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Re: Procedure for a Meniscus mirror

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  • Mark Christensen
    My question to you is WHY? In 9 inches you are not going to save that much weight. Slumping is used to reduce weight and to pre-form the concave. Neither is
    Message 1 of 18 , Jun 8, 2013
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      My question to you is WHY? In 9 inches you are not
      going to save that much weight. Slumping is used to
      reduce weight and to pre-form the concave. Neither is
      terribly important, it seems to me, in a 9 inch mirror.
       
      Mark Christensen
    • Joe
      to what extreme thinness could this be taken/ Joe Sig The Original Rolling Ball Clock Idle Tyme Idle-Tyme.com http://www.idle-tyme.com ... to what extreme
      Message 2 of 18 , Jun 8, 2013
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        to what extreme thinness could this be taken/

        Joe
        Sig
        The Original Rolling Ball Clock
        Idle Tyme
        Idle-Tyme.com
        http://www.idle-tyme.com
        On 6/8/2013 1:31 PM, paul valleli wrote:
        One thing missing here in the advice, One should grind the back convex side to a nine micron finish, then cast a Hydrostone or Hydrocal plaster holder to that side. 
        After drying, separate and cure, place thin pads in between, then make the concave side
        with the mirror lightly taped to the holder with masking tape. Tight tape will bend the mirror ! I use vinyl electrical tape because it is somewhat waterproof and stretches. It should be changed periodically to even out any de-center or stress. 
        The reason for fine grinding the back is to remove invisible sub-surface damage from the rough grinding operation that can cause a slow figure change after completion of the front, concave surface.  
        This process works well up to a D/Th aspect ratio of 12:1. For thinner mirrors, the back should be supported with pads of soft polishing pitch ( #55 Gugolz) 
        Starman Paul


        On Sat, Jun 8, 2013 at 8:39 AM, Jerry <wa4guu@...> wrote:
         

        Hello Christo,

        Except for the damage you might do to the mirror surface of the glass while working the back side, it doesn’t matter much which side you work first. Most (probably all) of the work on the convex side will probably be done TOT. This means the mirror side (concave) will be against some sort of support on the table.

        There might be (will be) some rubbing between support and bottom side of the mirror during stroking.

        I would prefer that this rubbing not be on the optical surface of the glass after rough grinding.

         

        I would rough grind both sides to the desired sag.  May as well do the back first and then the front. Reduce wedge while working toward sag.

         

        I would finish all the fine grinding on the back, and polish if desired.

         

        Then fine grind the front, reducing wedge at each grit size.

        Polish and figure.

         

        Jerry

         

         

         

        From: atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] On Behalf Of Christo vd Merwe
        Sent: Saturday, June 08, 2013 5:55 AM
        To: atm_free@yahoogroups.com
        Subject: [atm_free] Procedure for a Meniscus mirror

         




        I need some advice.

        What is the best procedure making a meniscus mirror?  Is starting from the front better, while the back is still flat? And then leaving polishing and figuring until the back of the mirror has been ground convex.

        Or should I start on the back first?

        The diameter is 225mm (just under 9”) and the aim is to make it f3 for a fast Newt with coma corrector. (thanks to Mel Bartels’ very convincing Zip Dob page: http://www.bbastrodesigns.com/ZipDob/ZipDob.html).

        Christo

         



      • Christo
        Mark That is an excellent question and I have to admit that I can t really answer it if this is the only mirror that I want to do in this way. I suppose I
        Message 3 of 18 , Jun 8, 2013
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          Mark

          That is an excellent question and I have to admit that I can't really answer it if this is the only mirror that I want to do in this way.

          I suppose I thought that I needed to practice on this 9" recycled glass blank first, before tackling a 12" plate glass mirror which is a lot thinner and which is earmarked to become a very fast mirror too.

          So, the 9" isn't that important to me. The lessons I learn with this relatively cheap mirror will be applied to my end goal, the 12" f3. I would hate to mess up the 12" through inexperience.

