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Alignment of guide scope and imaging scope: effect on guiding?

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  • Robert
    I am using a Casady tandem bar, and guide through a scope that is aimed via an adjustible saddle at the same point in the sky as the imaging scope (helps me
    Message 1 of 3 , May 3 7:50 PM
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      I am using a Casady tandem bar, and guide through a scope that is aimed via an adjustible saddle at the same point in the sky as the imaging scope (helps me navigate/wider field of view). In another forum, someone posted that this could cause "cone error". New term to me- my bottom line question: am I putting guiding accuracy at risk?
      Thanks
      Bob Hertel
    • Mike Dodd
      ... Iuse a separate guide scope on a Casady Triad Bar,and have never experienced any guiding errors I can attribute to cone error. I thought cone referred to
      Message 2 of 3 , May 3 8:37 PM
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        Robert wrote:
        > I am using a Casady tandem bar, and guide through a scope that is
        > aimed via an adjustible saddle at the same point in the sky as the
        > imaging scope (helps me navigate/wider field of view). In another
        > forum, someone posted that this could cause "cone error". New term
        > to me- my bottom line question: am I putting guiding accuracy at
        > risk? Thanks Bob Hertel

        Iuse a separate guide scope on a Casady Triad Bar,and have never
        experienced any guiding errors I can attribute to cone error. I thought
        cone referred to the scope being non-orthogonal to the mount, not to the
        main OTA and guide scope being non-parallel.

        I suspect that being even a degree or two off-parallel won't hurt
        guiding at all. I've never made any special attempt to ensure that my
        guide scope is pointing at exactly the same point in the sky.
        --
        Mike

        Mike Dodd
        Montpelier, VA USA
        http://astronomy.mdodd.com
        http://NotWhatIVotedFor.com
      • dmwmpd
        Hi Bob, I m not sure of the definition of cone error , but I know what happens when your guide and imaging scopes are not aligned. It s the same as using an
        Message 3 of 3 , May 5 9:58 AM
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          Hi Bob,

          I'm not sure of the definition of "cone error", but I know what happens when your guide and imaging scopes are not aligned. It's the same as using an off axis guider. By definition, the guide star will be kept to a point round star. The greater the angular distance from the guide star and the longer the image exposure can cause parts of the image to seem to rotate around the guide star.

          There are two conditions that cause elongated stars away from the GS: Polar mis-alignment and Atmospheric Differential Refraction. both of these effects were prominent when we were using film for very long exposures (long being more than 1 hour). With CCD camerras, and subs that are 20 minutes or so, what you might see is later images slightly rotated compared to earlier images. This effect is easily controlled with the alignment/stacking program.

          With large CCD chips like the KAF-16803 on a short focal length your FOV can be a couple degrees. It is always best to use a gyide star centered in the image FOV, but OAG's work fine too.

          BTW, the way to control image rotation (using off axis GS) from polar misalignment is to get better polar alignment (no surprise!). ADR is caused by refraction by the atmosphere and is stronger (worse) at lower altitude angles. ADR can be minimised by chosing a guide star at the same RA as the center of the target (choose a GS either higher or lower DEC). ADR also is almost non existent shooting 45° altitude and higher.

          Hope this helps,
          Don

          --- In ccd-newastro@yahoogroups.com, Mike Dodd <mike@...> wrote:
          >
          > Robert wrote:
          > > I am using a Casady tandem bar, and guide through a scope that is
          > > aimed via an adjustible saddle at the same point in the sky as the
          > > imaging scope (helps me navigate/wider field of view). In another
          > > forum, someone posted that this could cause "cone error". New term
          > > to me- my bottom line question: am I putting guiding accuracy at
          > > risk? Thanks Bob Hertel
          >
          > Iuse a separate guide scope on a Casady Triad Bar,and have never
          > experienced any guiding errors I can attribute to cone error. I thought
          > cone referred to the scope being non-orthogonal to the mount, not to the
          > main OTA and guide scope being non-parallel.
          >
          > I suspect that being even a degree or two off-parallel won't hurt
          > guiding at all. I've never made any special attempt to ensure that my
          > guide scope is pointing at exactly the same point in the sky.
          > --
          > Mike
          >
          > Mike Dodd
          > Montpelier, VA USA
          > http://astronomy.mdodd.com
          > http://NotWhatIVotedFor.com
          >
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