benoit.schillings@...> wrote:

                 

                Ok, so there seems to be a few paths which might work:

                - Astatic levers at the edge of the mirror, not too bad to build but another thing that can fail.
                - Double or triple sling so that the mirror will be supported at every angles.
                - 4 rest points around the edge, ball bearings or similar with as little distance possible between the edge and the rest point so that the mirror can settle agains 2 or 3 points.
                - N rest points with RTV filled gap around the edge.
                - Mercury bag (my wife would love this)

                And of course an 18 points cell for the back side.

                The primary is perforated (2 inch center hole), could the edge support be replaced by a central support point combined with the mirror cell? Teflon coated center hub or similar?


                -- benoit



              • Richard
                What kind of glue did you use when you glued the mirror to the cell? Epoxy? Contact cement? White glue? Cyanoacrylate crazy glue? Hide glue (horse or
                Message 7 of 17 , Feb 2, 2010
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                  What kind of glue did you use when you glued the mirror to the cell?  Epoxy?  Contact cement? White glue? Cyanoacrylate crazy glue?  Hide glue (horse or rabbit)?  Did you clamp the mirror.clips real tight while the glue dried?

                  Sent from Richard's iPhone

                  On Feb 1, 2010, at 6:04 AM, jacques savard <jacquessavard@...> wrote:

                   

                  a bit off topic

                  but do you make observation or photo whit this kind of big mirror
                  and do you compared the before and after said 8 point or 18 point cell

                  jack 47'n 71'W

                  Benoit,

                  I Had my first 20" mirror, 2" thickness, glued to the 18 points cell and
                  also had lots of astigmatism. I have no astimatism with this mirror, using
                  piano wires or rollerskate bearings, plaxced at 90° from each other and
                  touching the mirror in the COG plane. For my 1.5" thickness 20" mirror, I
                  use the same 18 pointscell and rollerbearings for edge support, without any
                  problem. Maybe for an equatorial mount, for bearings at 90° from each other
                  and at the COG plane might work?

                  Jan

                  http://members.ziggo.nl/ jhm.vangastel/ Astronomy/

                  ----- Origineel bericht -----
                  Van: Benoit Schillings <benoit.schillings@gmail.com>
                  Datum: maandag, februari 1, 2010 2:45
                  Onderwerp: [atm_free] thin mirror cell
                  Aan: atm_free@yahoogroups.com

                  > Hello Group, I did not post very often, but would like to get some advices
                  > from the group.
                  >
                  >
                  > I do have a 20 inch folded newtonian, which has a 20 inch f/4 nova
                  > primary,
                  > which is 1.5 inches thick. The scope is used on an equatorial mount
                  > (paramount) therefore can have any angle from zenith for the primary.
                  >
                  > The mirror has some issues, and will be re figured in the near
                  > future, but I
                  > am still worried by the requirements for the mirror mount for this
                  > kind of
                  > mirror thickness ratio.
                  >
                  > I currently built an 18 points cell using PLOP and had the mirror
                  > RTVed to
                  > the cell with 100 mil thick RTV pads, but after more work it looks like
                  > below 45 degrees elevation, I might start to have significant
                  > astigmatism.
                  >
                  > I do plan (after re figuring) to keep the 18 point cell, but instead
                  > of rtv
                  > to use some ball nose plungers (
                  > http://www.mcmaster .com/#ball- plungers/ =5mcw5p) both for the mirror
                  > cell 18
                  > points and for edge support (4 ball noise within a few mil from the
                  > edge of
                  > the mirror).
                  >
                  >
                  > Dobs obviously do have the advantage of having a single axis for
                  > elevation,
                  > so sling type design are easy, but I do not know if there any any sling
                  > design which would work for an equatorial, anybody has ideas?
                  >
                  >
                  > Finally, how critical is edge support really, anybody with scopes
                  > with this
                  > kind mirror thickness ratio using an RTV glued mirror ?
                  >
                  >
                  > thank you
                  >
                  >
                  > -- benoit
                  >

                  ------------ --------- --------- ------

                  Yahoo! Groups Links

                • Benoit Schillings
                  I used silicon type 1 from GE. I had spacers of about 0.1 inches to avoid having a thin layer and had the mirror upside down so that only the weight of the
                  Message 8 of 17 , Feb 2, 2010
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                    I used silicon type 1 from GE. I had spacers of about 0.1 inches to avoid having a thin layer and had the mirror upside down so that only the weight of the mirror cell would push down

                    Pads where about 0.5 inches in diameter for the 18 contact points.

