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Re: [William-Optics] Re:Megrez 80

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  • Richard Chalfan
    Kerry, With the extensive info you have posted here, I have a question and a thought or two. ... Are the anomalies areas where the coating is missing,
    Message 1 of 27 , May 9, 2002
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      Kerry,

      With the extensive info you have posted here, I
      have a question and a thought or two.

      > I also found that
      > what I thought was dust, were actually
      > anomalies in the coating. I
      > could tell that the cell has been removed at
      > some time in the past,
      > so I don't know if the coating problems
      > occurred then or not, but
      > they occur on all surfaces.

      Are the "anomalies" areas where the coating is
      missing, flaking, or just a different color but
      still coated? The first or third are probably
      stable and unnoticeable at the eyepiece. I could
      worry about the second.

      They are on the inside too, you mean between the
      elements? I assume the Megrez is air-spaced. If
      not - oops!
      >
      > I loosened the retaining ring on the lens cell
      > a bit & it helped
      > improve the star test, and the shield moves a
      > little easier too.
      > The optics are still a bit mis-collimated,
      > probably from shipping,
      > however it doesn't seem to have much of an
      > effect until around 185x,
      > where I can see the in-focus diffraction ring
      > slightly off to one
      > side, when looking at stars 3rd magnitude or
      > brighter.

      The mis-collimation, coupled with signs that the
      optic had been previously disassembled, and
      especially with the retaining ring too tight -
      suggests that someone who didn't know what they
      were doing might have disassembled the lens
      (shudder). Overtightning can cause astigmatism
      and do nasty things to the image. Though I
      suppose shipping vibration could have caused that
      too.

      > Also there's
      > a slight under-correction.

      The under-correction may actually be part of the
      aberration balance that the designer intended for
      the lens. A little bit is not uncommon to
      achieve the best overall correction in a given
      design. I wouldn't worry about it, especially
      since you say the views are very good at focus.

      >Otherwise, images
      > are great & it doesn't
      > seem to have any noticeable effect on lunar or
      > planetary either.
      > Secondary spectrum is well controlled despite
      > the minor effects seen
      > in the star-test. Almost no color on the
      > moon's limb or terminator,
      > and slight color on Jupiter. Low-power
      > wide-field views are great &
      > pinpoint sharp across the field. It's not an
      > APO, but heck, for $500
      > for the scope, diagonal, EP, carrying case,
      > it's a great deal.
      >
      > With respect to being simple, having dealt with
      > mostly reflectors, I
      > think collimating objectives and making sure
      > there is no false
      > centering in the elements is more complicated
      > than aligning the
      > mirrors of a Newtonian. But since there's no
      > push-pull adjustments
      > on the cell, I suppose a manual wouldn't help
      > much. Oh well, it's
      > still a nice little scope. Thanks again.
      >
      Lens centering is almost always left to the skill
      of the machinist who makes the cell, and the lens
      making process itself.

      Lens tilt is adjustable on most really high end
      scopes; but some very good instruments don't have
      collimation adjustments. The Tele-Vue Pronto
      (rather in the Megrez class I think) doesn't have
      collimation adjustment either. But if the cell
      is machined to high tolerances, and the threads
      are cut with high precision, adjustment may not
      be necessary and the instrument is probably more
      immune to vibration and shock that way.

      If yours is just a tad out, I would check to make
      sure that the threads and seating surfaces where
      the lens cell screws into the tube are absolutely
      clean and free of burrs or other problems. It is
      also (shudder) possible that the previous owner
      actually removed (shudder) the elements from the
      cell. In that case, a tiny bit of dirt between
      an element face and the cell could indeed tip the
      elements a bit.

      I suppose that with the string of little things
      you have mentioned, I would be tempted to send
      the scope to WYO and have them take a look at it.
      They might be able to fix things up a bit for
      you.

      But as you said, you did get a really good deal,
      and you might just rather go observe and not
      worry about it!

