Loading ...
Sorry, an error occurred while loading the content.

Re: More help needed with Astigmatism

Expand Messages
  • Ken Hunter
    ... on ... What is your setup? Is the turntable driven or do you walk around it? I m trying to imagine why you would rotate a turntable every 10 strokes unless
    Message 1 of 22 , Apr 1, 2004
    • 0 Attachment
      --- In atm_free@yahoogroups.com, "Craig V" <cbvkrv@c...> wrote:
      > I am working with a manual turntable. Every ten strokes, I rotate
      > the turntable one way and rotate the tool or mirror (whatever is
      on
      > top) the opposite way. I am doing what is shown on the Stellafane
      > site. Maybe I need to rotate more??

      What is your setup? Is the turntable driven or do you walk around
      it? I'm trying to imagine why you would rotate a turntable every 10
      strokes unless you were restricted to standing in one area.

      If that was the case, I'd make a few strokes, step sideways a bit
      and rotate the top disk, make some more strokes, step sideways and
      rotate the top disk etc until I got to the limit of my travel then
      rotate the bottom disk and start the dance over. I can't figure
      where a turntable comes into play here...

      Ken Hunter
    • Bob May
      How much is the turntable rotating? If you re doing some large value like 90deg., you may be grinding astigmatism into the mirror. The old method was to walk
      Message 2 of 22 , Apr 1, 2004
      • 0 Attachment
        How much is the turntable rotating? If you're doing some large value like
        90deg., you may be grinding astigmatism into the mirror.
        The old method was to walk in a slow shuffle about the post which means that
        the direction of the motion changes by about 10 deg. or so for each stroke
        and the mirror is being moved by about the same amount with each stroke.
        The strokes need to be somewhat random and in all of the different
        directions, period. Repeating a direction of a stroke allows for the
        creating of a pattern onto the surfaces. Automatic machines can and have
        made bad optics because the operator originally setup the machine so that
        the strokes and the rotation matched and that produces repeatable features
        on the surfaces and you thus get poor optical surfaces.
        Bob May
        http://nav.to/bobmay
        bobmay@...
        NEW! http://bobmay.astronomy.net
      • Craig V
        I have a manual turntable mounted on a table top. I typically sit, or stand in one spot. If I m polishing MOT, I put a piece of tape to mark my starting
        Message 3 of 22 , Apr 1, 2004
        • 0 Attachment
          I have a manual turntable mounted on a table top. I typically sit,
          or stand in one spot. If I'm polishing MOT, I put a piece of tape to
          mark my starting point. I will do about 10 strokes, then roate the
          turntable about a quarter turn clockwise (the tool rotates with the
          turntable), then I will rotate the mirror a little more than a
          quarter turn counter clockwise. If I am doing something like a W
          stroke, the total number of strokes could be more or less than 10. I
          keep doing this, stopping at 8 positions as marked by the tape on top
          of the mirror, and stop before the tape returns to the starting
          point. I call that one time around. For the next time around, I move
          the tape mark to a random location, then rotate the turntable so that
          the tape mark is at 12:00 and start again.

          When I polish TOT, I do something similar only this time I mark the
          turntable starting position. Every once in a while, I may rotate the
          tool more than 1/4 turn.

          That's what I'm doing. I may try to fix the position of the
          turntable and start walking around the table??

          Craig
        • Richard Schwartz
          I am not going to tell you how to measure astigmatism; I have posted plenty on this before, and it is mostly ignored. I don t think the popular strehl
          Message 4 of 22 , Apr 1, 2004
          • 0 Attachment
            I am not going to tell you how to measure astigmatism; I have posted plenty
            on this before, and it is mostly ignored. I don't think the popular
            strehl software considers astigmatism because it is mostly a problem with
            non-traditional thin mirror blanks.

            To CORRECT astigmatism, I did it on a thin mirror by polishing mirror on
            top of a full size pitch lap, pushing on the mirror only at its center so
            as to avoid any bending forces on the glass. I did plenty of cold
            pressing with no additional weight that might bend the glass.

