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Re: [sct-user] Moon Shine inside the OTA

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  • William Hamblen
    ... Another reason the primary on an SCT is a little larger than the corrector is to catch all the off-axis rays of light. Bud
    Message 1 of 10 , Aug 31, 2007
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      P T Chambers wrote:

      > Hmm, rereading what I said, I suppose it should be from the clear diam of
      > the corrector, not the primary which is smaller. That would make the
      > numbers closer to straight as you say. I have now confused myself. Is
      > the primary bigger or smaller than the corrector clear aperture??? (I
      > think I remember the primary is a bit bigger because the corrector
      > diverges the light a bit ??? or am I remembering all this incorrectly)

      Another reason the primary on an SCT is a little larger than the
      corrector is to catch all the off-axis rays of light.

      Bud
    • John Mahony
      ... Last I heard, only Meade makes the primary oversized. A tiny difference that Meade likes to make a big deal about. -John
      Message 2 of 10 , Sep 1, 2007
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        --- William Hamblen <wrhamblen@...> wrote:

        > P T Chambers wrote:
        >
        > > Hmm, rereading what I said, I suppose it should be from the clear diam of
        > > the corrector, not the primary which is smaller. That would make the
        > > numbers closer to straight as you say. I have now confused myself. Is
        > > the primary bigger or smaller than the corrector clear aperture??? (I
        > > think I remember the primary is a bit bigger because the corrector
        > > diverges the light a bit ??? or am I remembering all this incorrectly)
        >
        > Another reason the primary on an SCT is a little larger than the
        > corrector is to catch all the off-axis rays of light.

        Last I heard, only Meade makes the primary oversized. A tiny difference that
        Meade likes to make a big deal about.

        -John


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      • Cleon Wells
        The aperture diameter of the two old C8 scopes I own is 7.875 . The primary mirror is 8 in diameter, with a beveled edge, this reduces the mirror surface to
        Message 3 of 10 , Sep 4, 2007
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          The aperture diameter of the two old C8 scopes I own is 7.875". The
          primary mirror is 8" in diameter, with a beveled edge, this reduces
          the mirror surface to around 7.95".
          Getting back to the moonlight on the end of the rear cell mirror tube
          edge. When you center up the glow equally around this edge, the finder
          scope should point to the center of the light source. Statement , (The
          moon is not a point source of light, nor is Jupiter, like a star is
          with good seeing , so trying to ray trace the light inside the OTA
          doesn't work). Is this a correct statement? If you use a diaginal with
          the EP the primary mirror has to move closer to the secondary. This
          causes light from the margins of the primary mirror to strike the
          margins of the secondary. This increases the amount of light on the
          rear cell tube end. I've made up two donuts out of Masonite on my old
          wood lathe. The outer ring is 0.96 " wide plus the extra width to
          reach out to the corrector ring assembly, and an it has a inside
          diameter of around 6.9". The inner donut has an outside diameter 6.8"
          and an inner diameter that slips over the secondary assembly. I can
          stop the scope down using either donut for mirror area reduction. The
          smaller donut with an outside diameter of 6.8" increases the
          obstruction of the OTA which reduces contrast and resolution, see
          http://legault.club.fr/obstruction.html ----- . When I stop down the
          aperture, from 7.875" to 6.9" with the larger donut, the light
          illuminating the end of the rear cell tube is removed. Does this help
          increase the scopes contrast? ( I don't know if this is similar to a ½
          lb. nitrogen pressure change in the right rear that gets the 209 MPH
          race car to 210 MPH.) I still want to try a light absorbing donut on
          the end of the rear cell tube, with a diameter that is the same
          diameter as the primary mirror front metal flange. This diameter is
          smaller then the diameter of the secondary assembly.

