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Collimation confusion

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  • Tom Slater
    Hi List: A simple point but I still must ask. Some collimation instructions, and comments on list, state that a collimation screw must NEVER be loosened.
    Message 1 of 5 , Feb 1, 2004
      Hi List:

      A simple point but I still must ask. Some collimation instructions, and
      comments on list, state that a collimation screw must NEVER be loosened.
      Loosening a collimation screw will start the process of the secondary
      ?removal? with the eventual result that the secondary will *fall off* inside
      the scope.

      Can we loosen a collimation screw to correct the star image skew? I'm not
      talking about loosening a screw any more than one of the collimation screws
      should be tightened. Just that 1/16 to 1/8 turn.

      Cordially,
      Tom Slater
      tslater@...
      Adamsville, Ohio USA
    • John Mahony
      The secondary swivels on a center pivot, but it s not a captive swivel. The three screws are pull screws, so generally, when you loosen one screw, the
      Message 2 of 5 , Feb 1, 2004
        The secondary swivels on a center pivot, but it's not a "captive"
        swivel. The three screws are "pull" screws, so generally, when you
        loosen one screw, the other two should be tightened, or the secondary
        will flop around and the collimation won't be stable. Conversely, if
        you tighten one screw, you may need to loosen the other two. At the
        end of each step, before testing the image, all three screws should
        be snug. But not too snug, or you can crack the outer housing, which
        is usually plastic.
        If you were to loosen all three screws a few turns, the secondary
        would fall out of its housing. But having one screw a little loose
        in the middle of the process is not dangerous.
        A common method is to do a rough collimation by either tightening or
        loosening whichever screws you need, making sure that at the end all
        three screws are lightly to moderately snug. Then fine-tune using
        only tightening moves, so that it's snug at the end. But if you need
        too many fine-tuning moves, at some point the screws will get too
        tight, and you'll need to back all three of them off a bit.
        -John


        --- In sct-user@yahoogroups.com, "Tom Slater" <tslater@e...> wrote:
        > Hi List:
        >
        > A simple point but I still must ask. Some collimation
        instructions, and
        > comments on list, state that a collimation screw must NEVER be
        loosened.
        > Loosening a collimation screw will start the process of the
        secondary
        > ?removal? with the eventual result that the secondary will *fall
        off* inside
        > the scope.
        >
        > Can we loosen a collimation screw to correct the star image skew?
        I'm not
        > talking about loosening a screw any more than one of the
        collimation screws
        > should be tightened. Just that 1/16 to 1/8 turn.
        >
        > Cordially,
        > Tom Slater
        > tslater@e...
        > Adamsville, Ohio USA
      • gstrumol
        Take a look in the Files section for a jpg I placed there a long time ago called secondary mirror.jpg (near the bottom of the list). It shows a sketch of the
        Message 3 of 5 , Feb 1, 2004
          Take a look in the Files section for a jpg I placed there a long
          time ago called "secondary mirror.jpg" (near the bottom of the
          list). It shows a sketch of the secondary mirror assembly for my C8.

          Gary


          --- In sct-user@yahoogroups.com, "John Mahony" <jmmahony@h...> wrote:
          > The secondary swivels on a center pivot, but it's not a "captive"
          > swivel. The three screws are "pull" screws, so generally, when
          you
          > loosen one screw, the other two should be tightened, or the
          secondary
          > will flop around and the collimation won't be stable. Conversely,
          if
          > you tighten one screw, you may need to loosen the other two. At
          the
          > end of each step, before testing the image, all three screws
          should
          > be snug. But not too snug, or you can crack the outer housing,
          which
          > is usually plastic.
          > If you were to loosen all three screws a few turns, the secondary
          > would fall out of its housing. But having one screw a little
          loose
          > in the middle of the process is not dangerous.
          > A common method is to do a rough collimation by either tightening
          or
          > loosening whichever screws you need, making sure that at the end
          all
          > three screws are lightly to moderately snug. Then fine-tune using
          > only tightening moves, so that it's snug at the end. But if you
          need
          > too many fine-tuning moves, at some point the screws will get too
          > tight, and you'll need to back all three of them off a bit.
          > -John
          >
          >
          > --- In sct-user@yahoogroups.com, "Tom Slater" <tslater@e...> wrote:
          > > Hi List:
          > >
          > > A simple point but I still must ask. Some collimation
          > instructions, and
          > > comments on list, state that a collimation screw must NEVER be
          > loosened.
          > > Loosening a collimation screw will start the process of the
          > secondary
          > > ?removal? with the eventual result that the secondary will *fall
          > off* inside
          > > the scope.
          > >
          > > Can we loosen a collimation screw to correct the star image
          skew?
          > I'm not
          > > talking about loosening a screw any more than one of the
          > collimation screws
          > > should be tightened. Just that 1/16 to 1/8 turn.
          > >
          > > Cordially,
          > > Tom Slater
          > > tslater@e...
          > > Adamsville, Ohio USA
        • RMOLLISE@aol.com
          In a message dated 2/1/04 3:06:24 AM Central Standard Time, tslater@ee.net writes: Can we loosen a collimation screw to correct the star image skew? I m not
          Message 4 of 5 , Feb 1, 2004
            In a message dated 2/1/04 3:06:24 AM Central Standard Time, tslater@...
            writes:
            Can we loosen a collimation screw to correct the star image skew? I'm not
            talking about loosening a screw any more than one of the collimation screws
            should be tightened. Just that 1/16 to 1/8 turn.

            Hi:

            YES.

            Peace,
            Rod Mollise
            Author of _Choosing and Using a Schmidt Cassegrain Telescope_
            Like CATs? Try SCT User, the Yahoo Group for CAT Fanciers!
            http://members.aol.com/RMOLLISE/index.html


            [Non-text portions of this message have been removed]
          • P T Chambers
            Hi Tom Dont know where you read that but it is just not true. It is true that there needs to be tension on all of the screws since they have to pull the
            Message 5 of 5 , Feb 1, 2004
              Hi Tom
              Dont know where you read that but it is just not true. It is true that
              there needs to be tension on all of the screws since they have to "pull"
              the secondary back to a pivot point.

              But for collimation, if you have to loosen a screw, do it. Just remember
              that you have to tighten the other 2 a corresponding amount so that the
              tension stays even.

              If you keep tightening, the plastic housing will break.

              On most of the smaller scts (11 inch and smaller), the three screws are
              through the plastic holder and threaded into 3 holes in the aluminum back
              that is glued to the secondary mirror.

              So loosening is just as much a part of collimation as tightening with the
              caveat that you have to keep tension on the screws. So do it in small
              steps.

              Hope this helps
              ---------
              Phil Chambers [ptchamb@...] (S.F. Bay Area - Calif. USA)

              On Sun, 1 Feb 2004, Tom Slater wrote:

              > Hi List:
              >
              > A simple point but I still must ask. Some collimation instructions, and
              > comments on list, state that a collimation screw must NEVER be loosened.
              > Loosening a collimation screw will start the process of the secondary
              > ?removal? with the eventual result that the secondary will *fall off* inside
              > the scope.
              >
              > Can we loosen a collimation screw to correct the star image skew? I'm not
              > talking about loosening a screw any more than one of the collimation screws
              > should be tightened. Just that 1/16 to 1/8 turn.
              >
              > Cordially,
              > Tom Slater
              > tslater@...
              > Adamsville, Ohio USA
              >
              >
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