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Prism vs. Mirror Diagonals

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  • rjmodic
    I have a 80mm ZSFD (10th anniversary model) and am looking to buy a good 90 degree diagonal for it. I prefer a view that is not mirror reversed. Inverted
    Message 1 of 7 , Apr 29, 2007
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      I have a 80mm ZSFD (10th anniversary model) and am looking to
      buy a good 90 degree diagonal for it. I prefer a view that is not
      mirror reversed. Inverted images are not a problem for me, but a
      flipped image is. I've only had experience with lower quality
      correct image diagonals and these definitely degrade the image.

      1) Is it possible for a high quality prism diagonal to perform
      as well as a good mirror diagonal or is this theoretically
      impossible?

      2) Does anyone know if WO will offer a 1.25" version of their 2"
      correct image diagonal?

      3) Do the prisms in binoviewers degrade the image at all?

      4) What about using 2 mirror diagonals together to get a correct
      view?


      Bob
    • erlend_langsrud
      ... I believe it s theoretically impossible because the light passes through a lot of glass with flat entry and exit surfaces, which will introduce some
      Message 2 of 7 , Apr 30, 2007
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        > 1) Is it possible for a high quality prism diagonal to perform
        > as well as a good mirror diagonal or is this theoretically
        > impossible?

        I believe it's theoretically impossible because the light passes
        through a lot of glass with flat entry and exit surfaces, which will
        introduce some spherical abberation. The effect is more pronounced with
        fast scopes.

        Also, with a mirror diagonal there is only one reflection. With a 45
        deg correct image diagonal there are 3 reflections and 2 refractions.
        The light will be affected by optical surfaces 5 times. Theoretically
        not a problem, but...

        In fact, I did compare the WO 1.25" correct image diagonal with the WO
        2" mirror diagonal last time I was observing the moon. The 2" was
        clearly better at high magnifications.

        I'm not saying there's anything wrong withe correct image diagonal.
        It's great for low to medium power and terrestioal viewing, but
        it's just not my choice for high power observations.

        > 2) Does anyone know if WO will offer a 1.25" version of their 2"
        > correct image diagonal?

        They do, don't they? I've got one.

        >
        > 3) Do the prisms in binoviewers degrade the image at all?
        >
        > 4) What about using 2 mirror diagonals together to get a correct
        > view?
        For low to medium power, use a correct image diagonal, no problem. For
        high powers - I'm sorry, but my advice is to use a single mirror
        diagonal. You'll get used to it.

        Erlend

        >
        >
        > Bob
        >
      • Chris Lord
        What Eriend says is quite correct. The only exception is when observing the Sun in H-alpha. Mirror diagonals with multi-coated a/r surfaces or dielectric
        Message 3 of 7 , Apr 30, 2007
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          What Eriend says is quite correct. The only exception is when observing
          the Sun in H-alpha. Mirror diagonals with multi-coated a/r surfaces or
          dielectric surfaces produce narrow angle scatter, which although not a
          problem at night, even on the planets or bright stars, is a problem
          when observing the Sun in H-alpha. Because the beam needs to be slower
          than f/28 for a tail filter unit, a zenith prism is better (not an
          Amici roof prism), because it scatters less light, and at f/28 or
          slower spherical aberration is not a problem either.

          Chris Lord

          On 30 Apr 2007, at 13:52, erlend_langsrud wrote:

