Apochromatic Triplet Objective: Why oil-spaced lenses?

Why an oil-spaced triplet-lens?

TEC 140 APO mit ölgefügtem Triplet-Objektiv - völlig reflexfrei!

The first successful experiments with oil-spaced triplet-lenses were probably done by Wolfgang Busch from Ahrensburg, who had several discussions about this with Carl Zeiss Jena, back in the days of the German Democratic Republic. Even then, this technique was far superior to the standard technics like air-spaced or cemented lenses. But it was hard to keep the joining medium inside of the lens, because of insufficient synthetic oils and many other reasons. Because of this, the lens could freeze or dry out. It took Zeiss more than ten years of research to solve these difficulties and present the first APQ-lens. Besides Zeiss, only Astro-Physics and TEC were able to produce long-lasting oil-spaced lenses. It takes some tricks and knowledge to keep the oil (if the fluid can still be called oil) inside of the lens – at least over a long period of time with always-changing temperatures. But with the necessary craftsmanship and patience, it can be done.

Nevertheless, many people still think – wrongly – that an oil-spacing is especially prone to aging. This is surely true if you didn't master the technology, but we want to present a list of the pros and cons of oil-spaced lenses. Judge yourself!

Oil-spaced lenses by TEC, Astro Physics and Baader:

  • do not have to be protected from leaking oil. Horizontal storage is no problem, either. You only have to avoid temperature shocks. Never carry such an expensive instrument from the freezing cold outside at eg. -25°C into your cozy living room – this would mean a temperature difference of 50°C in a couple of minutes! But temperature differences of 25°C didn't cause any problems in tests over long periods of time. We are using a cool chamber in our production facilities to test dome parts, motors, circuit boards – and even lenses – at a temperature of -86°C to reliability of operation and perfect function under extreme conditions. Oil leakage is one of the few problems which can rarely affect lenses by Zeiss, Astro-Physics, TEC or by us – but only if they were mistreated. Here at Baader Planetarium, we have experienced opticians who have completed a multi-year training at Carl Zeiss Jena. We repair such oil leakages of lenses by Astro Physics/TEC and Zeiss since more than 20 years – quick and easy. So we think that the advantages of oil-spaced lenses are larger than the risks. We can do these works. Our company exists for more than 50 years, meanwhile in the third generation.
  • may "clatter" in their cells! This is no problem or disadvantage. The "clatter" is caused by a gap of a few micron between lens and holding ring. It avoids mechanical stress on the central lens element (which is made of fluorit), this way it is protected from deformations which result in a reduced optical quality. You may hear this clatter, sometimes a bit louder, while it will be almost inaudible at other time. But don't worry: Our lens cells are constructed so that the image will not move on the camera sensor. It is part of the design, and the tolerances are so tight that they meet even the high requirements of astrophotography.
  • are great for observing the sun – even and especially with our Cool Ceramic Herschel Wedge. The oil doesn't suffer or deteriorate under long usage (UV-radiation). We do not use a classical "oil". Carl Zeiss invested a decade of research into the development of the fluid which is used for these lenses today – and they gave a 30-year-warranty, which is now fulfilled by us.
  • adapt much faster to changing temperatures than air-spaced lenses. Each air-gap acts as an isolator; in case of a triplet lens, it protects both sides of the central element. That's why an air-spaced lens needs a much longer time to reach thermal equilibrium than a massive piece of glass (= an oil-spaced lens). If the temperature keeps changing, an air-spaced lense will never achieve its full performance, because the lens form is continously changing and does not fulfill the terms of the original optical design. What did work perfectly in the laboratoy, will fail under the stars. In contrast, an oil-spaced lens will keep its focus and is very good for photography with long exposure times.
  • are extremely well protected from fungal infestation or ageing, because all interior surfaces are sealed by the oil. An air-spaced lens "breathes": Whenever the temperature changes, warm air will be squeezed out of the gap between the lenses, and later cool air will be sucked in. This way, moisture and germs will gather between the lenses (unable to escape). That's why the coating on the inside of the lense is prone to ageing can downright go to waste – especially if it is not stored properly. This is the biggest danger to a lens – but it is no problem for an oil-spaced lens.
  • have got a much higher transmission. Whenever light passes the border between glass and air or between air and glass, some light is lost. Even the best coatings can't prevent these losses to a 100 percent. In an air-spaced lens, light moves six times from one medium to the next. An oil-spaced lens reduces this to two changes of the medium – when the light enters the lens, and when it leaves it. An oil-spaced triplet can be seen as one large piece of glass.
  • keep their collimation are better saved from shocks – because not every single lens has to be mounted and adjusted. Because of the oil's adhesion, the lenses act as one piece, which only has to be protected as a whole from lateral movements. Our temperature compensated lens cells are the result of long years of research and testing. The focus shift (caused by the changing of the lens forms and spacings) which is a common problem of air-spaced lenses, is almost no problem with oil-spaced triplets and only can be observed when the temperature changes very much.

Our conclusion:

Oil-spaced lenses are the best technology we are aware of to mount astronomical triplet-lenses – and to protect the inner surfaces of the lenses. It is very sophisticated and requires great knowledge and craftmanship which is the reason why it is done only by small manufactories, which have both a lot of experience and in-house production.


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