          Christo



          --- In atm_free@yahoogroups.com, "Mark Christensen" <mjcw500@...> wrote:
          >
          > My question to you is WHY? In 9 inches you are not
          > going to save that much weight. Slumping is used to
          > reduce weight and to pre-form the concave. Neither is
          > terribly important, it seems to me, in a 9 inch mirror.
          >
          > Mark Christensen
          >
        • Christo
          Thanks Zoran and Paul for your posts. I m taking notes as we go along. Great ideas and info from you guys. Christo
          Message 4 of 18 , Jun 8, 2013
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            Thanks Zoran and Paul for your posts. I'm taking notes as we go along.

            Great ideas and info from you guys.

            Christo



            --- In atm_free@yahoogroups.com, paul valleli <paulvalleli@...> wrote:
            >
            > One thing missing here in the advice, One should grind the back convex side
            > to a nine micron finish, then cast a Hydrostone or Hydrocal plaster holder
            > to that side.
            > After drying, separate and cure, place thin pads in between, then make the
            > concave side
            > with the mirror lightly taped to the holder with masking tape. Tight tape
            > will bend the mirror ! I use vinyl electrical tape because it is somewhat
            > waterproof and stretches. It should be changed periodically to even out any
            > de-center or stress.
            > The reason for fine grinding the back is to remove invisible sub-surface
            > damage from the rough grinding operation that can cause a slow figure
            > change after completion of the front, concave surface.
            > This process works well up to a D/Th aspect ratio of 12:1. For thinner
            > mirrors, the back should be supported with pads of soft polishing pitch (
            > #55 Gugolz)
            > Starman Paul
            >
            >
            > On Sat, Jun 8, 2013 at 8:39 AM, Jerry <wa4guu@...> wrote:
            >
            > > **
            > >
            > >
            > > Hello Christo,****
            > >
            > > Except for the damage you might do to the mirror surface of the glass
            > > while working the back side, it doesn't matter much which side you work
            > > first. Most (probably all) of the work on the convex side will probably be
            > > done TOT. This means the mirror side (concave) will be against some sort of
            > > support on the table.****
            > >
            > > There might be (will be) some rubbing between support and bottom side of
            > > the mirror during stroking.****
            > >
            > > I would prefer that this rubbing not be on the optical surface of the
            > > glass after rough grinding.****
            > >
            > > ** **
            > >
            > > I would rough grind both sides to the desired sag. May as well do the
            > > back first and then the front. Reduce wedge while working toward sag.****
            > >
            > > ** **
            > >
            > > I would finish all the fine grinding on the back, and polish if desired.**
            > > **
            > >
            > > ** **
            > >
            > > Then fine grind the front, reducing wedge at each grit size.****
            > >
            > > Polish and figure.****
            > >
            > > ** **
            > >
            > > Jerry****
            > >
            > > ** **
            > >
            > > ** **
            > >
            > > ** **
            > >
            > > *From:* atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] *On
            > > Behalf Of *Christo vd Merwe
            > > *Sent:* Saturday, June 08, 2013 5:55 AM
            > > *To:* atm_free@yahoogroups.com
            > > *Subject:* [atm_free] Procedure for a Meniscus mirror****
            > >
            > > ** **
            > >
            > >
            > >
            > >
            > > ****
            > >
            > > I need some advice. ****
            > >
            > > What is the best procedure making a meniscus mirror? Is starting from the
            > > front better, while the back is still flat? And then leaving polishing and
            > > figuring until the back of the mirror has been ground convex.****
            > >
            > > Or should I start on the back first?****
            > >
            > > The diameter is 225mm (just under 9") and the aim is to make it f3 for a
            > > fast Newt with coma corrector. (thanks to Mel Bartels' very convincing Zip
            > > Dob page: http://www.bbastrodesigns.com/ZipDob/ZipDob.html).****
            > >
            > > Christo****
            > >
            > > ** **
            > >
            > >
            > >
            >
          • Mark Christensen
            Christo, With a 9 inch at f/3 with a raw 36mm thick blank, unless you start thinning the blank generally, the most you ll lose in weight by convexing the back
            Message 5 of 18 , Jun 9, 2013
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              Christo,
               
              With a 9 inch at f/3 with a raw 36mm thick blank, unless you start
              thinning the blank generally, the most you'll lose in weight by convexing
              the back is less than 8%. Hardly seems worth the trouble (not to
              mention the complexity of tooling and the mirror mount for the curved
              back). And if you're trying to generally thin the blank by any material
              amount I suggest you just buy a thinner blank or live with the weight.
               
