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Largest FOV with UC22

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  • Stephan
    Hi, I own an Obsession 22UC and I wonder what is the widest FOV I can expect with it without any obstruction, vignetage... Right now, I am using a Nagler 5
    Message 1 of 6 , Jan 17, 2012
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      Hi,
      I own an Obsession 22UC and I wonder what is the widest FOV I can expect with it without any obstruction, vignetage...
      Right now, I am using a Nagler 5 31mm that give me 1° TFOV.
      Should I go for another eyepiece or a focal reducer?
    • Roger Greenwood
      I found the 31mm nagler and the 21mm Ethos have about the same FOV, but the Ethos has more of the space walk feeling with the higher Mag. I couldn t get used
      Message 2 of 6 , Jan 17, 2012
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        I found the 31mm nagler and the 21mm Ethos have about the same FOV, but the Ethos
        has more of the "space walk" feeling with the higher Mag. I couldn't get used to the
        41mm Panoptic as I could see some of the secondary obstruction and didn't like the
        lower Mag FOV. I think the Pan is very close to the FOV of the 31mm Nag. I haven't
        seen any views with the new 120 degree FOV from Explorer Scientific. I am not sure they
        are available yet. Cloudy nights has some conversations that are worth looking at.
        I have a friend that did get a view and said it was very impressive. Personally, I think
        the comfort level with the Explorers are not ideal as the bigger ones are extremely wide.

        -Roger
        18" Obsession
        www.astrocrumb.com

        On Jan 17, 2012, at 7:24 AM, Stephan wrote:

        > Hi,
        > I own an Obsession 22UC and I wonder what is the widest FOV I can expect with it without any obstruction, vignetage...
        > Right now, I am using a Nagler 5 31mm that give me 1° TFOV.
        > Should I go for another eyepiece or a focal reducer?
        >
        >
        >
        > ------------------------------------
        >
        > Yahoo! Groups Links
        >
        >
        >
      • Nils Olof Carlin
        The limiting factors are focal length of mirror, and field stop diameter of the EP used. And of course if you use a Paracorr or other coma corrector. Of the
        Message 3 of 6 , Jan 17, 2012
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          The limiting factors are focal length of mirror, and field stop
          diameter of the EP used. And of course if you use a Paracorr or other
          coma corrector.
          Of the Nagler EPs, your 31 mm has the widest realistic field stop with
          42 mm and without paracorr a 1.02 deg FOV, with it 0.89 deg. I don't
          think there are (or could be) any 2" EPs with significantly larger
          field stops. Anyway, the exit pupil would prevent much larger EPs.
          And a focal reducer wouldn't help, I suspect. So be happy with what
          you have... and get a smaller rich-field telescope (and/or birding
          binoculars) for the really wide fields...
          .
          Nils Olof

          I own an Obsession 22UC and I wonder what is the widest FOV I can
          expect with it without any obstruction, vignetage...
          Right now, I am using a Nagler 5 31mm that give me 1° TFOV.
          Should I go for another eyepiece or a focal reducer?
        • Stephan
          Thank you both for your informations and advices. I will stay with my actual eyepiece set (Nagler 31, Ethos 21, 13 and 8).
          Message 4 of 6 , Jan 17, 2012
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            Thank you both for your informations and advices.
            I will stay with my actual eyepiece set (Nagler 31, Ethos 21, 13 and 8).


            --- In bigdob@yahoogroups.com, "Stephan" <stecaro2002@...> wrote:
            >
            > Hi,
            > I own an Obsession 22UC and I wonder what is the widest FOV I can expect with it without any obstruction, vignetage...
            > Right now, I am using a Nagler 5 31mm that give me 1° TFOV.
            > Should I go for another eyepiece or a focal reducer?
            >
          • John Mahony
            ... 2 =50.8mm.  Allowing for thickness of the barrel wall, it s still possible.  Televue 55mm Plossl and 41 Pan both have a field stop of 46mm. ... At f/4.2
            Message 5 of 6 , Jan 22, 2012
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              > From: Nils Olof Carlin <nilsolof.carlin@...>
              >
              > The limiting factors are focal length of mirror, and field stop
              > diameter of the EP used. And of course if you use a Paracorr or other
              > coma corrector.
              > Of the Nagler EPs, your 31 mm has the widest realistic field stop with
              > 42 mm and without paracorr a 1.02 deg FOV, with it 0.89 deg. I don't
              > think there are (or could be) any 2" EPs with significantly larger
              > field stops.

              2"=50.8mm.  Allowing for thickness of the barrel wall, it's still possible.  Televue 55mm Plossl and 41 Pan both have a field stop of 46mm.

              > Anyway, the exit pupil would prevent much larger EPs.

              At f/4.2 he wouldn't want to use either of those.  I'm waiting for TV to come out with a lower power Nagler or Ethos with a 46mm field stop.

              > And a focal reducer wouldn't help, I suspect.

              Right.  You'd just get vignetting if you tried using it with a wide low power EP.

              -John


              > So be happy with what
              > you have... and get a smaller rich-field telescope (and/or birding
              > binoculars) for the really wide fields...
              > .
              > Nils Olof
              >
              > I own an Obsession 22UC and I wonder what is the widest FOV I can
              > expect with it without any obstruction, vignetage...
              > Right now, I am using a Nagler 5 31mm that give me 1° TFOV.
              > Should I go for another eyepiece or a focal reducer?
            • Nils Olof Carlin
              John, I wrote ... with 42 mm and without paracorr a 1.02 deg FOV, with it 0.89 deg. I don t ... field stops. ... possible.  Televue 55mm Plossl and 41 Pan
              Message 6 of 6 , Jan 22, 2012
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                John,


                I wrote
                >> Of the Nagler EPs, your 31 mm has the widest realistic field stop
                with 42 mm and without paracorr a 1.02 deg FOV, with it 0.89 deg. I
                don't
                > think there are (or could be) any 2" EPs with significantly larger
                field stops.>>>

                You replied:
                >2"=50.8mm.  Allowing for thickness of the barrel wall, it's still
                possible.  Televue 55mm Plossl and 41 Pan both have a field stop of
                46mm.

                ....  I'm waiting for TV to come out with a lower power Nagler or
                Ethos with a 46mm field stop.>

                I wonder if there is much room for improvement here. Neither the
                largest Plössl (useful above f/8) nor the Panoptic (useful above f/6)
                have negative field elements - a necessity for a well corrected very
                wide field EP such as any of the Nagler or Ethos designs. And in order
                not to demand unrealistic in travel of the focuser, this field lens
                must be mounted within the barrel, necessitating some 2 mm extra
                hardware, bringing the total down to some 42 mm of realistic usable
                field. Also, the virtual field stop is a bit inside the EP,
                necessitating some extra clearance not to vignet a fast light cone at
                the barrel.
                But it seems there are some eager designers out there, and who knows
                if one comes up with some trick to circumvent this likely design
                limit.

                Nils Olof
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