                    -- benoit

                    On Tue, Feb 2, 2010 at 9:02 AM, Richard <richard1941@...> wrote:
                     

                    What kind of glue did you use when you glued the mirror to the cell?  Epoxy?  Contact cement? White glue? Cyanoacrylate crazy glue?  Hide glue (horse or rabbit)?  Did you clamp the mirror.clips real tight while the glue dried?


                    Sent from Richard's iPhone

                    On Feb 1, 2010, at 6:04 AM, jacques savard <jacquessavard@...> wrote:

                     

                    a bit off topic

                    but do you make observation or photo whit this kind of big mirror
                    and do you compared the before and after said 8 point or 18 point cell

                    jack 47'n 71'W

                    Benoit,

                    I Had my first 20" mirror, 2" thickness, glued to the 18 points cell and
                    also had lots of astigmatism. I have no astimatism with this mirror, using
                    piano wires or rollerskate bearings, plaxced at 90° from each other and
                    touching the mirror in the COG plane. For my 1.5" thickness 20" mirror, I
                    use the same 18 pointscell and rollerbearings for edge support, without any
                    problem. Maybe for an equatorial mount, for bearings at 90° from each other
                    and at the COG plane might work?

                    Jan

                    http://members.ziggo.nl/jhm.vangastel/Astronomy/

                    ----- Origineel bericht -----
                    Van: Benoit Schillings <benoit.schillings@gmail.com>
                    Datum: maandag, februari 1, 2010 2:45
                    Onderwerp: [atm_free] thin mirror cell
                    Aan: atm_free@yahoogroups.com

                    > Hello Group, I did not post very often, but would like to get some advices
                    > from the group.
                    >
                    >
                    > I do have a 20 inch folded newtonian, which has a 20 inch f/4 nova
                    > primary,
                    > which is 1.5 inches thick. The scope is used on an equatorial mount
                    > (paramount) therefore can have any angle from zenith for the primary.
                    >
                    > The mirror has some issues, and will be re figured in the near
                    > future, but I
                    > am still worried by the requirements for the mirror mount for this
                    > kind of
                    > mirror thickness ratio.
                    >
                    > I currently built an 18 points cell using PLOP and had the mirror
                    > RTVed to
                    > the cell with 100 mil thick RTV pads, but after more work it looks like
                    > below 45 degrees elevation, I might start to have significant
                    > astigmatism.
                    >
                    > I do plan (after re figuring) to keep the 18 point cell, but instead
                    > of rtv
                    > to use some ball nose plungers (
                    > http://www.mcmaster.com/#ball-plungers/=5mcw5p) both for the mirror
                    > cell 18
                    > points and for edge support (4 ball noise within a few mil from the
                    > edge of
                    > the mirror).
                    >
                    >
                    > Dobs obviously do have the advantage of having a single axis for
                    > elevation,
                    > so sling type design are easy, but I do not know if there any any sling
                    > design which would work for an equatorial, anybody has ideas?
                    >
                    >
                    > Finally, how critical is edge support really, anybody with scopes
                    > with this
                    > kind mirror thickness ratio using an RTV glued mirror ?
                    >
                    >
                    > thank you
                    >
                    >
                    > -- benoit
                    >

                    ------------------------------------

                    Yahoo! Groups Links


                  • Richard
                    I never buil a dob, but computer modeling long ago revealed that two pieces of duct tape stuck to the back at the 50% zone do a good job of lateral support. I
                    Message 9 of 17 , Feb 2, 2010
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                      I never buil a dob, but computer modeling long ago revealed that two pieces of duct tape stuck to the back at the 50% zone do a good job of lateral support.  

