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Restriction to 30' field of view for a 12 inch LX90 ???

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  • time_to_enjoy_life
    That s what someone replied when I posted this problem on another forum and I am skeptical: I have a 12 inch LX90 with a 1.25 inch WO diagonal and a Borg fine
    Message 1 of 4 , Sep 1 6:50 AM
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      That's what someone replied when I posted this problem on another
      forum and I am skeptical:

      I have a 12 inch LX90 with a 1.25 inch WO diagonal and a Borg fine
      focuser mounted between the diagonal and the EP.

      A few nights ago I was observing the full moon with a Tele Vue 32 mm
      Plossl. With a 12 inch LX90, the TV has a Field of view of 31', so
      I should just about be able to fit the whole full moon in the field
      of view.

      It looked like I was only seeing about half of it, so I attached my
      Atik focal reducer to the Tele Vue. The Atik should give reduction
      of .44 to .60, so you should see a field of view of 52 to 70'.

      When I tried it on the moon, I folound that I could barely fit the
      full moon into the field of view, suggesting that the field of view
      was now only about 33 degrees?

      This person claims that an f/10 scope with 6 inches of light path -
      in reality more like 3 or 4 - through stuff hanging on the back of
      the scope is actually operating about f/12. Even if that were true,
      would going from f/10 to f/12 reduce the field of view by 50% ?

      They also claim that the only way to go beyond the 30' restriction
      in field of view is to replace the visual back with an Eye Opener,
      get a 2 inch diagonal, a Starizona 2-speed focuser and dump all of
      my 1.25 inch Ep's for 2 inch EP's.

      Are they correct?

      Has anyone out there measured the field of view of a 12 inch LX90
      with 1.25 inch EP's?

      I would greatly appreciate any advise you can give me.


      Enjoy the day........Mike
    • barringtonri
      Hi Mike, They re somewhat correct. Your scope has a 3000-something focal length, and that s pretty long. Field of view is a function of focal length and the
      Message 2 of 4 , Sep 1 11:41 AM
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        Hi Mike,

        They're somewhat correct. Your scope has a 3000-something focal
        length, and that's pretty long. Field of view is a function of focal
        length and the size of the eyepiece field stop. I developed the
        EyeOpener about 7 years ago as the best way to get the widest
        possible field of view out of my 10" LX200 Classic. There's a full
        explaination of what's happening, including a short paper by Doc G,
        to be found at
        [Url]http://www.petersonengineering.com/sky/EO.htm%5b/Url]

        Yes, using a focal reducer will give you a wider fov with 1-1/4"
        eyepieces, but you'll get severe vignetting, plus the additional
        light loss and optical distortion associated with putting extra glass
        in the optical path.

        With a 1-1/4" eyepiece at f/10 the best you can do is an actual fov
        of about 32'. With a "full 2" eyepiece you an fov of about 50'.
        Even with a full sized 2" eyepiece (one with a 2" field stop) you'll
        lose more than a magnitude toward the edge of the fov unless you open
        up your optical path to 2" - which is why I developed the EyeOpener.

        Incidently, I developed this for my personal use. Only later did it
        occur to me that 1) other astronomers would need this and 2) I
        already had a manufacturing business dealing with precision machined
        parts so I might as well make it available to others.

        2" eyepieces are a fairly recent development, and the long and the
        short of it is 2 INCH EYEPIECES ARE WHERE IT'S AT for wide to medium
        field viewing.

        Clear skies,

        Pete Peterson

        ps. Sorry if this sounds like a commercial. Study the charts &
        graphs and you'll understand better.



        --- In lx90@yahoogroups.com, "time_to_enjoy_life" <mta472@...> wrote:
        >
        > That's what someone replied when I posted this problem on another
        > forum and I am skeptical:
        >
        > I have a 12 inch LX90 with a 1.25 inch WO diagonal and a Borg fine
        > focuser mounted between the diagonal and the EP.
        >
        > A few nights ago I was observing the full moon with a Tele Vue 32
        mm
        > Plossl. With a 12 inch LX90, the TV has a Field of view of 31', so
        > I should just about be able to fit the whole full moon in the field
        > of view.
        >
        > It looked like I was only seeing about half of it, so I attached my
        > Atik focal reducer to the Tele Vue. The Atik should give reduction
        > of .44 to .60, so you should see a field of view of 52 to 70'.
        >
        > When I tried it on the moon, I folound that I could barely fit the
        > full moon into the field of view, suggesting that the field of view
        > was now only about 33 degrees?
        >
        > This person claims that an f/10 scope with 6 inches of light path -
        > in reality more like 3 or 4 - through stuff hanging on the back of
        > the scope is actually operating about f/12. Even if that were
        true,
        > would going from f/10 to f/12 reduce the field of view by 50% ?
        >
        > They also claim that the only way to go beyond the 30' restriction
        > in field of view is to replace the visual back with an Eye Opener,
        > get a 2 inch diagonal, a Starizona 2-speed focuser and dump all of
        > my 1.25 inch Ep's for 2 inch EP's.
        >
        > Are they correct?
        >
        > Has anyone out there measured the field of view of a 12 inch LX90
        > with 1.25 inch EP's?
        >
        > I would greatly appreciate any advise you can give me.
        >
        >
        > Enjoy the day........Mike
        >
      • stargazerlx90
        Hi all, Hope you don t mind my question about the EyeOpener , but would not the 1209 Microfocuser reduce the aperture back to less than 2 anyway?. As i have
        Message 3 of 4 , Sep 2 2:06 AM
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          Hi all,
          Hope you don't mind my question about the "EyeOpener", but would not
          the 1209 Microfocuser reduce the aperture back to less than 2"
          anyway?.
          As i have a 12" LX90 with Microfocuser i'm finding this very
          interesting.
          Regards.
          Jeff.