            . . . Richard
          • Richard Schwartz
            You need to turn the mirror CONTINUOUSLY. You don t need to turn the tool. ... From: To: atm_free@yahoogroups.com Sent: 3/31/2004 2:26:23 AM Subject: Re:
            Message 5 of 22 , Apr 1, 2004
            • 0 Attachment
              You need to turn the mirror CONTINUOUSLY.   You don't need to turn the tool.
               
               
              ----- Original Message -----
              From:
              Sent: 3/31/2004 2:26:23 AM
              Subject: Re: [atm_free] Re: More help needed with Astigmatism

              Craig,
              It sounds to me like one must turn the tool or mirror almost every stroke, from the replies you are receiving. I did not read any info anywhere before where constant tool or mirror rotation was necessary.
                             John
            • Ken Hunter
              Craig... If you are NOT ROTATING THE DISK IN RELATION TO THE TURNTABLE, you are grinding/polishing in whatever astigmatism is present in the turntable. Ditch
              Message 6 of 22 , Apr 1, 2004
              • 0 Attachment
                Craig...

                If you are NOT ROTATING THE DISK IN RELATION TO THE TURNTABLE, you
                are grinding/polishing in whatever astigmatism is present in the
                turntable.

                Ditch the turntable, put some cleats on the table (unless it's your
                wife's favorite grandmother's heirloom) and do what you've been doing
                together with the "waltz" around the table...

                One hint... Make the cleats (3 of them) 3/8 inch (or so) high on a
                circle 1/4 inch larger than the mirror/tool disk and use wooden
                wedges (from wooden clothespins if you can still find them) to hold
                the mirror/tool.

                Do NOT drive them in so tight that you distort the mirror/tool. Just
                tight enough to keep the mirror/tool from moving back and forth with
                the strokes. If they rotate a bit, that's OK too, you were going to
                rotate them every 3-4 minutes anyway!

                Ken Hunter





                --- In atm_free@yahoogroups.com, "Craig V" <cbvkrv@c...> wrote:
                >
                > I have a manual turntable mounted on a table top. I typically sit,
                > or stand in one spot. If I'm polishing MOT, I put a piece of tape
                to
                > mark my starting point. I will do about 10 strokes, then roate the
                > turntable about a quarter turn clockwise (the tool rotates with the
                > turntable), then I will rotate the mirror a little more than a
                > quarter turn counter clockwise. If I am doing something like a W
                > stroke, the total number of strokes could be more or less than 10.
                I
                > keep doing this, stopping at 8 positions as marked by the tape on
                top
                > of the mirror, and stop before the tape returns to the starting
                > point. I call that one time around. For the next time around, I
                move
                > the tape mark to a random location, then rotate the turntable so
                that
                > the tape mark is at 12:00 and start again.
                >
                > When I polish TOT, I do something similar only this time I mark the
                > turntable starting position. Every once in a while, I may rotate
                the
                > tool more than 1/4 turn.
                >
                > That's what I'm doing. I may try to fix the position of the
                > turntable and start walking around the table??
                >
                > Craig
              • Craig V
                Thanks Richard. I read your previous posting on measureing astigmatism and did something similar by using a mask with 2 sets of cutouts 90 degrees apart,
                Message 7 of 22 , Apr 1, 2004
                • 0 Attachment
                  Thanks Richard. I read your previous posting on measureing
                  astigmatism and did something similar by using a mask with 2 sets of
                  cutouts 90 degrees apart, doing measurements, then rotating the mask
                  by 45 degrees. I tried your suggestion MOT pushing from the center,
                  but I need to push from the edges to get it to move.