                      I do not recommed this, however, because experts on the list will tell you it is a wrong idea.

                      Sent from Richard's iPhone

                      On Feb 1, 2010, at 2:06 AM, "Nils Olof Carlin" <nilsolof.carlin@...> wrote:

                       

                      Benoit wrote:

                      ....>>>Dobs obviously do have the advantage of having a single axis for
                      elevation, so sling type design are easy, but I do not know if there any any
                      sling design which would work for an equatorial, anybody has ideas?

                      Finally, how critical is edge support really, anybody with scopes with this
                      kind mirror thickness ratio using an RTV glued mirror ?

                      There are some known very good schemes for support of thin mirrors - PLOP
                      for support on the back, and Robert Houdart's edge support calculator - the
                      latter will calculate some selected configurations.

                      There are some schemes that are good for avoiding primary astigmatism, the
                      single most important deformation. They do so by balancing the vertical
                      support forces by horizontal forces, ideally pushing the mirror edges in a
                      horizontal direction equal to the vertical forces by direct gravity.

                      The 180 deg sling is the theoretically best (of the "simple" solutions!),
                      and the 2 point (at +-45 deg to the vertical) support is as good (though it
                      will give more higher order deformations) - a 4 point whiffletree
                      arrangement seems to perform midways or rather closer to the sling, and
                      possibly is easier to implement well. But one fundamental requirement is a
                      correct balance between vertical and horizontal forces - merely increasing
                      the number of supports won't change this, and neither would nitrogen
                      pressure around the perimeter unless you can use graded pressure (and have
                      some way of modeling the pressure distribution) .

                      For an optimum support, it seems essential that the back and edge supports
                      are both optimized - the edge supports cannot readily be moved to the back
                      of the mirror without loss of performance (see Robert's calculator) - and
                      besides, a multi-point (e.g. 18) support of the back is not in general
                      balanced for "sideways" forces and may not have a predictable performance at
                      all. Even so, you may find the performance good enough for your personal
                      taste.

                      The fundamental problem here is of course that with an EQ mount, the edge
                      forces cannot be distributed in a way that is independent of the orientation
                      of the mirror cell (possibly excepting the RTV-ed multipoint cell with its
                      own problems). Of course, rotating the tube (or just the cell!) would solve
                      this by introducing other perhaps more profound mechanical design problems.

                      I don't believe (but would really like to be falsified ;-) that there is a
                      simple, passive arrangement for an eq mount that can come close to the ideal
                      situation of an optimized alt-az type support. It can no doubt be done with
                      computers, or perhaps with purely mechanical means that I can't visualise.
                      Else, you need some less than ideal but hopefully good enough solution.
                      Like, perhaps, a multi-point RTV-ed cell with a greater number of support
                      points than required by PLOP for merely back support, and if possible
                      designed for balanced supports also within the plane of the mirror. Or
                      perhaps four 90 deg slings, adjusted for a bare minimum of play, giving
                      ideal support at 4 orientations and perhaps acceptable performance in
                      between.

                      More ideas to investigate?

                      Nils Olof

                    • Richard
                      You did it right. It should have worked. Perhaps some problem in the mirror cel pivots? Were they also silicone flexures? Sent from Richard s iPhone On Feb
                      Message 10 of 17 , Feb 2, 2010
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                        You did it right.  It should have worked. Perhaps some problem in the mirror cel pivots?  Were they also silicone flexures?

                        Sent from Richard's iPhone

                        On Feb 2, 2010, at 9:06 AM, Benoit Schillings <benoit.schillings@...> wrote:

                         

                        I used silicon type 1 from GE. I had spacers of about 0.1 inches to avoid having a thin layer and had the mirror upside down so that only the weight of the mirror cell would push down


                        Pads where about 0.5 inches in diameter for the 18 contact points.

                        -- benoit

                        On Tue, Feb 2, 2010 at 9:02 AM, Richard <richard1941@gmail.com> wrote:
                         

                        What kind of glue did you use when you glued the mirror to the cell?  Epoxy?  Contact cement? White glue? Cyanoacrylate crazy glue?  Hide glue (horse or rabbit)?  Did you clamp the mirror.clips real tight while the glue dried?


