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Re: Is there something wrong with my scope?

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  • starcrazed28056
    Rod, You are da man! With clear skies last evening I proceeded with my experiment as promised. First collimating to the shape of the donut. Saturn s rings
    Message 1 of 22 , Mar 1, 2002
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      Rod,

      You are da man! With clear skies last evening I proceeded with my
      experiment as promised. First collimating to the shape of the
      donut. Saturn's rings looked good, but not all that impressive.
      Went back to Alderbaran and collimated on the tiny diffraction rings
      and centered the dot at high mag. Bingo! Cassini's division was
      clearly visible all the way around. I don't remember ever seeing it
      so plainly! Seeing was pretty good too, which helped some.

      In retrospect here is what I was doing wrong: I started the process
      at a moderately high mag, 200x. At this mag the donut looks outta
      wack when the tiny rings are concentric. When I went back to low
      power, say 77x, the donut looked pretty good. The other thing that
      made a *big* difference was fine tuning the collimation at even
      higher mag. With my Barlow in I centered that dot dead on at 400x.

      If anyone doubts Rod's advice concerning the use of the tiny rings,
      try this experiment. Collimate on the tiny rings at high mag. Now
      take the focus out till you see the big donut. In my case the donut
      is skewed to oneside. Now bring the focus in ever so slowly. What I
      see is the big donut collapses in to form a bright ring which is
      skewed relative to the concentric tiny rings inside. Now if you
      focus in ever so slowly you will see that eventually this bright
      outer ring begins to take on the shape of the tiny inner rings, and
      just before critical focus you will see a single bright ring
      surrounding a central dot (no comet tails). Focus a little more and
      you see the airy disk with a faint diffraction ring around it (if
      seeing is good). It doesn't get any better than this.

      So I guess the answer to my question, is there something wrong with
      my scope, is a definite no. At least I'm happy with the quality of
      viewing. Still the purist in me wonders why that big donut looks
      skewed when the optics are perfectly aligned? Is this an indication
      that the optics are mis-shapen? (I've heard reports of C8s
      manufactured in the mid 80's having quality problems...I bought mine
      in early 85). Is this skewed donut a common thing with properly
      collimated SCTs?

      If you guys think I have a turkey, give it to me straight, it won't
      hurt my feelings.

      Thankx to all you guys who chimed in with questions and
      recommendations. I appreciate all the help, you are a great group!

      Clear skies, Frank
      --- In sct-user@y..., RMOLLISE@a... wrote:
      > Your words are encouraging, but now I'm getting confused. The
      > previous post, from Mr GNowellSct, says to completely ignore the
      tiny
      > rings that 'distract' from proper collimation and you advocate
      using
      > the tiny rings whilst forgetting about the fat donut. I've done
      alot
      > of reading on collimation (even been to your site ;-) and they all
      > start out with centering the fat donut, and then focus in with
      higher
      > mag. Ok, I've centered the fat donut, and now I move to higher mag
      > and focus in to see the tiny rings.
      >
      > Hi Frank:
      >
      > Again, use the diffraction rings! Use the "donut" only as a rough
      guide to collimation!
      >
      > Peace,
      > Rod
    • RMOLLISE@aol.com
      ... my scope, is a definite no. At least I m happy with the quality of viewing. Still the purist in me wonders why that big donut looks skewed when the optics
      Message 2 of 22 , Mar 1, 2002
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        >>>>>So I guess the answer to my question, is there something wrong with
        my scope, is a definite no. At least I'm happy with the quality of
        viewing. Still the purist in me wonders why that big donut looks
        skewed when the optics are perfectly aligned? Is this an indication
        that the optics are mis-shapen? (I've heard reports of C8s
        manufactured in the mid 80's having quality problems...I bought mine
        in early 85). Is this skewed donut a common thing with properly
        collimated SCTs?
        >>>

        Hi Frank:

        The main reason the "big donut" looks skewed is the strongly curved shape of the SCT's field. That's why all collimation instructions emphasize the need to carefully center the slightly out of focus star when looking at its diffraction rings...

        Peace,
        Rod
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