          > > 1) Is it possible for a high quality prism diagonal to perform
          > > as well as a good mirror diagonal or is this theoretically
          > > impossible?
          >
          > I believe it's theoretically impossible because the light passes
          > through a lot of glass with flat entry and exit surfaces, which will
          > introduce some spherical abberation. The effect is more pronounced
          > with
          > fast scopes.
          >
          > Also, with a mirror diagonal there is only one reflection. With a 45
          > deg correct image diagonal there are 3 reflections and 2 refractions.
          > The light will be affected by optical surfaces 5 times. Theoretically
          > not a problem, but...
          >
          > In fact, I did compare the WO 1.25" correct image diagonal with the WO
          > 2" mirror diagonal last time I was observing the moon. The 2" was
          > clearly better at high magnifications.
          >
          > I'm not saying there's anything wrong withe correct image diagonal.
          > It's great for low to medium power and terrestioal viewing, but
          > it's just not my choice for high power observations.
          >
          > > 2) Does anyone know if WO will offer a 1.25" version of their 2"
          > > correct image diagonal?
          >
          > They do, don't they? I've got one.
          >
          > >
          > > 3) Do the prisms in binoviewers degrade the image at all?
          > >
          > > 4) What about using 2 mirror diagonals together to get a correct
          > > view?
          > For low to medium power, use a correct image diagonal, no problem. For
          > high powers - I'm sorry, but my advice is to use a single mirror
          > diagonal. You'll get used to it.
          >
          > Erlend
          >
          > >
          > >
          > > Bob
          > >
          >
          >
          >
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        • Chris Lord
          What Eriend says is quite correct. The only exception is when observing the Sun in H-alpha. Mirror diagonals with multi-coated a/r surfaces or dielectric
          Message 4 of 7 , Apr 30, 2007
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            What Eriend says is quite correct. The only exception is when observing
            the Sun in H-alpha. Mirror diagonals with multi-coated a/r surfaces or
            dielectric surfaces produce narrow angle scatter, which although not a
            problem at night, even on the planets or bright stars, is a problem
            when observing the Sun in H-alpha. Because the beam needs to be slower
            than f/28 for a tail filter unit, a zenith prism is better (not an
            Amici roof prism), because it scatters less light, and at f/28 or
            slower spherical aberration is not a problem either.

            Chris Lord

            On 30 Apr 2007, at 13:52, erlend_langsrud wrote:

            > > 1) Is it possible for a high quality prism diagonal to perform
            > > as well as a good mirror diagonal or is this theoretically
            > > impossible?
            >
            > I believe it's theoretically impossible because the light passes
            > through a lot of glass with flat entry and exit surfaces, which will
            > introduce some spherical abberation. The effect is more pronounced
            > with
            > fast scopes.
            >
            > Also, with a mirror diagonal there is only one reflection. With a 45
            > deg correct image diagonal there are 3 reflections and 2 refractions.
            > The light will be affected by optical surfaces 5 times. Theoretically
            > not a problem, but...
            >
            > In fact, I did compare the WO 1.25" correct image diagonal with the WO
            > 2" mirror diagonal last time I was observing the moon. The 2" was
            > clearly better at high magnifications.
            >
            > I'm not saying there's anything wrong withe correct image diagonal.
            > It's great for low to medium power and terrestioal viewing, but
            > it's just not my choice for high power observations.
            >
            > > 2) Does anyone know if WO will offer a 1.25" version of their 2"
            > > correct image diagonal?
            >
            > They do, don't they? I've got one.
            >
            > >
            > > 3) Do the prisms in binoviewers degrade the image at all?
            > >
            > > 4) What about using 2 mirror diagonals together to get a correct
            > > view?
            > For low to medium power, use a correct image diagonal, no problem. For
            > high powers - I'm sorry, but my advice is to use a single mirror
            > diagonal. You'll get used to it.
            >
            > Erlend
            >
            > >
            > >
            > > Bob
            > >
            >
            >
            >
            > Messages in this topic (0) Reply (via web post) | Start a new
            > topic
            >
            > Messages | Files | Photos | Links | Database | Polls | Calendar
            >                              www.william-optics.com
            > <ma_grp_160.gif>
            > Change settings via the Web (Yahoo! ID required)
            > Change settings via email: Switch delivery to Daily Digest | Switch
            > format to Traditional
            > Visit Your Group | Yahoo! Groups Terms of Use | Unsubscribe
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            ________________________________________________________
            ========================================================
            Chris Lord
            Brayebrook Observatory
            30 Harlton Road
            Little Eversden
            Cambridge
            CB3 7HB
            tel (+44) 01223 263481
            http://www.brayebrookobservatory.org/
            =========================================================
            _________________________________________________________
          • Scott Walker
            Here s the straight poop. If you use a standard 90 degree prism that was perfectly made, you will see some chromatic aberation caused by the prism. The
            Message 5 of 7 , Apr 30, 2007
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              Here's the straight poop. If you use a standard 90 degree prism that was perfectly made, you will see some chromatic aberation caused by the prism. The diagonal manufacturer could remove a lot if not all of this color aberation with a correction lens, but I do not believe many do. If one uses a prism like a 45 degree prism which produces a fully correct image another errors is generated. This error are caused by slitting the path. This addition errors greatly reduce the resolution limit of the scope in one axis. This error can be at least reduced by using what is called "phase coatings". Phase coatings are common in better in roof prism binoculars, but I have not seen them used in telescope dianganals. The 45 degree prisms also have longer path length than a simple 90 degree prism so the color error well be more. I believe "upper-end" binoviewers have non roof prisms, so they do not need phase coating. I have also heard that they use correction lenses to cancel the color error due to the path length.
              A high-end scoping scope using prisms but the whole design is done with the prisms in mind so the prisms do not cause large errors. A 45 degree prism can also generate a spike due to the edge of the slit. This is dependent on the quality of the prism.
               