                        Sent from Richard's iPhone

                        On Feb 1, 2010, at 6:04 AM, jacques savard <jacquessavard@ videotron. ca> wrote:

                         

                        a bit off topic

                        but do you make observation or photo whit this kind of big mirror
                        and do you compared the before and after said 8 point or 18 point cell

                        jack 47'n 71'W

                        Benoit,

                        I Had my first 20" mirror, 2" thickness, glued to the 18 points cell and
                        also had lots of astigmatism. I have no astimatism with this mirror, using
                        piano wires or rollerskate bearings, plaxced at 90° from each other and
                        touching the mirror in the COG plane. For my 1.5" thickness 20" mirror, I
                        use the same 18 pointscell and rollerbearings for edge support, without any
                        problem. Maybe for an equatorial mount, for bearings at 90° from each other
                        and at the COG plane might work?

                        Jan

                        http://members.ziggo.nl/jhm.vangastel/ Astronomy/

                        ----- Origineel bericht -----
                        Van: Benoit Schillings <benoit.schillings@gmail.com>
                        Datum: maandag, februari 1, 2010 2:45
                        Onderwerp: [atm_free] thin mirror cell
                        Aan: atm_free@yahoogroups.com

                        > Hello Group, I did not post very often, but would like to get some advices
                        > from the group.
                        >
                        >
                        > I do have a 20 inch folded newtonian, which has a 20 inch f/4 nova
                        > primary,
                        > which is 1.5 inches thick. The scope is used on an equatorial mount
                        > (paramount) therefore can have any angle from zenith for the primary.
                        >
                        > The mirror has some issues, and will be re figured in the near
                        > future, but I
                        > am still worried by the requirements for the mirror mount for this
                        > kind of
                        > mirror thickness ratio.
                        >
                        > I currently built an 18 points cell using PLOP and had the mirror
                        > RTVed to
                        > the cell with 100 mil thick RTV pads, but after more work it looks like
                        > below 45 degrees elevation, I might start to have significant
                        > astigmatism.
                        >
                        > I do plan (after re figuring) to keep the 18 point cell, but instead
                        > of rtv
                        > to use some ball nose plungers (
                        > http://www.mcmaster.com/#ball-plungers /=5mcw5p) both for the mirror
                        > cell 18
                        > points and for edge support (4 ball noise within a few mil from the
                        > edge of
                        > the mirror).
                        >
                        >
                        > Dobs obviously do have the advantage of having a single axis for
                        > elevation,
                        > so sling type design are easy, but I do not know if there any any sling
                        > design which would work for an equatorial, anybody has ideas?
                        >
                        >
                        > Finally, how critical is edge support really, anybody with scopes
                        > with this
                        > kind mirror thickness ratio using an RTV glued mirror ?
                        >
                        >
                        > thank you
                        >
                        >
                        > -- benoit
                        >

                        ------------ --------- --------- ------

                        Yahoo! Groups Links


                      • Benoit Schillings
                        Pivots are ball joint, really very smooth. Of course, the mirror is very suspicious, this is why it will be refigured. Astigmatism & spherical was present when
                        Message 11 of 17 , Feb 2, 2010
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                          Pivots are ball joint, really very smooth. Of course, the mirror is very suspicious, this is why it will be refigured. Astigmatism & spherical was present when pointing @ zenith, I am looking when the mirror is re figured to put all the care on my side on how the mirror will be supported.

                          -- benoit

                          On Tue, Feb 2, 2010 at 10:41 AM, Richard <richard1941@...> wrote:
                           

                          You did it right.  It should have worked. Perhaps some problem in the mirror cel pivots?  Were they also silicone flexures?


                          Sent from Richard's iPhone

                          On Feb 2, 2010, at 9:06 AM, Benoit Schillings <benoit.schillings@...> wrote:

                           

                          I used silicon type 1 from GE. I had spacers of about 0.1 inches to avoid having a thin layer and had the mirror upside down so that only the weight of the mirror cell would push down


                          Pads where about 0.5 inches in diameter for the 18 contact points.

