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Air-spaced versus oil-spaced lens elements?

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  • rfrisk
    Could someone please direct me to a resource that discusses this topic? I am looking to purchase a 105 in the future and believe I read somewhere about
    Message 1 of 12 , Jan 21, 2006
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      Could someone please direct me to a resource that discusses this
      topic? I am looking to purchase a 105 in the future and believe I read
      somewhere about thermal efficiency of one or the other. I really want
      to perform due diligence before making a purchase decision.

      Thanks for your comments.

      raf
    • Kurt Friedrich
      My suggestion is to join the Yahoo Astro-Physics group and then search their message archieves. I consider Roland to be the expert in design, and he has done
      Message 2 of 12 , Jan 21, 2006
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        My suggestion is to join the Yahoo Astro-Physics group
        and then search their message archieves. I consider
        Roland to be the expert in design, and he has done
        both kinds. This has been discussed there many times.
        Seems like he used to do more oil, but lately had gone
        to air.

        Kurt

        --- rfrisk <rfrisk@...> wrote:

        > Could someone please direct me to a resource that
        > discusses this
        > topic? I am looking to purchase a 105 in the future
        > and believe I read
        > somewhere about thermal efficiency of one or the
        > other. I really want
        > to perform due diligence before making a purchase
        > decision.
        >
        > Thanks for your comments.
        >
        > raf
        >
        >
        >
        >


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      • rfrisk
        Hi Curt -thanks for taking the time to respond, I appreciate it. Will check out the AP group. Cheers
        Message 3 of 12 , Jan 21, 2006
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          Hi Curt -thanks for taking the time to respond, I appreciate it. Will
          check out the AP group.

          Cheers

          --- In William-Optics@yahoogroups.com, Kurt Friedrich
          <kurt_friedrich@y...> wrote:
          >
          > My suggestion is to join the Yahoo Astro-Physics group
          > and then search their message archieves.
        • ronbee
          Here are some answers to your questions. http://www.astromart.com/forums/viewpost.asp?forum_post_id=384424&poll_id=&news_id=&page=
          Message 4 of 12 , Jan 21, 2006
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            Here are some answers to your questions.

            http://www.astromart.com/forums/viewpost.asp?forum_post_id=384424&poll_id=&news_id=&page=
            http://www.astromart.com/forums/viewpost.asp?forum_post_id=405916&poll_id=&news_id=&page=
            http://www.astromart.com/forums/viewpost.asp?forum_post_id=408311&poll_id=&news_id=&page=

            Ron B[ee]

            --- In William-Optics@yahoogroups.com, "rfrisk" <rfrisk@G...> wrote:

            >
            > Could someone please direct me to a resource that discusses this
            > topic? I am looking to purchase a 105 in the future and believe I read
            > somewhere about thermal efficiency of one or the other. I really want
            > to perform due diligence before making a purchase decision.
            >
            > Thanks for your comments.
            >
            > raf
            >

