7168Re: Air-spaced versus oil-spaced lens elements?
- Jan 23, 2006It'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.
--- In William-Optics@yahoogroups.com, Robin Retzlaff
>want. FWIW I'll share with you some of my gleanings and hope for
> We could try and start a discussion of this issue here if you
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-spacedfor 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 airspaces. 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
> lens the thicker the lens elements must be and the thicker theelements 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 possiblein 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 reallywant
> to perform due diligence before making a purchase decision.Service.
> Thanks for your comments.
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