                  Craig


                  --- In atm_free@yahoogroups.com, "Richard Schwartz" <richas@e...>
                  wrote:
                  > I am not going to tell you how to measure astigmatism; I have
                  posted plenty
                  > on this before, and it is mostly ignored. I don't think the
                  popular
                  > strehl software considers astigmatism because it is mostly a
                  problem with
                  > non-traditional thin mirror blanks.
                  >
                  > To CORRECT astigmatism, I did it on a thin mirror by polishing
                  mirror on
                  > top of a full size pitch lap, pushing on the mirror only at its
                  center so
                  > as to avoid any bending forces on the glass. I did plenty of cold
                  > pressing with no additional weight that might bend the glass.
                  >
                  > . . . Richard
                • Craig V
                  Ken, thanks for the advice. I locked down the turntable so that it won t turn (it already has cleats) and started walking around the table. I did a TOT COC
                  Message 8 of 22 , Apr 1, 2004
                  • 0 Attachment
                    Ken,

                    thanks for the advice. I locked down the turntable so that it won't
                    turn (it already has cleats) and started walking around the table. I
                    did a TOT COC stroke and I can see that this technique is a lot more
                    random than my turntable technique. It took about 25-30 strokes to
                    make it all the way around. It feels a little strange now and I am
                    not used to rotating the tool yet, but I think this has
                    possibilities.

                    The W strokes will really be strange. When I was working with the
                    turntable, a W was really a W since I was not moving. Now if I do a
                    W while walking around, who knows what letter it will turn into. I
                    will give it a shot because what I was doing before was not working
                    for me.

                    thanks agian

                    Craig

                    --- In atm_free@yahoogroups.com, "Ken Hunter" <atm_ken_hunter@y...>
                    wrote:
                    > Craig...
                    >
                    > If you are NOT ROTATING THE DISK IN RELATION TO THE TURNTABLE, you
                    > are grinding/polishing in whatever astigmatism is present in the
                    > turntable.
                    >
                    > Ditch the turntable, put some cleats on the table (unless it's your
                    > wife's favorite grandmother's heirloom) and do what you've been
                    doing
                    > together with the "waltz" around the table...
                    >
                    > One hint... Make the cleats (3 of them) 3/8 inch (or so) high on a
                    > circle 1/4 inch larger than the mirror/tool disk and use wooden
                    > wedges (from wooden clothespins if you can still find them) to hold
                    > the mirror/tool.
                    >
                    > Do NOT drive them in so tight that you distort the mirror/tool.
                    Just
                    > tight enough to keep the mirror/tool from moving back and forth
                    with
                    > the strokes. If they rotate a bit, that's OK too, you were going to
                    > rotate them every 3-4 minutes anyway!
                    >
                    > Ken Hunter
                    >
                    >
                    >
                    >
                    >
                    > --- In atm_free@yahoogroups.com, "Craig V" <cbvkrv@c...> wrote:
                    > >
                    > > I have a manual turntable mounted on a table top. I typically
                    sit,
                    > > or stand in one spot. If I'm polishing MOT, I put a piece of
                    tape
                    > to
                    > > mark my starting point. I will do about 10 strokes, then roate
                    the
                    > > turntable about a quarter turn clockwise (the tool rotates with
                    the
                    > > turntable), then I will rotate the mirror a little more than a
                    > > quarter turn counter clockwise. If I am doing something like a W
                    > > stroke, the total number of strokes could be more or less than
                    10.
                    > I
                    > > keep doing this, stopping at 8 positions as marked by the tape on
                    > top
                    > > of the mirror, and stop before the tape returns to the starting
                    > > point. I call that one time around. For the next time around, I
                    > move
                    > > the tape mark to a random location, then rotate the turntable so
                    > that
                    > > the tape mark is at 12:00 and start again.
                    > >
                    > > When I polish TOT, I do something similar only this time I mark
                    the
                    > > turntable starting position. Every once in a while, I may rotate
                    > the
                    > > tool more than 1/4 turn.
                    > >
                    > > That's what I'm doing. I may try to fix the position of the
                    > > turntable and start walking around the table??
                    > >
                    > > Craig
                  • brock.family
                    Craig wrote:- ... I ve been doing W strokes without moving on all my mirrors with no problems. I do a W with about 5 forward and 5 backward strokes then take
                    Message 9 of 22 , Apr 1, 2004
                    • 0 Attachment
                      Craig wrote:-
                      > The W strokes will really be strange. When I was working with the
                      > turntable, a W was really a W since I was not moving. Now if I do a
                      > W while walking around, who knows what letter it will turn into.