          • Robin Retzlaff
            We could try and start a discussion of this issue here if you want. FWIW I ll share with you some of my gleanings and hope for someone more knowledgeable to
            Message 5 of 12 , Jan 22, 2006
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                   We could try and start a discussion of this issue here if you want. FWIW I'll share with you some of my gleanings and hope for someone more knowledgeable to post........
                   The curvature radiuses of the mating surfaces in an oil-spaced design must be identical since usually only a drop or so of oil is used as an immersion. This leaves less freely variable parameters for the lens designer than an air-spaced one because you can only vary the surrounding surfaces and nothing else. Theoretically then, air-spaced lenses can be better corrected for optical aberration than oil-spaced. But in practice I understand that at least at f/6 and above there are still enough free parameters in an oil-spaced triplet to make it "practically" as well corrected as a similar air-spaced one. However, with an oil-spaced doublet apparently there aren't enough free parameters to correct for spherical aberration, coma, and chromatic aberration at the same time.....which is why you don't find any on the market. Oil-spaced lenses have only 2 glass to air surfaces which reduces the possibility of internal light reflections which can cause ghosting problems. The light throughput and contrast are meant to be better in an oil-spaced for this reason too. Both air-spaced and oil-spaced triplets can be used to make optically highly corrected lenses. There is supposed to be a better built-in tolerance of manufacturing errors with an oil-spaced design since curvature radiuses and lens member thicknesses can vary slightly from theoretical values without much effect......so having a lesser degree of design freedom also means the manufacturer has an equally small chance to produce a bad lens. Air-spaced lenses are supposed to be extremely sensitive to changes in the radii of the 4 surfaces facing the air spaces and the parallel positioning of the surfaces themselves. Changing one can cause severe spherical aberration problems. With an oil-spaced design manufacturing deviances are supposedly better tolerated. I reckon with todays automated manufacturing processes this isn't much of a problem though. 
                   The main thermal problem with air-spaced lenses are the air spaces. Most air-spaced triplets have rather wide air-spaces because you can reduce the sphero-chromatism of an apo triplet lens this way.With a doublet however its supposedly more complicated because you must sacrifice some coma correction to do it. (by the same token with an oil-spaced design you cannot control spherochromatism this way at all and in this way they can be less than optimal) Unfortunately air is also a very poor heat conductor (1000x worse than oil) and if the all important central element of a triplet is thermally isolated from the others by wide air spaces then the only place it can transfer heat is its edge where it contacts the lens cell. Also, the center thickness of this element is much thicker and can take alot longer to to cool down than the edge. Oil-spaced lenses therefore should cool faster and in a more homogeneous way.This problem is accentuated by fast focal ratios because the faster the lens the thicker the lens elements must be and the thicker the elements the slower the cooldown. However, the faster the focal ratio the more the possible improved corrections of an air-spaced design become significant. Below f/6.5 or so I'm told the improved spherochromatism of a good airspaced lens can produce visibly less false color. I'm also told the cool-down issue is a much more significant one with larger apertures and for critical planetary observation. If I were considering a 5 or 6 inch scope for critical planetary observing I would look for a long focus oil-spaced design for it's superior constrast, better thermal performance, and lack of need for barlows to achieve adequate magnification. For a 105mm scope where the thermal issues are greatly lessened, and at fast focal ratios for widefield use a good air-spaced design might serve you better. William Optics products come highly recommended. The amazing Flt-110, by the way has all the advantages of an oil-spaced design and with its f ratio is perhaps near the limit of whats possible in an oil-spaced. If I were looking for a fast scope in that aperture I think you probably can't do much better. And the price seems like an absolute bargain to me. Tom Trussock has a good review of it on cloudy nights. Good luck with the research....the ideas and the learning is half the fun. 

              rfrisk <rfrisk@...> wrote:
              Could someone please direct me to a resource that discusses this
              topic?  I am looking to purchase a 105 in the future and believe I read
              somewhere about thermal efficiency of one or the other.  I really want
              to perform due diligence before making a purchase decision.

              Thanks for your comments.

              raf





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            • rfrisk
              Great Scott, Robin - you just addressed about 99% of my questions in one post!! My reason for purchasing this scope is casual planetary observation and serious
              Message 6 of 12 , Jan 22, 2006
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                Great Scott, Robin - you just addressed about 99% of my questions in
                one post!! My reason for purchasing this scope is casual planetary
                observation and serious DSO observation and imaging. I have been
                strongly favoring the WO FLT-110 and appreciate the heads up for
                Tom's review. I really appreciate you taking the time to reply with
                such an informative response. It helped me out more than you know.

                Thanks again.

                raf


                --- In William-Optics@yahoogroups.com, Robin Retzlaff
                <robinretzlaff@y...> wrote:
                >
                > We could try and start a discussion of this issue here if you
                want. FWIW I'll share with you some of my gleanings and hope for
                someone more knowledgeable to post........
                >The curvature radiuses of the mating surfaces in an oil-spaced
                design must be .....
                >The main thermal problem with air-spaced lenses are the .....
                >The amazing Flt-110, by the way has all the advantages of an oil-
                spaced design and with its......
              • Robin Retzlaff
                Glad to help. Hopefully we ll get opinions from others and mix it up a bit. rfrisk wrote: Great Scott, Robin - you just addressed about 99%
                Message 7 of 12 , Jan 22, 2006
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                  Glad to help. Hopefully we'll get opinions from others and mix it up a bit.

                  rfrisk <rfrisk@...> wrote:
                  Great Scott, Robin - you just addressed about 99% of my questions in
                  one post!! My reason for purchasing this scope is casual planetary
                  observation and serious DSO observation and imaging. I have been
                  strongly favoring the WO FLT-110 and appreciate the heads up for
                  Tom's review. I really appreciate you taking the time to reply with
                  such an informative response. It helped me out more than you know.