              With a 12 inch at f/3 you'll reduce the weight (assuming a 36 inch thick
              blank again) by about the same percentage, about 10%, which again
              hardly seems worth the trouble.
               
              If you do insist on doing this, I suggest you get a diamond cutter (cheap
              on Ebay from Hong Kong), make a sine table from plywood, and use a
              drill press to rough it out (as well as the concave). You'll find descriptions
              on the web and about a year or so ago in Sky and Telescope. Or send money
              and your blank to someplace like Newport Glass (it will be about $100 or so)
              and have them do it for you.
               
              By the way, 36mm is not a thin blank for a nine inch and not that thin
              for a 12 inch (1 inch is very common). Since it is recycled glass (whatever
              that really means hope you have a sense of how well the strains have been
              annealed out.
               
              Best Wishes,
               
              Mark C.
            • Roman Toledo Casabona
              Has anyone ever gotten a response back from Newport ?  Because 3 years ago when I got into the ATM madness I tried and ended just giving up. I still have not
              Message 6 of 18 , Jun 10, 2013
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                Has anyone ever gotten a response back from Newport ?  Because 3 years ago when I got into the ATM
                madness I tried and ended just giving up.

                I still have not gone back to finishing my 12" pieces, just fizzled out I guess.
              • paul valleli
                Christo, A 9-inch diameter F/3 has a FL of 27-inches and a RoC of 54-inches The sagittal Z = y^2/2R or 4.584.5/2/108 Z = 0.1875 or 3/16ths inch = 4.7mm That is
                Message 7 of 18 , Jun 11, 2013
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                  Christo,
                  A 9-inch diameter F/3 has a FL of 27-inches and a RoC of 54-inches
                  The sagittal Z = y^2/2R or 4.584.5/2/108
                   
                  Z = 0.1875 or 3/16ths inch
                  = 4.7mm 
                  That is hardly worth the effort for lightweighting unless you reduce the Central Thickness
                  to less than 3/4 inch.
                  Diamond Coring holes in the back and keeping the 36mm thickness will only reduce the weight by about
                  30% but the disk will retain most of the rigidness.
                  The most effective lightweighting method is to machine out triangular pockets leaving only thin webs.
                  This must be done very carefully on a CNC milling machine with diamond tools and coolant flow.
                  The lightest known mirror was 1.5 meters in diameter and only 10% of the weight of the original cylinder.
                  It was achieved by a combination of glass machining and chemical etching.
                  Starman Paul 


                  On Mon, Jun 10, 2013 at 11:12 PM, Roman Toledo Casabona <rtoledo2002@...> wrote:
                   

                  Has anyone ever gotten a response back from Newport ?  Because 3 years ago when I got into the ATM
                  madness I tried and ended just giving up.

                  I still have not gone back to finishing my 12" pieces, just fizzled out I guess.


                • Christo
                  Hi Paul Thanks for confirming what Mark pointed out previously. This 9 mirror will not benefit much from being made meniscus shape due to the (excessive)
                  Message 8 of 18 , Jun 12, 2013
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                    Hi Paul

                    Thanks for confirming what Mark pointed out previously. This 9" mirror will not benefit much from being made meniscus shape due to the (excessive) thickness and small diameter. So I have given up on the idea.

                    I still want to make my 12" plate glass mirror into a meniscus-shape, but the front has already been ground close to target sagitta (currently f3.4). I'm trying to figure out how to proceed. It seems that making a flat plaster base for the front is the way to go, before doing the back and then fine grinding and polishing the front.

