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Re: [sct-user] Is it normal?

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  • David Cochran
    Thanks Chris and Phil! It s not terrible, the movement is slight and as soon as I detect it i go the other way and go back to the direction I was going at it
    Message 1 of 13 , Sep 2, 2008
      Thanks Chris and Phil!

      It's not terrible, the movement is slight and as soon as I detect it i go the other way and go back to the direction I was going at it seems fine. I'm used to the Crayford more than anything.

      Have any of you used a crayford on a SCT? How is the experience?

      peace

      David



      ----- Original Message ----
      From: P T Chambers <ptchamb@...>
      To: sct-user@yahoogroups.com
      Sent: Tuesday, September 2, 2008 10:18:31 PM
      Subject: Re: [sct-user] Is it normal?


      Hi
      Most SCTs focus by moving the primary mirror back and forth. It is
      mounted to what I call, a "slider" and it moves back and forth on the
      primary baffle.

      So their has to be a bit of clearence between the two parts.

      Now, notice that the focus knob is offset from the center of the scope.
      It is connected to the slider and it pushes and pulls the primary back and
      forth.

      Because it is offset, and there must be some clearence for the parts to
      move, the primary will wobble a little bit as you change directions.
      Called "focus shift". If you set down with a calculator and figure how
      much shift you are seeing (probably a few arc seconds) and back calculate
      how much clearance you must have to get that, you will find it is small,
      indeed.

      The effect is minimized by always finishing your focusing by turning the
      knob so that the primary moves "uphill". This requires a bit of getting
      used to but will quickly become second nature.

      ---------
      Phil Chambers [ptchamb-at- svpal.org] (S.F. Bay Area - Calif. USA)

      On Wed, 3 Sep 2008, David Cochran wrote:

      > Hi all
      >
      > Is it normal to have an object move inside the eyepiece while focusing?
      >
      > It seems that at some points the mirror shifts.
      >
      > peace
      >
      > David
      >
      >
      > ------------ --------- --------- ------
      >
      > Visit the sct-user home page at:
      >
      >
      >
      > http://members. aol.com/RMOLLISE /index4.htmlYaho o! Groups Links
      >
      >
      >
      >





      [Non-text portions of this message have been removed]
    • P T Chambers
      Hi Crayfords are pretty standard with imagers. Even I use one when using a webcam. No focus shift is nice under those conditions when your chip is so small
      Message 2 of 13 , Sep 2, 2008
        Hi
        Crayfords are pretty standard with imagers. Even I use one when using a
        webcam. No focus shift is nice under those conditions when your chip is
        so small that the object -does- leave the field of view as with a webcam.

        But for visual, the "always focus uphill" is very repeatable. The
        reason of course, is that gravity is pulling the primary toward you (I do
        assume you are looking up :-) and when you quit moving the knob the
        primary is in a stable position and does not continue to move. If you
        reach focus moving "downhill" then the primary will shift toward you until
        the play is taken up. It sometimes happens slowly so your apparent focus
        moves out not to mention the shift occurs then too.

        So for visual, they really are not necessary.

        ---------
        Phil Chambers [ptchamb-at-svpal.org] (S.F. Bay Area - Calif. USA)

        On Tue, 2 Sep 2008, David Cochran wrote:

        > Thanks Chris and Phil!
        >
        > It's not terrible, the movement is slight and as soon as I detect it i go the other way and go back to the direction I was going at it seems fine. I'm used to the Crayford more than anything.
        >
        > Have any of you used a crayford on a SCT? How is the experience?
        >
        > peace
        >
        > David
        >
        >
        >
        > ----- Original Message ----
        > From: P T Chambers <ptchamb@...>
        > To: sct-user@yahoogroups.com
        > Sent: Tuesday, September 2, 2008 10:18:31 PM
        > Subject: Re: [sct-user] Is it normal?
        >
        >
        > Hi
        > Most SCTs focus by moving the primary mirror back and forth. It is
        > mounted to what I call, a "slider" and it moves back and forth on the
        > primary baffle.
        >
        > So their has to be a bit of clearence between the two parts.
        >
        > Now, notice that the focus knob is offset from the center of the scope.
        > It is connected to the slider and it pushes and pulls the primary back and
        > forth.
        >
        > Because it is offset, and there must be some clearence for the parts to
        > move, the primary will wobble a little bit as you change directions.
        > Called "focus shift". If you set down with a calculator and figure how
        > much shift you are seeing (probably a few arc seconds) and back calculate
        > how much clearance you must have to get that, you will find it is small,
        > indeed.
        >
        > The effect is minimized by always finishing your focusing by turning the
        > knob so that the primary moves "uphill". This requires a bit of getting
        > used to but will quickly become second nature.
        >
        > ---------
        > Phil Chambers [ptchamb-at- svpal.org] (S.F. Bay Area - Calif. USA)
        >
        > On Wed, 3 Sep 2008, David Cochran wrote:
        >
        >> Hi all
        >>
        >> Is it normal to have an object move inside the eyepiece while focusing?
        >>
        >> It seems that at some points the mirror shifts.
        >>
        >> peace
        >>
        >> David
        >>
        >>
        >> ------------ --------- --------- ------
        >>
        >> Visit the sct-user home page at:
        >>
        >>
        >>
        >> http://members. aol.com/RMOLLISE /index4.htmlYaho o! Groups Links
        >>
        >>
        >>
        >>
        >
        >
        >
        >
        >
        > [Non-text portions of this message have been removed]
        >
        >
        > ------------------------------------
        >
        > Visit the sct-user home page at:
        >
        >
        >
        > http://members.aol.com/RMOLLISE/index4.htmlYahoo! Groups Links
        >
        >
        >
        >
      • Chris Peterson
        I like a Crayford for visual use. For imaging, I prefer to remove any external focuser and use the primary focus knob (motorized, of course). Chris
        Message 3 of 13 , Sep 2, 2008
          I like a Crayford for visual use. For imaging, I prefer to remove any
          external focuser and use the primary focus knob (motorized, of course).

          Chris

          *****************************************
          Chris L Peterson
          Cloudbait Observatory
          http://www.cloudbait.com


          ----- Original Message -----
          From: "David Cochran" <davidcochran@...>
          To: <sct-user@yahoogroups.com>
          Sent: Tuesday, September 02, 2008 9:43 PM
          Subject: Re: [sct-user] Is it normal?


          > Thanks Chris and Phil!
          >
          > It's not terrible, the movement is slight and as soon as I detect it i go
          > the other way and go back to the direction I was going at it seems fine.
          > I'm used to the Crayford more than anything.
          >
          > Have any of you used a crayford on a SCT? How is the experience?
          >
          > peace
          >
          > David
        • Chris Peterson
          More generally, it should be focus against the normal force . Some SCTs (the 12 Meades, at least), use a spring between the mirror cell and back plate. With
          Message 4 of 13 , Sep 2, 2008
            More generally, it should be "focus against the normal force". Some SCTs
            (the 12" Meades, at least), use a spring between the mirror cell and back
            plate. With these scopes you need to finish the focus downhill.

            BTW, I agree that a Crayford isn't necessary. I just find them a bit more
            comfortable to use for visual. For imaging I like using the primary better
            because it puts the sensor in an optically better position, and because a
            short equipment train allows the camera to clear the base and also performs
            better mechanically because of the reduced moment of inertia. Focus shift is
            a non-issue when you use a motorized focuser on the primary, and whether
            mirror flop is a problem depends on the individual scope, as well as whether
            you are guiding through the main optics or not.

            Clearly, though, the choice of focuser type is more about individual
            preference than anything else.

            Chris

            *****************************************
            Chris L Peterson
            Cloudbait Observatory
            http://www.cloudbait.com


            ----- Original Message -----
            From: "P T Chambers" <ptchamb@...>
            To: <sct-user@yahoogroups.com>
            Sent: Tuesday, September 02, 2008 9:55 PM
            Subject: Re: [sct-user] Is it normal?