                  Thanks again.

                  raf


                  --- In William-Optics@yahoogroups.com, Robin Retzlaff
                  <robinretzlaff@y...> wrote:
                  >
                  > We could try and start a discussion of this issue here if you
                  want. FWIW I'll share with you some of my gleanings and hope for
                  someone more knowledgeable to post........
                  >The curvature radiuses of the mating surfaces in an oil-spaced
                  design must be .....
                  >The main thermal problem with air-spaced lenses are the .....
                  >The amazing Flt-110, by the way has all the advantages of an oil-
                  spaced design and with its......








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                • Rick Jack
                  Hi, How does the oil hold up to time and elements? I know some oils used in microscope optics discolor and reduce light transmission. I ve been using a 6 f15
                  Message 8 of 12 , Jan 22, 2006
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                    Hi,
                     
                       How does the oil hold up to time and elements?  I know some oils used in microscope optics discolor and reduce light transmission. I've been using a 6" f15 air spaced objective for years without a hitch.
                    Regards,
                    Rick

                    Robin Retzlaff <robinretzlaff@...> wrote:
                    Glad to help. Hopefully we'll get opinions from others and mix it up a bit.

                    rfrisk <rfrisk@...> wrote:
                    Great Scott, Robin - you just addressed about 99% of my questions in
                    one post!! My reason for purchasing this scope is casual planetary
                    observation and serious DSO observation and imaging. I have been
                    strongly favoring the WO FLT-110 and appreciate the heads up for
                    Tom's review. I really appreciate you taking the time to reply with
                    such an informative response. It helped me out more than you know.

                    Thanks again.

                    raf


                    --- In William-Optics@yahoogroups.com, Robin Retzlaff
                    <robinretzlaff@y...> wrote:
                    >
                    > We could try and start a discussion of this issue here if you
                    want. FWIW I'll share with you some of my gleanings and hope for
                    someone more knowledgeable to post........
                    >The curvature radiuses of the mating surfaces in an oil-spaced
                    design must be .....
                    >The main thermal problem with air-spaced lenses are the .....
                    >The amazing Flt-110, by the way has all the advantages of an oil-
                    spaced design and with its......








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                  • Timm Bottoni
                    Excellent information. I will assume it all to be accurate. Between this and the post from Ron, I may be able to consolidate some of this into the FAQs
                    Message 9 of 12 , Jan 22, 2006
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                      Excellent information. I will assume it all to be accurate. Between
                      this and the post from Ron, I may be able to consolidate some of
                      this into the FAQs document I am working on.

