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

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  • 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 1 of 10 , Sep 1, 2007
      --- 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 2 of 10 , Sep 4, 2007
        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
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        > >>>
        > >>>
        > >>
        > >>
        > >> Visit the sct-user home page at:
        > >>
        > >>
        > >>
        > >> http://members.aol.com/RMOLLISE/index4.html
        > >> Yahoo! Groups Links
        > >>
        > >>
        > >>
        > >>
        > >
        > >
        > >
        > >
        > >
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