            > Hi
            > Crayfords are pretty standard with imagers. Even I use one when using a
            > webcam. No focus shift is nice under those conditions when your chip is
            > so small that the object -does- leave the field of view as with a webcam.
            >
            > But for visual, the "always focus uphill" is very repeatable. The
            > reason of course, is that gravity is pulling the primary toward you (I do
            > assume you are looking up :-) and when you quit moving the knob the
            > primary is in a stable position and does not continue to move. If you
            > reach focus moving "downhill" then the primary will shift toward you until
            > the play is taken up. It sometimes happens slowly so your apparent focus
            > moves out not to mention the shift occurs then too.
            >
            > So for visual, they really are not necessary.
          • P T Chambers
            Of course Chris just replied that he doesnt use one.... LOL. One of issues is backfocus distance. Not only does it move the scope away from its design
            Message 5 of 13 , Sep 2, 2008
              Of course Chris just replied that he doesnt use one.... LOL.

              One of issues is backfocus distance. Not only does it move the scope
              away from its design optimum primary-corrector distance and introduce some
              spherical abberations, it also causes imagers to worry about all the stuff
              hanging on the back of the scope. You have the camera, filter wheels,
              possibly a prism pick off for guiding, splitters for autofocus devices,
              and various and sundry assorted adapters.

              This causes the backfocus issue in spades. Also the new cameras have
              large chips so focus shift is not as much of an issue.

              ---------
              Phil Chambers [ptchamb-at-svpal.org] (S.F. Bay Area - Calif. USA)

              On Tue, 2 Sep 2008, P T Chambers wrote:

              > Hi
              > Crayfords are pretty standard with imagers. Even I use one when using a
              > webcam. No focus shift is nice under those conditions when your chip is
              > so small that the object -does- leave the field of view as with a webcam.
              >
              > But for visual, the "always focus uphill" is very repeatable. The
              > reason of course, is that gravity is pulling the primary toward you (I do
              > assume you are looking up :-) and when you quit moving the knob the
              > primary is in a stable position and does not continue to move. If you
              > reach focus moving "downhill" then the primary will shift toward you until
              > the play is taken up. It sometimes happens slowly so your apparent focus
              > moves out not to mention the shift occurs then too.
              >
              > So for visual, they really are not necessary.
              >
              > ---------
              > Phil Chambers [ptchamb-at-svpal.org] (S.F. Bay Area - Calif. USA)
              >
              > On Tue, 2 Sep 2008, David Cochran wrote:
              >
              >> Thanks Chris and Phil!
              >>
              >> It's not terrible, the movement is slight and as soon as I detect it i go the other way and go back to the direction I was going at it seems fine. I'm used to the Crayford more than anything.
              >>
              >> Have any of you used a crayford on a SCT? How is the experience?
              >>
              >> peace
              >>
              >> David
              >>
              >>
              >>
              >> ----- Original Message ----
              >> From: P T Chambers <ptchamb@...>
              >> To: sct-user@yahoogroups.com
              >> Sent: Tuesday, September 2, 2008 10:18:31 PM
              >> Subject: Re: [sct-user] Is it normal?
              >>
              >>
              >> Hi
              >> Most SCTs focus by moving the primary mirror back and forth. It is
              >> mounted to what I call, a "slider" and it moves back and forth on the
              >> primary baffle.
              >>
              >> So their has to be a bit of clearence between the two parts.
              >>
              >> Now, notice that the focus knob is offset from the center of the scope.
              >> It is connected to the slider and it pushes and pulls the primary back and
              >> forth.
              >>
              >> Because it is offset, and there must be some clearence for the parts to
              >> move, the primary will wobble a little bit as you change directions.
              >> Called "focus shift". If you set down with a calculator and figure how
              >> much shift you are seeing (probably a few arc seconds) and back calculate
              >> how much clearance you must have to get that, you will find it is small,
              >> indeed.
              >>
              >> The effect is minimized by always finishing your focusing by turning the
              >> knob so that the primary moves "uphill". This requires a bit of getting
              >> used to but will quickly become second nature.
              >>
              >> ---------
              >> Phil Chambers [ptchamb-at- svpal.org] (S.F. Bay Area - Calif. USA)
              >>
              >> On Wed, 3 Sep 2008, David Cochran wrote:
              >>
              >>> Hi all
              >>>
              >>> Is it normal to have an object move inside the eyepiece while focusing?
              >>>
              >>> It seems that at some points the mirror shifts.
              >>>
              >>> peace
              >>>
              >>> David
              >>>
              >>>
              >>> ------------ --------- --------- ------
              >>>
              >>> Visit the sct-user home page at:
              >>>
              >>>
              >>>
              >>> http://members. aol.com/RMOLLISE /index4.htmlYaho o! Groups Links
              >>>
              >>>
              >>>
              >>>
              >>
              >>
              >>
              >>
              >>
              >> [Non-text portions of this message have been removed]
              >>
              >>
              >> ------------------------------------
              >>
              >> Visit the sct-user home page at:
              >>
              >>
              >>
              >> http://members.aol.com/RMOLLISE/index4.htmlYahoo! Groups Links
              >>
              >>
              >>
              >>
              >
              > ------------------------------------
              >
              > Visit the sct-user home page at:
              >
              >
              >
              > http://members.aol.com/RMOLLISE/index4.htmlYahoo! Groups Links
              >
              >
              >
              >
            • John Mahony
              ... That depends on the model/year. On my first SCT, an orange tube C8 in 1980, the image shift would easily move an object out of a high power view. But
              Message 6 of 13 , Sep 3, 2008
                ----- Original Message ----