                      Thanks

                      Timm

                      --- In William-Optics@yahoogroups.com, Robin Retzlaff
                      <robinretzlaff@y...> wrote:
                      >
                      > We could try and start a discussion of this issue here if you
                      want. FWIW I'll share with you some of my gleanings and hope for
                      someone more knowledgeable to post........
                      > The curvature radiuses of the mating surfaces in an oil-
                      spaced design must be identical since usually only a drop or so of
                      oil is used as an immersion. This leaves less freely variable
                      parameters for the lens designer than an air-spaced one because you
                      can only vary the surrounding surfaces and nothing else.
                      Theoretically then, air-spaced lenses can be better corrected for
                      optical aberration than oil-spaced. But in practice I understand
                      that at least at f/6 and above there are still enough free
                      parameters in an oil-spaced triplet to make it "practically" as well
                      corrected as a similar air-spaced one. However, with an oil-spaced
                      doublet apparently there aren't enough free parameters to correct
                      for spherical aberration, coma, and chromatic aberration at the same
                      time.....which is why you don't find any on the market. Oil-spaced
                      lenses have only 2 glass to air surfaces which reduces the
                      possibility of internal light reflections which can cause ghosting
                      problems. The light
                      > throughput and contrast are meant to be better in an oil-spaced
                      for this reason too. Both air-spaced and oil-spaced triplets can be
                      used to make optically highly corrected lenses. There is supposed to
                      be a better built-in tolerance of manufacturing errors with an oil-
                      spaced design since curvature radiuses and lens member thicknesses
                      can vary slightly from theoretical values without much
                      effect......so having a lesser degree of design freedom also means
                      the manufacturer has an equally small chance to produce a bad lens.
                      Air-spaced lenses are supposed to be extremely sensitive to changes
                      in the radii of the 4 surfaces facing the air spaces and the
                      parallel positioning of the surfaces themselves. Changing one can
                      cause severe spherical aberration problems. With an oil-spaced
                      design manufacturing deviances are supposedly better tolerated. I
                      reckon with todays automated manufacturing processes this isn't much
                      of a problem though.
                      > The main thermal problem with air-spaced lenses are the air
                      spaces. Most air-spaced triplets have rather wide air-spaces because
                      you can reduce the sphero-chromatism of an apo triplet lens this
                      way.With a doublet however its supposedly more complicated because
                      you must sacrifice some coma correction to do it. (by the same token
                      with an oil-spaced design you cannot control spherochromatism this
                      way at all and in this way they can be less than optimal)
                      Unfortunately air is also a very poor heat conductor (1000x worse
                      than oil) and if the all important central element of a triplet is
                      thermally isolated from the others by wide air spaces then the only
                      place it can transfer heat is its edge where it contacts the lens
                      cell. Also, the center thickness of this element is much thicker and
                      can take alot longer to to cool down than the edge. Oil-spaced
                      lenses therefore should cool faster and in a more homogeneous
                      way.This problem is accentuated by fast focal ratios because the
                      faster the
                      > lens the thicker the lens elements must be and the thicker the
                      elements the slower the cooldown. However, the faster the focal
                      ratio the more the possible improved corrections of an air-spaced
                      design become significant. Below f/6.5 or so I'm told the improved
                      spherochromatism of a good airspaced lens can produce visibly less
                      false color. I'm also told the cool-down issue is a much more
                      significant one with larger apertures and for critical planetary
                      observation. If I were considering a 5 or 6 inch scope for critical
                      planetary observing I would look for a long focus oil-spaced design
                      for it's superior constrast, better thermal performance, and lack of
                      need for barlows to achieve adequate magnification. For a 105mm
                      scope where the thermal issues are greatly lessened, and at fast
                      focal ratios for widefield use a good air-spaced design might serve
                      you better. William Optics products come highly recommended. The
                      amazing Flt-110, by the way has all the advantages of an oil-spaced
                      design
                      > and with its f ratio is perhaps near the limit of whats possible
                      in an oil-spaced. If I were looking for a fast scope in that
                      aperture I think you probably can't do much better. And the price
                      seems like an absolute bargain to me. Tom Trussock has a good review
                      of it on cloudy nights. Good luck with the research....the ideas and
                      the learning is half the fun.
                      >
                      > rfrisk <rfrisk@G...> wrote:
                      > Could someone please direct me to a resource that discusses this
                      > topic? I am looking to purchase a 105 in the future and believe I
                      read
                      > somewhere about thermal efficiency of one or the other. I really
                      want
                      > to perform due diligence before making a purchase decision.
                      >
                      > Thanks for your comments.
                      >
                      > raf
                      >
                      >
                      >
                      >
                      >
                      > SPONSORED LINKS
                      > Astronomy telescope Fun Telescope
                      >
                      > ---------------------------------
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                      >
                      > Visit your group "William-Optics" on the web.
                      >
                      > To unsubscribe from this group, send an email to:
                      > William-Optics-unsubscribe@yahoogroups.com
                      >
                      > Your use of Yahoo! Groups is subject to the Yahoo! Terms of
                      Service.
                      >
                      >
                      > ---------------------------------
                      >
                      >
                      >
                      >
                      >
                      >
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                    • erlend_langsrud
                      It s possible to make an aplanatic Oil-spaced doublet or triplet if one of the surfaces is aspherical. This means that the lens is corrected for coma and
                      Message 10 of 12 , Jan 23, 2006
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                        It's possible to make an aplanatic Oil-spaced doublet or triplet if
                        one of the surfaces is aspherical. This means that the lens is
                        corrected for coma and off-axis astigmatism, just like a high
                        quality air-spaced doublet (Steinheil or Frauenhofer). The level of
                        color correction could be better than an air-spaced doublet.

                        Another good thing about oil-spaced triplets is that only the outer
                        surfaces are very critical. In other words, it should be easier to
                        make good optics (except for the aspherical surface.) Also, the
                        light throughput is better.