      Richard


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    • ckwastro
      Hi Richard, Thanks for the insight, especailly about the undercorrection. On the coating, it s difficult to tell for sure, but I don t think it s flaking off.
      Message 2 of 27 , May 10, 2002
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        Hi Richard,

        Thanks for the insight, especailly about the undercorrection.

        On the coating, it's difficult to tell for sure, but I don't think
        it's flaking off. The specks where the coating appears to missing
        are not jagged around the boundary like you would expect from random
        flaking. It looks more like an "overspray" from some liquid that got
        on the lenses, and the spots are fairly uniform in size and circular
        in shape.

        Yes, the objective is an air-spaced doublet, and there are definitely
        spots on the inside sections of the lenses. It definitely seems like
        the lenses were removed from the cell at some point.

        I was rather alarmed when I first saw this, however as I stated, the
        scope still performs well. As good or better than most Pronto's I've
        used, so as long as everything stays as it is now, I think I'll be
        O.K.

        David is checking into the alignment for me now to see if there's any
        way to get it right on without sending it back to WYO.

        Take care,

        Kerry


        --- In William-Optics@y..., Richard Chalfan <rchalfan@y...> wrote:
        > Kerry,
        >
        > With the extensive info you have posted here, I
        > have a question and a thought or two.
        >
        > Are the "anomalies" areas where the coating is
        > missing, flaking, or just a different color but
        > still coated? The first or third are probably
        > stable and unnoticeable at the eyepiece. I could
        > worry about the second.
        >
        > They are on the inside too, you mean between the
        > elements? I assume the Megrez is air-spaced. If
        > not - oops!>
        > Richard
        >
        >
        > __________________________________________________
        > Do You Yahoo!?
        > Yahoo! Shopping - Mother's Day is May 12th!
        > http://shopping.yahoo.com
      • poziovahim2001
        I m not real hip on refractors but when these little scopes first came out last year, some of them had pinched optics! Contact Anacortes and I m sure they can
        Message 3 of 27 , May 13, 2002
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          I'm not real hip on refractors but when these little scopes first
          came out last year, some of them had pinched optics! Contact
          Anacortes and I'm sure they can help! buen cielo, Dan


          --- In William-Optics@y..., "ckwastro" <ckw1@e...> wrote:
          > Hi Richard,
          >
          > Thanks for the insight, especailly about the undercorrection.
          >
          > On the coating, it's difficult to tell for sure, but I don't think
          > it's flaking off. The specks where the coating appears to missing
          > are not jagged around the boundary like you would expect from
          random
          > flaking. It looks more like an "overspray" from some liquid that
          got
          > on the lenses, and the spots are fairly uniform in size and
          circular
          > in shape.
          >
          > Yes, the objective is an air-spaced doublet, and there are
          definitely
          > spots on the inside sections of the lenses. It definitely seems
          like
          > the lenses were removed from the cell at some point.
          >
          > I was rather alarmed when I first saw this, however as I stated,
          the
          > scope still performs well. As good or better than most Pronto's
          I've
          > used, so as long as everything stays as it is now, I think I'll be
          > O.K.
          >
          > David is checking into the alignment for me now to see if there's
          any
          > way to get it right on without sending it back to WYO.
          >
          > Take care,
          >
          > Kerry
          >
          >
          > --- In William-Optics@y..., Richard Chalfan <rchalfan@y...> wrote:
          > > Kerry,
          > >
          > > With the extensive info you have posted here, I
          > > have a question and a thought or two.
          > >
          > > Are the "anomalies" areas where the coating is
          > > missing, flaking, or just a different color but
          > > still coated? The first or third are probably
          > > stable and unnoticeable at the eyepiece. I could
          > > worry about the second.
          > >
          > > They are on the inside too, you mean between the
          > > elements? I assume the Megrez is air-spaced. If
          > > not - oops!>
          > > Richard
          > >
          > >
          > > __________________________________________________
          > > Do You Yahoo!?
          > > Yahoo! Shopping - Mother's Day is May 12th!
          > > http://shopping.yahoo.com
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