                      I've been doing W strokes without moving on all my mirrors with no
                      problems. I do a W with about 5 forward and 5 backward strokes then take a
                      small step anticlockwise and rotate the top glass/lap/mirror a small amount
                      clockwise and repeat etc. I have not had any problems with astigmatism. This
                      is when smooth grinding and general polishing. Also, when doing certain
                      corrective strokes (eg accentuated pressure using the edge of the lap to
                      reduce a high zone ) I can't see how continually moving can give the result
                      required. I do however move continuously when using a small lap with a
                      circular stroke on a high zone, or a sub-diameter lap and long stroke for
                      general parabolising.
                      Just my 2c worth.

                      Dave Brock.
                    • Richard Schwartz
                      By working TOT and not rotating the mirror on its platform, you are inviting astigmatism.
                      Message 10 of 22 , Apr 2, 2004
                      • 0 Attachment
                        By working TOT and not rotating the mirror on its platform, you are
                        inviting astigmatism.


                        > [Original Message]
                        > From: Craig V <cbvkrv@...>
                        > To: <atm_free@yahoogroups.com>
                        > Date: 4/1/2004 7:58:16 PM
                        > Subject: [atm_free] Re: More help needed with Astigmatism
                        >
                        > Ken,
                        >
                        > thanks for the advice. I locked down the turntable so that it won't
                        > turn (it already has cleats) and started walking around the table. I
                        > did a TOT COC stroke and I can see that this technique is a lot more
                        > random than my turntable technique. It took about 25-30 strokes to
                        > make it all the way around. It feels a little strange now and I am
                        > not used to rotating the tool yet, but I think this has
                        > possibilities.
                        >
                        > The W strokes will really be strange. When I was working with the
                        > turntable, a W was really a W since I was not moving. Now if I do a
                        > W while walking around, who knows what letter it will turn into. I
                        > will give it a shot because what I was doing before was not working
                        > for me.
                        >
                        > thanks agian
                        >
                        > Craig
                        >
                        > --- In atm_free@yahoogroups.com, "Ken Hunter" <atm_ken_hunter@y...>
                        > wrote:
                        > > Craig...
                        > >
                        > > If you are NOT ROTATING THE DISK IN RELATION TO THE TURNTABLE, you
                        > > are grinding/polishing in whatever astigmatism is present in the
                        > > turntable.
                        > >
                        > > Ditch the turntable, put some cleats on the table (unless it's your
                        > > wife's favorite grandmother's heirloom) and do what you've been
                        > doing
                        > > together with the "waltz" around the table...
                        > >
                        > > One hint... Make the cleats (3 of them) 3/8 inch (or so) high on a
                        > > circle 1/4 inch larger than the mirror/tool disk and use wooden
                        > > wedges (from wooden clothespins if you can still find them) to hold
                        > > the mirror/tool.
                        > >
                        > > Do NOT drive them in so tight that you distort the mirror/tool.
                        > Just
                        > > tight enough to keep the mirror/tool from moving back and forth
                        > with
                        > > the strokes. If they rotate a bit, that's OK too, you were going to
                        > > rotate them every 3-4 minutes anyway!
                        > >
                        > > Ken Hunter
                        > >
                        > >
                        > >
                        > >
                        > >
                        > > --- In atm_free@yahoogroups.com, "Craig V" <cbvkrv@c...> wrote:
                        > > >
                        > > > I have a manual turntable mounted on a table top. I typically
                        > sit,
                        > > > or stand in one spot. If I'm polishing MOT, I put a piece of
                        > tape
                        > > to
                        > > > mark my starting point. I will do about 10 strokes, then roate
                        > the
                        > > > turntable about a quarter turn clockwise (the tool rotates with
                        > the
                        > > > turntable), then I will rotate the mirror a little more than a
                        > > > quarter turn counter clockwise. If I am doing something like a W
                        > > > stroke, the total number of strokes could be more or less than
                        > 10.
                        > > I
                        > > > keep doing this, stopping at 8 positions as marked by the tape on
                        > > top
                        > > > of the mirror, and stop before the tape returns to the starting
                        > > > point. I call that one time around. For the next time around, I
                        > > move
                        > > > the tape mark to a random location, then rotate the turntable so
                        > > that
                        > > > the tape mark is at 12:00 and start again.
                        > > >
                        > > > When I polish TOT, I do something similar only this time I mark
                        > the
                        > > > turntable starting position. Every once in a while, I may rotate
                        > > the
                        > > > tool more than 1/4 turn.
                        > > >
                        > > > That's what I'm doing. I may try to fix the position of the
                        > > > turntable and start walking around the table??
                        > > >
                        > > > Craig
                        >
                        >
                        >
                        >
                        > Yahoo! Groups Links
                        >
                        >
                        >
                        >
                      • Richard Schwartz
                        It is ok to push on the edges to help things move. Just don t exert any vertical force there. Try to visualize that you are polishing a piece of saran wrap
                        Message 11 of 22 , Apr 2, 2004
                        • 0 Attachment
                          It is ok to push on the edges to help things move. Just don't exert any
                          vertical force there. Try to visualize that you are polishing a piece of
                          saran wrap and that you have to be extra careful not to bend it over the
                          edge of the pitch lap. Alternatives exist: (1) attach a pushing knob
                          to the back center of the mirror with pitch, (2) attach a pushing fixture
                          to the back of the mirror with wire coat hangar hooks that go over the
                          mirror's edge in at least five places to apply horizontal force.