                > From: Chris Peterson <cpeterson@...>
                >
                > This happens with all SCTs. But it shouldn't move enough to be a problem
                > (that is, even at the highest magnifications, the object shouldn't move out
                > of the field, or even too far from center).

                That depends on the model/year. On my first SCT, an orange tube C8 in 1980, the image shift would easily move an object out of a high power view. But newer SCTs seem much better.

                -John


                > ----- Original Message -----
                > From: "David Cochran"
                > >
                > > Is it normal to have an object move inside the eyepiece while focusing?
                > >
                > > It seems that at some points the mirror shifts.
              • Mark Whitaker
                My 1981 C8 had so little image shift, I never knew what it was until I bought a 12 LX200. That scope would shift a star halfway across the field on a 26mm
                Message 7 of 13 , Sep 3, 2008
                  My 1981 C8 had so little image shift, I never knew what it was until I bought a 12" LX200. That scope would shift a star halfway across the field on a 26mm eyepiece and out of the field on shorter fl eyepieces. Redistributing the grease helped quite a bit but did not last long. I have to redistribute it every few months. Going back to the C8, I now notice very slight shift on highest powers.

                  Mark



                  ----- Original Message ----
                  From: John Mahony <jmmahony@...>
                  To: sct-user@yahoogroups.com
                  Sent: Wednesday, September 3, 2008 7:58:13 AM
                  Subject: Re: [sct-user] Is it normal?


                  ----- Original Message ----

                  > From: Chris Peterson <cpeterson@earthlink .net>
                  >
                  > This happens with all SCTs. But it shouldn't move enough to be a problem
                  > (that is, even at the highest magnifications, the object shouldn't move out
                  > of the field, or even too far from center).

                  That depends on the model/year. On my first SCT, an orange tube C8 in 1980, the image shift would easily move an object out of a high power view. But newer SCTs seem much better.

                  -John

                  > ----- Original Message -----
                  > From: "David Cochran"
                  > >
                  > > Is it normal to have an object move inside the eyepiece while focusing?
                  > >
                  > > It seems that at some points the mirror shifts.






                  [Non-text portions of this message have been removed]
                • Tanveer Gani
                  The sad thing is that Celestron had the mirror shift problem completely solved in the pre-C8 with the belt driven focuser but abandoned that for the current
                  Message 8 of 13 , Sep 3, 2008
                    The sad thing is that Celestron had the mirror shift problem
                    completely solved in the pre-C8 with the belt driven focuser but
                    abandoned that for the current mechanism in the name of cutting costs
                    (and we amateurs are partly to blame for this). I wouldn't mind paying
                    an extra $100 or two to get the belt-driven focuser were it made an
                    option. Intes-Micro has also solved the mirror shift problem by spring
                    loading the mirror and using gears but I think Celestron's method is
                    more scalable to larger sizes.

                    Regards,

                    Tanveer.

