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Hartman mask question

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  • Aurigema, Andrew N
    Hi, I am having trouble aligning the mirrors of my home made cassegrain scope. Camera is ST8i, scope is 25 f/5.1 RC I have put a laser alignment tool in place
    Message 1 of 3 , Dec 2, 2005
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      Hi,

      I am having trouble aligning the mirrors of my home made cassegrain scope.
      Camera is ST8i, scope is 25" f/5.1 RC

      I have put a laser alignment tool in place of the CCD and have adjusted the secondary mirror's tip/tilt until the secondary is laser bounce aligned and
      centered with the mechanical focusing axis. I have star tested using the CCD and adjusted the primary mirror's tip/tilt until the shadow of the secondary
      is perfectly centered in the out of focus star. I get really good looking CCD intra-focal donut images ( perfectly centered donut holes ) but as I focus
      down towards best focus the image just smears and goes elliptical then Mercury Capsule shaped. I am using a star in the very center of the chip for all
      testing.

      So I made a Hartman mask ( with three little triangles at 120 degrees apart ) for the mirror and the resulting focusing test has me confused. I understand
      the principle of how a single star will be seen as three until they converge at perfect focus.

      Two of the resulting little stars are bright and round and converge towards a common center as I focus down towards best focus. The third little star is
      smeared, dim and never really heads towards the common center. If I converge the two brighter little stars into a single bright star the third is hovering
      above and a fuzzy mess.

      If I cover one of the holes that makes one of the brighter little stars, then focus to converge the dim and bright star, they will merge, but not at best
      focus. If I uncover the hole that makes the other little bright star, the two bright stars are not converged. I can not get all three stars to merge at
      the same time with a common focus.

      I thought all three little stars of a tri-star Hartman mask are supposed to converge at the same focus point ???

      Is this telling me that my primary mirror is still tipped / tilted a little wrong ???

      Or is it telling me that my primary mirror is not centered with respect to the focusing axis???

      Or is it telling me to find a new hobby that can actually be understood :_))))

      Please help.

      Andrew




      [Non-text portions of this message have been removed]
    • Lou Jackson
      Take a good look at your mirror support. Assuming you have a good mirror, it sounds like it might be pinched on one side. It doesn t take all that much stress
      Message 2 of 3 , Dec 2, 2005
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        Take a good look at your mirror support. Assuming you have a good
        mirror, it sounds like it might be pinched on one side. It doesn't
        take all that much stress on the mirror to cause what you are
        describing.

        Try repeating the test with the OTA as close to vertical as possible.
        That should alleviate any issues with weight-induced stress on the
        mirror.

