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7165Re: Air-spaced versus oil-spaced lens elements?

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  • Timm Bottoni
    Jan 22, 2006
      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.



      --- 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
      > 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
      > somewhere about thermal efficiency of one or the other. I really
      > to perform due diligence before making a purchase decision.
      > Thanks for your comments.
      > raf
      > Astronomy telescope Fun Telescope
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