          -- In lx90@yahoogroups.com, "barringtonri" <pecorp@...> wrote:
          >
          > Hi Mike,
          >
          > They're somewhat correct. Your scope has a 3000-something focal
          > length, and that's pretty long. Field of view is a function of
          focal
          > length and the size of the eyepiece field stop. I developed the
          > EyeOpener about 7 years ago as the best way to get the widest
          > possible field of view out of my 10" LX200 Classic. There's a full
          > explaination of what's happening, including a short paper by Doc G,
          > to be found at
          > [Url]http://www.petersonengineering.com/sky/EO.htm%5b/Url]
          >
          > Yes, using a focal reducer will give you a wider fov with 1-1/4"
          > eyepieces, but you'll get severe vignetting, plus the additional
          > light loss and optical distortion associated with putting extra
          glass
          > in the optical path.
          >
          > With a 1-1/4" eyepiece at f/10 the best you can do is an actual fov
          > of about 32'. With a "full 2" eyepiece you an fov of about 50'.
          > Even with a full sized 2" eyepiece (one with a 2" field stop)
          you'll
          > lose more than a magnitude toward the edge of the fov unless you
          open
          > up your optical path to 2" - which is why I developed the EyeOpener.
          >
          > Incidently, I developed this for my personal use. Only later did
          it
          > occur to me that 1) other astronomers would need this and 2) I
          > already had a manufacturing business dealing with precision
          machined
          > parts so I might as well make it available to others.
          >
          > 2" eyepieces are a fairly recent development, and the long and the
          > short of it is 2 INCH EYEPIECES ARE WHERE IT'S AT for wide to
          medium
          > field viewing.
          >
          > Clear skies,
          >
          > Pete Peterson
          >
          > ps. Sorry if this sounds like a commercial. Study the charts &
          > graphs and you'll understand better.
          >
          >
          >
          > --- In lx90@yahoogroups.com, "time_to_enjoy_life" <mta472@> wrote:
          > >
          > > That's what someone replied when I posted this problem on another
          > > forum and I am skeptical:
          > >
          > > I have a 12 inch LX90 with a 1.25 inch WO diagonal and a Borg
          fine
          > > focuser mounted between the diagonal and the EP.
          > >
          > > A few nights ago I was observing the full moon with a Tele Vue 32
          > mm
          > > Plossl. With a 12 inch LX90, the TV has a Field of view of 31',
          so
          > > I should just about be able to fit the whole full moon in the
          field
          > > of view.
          > >
          > > It looked like I was only seeing about half of it, so I attached
          my
          > > Atik focal reducer to the Tele Vue. The Atik should give
          reduction
          > > of .44 to .60, so you should see a field of view of 52 to 70'.
          > >
          > > When I tried it on the moon, I folound that I could barely fit
          the
          > > full moon into the field of view, suggesting that the field of
          view
          > > was now only about 33 degrees?
          > >
          > > This person claims that an f/10 scope with 6 inches of light
          path -
          > > in reality more like 3 or 4 - through stuff hanging on the back
          of
          > > the scope is actually operating about f/12. Even if that were
          > true,
          > > would going from f/10 to f/12 reduce the field of view by 50% ?
          > >
          > > They also claim that the only way to go beyond the 30'
          restriction
          > > in field of view is to replace the visual back with an Eye
          Opener,
          > > get a 2 inch diagonal, a Starizona 2-speed focuser and dump all
          of
          > > my 1.25 inch Ep's for 2 inch EP's.
          > >
          > > Are they correct?
          > >
          > > Has anyone out there measured the field of view of a 12 inch LX90
          > > with 1.25 inch EP's?
          > >
          > > I would greatly appreciate any advise you can give me.
          > >
          > >
          > > Enjoy the day........Mike
          > >
          >
        • barringtonri
          The Microfocuser has a 2 optical path so there s no problem. Pete Peterson ... not ... full ... G, ... fov ... EyeOpener. ... the ... another ... 32 ... 31 ,
          Message 4 of 4 , Sep 3 2:01 PM
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            The Microfocuser has a 2" optical path so there's no problem.