                        Erlend



                        --- In William-Optics@yahoogroups.com, Robin Retzlaff
                        <robinretzlaff@y...> wrote:
                        >
                        > We could try and start a discussion of this issue here if you
                        want. FWIW I'll share with you some of my gleanings and hope for
                        someone more knowledgeable to post........
                        > The curvature radiuses of the mating surfaces in an oil-
                        spaced design must be identical since usually only a drop or so of
                        oil is used as an immersion. This leaves less freely variable
                        parameters for the lens designer than an air-spaced one because you
                        can only vary the surrounding surfaces and nothing else.
                        Theoretically then, air-spaced lenses can be better corrected for
                        optical aberration than oil-spaced. But in practice I understand
                        that at least at f/6 and above there are still enough free
                        parameters in an oil-spaced triplet to make it "practically" as well
                        corrected as a similar air-spaced one. However, with an oil-spaced
                        doublet apparently there aren't enough free parameters to correct
                        for spherical aberration, coma, and chromatic aberration at the same
                        time.....which is why you don't find any on the market. Oil-spaced
                        lenses have only 2 glass to air surfaces which reduces the
                        possibility of internal light reflections which can cause ghosting
                        problems. The light
                        > throughput and contrast are meant to be better in an oil-spaced
                        for this reason too. Both air-spaced and oil-spaced triplets can be
                        used to make optically highly corrected lenses. There is supposed to
                        be a better built-in tolerance of manufacturing errors with an oil-
                        spaced design since curvature radiuses and lens member thicknesses
                        can vary slightly from theoretical values without much
                        effect......so having a lesser degree of design freedom also means
                        the manufacturer has an equally small chance to produce a bad lens.
                        Air-spaced lenses are supposed to be extremely sensitive to changes
                        in the radii of the 4 surfaces facing the air spaces and the
                        parallel positioning of the surfaces themselves. Changing one can
                        cause severe spherical aberration problems. With an oil-spaced
                        design manufacturing deviances are supposedly better tolerated. I
                        reckon with todays automated manufacturing processes this isn't much
                        of a problem though.
                        > The main thermal problem with air-spaced lenses are the air
                        spaces. Most air-spaced triplets have rather wide air-spaces because
                        you can reduce the sphero-chromatism of an apo triplet lens this
                        way.With a doublet however its supposedly more complicated because
                        you must sacrifice some coma correction to do it. (by the same token
                        with an oil-spaced design you cannot control spherochromatism this
                        way at all and in this way they can be less than optimal)
                        Unfortunately air is also a very poor heat conductor (1000x worse
                        than oil) and if the all important central element of a triplet is
                        thermally isolated from the others by wide air spaces then the only
                        place it can transfer heat is its edge where it contacts the lens
                        cell. Also, the center thickness of this element is much thicker and
                        can take alot longer to to cool down than the edge. Oil-spaced
                        lenses therefore should cool faster and in a more homogeneous
                        way.This problem is accentuated by fast focal ratios because the
                        faster the
                        > lens the thicker the lens elements must be and the thicker the
                        elements the slower the cooldown. However, the faster the focal
                        ratio the more the possible improved corrections of an air-spaced
                        design become significant. Below f/6.5 or so I'm told the improved
                        spherochromatism of a good airspaced lens can produce visibly less
                        false color. I'm also told the cool-down issue is a much more
                        significant one with larger apertures and for critical planetary
                        observation. If I were considering a 5 or 6 inch scope for critical
                        planetary observing I would look for a long focus oil-spaced design
                        for it's superior constrast, better thermal performance, and lack of
                        need for barlows to achieve adequate magnification. For a 105mm
                        scope where the thermal issues are greatly lessened, and at fast
                        focal ratios for widefield use a good air-spaced design might serve
                        you better. William Optics products come highly recommended. The
                        amazing Flt-110, by the way has all the advantages of an oil-spaced
                        design
                        > and with its f ratio is perhaps near the limit of whats possible
                        in an oil-spaced. If I were looking for a fast scope in that
                        aperture I think you probably can't do much better. And the price
                        seems like an absolute bargain to me. Tom Trussock has a good review
                        of it on cloudy nights. Good luck with the research....the ideas and
                        the learning is half the fun.
                        >
                        > rfrisk <rfrisk@G...> wrote:
                        > Could someone please direct me to a resource that discusses this
                        > topic? I am looking to purchase a 105 in the future and believe I
                        read
                        > somewhere about thermal efficiency of one or the other. I really
                        want
                        > to perform due diligence before making a purchase decision.
                        >
                        > Thanks for your comments.
                        >
                        > raf
                        >
                        >
                        >
                        >
                        >
                        > SPONSORED LINKS
                        > Astronomy telescope Fun Telescope
                        >
                        > ---------------------------------
                        > YAHOO! GROUPS LINKS
                        >
                        >
                        > Visit your group "William-Optics" on the web.
                        >
                        > To unsubscribe from this group, send an email to:
                        > William-Optics-unsubscribe@yahoogroups.com
                        >
                        > Your use of Yahoo! Groups is subject to the Yahoo! Terms of
                        Service.
                        >
                        >
                        > ---------------------------------
                        >
                        >
                        >
                        >
                        >
                        >
                        > ---------------------------------
                        > Yahoo! Photos
                        > Got holiday prints? See all the ways to get quality prints in
                        your hands ASAP.
                        >
                      • Robin Retzlaff
                        It s an interesting point you bring up Erlend, because I ve come to understand that the maker of an oil-spaced apochromatic triplet can also use aspheric
                        Message 11 of 12 , Jan 23, 2006
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                          It's an interesting point you bring up Erlend, because I've come to understand that the maker of an oil-spaced apochromatic triplet can also use aspheric figuring to correct for the spherical aberrations that start to rapidly increase below f/6 or so. But apparently it's very time consuming and expensive so almost no one does it. Supposedly Roland Christen slightly aspherizes his lenses.