        Lou


        --- In ccd-newastro@yahoogroups.com, "Aurigema, Andrew N"
        <eosraptor@g...> wrote:
        >
        > Hi,
        >
        > I am having trouble aligning the mirrors of my home made cassegrain
        scope.
        > Camera is ST8i, scope is 25" f/5.1 RC
        >
        > I have put a laser alignment tool in place of the CCD and have
        adjusted the secondary mirror's tip/tilt until the secondary is laser
        bounce aligned and
        > centered with the mechanical focusing axis. I have star tested
        using the CCD and adjusted the primary mirror's tip/tilt until the
        shadow of the secondary
        > is perfectly centered in the out of focus star. I get really good
        looking CCD intra-focal donut images ( perfectly centered donut
        holes ) but as I focus
        > down towards best focus the image just smears and goes elliptical
        then Mercury Capsule shaped. I am using a star in the very center of
        the chip for all
        > testing.
        >
        > So I made a Hartman mask ( with three little triangles at 120
        degrees apart ) for the mirror and the resulting focusing test has me
        confused. I understand
        > the principle of how a single star will be seen as three until they
        converge at perfect focus.
        >
        > Two of the resulting little stars are bright and round and converge
        towards a common center as I focus down towards best focus. The
        third little star is
        > smeared, dim and never really heads towards the common center. If
        I converge the two brighter little stars into a single bright star
        the third is hovering
        > above and a fuzzy mess.
        >
        > If I cover one of the holes that makes one of the brighter little
        stars, then focus to converge the dim and bright star, they will
        merge, but not at best
        > focus. If I uncover the hole that makes the other little bright
        star, the two bright stars are not converged. I can not get all
        three stars to merge at
        > the same time with a common focus.
        >
        > I thought all three little stars of a tri-star Hartman mask are
        supposed to converge at the same focus point ???
        >
        > Is this telling me that my primary mirror is still tipped / tilted
        a little wrong ???
        >
        > Or is it telling me that my primary mirror is not centered with
        respect to the focusing axis???
        >
        > Or is it telling me to find a new hobby that can actually be
        understood :_))))
        >
        > Please help.
        >
        > Andrew
        >
        >
        >
        >
        > [Non-text portions of this message have been removed]
        >
      • Roger Hamlett
        ... If this is an RC as you say, then you are missing a stage at the beginning. With a normal SCT, the primary is spherical, and the secondary very close to
        Message 3 of 3 , Dec 2, 2005
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          > Hi,
          >
          > I am having trouble aligning the mirrors of my home made cassegrain
          > scope.
          > Camera is ST8i, scope is 25" f/5.1 RC
          >
          > I have put a laser alignment tool in place of the CCD and have adjusted
          > the secondary mirror's tip/tilt until the secondary is laser bounce
          > aligned and
          > centered with the mechanical focusing axis. I have star tested using
          > the CCD and adjusted the primary mirror's tip/tilt until the shadow of
          > the secondary
          > is perfectly centered in the out of focus star. I get really good
          > looking
          > CCD intra-focal donut images ( perfectly centered donut holes ) but as I
          > focus
          > down towards best focus the image just smears and goes elliptical then
          > Mercury
          > Capsule shaped. I am using a star in the very center of the chip for all
          > testing.
          >
          > So I made a Hartman mask ( with three little triangles at 120 degrees
          > apart )
          > for the mirror and the resulting focusing test has me confused. I
          > understand
          > the principle of how a single star will be seen as three until they
          > converge
          > at perfect focus.
          >
          > Two of the resulting little stars are bright and round and converge
          > towards
          > a common center as I focus down towards best focus. The third little
          > star is
          > smeared, dim and never really heads towards the common center. If I
          > converge
          > the two brighter little stars into a single bright star the third is
          > hovering
          > above and a fuzzy mess.
          >
          > If I cover one of the holes that makes one of the brighter little stars,
          > then
          > focus to converge the dim and bright star, they will merge, but not at
          > best
          > focus. If I uncover the hole that makes the other little bright star,
          > the two
          > bright stars are not converged. I can not get all three stars to merge
          > at
          > the same time with a common focus.
          >
          > I thought all three little stars of a tri-star Hartman mask are supposed
          > to
          > converge at the same focus point ???
          >
          > Is this telling me that my primary mirror is still tipped / tilted a
          > little
          > wrong ???
          >
          > Or is it telling me that my primary mirror is not centered with respect
          > to the
          > focusing axis???
          >
          > Or is it telling me to find a new hobby that can actually be understood
          > :_))))
          >
          > Please help.
          >
          > Andrew
          If this is an RC as you say, then you are missing a stage at the beginning.
          With a normal SCT, the primary is spherical, and the secondary very close
          to spherical. As a result, you can correct for a radial misalignment of a
          component, by tilting the one component that is normally adjustable. With a
          RC, the situation is different. Both mirrors are complex hyperboloids, and
          there is only one alignment of all the components, that will actually work
          properly. With this configuration, if you start, by adjusting the tilt of
          the secondary, to return the laser to the source, _unless_ the laser is
          hitting the exact centre of the mirror,you will end up tilting this mirror.
          Then if you align the primary to this, you now have both mirrors tilted to
          try to correct for each other in the centre of the field, but the
          correction everywhere else will be from the wrong parts of each mirror.
          This is why on the RC, it is vital to start by mechanically aligning the
          centre of the secondary, to the centre of the focusser tube. Only once this
          is done, can you adjust the angle to place this mirror close to the right
          angle. Then you align the primary. At this point, if everything is correct,
          the view if you look from the eyepiece location, should show everything as
          concentric. If it isn't, you start again, and hopefully improve the
          settings. Some successive approximation will be needed, unless the
          secondary was _exactly_ on the focusser axis on the initial pass.
          If your secondary is centre spotted, you can work by putting the mirror
          close to flat, and then centering it till the laser hits the centre spot,
          and then adjusting the angle. The Takahashi collimation scope, is better
          than a laser on these scopes, allowing you to centre the mirror, and see
          the position of the outer edge at the same time.
          Also if your scope allows either the primary or secondary to move for
          focussing, remember that unless your eyepiece is exactly at the design
          distance behind the scope, the mirror seperation will be wrong, and the
          scope will start to show aberrations, with the defocus rings becoming
          assymetric each side of focus. If the mirrors are both fixed, and the scope
          shows this, then the spacing is wrong, and will need adjustment.
          Yes, I think your scope is telling you that you have started with the
          secondary 'off centre', and tried to collimate in this position.

          Best Wishes
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