                    On Wed, Sep 3, 2008 at 6:48 AM, Mark Whitaker <tnut55@...> wrote:
                    >
                    > My 1981 C8 had so little image shift, I never knew what it was until I bought a 12" LX200. That scope would shift a star halfway across the field on a 26mm eyepiece and out of the field on shorter fl eyepieces. Redistributing the grease helped quite a bit but did not last long. I have to redistribute it every few months. Going back to the C8, I now notice very slight shift on highest powers.
                    >
                    > Mark
                    >
                    > ----- Original Message ----
                    > From: John Mahony <jmmahony@...>
                    > To: sct-user@yahoogroups.com
                    > Sent: Wednesday, September 3, 2008 7:58:13 AM
                    > Subject: Re: [sct-user] Is it normal?
                    >
                    > ----- Original Message ----
                    >
                    > > From: Chris Peterson <cpeterson@earthlink .net>
                    > >
                    > > This happens with all SCTs. But it shouldn't move enough to be a problem
                    > > (that is, even at the highest magnifications, the object shouldn't move out
                    > > of the field, or even too far from center).
                    >
                    > That depends on the model/year. On my first SCT, an orange tube C8 in 1980, the image shift would easily move an object out of a high power view. But newer SCTs seem much better.
                    >
                    > -John
                    >
                    > > ----- Original Message -----
                    > > From: "David Cochran"
                    > > >
                    > > > Is it normal to have an object move inside the eyepiece while focusing?
                    > > >
                    > > > It seems that at some points the mirror shifts.
                    >
                    > [Non-text portions of this message have been removed]
                    >
                    >
                  • carter646831
                    Hi David, Unfortunately, it is normal for an image to shift while you focus. That is what is called image shift. The higher power of the eyepiece, the more
                    Message 9 of 13 , Sep 3, 2008
                      Hi David,

                      Unfortunately, it is normal for an image to shift while you focus. That
                      is what is called "image shift." The higher power of the eyepiece, the more the
                      image will shift.

                      The best way to avoid this problem would be to use parfocal eyepieces so
                      you only have to make minor focus changes.

                      Tandy W. Carter Jr.
                      carter646831@...
                      http://home.att.net/~tandy.carter/index.htm



                      > -----Original Message-----
                      > From: sct-user@yahoogroups.com [mailto:sct-user@yahoogroups.com] On Behalf Of
                      > David Cochran
                      > Sent: Tuesday, September 02, 2008 10:52 PM
                      > To: sct-user@yahoogroups.com
                      > Subject: [sct-user] Is it normal?
                      >
                      > Hi all
                      >
                      > Is it normal to have an object move inside the eyepiece while focusing?
                      >
                      > It seems that at some points the mirror shifts.
                      >
                      > peace
                      >
                      > David
                    • Dennis Persyk
                      Hello David, Perhaps it would be instructive to estimate the image shift so group members could comment on its magnitude. Of the six SCTs I ve owned that had
                      Message 10 of 13 , Sep 3, 2008
                        Hello David,

                        Perhaps it would be instructive to estimate the image shift so group
                        members could comment on its magnitude. Of the six SCTs I've owned
                        that had move-the-primary-focusing, none had image shift exceeding 6
                        arc minutes. My C-11 and LX-200GPS 14" had the least, with about 3
                        arc minutes. It was merely an annoyance with high powers, 300x or
                        more. At the time I didn't have any wide FOV, high-power parfocal
                        eyepieces.

                        To estimate image shift in arc minutes, look up the FOV of the
                        eyepiece you are using. Then eyeball the displacement in the eyepiece
                        to get an estimate of the image shift.

                        Clear skies,

                        Dennis Persyk
                        Igloo Observatory Home Page http://dpersyk.home.att.net
                        Hampshire, IL
                        Pier Design http://home.att.net/~dpersyk/Pier_Design.htm


                        --- In sct-user@yahoogroups.com, "David Cochran" <davidcochran@...>
                        wrote:
                        >
                        > Hi all
                        >
                        > Is it normal to have an object move inside the eyepiece while
                        focusing?
                        >
                        > It seems that at some points the mirror shifts.
                        >
                        > peace
                        >
                        > David
                        >
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