          Cleon

          PS. If you're over 50 orbits old check your PSA level. It should be no
          more then 2.0.
          --- In sct-user@yahoogroups.com, P T Chambers <ptchamb@...> wrote:
          >
          > Thanks John
          > That cleared up my muddled thinking on why the primary is bigger.
          >
          > ---------
          > Phil Chambers [ptchamb-at-svpal.org] (S.F. Bay Area - Calif. USA)
          >
          > On Fri, 31 Aug 2007, John Mahony wrote:
          >
          > > --- P T Chambers <ptchamb@...> wrote:
          > >
          > >> Hi
          > >>
          > >> My 8 inch has an outside diam (where the dewshield attaches) of 9
          inches.
          > >> The C11 is about 12.4 inches so it varies a bit per size of scope.
          > >>
          > >> However, the largest fov of an 8 inch is about at 1.5 deg. That
          cone
          > >> angle is so small that you can be very close to the primary
          diameter for a
          > >> ways in front of the scope. I used to have the numbers in my head
          > >> but am too lazy to reconstruct them.
          > >
          > > For small angles just use the small angle approximations to
          convert to simple
          > > ratios: convert 1.5 deg to radian measure, 1.5 X pi/180 = 0.026.
          So multiply
          > > the length of the dewshield by that, and you get the amount by
          which the front
          > > of the dewshield should exceed the aperture. So a 24" dewshield
          on an 8" SCT
          > > should be about 8.63" wide at the front. That's a bit less than
          the 9" OD of
          > > the OTA, so yes, it could be shaped like an inverted cone, but not
          by much.
          > >
          > >> Hmm, rereading what I said, I suppose it should be from the clear
          diam of
          > >> the corrector, not the primary which is smaller. That would make the
          > >> numbers closer to straight as you say. I have now confused
          myself. Is
          > >> the primary bigger or smaller than the corrector clear aperture??? (I
          > >> think I remember the primary is a bit bigger because the corrector
          > >> diverges the light a bit ??? or am I remembering all this
          incorrectly)
          > >
          > > The corrector aperture is usually the stated aperture of the
          scope. Meade
          > > makes the primary slightly oversized, but not because of any
          divergence by the
          > > corrector (which would be tiny), but because if you want the
          primary to get 8"
          > > of light even from off-axis stars, it has to be larger than the 8"
          corrector
          > > (forget the finer details of the corrector and just picture it as
          an 8" window
          > > placed about 16" in front of the primary. Off-axis light going
          through that 8"
          > > window would not be centered on the primary, so if the primary was
          the same
          > > size as the corrector, some of the light would fall beyond the
          edge of the
          > > primary. You can work the numbers just as above: 16X.026= .16",
          so the primary
          > > should be at least 8.16". Meade uses 8.25".
          > >
          > > -John
          > >
          > >
          > >> In any case, whichever diam you use:
          > >>
          > >> If you use 2 deg, that should give you plenty of room. The sin
          of 1 deg
          > >> is .017 or so. So the front of a 16 inch dewshield (probably
          longer than
          > >> most at 2 diameters) only needs to be about 1/2 inch larger (diam)
          > >> due to cone angle and you are starting from a bigger diam by 1
          inch or
          > >> better.
          > >>
          > >> But it would have to be centered very well which could be an issue.
          > >>
          > >> Some of the dewshields have the reverse cone built in. I have
          one for my
          > >> C8 like that.
          > >>
          > >> I didnt mean to imply larger angles.
          > >>
          > >> ---------
          > >> Phil Chambers [ptchamb-at-svpal.org] (S.F. Bay Area - Calif. USA)
          > >>
          > >> On Wed, 29 Aug 2007, John Mahony wrote:
          > >>
          > >>> --- P T Chambers <ptchamb@...> wrote:
          > >>>
          > >>>> Hi
          > >>>> Those reflections are the reason some people flock the inside
          of the OTA.
          > >>>> I believe a good flocked dew shield will do as well. Also the
          dew shield
          > >>>> should have a reverse cone so that the front is smaller than
          the back (the
          > >>>> outside of the scope is an inch or 2 bigger than the primary).
          > >>>
          > >>> It's usually only about 1" larger diameter. And remember, you
          need the
          > >>> off-axis light for objects at the edge of the FOV. The geometry is
          > >>> straightforward- figure your largest FOV, and make sure the
          forward end of
          > >> the
          > >>> dewshield is larger than the aperture by enough to allow light
          at that
          > >> angle
          > >>> (radius of your FOV) to get to the corrector.
          > >>>
          > >>> -John
          > >>>
          > >>>
          > >>>> That light
          > >>>> you see can bounce around and scatter. The effect at the ep is
          that it
          > >>>> reduces contrast (it basically increases the "black level" if
          you are a
          > >>>> CRT kind of guy).
          > >>>>
          > >>>>
          > >>>>
          > >>>> ---------
          > >>>> Phil Chambers [ptchamb-at-svpal.org] (S.F. Bay Area - Calif. USA)
          > >>>>
          > >>>> On Thu, 30 Aug 2007, Cleon Wells wrote:
          > >>>>
          > >>>>> While standing to the side and in front of the OTA when the
          scope is
          > >>>>> pointing at the Moon I noticed down in the OTA through the
          corrector
          > >>>>> lens, light on the end of the rear cell tube. This can be seen
          with
          > >>>>> the scope pointed at the edge of the full moon on my C8. I can
          center
          > >>>>> the tube so this almost all the light goes away. Is the length
          of the
          > >>>>> tube a little long for this focus point ( mirror close to the
          > >>>>> secondary ) ? What happens to all the light that is reflected
          off the
          > >>>>> inside of the grooved tub traveling towards the EP? My Orange
          C8 does
          > >>>>> the same thing.
          > >>>>>
          > >>>>> Cleon
          > >>>>>
          > >>>>>
          > >>>>>
          > >>>>>
          > >>>>> Visit the sct-user home page at:
          > >>>>>
          > >>>>>
          > >>>>>
          > >>>>> http://members.aol.com/RMOLLISE/index4.html
          > >>>>> Yahoo! Groups Links
          > >>>>>
          > >>>>>
          > >>>>>
          > >>>>>
          > >>>>
          > >>>>
          > >>>> Visit the sct-user home page at:
          > >>>>
          > >>>>
          > >>>>
          > >>>> http://members.aol.com/RMOLLISE/index4.html
          > >>>> Yahoo! Groups Links
          > >>>>
          > >>>>
          > >>>>
          > >>>>
          > >>>
          > >>>
          > >>>
          > >>>
          > >>>
          > >>
          > >
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          > >>> Visit the sct-user home page at:
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          > >>> http://members.aol.com/RMOLLISE/index4.html
          > >>> Yahoo! Groups Links
          > >>>
          > >>>
          > >>>
          > >>>
          > >>
          > >>
          > >> Visit the sct-user home page at:
          > >>
          > >>
          > >>
          > >> http://members.aol.com/RMOLLISE/index4.html
          > >> Yahoo! Groups Links
          > >>
          > >>
          > >>
          > >>
          > >
          > >
          > >
          > >
          > >
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