              I believe that 90 degree "correct" image diagonals also use a roof prism so they will have the same error as a 45 degree diagonal. I am not sure of this so if some one knows for sure, please let the group know.
               
              4) I am not sure if you can do it with just two diagonals. In binoculars 4 reflective surfaces are used. 
               
              I think the bottom line is if you real need a " fully corrected" image and you want the very good image quality buy a spotting scope. If you want the best quality image buy a telescope, but it will not be a "fully corrected" image.
               
              I hope this helps
               
              Scott Walker
              ----- Original Message -----
              From: rjmodic
              Sent: Sunday, April 29, 2007 10:09 PM
              Subject: [William-Optics] Prism vs. Mirror Diagonals

              I have a 80mm ZSFD (10th anniversary model) and am looking to
              buy a good 90 degree diagonal for it. I prefer a view that is not
              mirror reversed. Inverted images are not a problem for me, but a
              flipped image is. I've only had experience with lower quality
              correct image diagonals and these definitely degrade the image.

              1) Is it possible for a high quality prism diagonal to perform
              as well as a good mirror diagonal or is this theoretically
              impossible?

              2) Does anyone know if WO will offer a 1.25" version of their 2"
              correct image diagonal?

              3) Do the prisms in binoviewers degrade the image at all?

              4) What about using 2 mirror diagonals together to get a correct
              view?

              Bob

            • rjmodic
              Thanks everyone for the replies. I m looking to bend the light 90 degrees so I won t be using a 45 degree diagonal. Since the 80mm ZSFD is f/7, I m guessing
              Message 6 of 7 , May 1, 2007
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                Thanks everyone for the replies. I'm looking to bend the light 90
                degrees so I won't be using a 45 degree diagonal. Since the 80mm ZSFD
                is f/7, I'm guessing a prism would produce some aberration. I may
                experiment using two good quality mirror diagonals. The main concern
                here is to avoid vignetting by stacking the two diagonals. Maybe if I
                paired 1.25" and 2" mirror diagonals I can avoid vignetting.

                Bob
              • williamopticsmarketing
                ... Practically impossible. ... We already do: http://www.williamoptics.com/diagonalPrism/diaPrism_erecting.htm If you mean looking like the 2 , then consider
                Message 7 of 7 , May 1, 2007
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                  --- In William-Optics@yahoogroups.com, "rjmodic" <rjmodic@...> wrote:
                  >
                  > I have a 80mm ZSFD (10th anniversary model) and am looking to
                  > buy a good 90 degree diagonal for it. I prefer a view that is not
                  > mirror reversed. Inverted images are not a problem for me, but a
                  > flipped image is. I've only had experience with lower quality
                  > correct image diagonals and these definitely degrade the image.
                  >
                  > 1) Is it possible for a high quality prism diagonal to perform
                  > as well as a good mirror diagonal or is this theoretically
                  > impossible?

                  Practically impossible.

                  > 2) Does anyone know if WO will offer a 1.25" version of their 2"
                  > correct image diagonal?

                  We already do:

                  http://www.williamoptics.com/diagonalPrism/diaPrism_erecting.htm

                  If you mean "looking like the 2", then consider this one from a very
                  old batch:

                  http://tinyurl.com/2lpn5w

                  (yes this item is from William Optics!)

                  If you mean 90 deg., the answer is no.


                  > 3) Do the prisms in binoviewers degrade the image at all?

                  Yes they do.



                  > 4) What about using 2 mirror diagonals together to get a correct
                  > view?

                  mmmmm...

                  Why not simply live with it and get a higher quality prism such as
                  this:

                  www.williamoptics.com/diagonalPrism/erectPrism2in90deg_features.htm

                  This prism is 90 deg. and performs very well even for astronomy.

                  Personally though, I would simply go with a good quality dielectric
                  diagonal...
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