                    Thanks
                    Christo


                    --- In atm_free@yahoogroups.com, paul valleli <paulvalleli@...> wrote:
                    >
                    > Christo,
                    > A 9-inch diameter F/3 has a FL of 27-inches and a RoC of 54-inches
                    > The sagittal Z = y^2/2R or 4.584.5/2/108
                    >
                    > Z = 0.1875 or 3/16ths inch
                    > = 4.7mm
                    > That is hardly worth the effort for lightweighting unless you reduce the
                    > Central Thickness
                    > to less than 3/4 inch.
                    > Diamond Coring holes in the back and keeping the 36mm thickness will only
                    > reduce the weight by about
                    > 30% but the disk will retain most of the rigidness.
                    > The most effective lightweighting method is to machine out triangular
                    > pockets leaving only thin webs.
                    > This must be done very carefully on a CNC milling machine with diamond
                    > tools and coolant flow.
                    > The lightest known mirror was 1.5 meters in diameter and only 10% of the
                    > weight of the original cylinder.
                    > It was achieved by a combination of glass machining and chemical etching.
                    > Starman Paul
                    >
                    >
                    > On Mon, Jun 10, 2013 at 11:12 PM, Roman Toledo Casabona <
                    > rtoledo2002@...> wrote:
                    >
                    > > **
                    > >
                    > >
                    > > Has anyone ever gotten a response back from Newport ? Because 3 years ago
                    > > when I got into the ATM
                    > > madness I tried and ended just giving up.
                    > >
                    > > I still have not gone back to finishing my 12" pieces, just fizzled out I
                    > > guess.
                    > >
                    > >
                    > >
                    >
                  • Mark Christensen
                    Go to the Newport Glass (not Optical) website and get the phone number so you can call if they don t respond to emails. I had no trouble getting their
                    Message 9 of 18 , Jun 12, 2013
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                      Go to the Newport Glass (not Optical) website and get the
                      phone number so you can call if they don't respond to emails.
                       
                      I had no trouble getting their attention when I needed
                      some curves generated last year. Just be brief and specific
                      on what you want done and what you're criteria (not end
                      use) is. After all, they are in it to make a living.
                       
                      Regards,
                       
                      Mark Christensen
                    • paul valleli
                      Christo, A mirror does not have to be a concentric meniscus. It can have a strongly convex back surface that curves to a thin edge thickness of as little as
                      Message 10 of 18 , Jun 12, 2013
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                        Christo,
                        A mirror does not have to be a concentric meniscus.
                        It can have a strongly convex back surface that curves to 
                        a thin edge thickness of as little as 1/4-inch. 
                        For a 12-inch mirror of 1.75-inch thickness, it could have a 
                        rear Radius of Curvature of 12-inches convex and a sag 
                        of 1.5-inches. 
                        These mirrors are hub, post, or center mounted like an SCT Primary.
                        With a special jig, you could grind out the curvature, leaving a 
                        conical surface and useful lightweighting of 25 to 35%.
                        Starman Paul 
                      • Christo vd Merwe
                        Paul It never occurred to me. It s a great idea and I might just do that, thanks. My calculation suggests that I can save about 30% weight by making a strongly
                        Message 11 of 18 , Jun 12, 2013
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                          Paul

                          It never occurred to me. It’s a great idea and I might just do that, thanks.

                          My calculation suggests that I can save about 30% weight by making a strongly convex curve with edge thickness of 6mm.

                          Christo

                           

                           

                           

                          From: atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] On Behalf Of paul valleli
                          Sent: 12 June 2013 11:00 PM
                          To: atm_free@yahoogroups.com
                          Subject: [atm_free] Re: Procedure for a Meniscus mirror

                           

                           

                          Christo,

                          A mirror does not have to be a concentric meniscus.

                          It can have a strongly convex back surface that curves to 

                          a thin edge thickness of as little as 1/4-inch. 

                          For a 12-inch mirror of 1.75-inch thickness, it could have a 

                          rear Radius of Curvature of 12-inches convex and a sag 

                          of 1.5-inches. 

                          These mirrors are hub, post, or center mounted like an SCT Primary.

                          With a special jig, you could grind out the curvature, leaving a 

                          conical surface and useful lightweighting of 25 to 35%.

                          Starman Paul 

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