                          -- benoit

                          On Tue, Feb 2, 2010 at 9:02 AM, Richard <richard1941@gmail.com> wrote:
                           

                          What kind of glue did you use when you glued the mirror to the cell?  Epoxy?  Contact cement? White glue? Cyanoacrylate crazy glue?  Hide glue (horse or rabbit)?  Did you clamp the mirror.clips real tight while the glue dried?


                          Sent from Richard's iPhone

                          On Feb 1, 2010, at 6:04 AM, jacques savard <jacquessavard@...> wrote:

                           

                          a bit off topic

                          but do you make observation or photo whit this kind of big mirror
                          and do you compared the before and after said 8 point or 18 point cell

                          jack 47'n 71'W

                          Benoit,

                          I Had my first 20" mirror, 2" thickness, glued to the 18 points cell and
                          also had lots of astigmatism. I have no astimatism with this mirror, using
                          piano wires or rollerskate bearings, plaxced at 90° from each other and
                          touching the mirror in the COG plane. For my 1.5" thickness 20" mirror, I
                          use the same 18 pointscell and rollerbearings for edge support, without any
                          problem. Maybe for an equatorial mount, for bearings at 90° from each other
                          and at the COG plane might work?

                          Jan

                          http://members.ziggo.nl/jhm.vangastel/Astronomy/

                          ----- Origineel bericht -----
                          Van: Benoit Schillings <benoit.schillings@gmail.com>
                          Datum: maandag, februari 1, 2010 2:45
                          Onderwerp: [atm_free] thin mirror cell
                          Aan: atm_free@yahoogroups.com

                          > Hello Group, I did not post very often, but would like to get some advices
                          > from the group.
                          >
                          >
                          > I do have a 20 inch folded newtonian, which has a 20 inch f/4 nova
                          > primary,
                          > which is 1.5 inches thick. The scope is used on an equatorial mount
                          > (paramount) therefore can have any angle from zenith for the primary.
                          >
                          > The mirror has some issues, and will be re figured in the near
                          > future, but I
                          > am still worried by the requirements for the mirror mount for this
                          > kind of
                          > mirror thickness ratio.
                          >
                          > I currently built an 18 points cell using PLOP and had the mirror
                          > RTVed to
                          > the cell with 100 mil thick RTV pads, but after more work it looks like
                          > below 45 degrees elevation, I might start to have significant
                          > astigmatism.
                          >
                          > I do plan (after re figuring) to keep the 18 point cell, but instead
                          > of rtv
                          > to use some ball nose plungers (
                          > http://www.mcmaster.com/#ball-plungers/=5mcw5p) both for the mirror
                          > cell 18
                          > points and for edge support (4 ball noise within a few mil from the
                          > edge of
                          > the mirror).
                          >
                          >
                          > Dobs obviously do have the advantage of having a single axis for
                          > elevation,
                          > so sling type design are easy, but I do not know if there any any sling
                          > design which would work for an equatorial, anybody has ideas?
                          >
                          >
                          > Finally, how critical is edge support really, anybody with scopes
                          > with this
                          > kind mirror thickness ratio using an RTV glued mirror ?
                          >
                          >
                          > thank you
                          >
                          >
                          > -- benoit
                          >

                          ------------------------------------

                          Yahoo! Groups Links



                        • Richard
                          Is also important hoe you support mirror during test. Sent from Richard s iPhone On Feb 2, 2010, at 10:49 AM, Benoit Schillings
                          Message 12 of 17 , Feb 2, 2010
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                            Is also important hoe you support mirror during test.

                            Sent from Richard's iPhone

                            On Feb 2, 2010, at 10:49 AM, Benoit Schillings <benoit.schillings@...> wrote:

                             

                            Pivots are ball joint, really very smooth. Of course, the mirror is very suspicious, this is why it will be refigured. Astigmatism & spherical was present when pointing @ zenith, I am looking when the mirror is re figured to put all the care on my side on how the mirror will be supported.

