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7149Re: [William-Optics] Air-spaced versus oil-spaced lens elements?

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  • Robin Retzlaff
    Jan 22, 2006
           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.


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