                          Most important is to learn by doing. You will soon discover that many on
                          the ATM list have no knowledge of surface correction beyond what they have
                          learned in books (books they believe in); they have not actually figured a
                          mirror. John Dobson has said that there is no mistake you can make that
                          cannot be undone. Even if you smash the glass, there is another piece out
                          there somewhere with your name on it. (Dobson recommends that if you
                          smash the glass, you do so during rough grinding, not during final
                          figuring!) And Richard preaches that the glass KNOWS it all; cast aside
                          what is written in authoritative books, open your mind, and let the glass
                          teach you.

                          . . . Richard


                          > [Original Message]
                          > From: Craig V <cbvkrv@...>
                          > To: <atm_free@yahoogroups.com>
                          > Date: 4/1/2004 7:48:33 PM
                          > Subject: [atm_free] Re: More help needed with Astigmatism
                          >
                          > Thanks Richard. I read your previous posting on measureing
                          > astigmatism and did something similar by using a mask with 2 sets of
                          > cutouts 90 degrees apart, doing measurements, then rotating the mask
                          > by 45 degrees. I tried your suggestion MOT pushing from the center,
                          > but I need to push from the edges to get it to move.
                          >
                          > Craig
                          >
                          >
                          > --- In atm_free@yahoogroups.com, "Richard Schwartz" <richas@e...>
                          > wrote:
                          > > I am not going to tell you how to measure astigmatism; I have
                          > posted plenty
                          > > on this before, and it is mostly ignored. I don't think the
                          > popular
                          > > strehl software considers astigmatism because it is mostly a
                          > problem with
                          > > non-traditional thin mirror blanks.
                          > >
                          > > To CORRECT astigmatism, I did it on a thin mirror by polishing
                          > mirror on
                          > > top of a full size pitch lap, pushing on the mirror only at its
                          > center so
                          > > as to avoid any bending forces on the glass. I did plenty of cold
                          > > pressing with no additional weight that might bend the glass.
                          > >
                          > > . . . Richard
                          >
                          >
                          >
                          >
                          >
                          > Yahoo! Groups Links
                          >
                          >
                          >
                          >
                        • Bob May
                          Sounds like the near perfect receipie for astigmatism. 90 deg. plus 90 deg. equals 180 deg.! You want to do only about 5-10 degrees of rotation to emulate a
                          Message 12 of 22 , Apr 2, 2004
                          • 0 Attachment
                            Sounds like the near perfect receipie for astigmatism. 90 deg. plus 90 deg.
                            equals 180 deg.! You want to do only about 5-10 degrees of rotation to
                            emulate a slow walk about the post as was done in the old days.
                            Bob May
                            http://nav.to/bobmay
                            bobmay@...
                            NEW! http://bobmay.astronomy.net
                          • Ken Hunter
                            ... wrote: cast aside what is written in authoritative books, open your mind, and let the glass teach you. ... Be one with the force young Skywalker....
                            Message 13 of 22 , Apr 2, 2004
                            • 0 Attachment
                              --- In atm_free@yahoogroups.com, "Richard Schwartz" <richas@e...>
                              wrote:
                              cast aside what is written in authoritative books, open your mind,
                              and let the glass teach you.
                              >
                              > . . . Richard