                          erlend_langsrud <erlend_langsrud@...> wrote:
                          It's possible to make an aplanatic Oil-spaced doublet or triplet if
                          one of the surfaces is aspherical. This means that the lens is
                          corrected for coma and off-axis astigmatism, just like a high
                          quality air-spaced doublet (Steinheil or Frauenhofer). The level of
                          color correction could be better than an air-spaced doublet.

                          Another good thing about oil-spaced triplets is that only the outer
                          surfaces are very critical. In other words, it should be easier to
                          make good optics (except for the aspherical surface.) Also, the
                          light throughput is better.

                          Erlend



                          --- In William-Optics@yahoogroups.com, Robin Retzlaff
                          <robinretzlaff@y...> wrote:
                          >
                          >      We could try and start a discussion of this issue here if you
                          want. FWIW I'll share with you some of my gleanings and hope for
                          someone more knowledgeable to post........
                          >        The curvature radiuses of the mating surfaces in an oil-
                          spaced design must be identical since usually only a drop or so of
                          oil is used as an immersion. This leaves less freely variable
                          parameters for the lens designer than an air-spaced one because you
                          can only vary the surrounding surfaces and nothing else.
                          Theoretically then, air-spaced lenses can be better corrected for
                          optical aberration than oil-spaced. But in practice I understand
                          that at least at f/6 and above there are still enough free
                          parameters in an oil-spaced triplet to make it "practically" as well
                          corrected as a similar air-spaced one. However, with an oil-spaced
                          doublet apparently there aren't enough free parameters to correct
                          for spherical aberration, coma, and chromatic aberration at the same
                          time.....which is why you don't find any on the market. Oil-spaced
                          lenses have only 2 glass to air surfaces which reduces the
                          possibility of internal light reflections which can cause ghosting
                          problems. The light
                          >  throughput and contrast are meant to be better in an oil-spaced
                          for this reason too. Both air-spaced and oil-spaced triplets can be
                          used to make optically highly corrected lenses. There is supposed to
                          be a better built-in tolerance of manufacturing errors with an oil-
                          spaced design since curvature radiuses and lens member thicknesses
                          can vary slightly from theoretical values without much
                          effect......so having a lesser degree of design freedom also means
                          the manufacturer has an equally small chance to produce a bad lens.
                          Air-spaced lenses are supposed to be extremely sensitive to changes
                          in the radii of the 4 surfaces facing the air spaces and the
                          parallel positioning of the surfaces themselves. Changing one can
                          cause severe spherical aberration problems. With an oil-spaced
                          design manufacturing deviances are supposedly better tolerated. I
                          reckon with todays automated manufacturing processes this isn't much
                          of a problem though.
                          >        The main thermal problem with air-spaced lenses are the air
                          spaces. Most air-spaced triplets have rather wide air-spaces because
                          you can reduce the sphero-chromatism of an apo triplet lens this
                          way.With a doublet however its supposedly more complicated because
                          you must sacrifice some coma correction to do it. (by the same token
                          with an oil-spaced design you cannot control spherochromatism this
                          way at all and in this way they can be less than optimal)
                          Unfortunately air is also a very poor heat conductor (1000x worse
                          than oil) and if the all important central element of a triplet is
                          thermally isolated from the others by wide air spaces then the only
                          place it can transfer heat is its edge where it contacts the lens
                          cell. Also, the center thickness of this element is much thicker and
                          can take alot longer to to cool down than the edge. Oil-spaced
                          lenses therefore should cool faster and in a more homogeneous
                          way.This problem is accentuated by fast focal ratios because the
                          faster the
                          >  lens the thicker the lens elements must be and the thicker the
                          elements the slower the cooldown. However, the faster the focal