            Pete Peterson

            --- In lx90@yahoogroups.com, "stargazerlx90" <stargazerlx90@...>
            wrote:
            >
            > Hi all,
            > Hope you don't mind my question about the "EyeOpener", but would
            not
            > the 1209 Microfocuser reduce the aperture back to less than 2"
            > anyway?.
            > As i have a 12" LX90 with Microfocuser i'm finding this very
            > interesting.
            > Regards.
            > Jeff.
            >
            >
            >
            > -- In lx90@yahoogroups.com, "barringtonri" <pecorp@> wrote:
            > >
            > > Hi Mike,
            > >
            > > They're somewhat correct. Your scope has a 3000-something focal
            > > length, and that's pretty long. Field of view is a function of
            > focal
            > > length and the size of the eyepiece field stop. I developed the
            > > EyeOpener about 7 years ago as the best way to get the widest
            > > possible field of view out of my 10" LX200 Classic. There's a
            full
            > > explaination of what's happening, including a short paper by Doc
            G,
            > > to be found at
            > > [Url]http://www.petersonengineering.com/sky/EO.htm%5b/Url]
            > >
            > > Yes, using a focal reducer will give you a wider fov with 1-1/4"
            > > eyepieces, but you'll get severe vignetting, plus the additional
            > > light loss and optical distortion associated with putting extra
            > glass
            > > in the optical path.
            > >
            > > With a 1-1/4" eyepiece at f/10 the best you can do is an actual
            fov
            > > of about 32'. With a "full 2" eyepiece you an fov of about 50'.
            > > Even with a full sized 2" eyepiece (one with a 2" field stop)
            > you'll
            > > lose more than a magnitude toward the edge of the fov unless you
            > open
            > > up your optical path to 2" - which is why I developed the
            EyeOpener.
            > >
            > > Incidently, I developed this for my personal use. Only later did
            > it
            > > occur to me that 1) other astronomers would need this and 2) I
            > > already had a manufacturing business dealing with precision
            > machined
            > > parts so I might as well make it available to others.
            > >
            > > 2" eyepieces are a fairly recent development, and the long and
            the
            > > short of it is 2 INCH EYEPIECES ARE WHERE IT'S AT for wide to
            > medium
            > > field viewing.
            > >
            > > Clear skies,
            > >
            > > Pete Peterson
            > >
            > > ps. Sorry if this sounds like a commercial. Study the charts &
            > > graphs and you'll understand better.
            > >
            > >
            > >
            > > --- In lx90@yahoogroups.com, "time_to_enjoy_life" <mta472@>
            wrote:
            > > >
            > > > That's what someone replied when I posted this problem on
            another
            > > > forum and I am skeptical:
            > > >
            > > > I have a 12 inch LX90 with a 1.25 inch WO diagonal and a Borg
            > fine
            > > > focuser mounted between the diagonal and the EP.
            > > >
            > > > A few nights ago I was observing the full moon with a Tele Vue
            32
            > > mm
            > > > Plossl. With a 12 inch LX90, the TV has a Field of view of
            31',
            > so
            > > > I should just about be able to fit the whole full moon in the
            > field
            > > > of view.
            > > >
            > > > It looked like I was only seeing about half of it, so I
            attached
            > my
            > > > Atik focal reducer to the Tele Vue. The Atik should give
            > reduction
            > > > of .44 to .60, so you should see a field of view of 52 to 70'.
            > > >
            > > > When I tried it on the moon, I folound that I could barely fit
            > the
            > > > full moon into the field of view, suggesting that the field of
            > view
            > > > was now only about 33 degrees?
            > > >
            > > > This person claims that an f/10 scope with 6 inches of light
            > path -
            > > > in reality more like 3 or 4 - through stuff hanging on the back
            > of
            > > > the scope is actually operating about f/12. Even if that were
            > > true,
            > > > would going from f/10 to f/12 reduce the field of view by 50% ?
            > > >
            > > > They also claim that the only way to go beyond the 30'
            > restriction
            > > > in field of view is to replace the visual back with an Eye
            > Opener,
            > > > get a 2 inch diagonal, a Starizona 2-speed focuser and dump all
            > of
            > > > my 1.25 inch Ep's for 2 inch EP's.
            > > >
            > > > Are they correct?
            > > >
            > > > Has anyone out there measured the field of view of a 12 inch
            LX90
            > > > with 1.25 inch EP's?
            > > >
            > > > I would greatly appreciate any advise you can give me.
            > > >
            > > >
            > > > Enjoy the day........Mike
            > > >
            > >
            >
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