                              Be one with the force young Skywalker.... errr... atm'er

                              Ken Hunter
                            • Peter
                              Richard, How thin and how large was your thin mirror? ... From: Richard Schwartz [mailto:richas@earthlink.net] Sent: Thursday, April 01, 2004 7:58 PM To:
                              Message 14 of 22 , Apr 3, 2004
                              • 0 Attachment
                                Richard,
                                 
                                How thin and how large was your thin mirror?
                                -----Original Message-----
                                From: Richard Schwartz [mailto:richas@...]
                                Sent: Thursday, April 01, 2004 7:58 PM
                                To: atm_free@yahoogroups.com
                                Subject: RE: [atm_free] More help needed with Astigmatism

                                I am not going to tell you how to measure astigmatism; I have posted plenty
                                on this before, and it is mostly ignored.   I don't think the popular
                                strehl software considers astigmatism because it is mostly a problem with
                                non-traditional thin mirror blanks.

                                To CORRECT astigmatism, I did it on a thin mirror by polishing mirror on
                                top of a full size pitch lap, pushing on the mirror only at its center so
                                as to avoid any bending forces on the glass.   I did plenty of cold
                                pressing with no additional weight that might bend the glass.

                                . . . Richard


                              • Richard Schwartz
                                That mirror was 8 x 3.8 thick, curve formed by slumping. ... From: Peter To: atm_free@yahoogroups.com Sent: 4/3/2004 5:01:21 AM Subject: RE: [atm_free] More
                                Message 15 of 22 , Apr 3, 2004
                                • 0 Attachment
                                  That mirror was 8" x 3.8" thick, curve formed by slumping.
                                   
                                   
                                  ----- Original Message -----
                                  From: Peter
                                  Sent: 4/3/2004 5:01:21 AM
                                  Subject: RE: [atm_free] More help needed with Astigmatism

                                  Richard,
                                   
                                  How thin and how large was your thin mirror?
                                  -----Original Message-----
                                  From: Richard Schwartz [mailto:richas@...]
                                  Sent: Thursday, April 01, 2004 7:58 PM
                                  To: atm_free@yahoogroups.com
                                  Subject: RE: [atm_free] More help needed with Astigmatism

                                  I am not going to tell you how to measure astigmatism; I have posted plenty
                                  on this before, and it is mostly ignored.   I don't think the popular
                                  strehl software considers astigmatism because it is mostly a problem with
                                  non-traditional thin mirror blanks.

                                  To CORRECT astigmatism, I did it on a thin mirror by polishing mirror on
                                  top of a full size pitch lap, pushing on the mirror only at its center so
                                  as to avoid any bending forces on the glass.   I did plenty of cold
                                  pressing with no additional weight that might bend the glass.

                                  . . . Richard


                                Your message has been successfully submitted and would be delivered to recipients shortly.