                          ratio the more the possible improved corrections of an air-spaced
                          design become significant. Below f/6.5 or so I'm told the improved
                          spherochromatism of a good airspaced lens can produce visibly less
                          false color. I'm also told the cool-down issue is a much more
                          significant one with larger apertures and for critical planetary
                          observation. If I were considering a 5 or 6 inch scope for critical
                          planetary observing I would look for a long focus oil-spaced design
                          for it's superior constrast, better thermal performance, and lack of
                          need for barlows to achieve adequate magnification. For a 105mm
                          scope where the thermal issues are greatly lessened, and at fast
                          focal ratios for widefield use a good air-spaced design might serve
                          you better. William Optics products come highly recommended. The
                          amazing Flt-110, by the way has all the advantages of an oil-spaced
                          design
                          >  and with its f ratio is perhaps near the limit of whats possible
                          in an oil-spaced. If I were looking for a fast scope in that
                          aperture I think you probably can't do much better. And the price
                          seems like an absolute bargain to me. Tom Trussock has a good review
                          of it on cloudy nights. Good luck with the research....the ideas and
                          the learning is half the fun.
                          >
                          > rfrisk <rfrisk@G...> wrote:
                          >   Could someone please direct me to a resource that discusses this
                          > topic?  I am looking to purchase a 105 in the future and believe I
                          read
                          > somewhere about thermal efficiency of one or the other.  I really
                          want
                          > to perform due diligence before making a purchase decision.
                          >
                          > Thanks for your comments.
                          >
                          > raf
                          >
                          >
                          >
                          >
                          >
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                        • Robin Retzlaff
                          The lens designer I know uses a synthetic oil with a refractive index carefully matched to the glass. Protected from oxygen between the glass elements he
                          Message 12 of 12 , Jan 23, 2006
                          • 0 Attachment
                            The lens designer I know uses a synthetic oil with a refractive index carefully matched to the glass. Protected from oxygen between the glass elements he doesn't think that deterioration over time is much of a problem. But even if after half a century an oil-spaced lens needed servicing it seems like a good compromise to me. Another myth is that they'll leak oil. Truth is that the cohesion forces of such a thin film of oil prevent this from happening even under pressure. Sealing the edges is largely unnecesary but they do it anyway. Is your f/15 a nice achromat? I'd relish a look through it.......

                            Rick Jack <jack11973@...> wrote:
                            Hi,
                             
                               How does the oil hold up to time and elements?  I know some oils used in microscope optics discolor and reduce light transmission. I've been using a 6" f15 air spaced objective for years without a hitch.
                            Regards,
                            Rick

                            Robin Retzlaff <robinretzlaff@...> wrote:
                            Glad to help. Hopefully we'll get opinions from others and mix it up a bit.

                            rfrisk <rfrisk@...> wrote:
                            Great Scott, Robin - you just addressed about 99% of my questions in
                            one post!! My reason for purchasing this scope is casual planetary
                            observation and serious DSO observation and imaging. I have been
                            strongly favoring the WO FLT-110 and appreciate the heads up for
                            Tom's review. I really appreciate you taking the time to reply with
                            such an informative response. It helped me out more than you know.

                            Thanks again.

                            raf


                            --- In William-Optics@yahoogroups.com, Robin Retzlaff
                            <robinretzlaff@y...> wrote:
                            >
                            > We could try and start a discussion of this issue here if you
                            want. FWIW I'll share with you some of my gleanings and hope for
                            someone more knowledgeable to post........
                            >The curvature radiuses of the mating surfaces in an oil-spaced
                            design must be .....
                            >The main thermal problem with air-spaced lenses are the .....
                            >The amazing Flt-110, by the way has all the advantages of an oil-
                            spaced design and with its......








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