                            -- benoit

                            On Tue, Feb 2, 2010 at 10:41 AM, Richard <richard1941@gmail.com> wrote:
                             

                            You did it right.  It should have worked. Perhaps some problem in the mirror cel pivots?  Were they also silicone flexures?


                            Sent from Richard's iPhone

                            On Feb 2, 2010, at 9:06 AM, Benoit Schillings <benoit.schillings@gmail.com> wrote:

                             

                            I used silicon type 1 from GE. I had spacers of about 0.1 inches to avoid having a thin layer and had the mirror upside down so that only the weight of the mirror cell would push down


                            Pads where about 0.5 inches in diameter for the 18 contact points.

                            -- benoit

                            On Tue, Feb 2, 2010 at 9:02 AM, Richard <richard1941@gmail.com> wrote:
                             

                            What kind of glue did you use when you glued the mirror to the cell?  Epoxy?  Contact cement? White glue? Cyanoacrylate crazy glue?  Hide glue (horse or rabbit)?  Did you clamp the mirror.clips real tight while the glue dried?


                            Sent from Richard's iPhone

                            On Feb 1, 2010, at 6:04 AM, jacques savard <jacquessavard@ videotron. ca> wrote:

                             

                            a bit off topic

                            but do you make observation or photo whit this kind of big mirror
                            and do you compared the before and after said 8 point or 18 point cell

                            jack 47'n 71'W

                            Benoit,

                            I Had my first 20" mirror, 2" thickness, glued to the 18 points cell and
                            also had lots of astigmatism. I have no astimatism with this mirror, using
                            piano wires or rollerskate bearings, plaxced at 90° from each other and
                            touching the mirror in the COG plane. For my 1.5" thickness 20" mirror, I
                            use the same 18 pointscell and rollerbearings for edge support, without any
                            problem. Maybe for an equatorial mount, for bearings at 90° from each other
                            and at the COG plane might work?

                            Jan

                            http://members.ziggo.nl/jhm.vangastel/ Astronomy/

                            ----- Origineel bericht -----
                            Van: Benoit Schillings <benoit.schillings@gmail.com>
                            Datum: maandag, februari 1, 2010 2:45
                            Onderwerp: [atm_free] thin mirror cell
                            Aan: atm_free@yahoogroups.com

                            > Hello Group, I did not post very often, but would like to get some advices
                            > from the group.
                            >
                            >
                            > I do have a 20 inch folded newtonian, which has a 20 inch f/4 nova
                            > primary,
                            > which is 1.5 inches thick. The scope is used on an equatorial mount
                            > (paramount) therefore can have any angle from zenith for the primary.
                            >
                            > The mirror has some issues, and will be re figured in the near
                            > future, but I
                            > am still worried by the requirements for the mirror mount for this
                            > kind of
                            > mirror thickness ratio.
                            >
                            > I currently built an 18 points cell using PLOP and had the mirror
                            > RTVed to
                            > the cell with 100 mil thick RTV pads, but after more work it looks like
                            > below 45 degrees elevation, I might start to have significant
                            > astigmatism.
                            >
                            > I do plan (after re figuring) to keep the 18 point cell, but instead
                            > of rtv
                            > to use some ball nose plungers (
                            > http://www.mcmaster.com/#ball-plungers /=5mcw5p) both for the mirror
                            > cell 18
                            > points and for edge support (4 ball noise within a few mil from the
                            > edge of
                            > the mirror).
                            >
                            >
                            > Dobs obviously do have the advantage of having a single axis for
                            > elevation,
                            > so sling type design are easy, but I do not know if there any any sling
                            > design which would work for an equatorial, anybody has ideas?
                            >
                            >
                            > Finally, how critical is edge support really, anybody with scopes
                            > with this
                            > kind mirror thickness ratio using an RTV glued mirror ?
                            >
                            >
                            > thank you
                            >
                            >
                            > -- benoit
                            >

                            ------------ --------- --------- ------

                            Yahoo! Groups Links



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