Oil separated vs air separated?

[amgray04]

Can anyone explain the difference between air/oil separation in refractors?

Sent from my iPhone using Tapatalk

[John]

Good question !

I'll be interested in input to this one myself.

I've only used air airspaced refractors so I don't have any experience of oil spaced to contribute I'm afriad   

[Oddsocks]

Oil spacing was quite useful in the past, before exotic multi-coating of lenses became the norm.

Before good multicoating was possible the air-spaced triplet would suffer from ghosting, increasing the further off-axis that the object appeared in the field of view. This was due to the differences between the radii of both surfaces of the central lens element of the objective compared to the inner radii of both outer lens elements, looking from the side, the air space between the central lens element and outer lens elements is smallest on the central axis and largest towards the edges of the lens assembly. As the light ray refracts when passing from the high density glass to the low density air in the air gap so the ray path becomes longer and more acute towards the edge of the objective assembly and this allows multiple reflections to form causing ghosting which appears as a loss of definition or sharpness.

Oil spacing was used to defeat the ghosting problem. In an oil-spaced objective the outer radii of both surfaces of the central lens element exactly match the inner radii of both outer elements, looking from the side the gap between lens elements is the same on the central axis as it is at the edges of the lens group, in addition, oil is denser than air so the light ray leaving the first element is refracted less before it enters the central element and again less as it leaves the central element and enters the final element.

The result is that ghosting is virtually eliminated, the image formed by an oil-spaced objective will be pin sharp across the whole field of view. The disadvantage of oil spacing to the designer is that he is constrained by the need to have matching radii on the four internal lens surfaces of the objective and this makes it really hard to correct for chromatic and spherical aberation, a compromise has to be made between good chromatic correction and weaker spherical correction or vice versa depending on the goals of the designer.

Because of the close coupling between lens elements and the reduction of internal reflections light transmission of a oil-spaced objective is around 8% higher than an equivalent air-spaced objective of the same aperture and focal length.

During cool down, because the three lens elements of the objective are oil coupled then the whole lens group reaches equilibrium quite quickly compared to an air spaced objective but if the temperature change is very great then quite high stresses can build up in the objective assembly causing astigmatism, once at thermal equilbrium the astigmatism will usually disappear.

Oil spacing was also used at one time to improve the chromatic correction in doublet objectives but is seldom used now due to the extra costs involved in producing a really good leakproof lens cell, a normal air-spaced triplet is less costly to produce and would out perform an oil-spaced doublet.

With the advent of sophisticated multi-coating the control of internal reflections in the air-spaced doublet or triplet objective is pretty close to that achieved by oil spacing and because an air-spaced lens cell is cheaper to manufacture the extra light transmission of the oil-spaced objective can be negated by increasing the aperture of the air-spaced objective and still end up with a less costly design.

In the air-spaced objective freedom from the need to match internal radii of the lens elements means both chromatic and spherical aberrations can be corrected for.

Perhaps ten or twenty years ago, ignoring spherical aberrations it would have been quite easy to see the superior sharpness and chromatic performance of an oil-spaced objective against an air-spaced objective of the same aperture and focal length but given improvements to the design of air-spaced objectives, and a product from a really good designer and manufacturer you would be hard pushed to see the difference, add in the easier-to-achieve spherical correction of the air-spaced objective and you can see why so few oil-spaced objectives are produced today.

[timwetherell]

Yes, as you say, oiil spaced objectives do come to thermal equilibrium faster than air spaced because they are essentially one solid lump. Air spaced objectives are like a double glazed window so act as an insulator.  I believe that TEC are the only manufacturer to routinely use oil these days though AP have in the past.

Personally I don't see it as an either/or better/worse. Each has its own slight advantages and disadvantages but two well designed objectives of each type will both perform superbly

[Peter Drew]

The post from Oddsocks pretty much says it all. In a nutshell, oil spacing is an alternative to the common practice of cementing small objectives such as those in a binocular to reduce ghosting and light loss without the potential strains that occur. Oil spacing was often referred to as "soft" cementing. I have a 5" triplet figured by Horace Dall with oil spaced central element. 

[DaveS]

I think CFF in Hungary are still making oil spaced triplets, but each one is hand figured and aspherised to correct spherical abberation.

[Physopto]

As an example of a solid lens system:

I still have an old (now) Solid Cat 600mm lens. It was sold by Vivitar way back around the late 1970s. It was an f8  aperture but because of the coatings only let through at f11. I bought mine specifically to photograph the Tall Ships coming into in the Tyne using  an F2AS Nikon.

The lens is a series of different elements all glued together to remove any ghosting. It is very heavy so needs supporting separately to the camera. By todays standards a bit soft, but it was very small by yesterdays criteria.

Quote "This lens was made in the USA by Perkin-Elmer, the same company that made the optical system for the Hubble Space Telescope. Because the lens is made of solid glass, it is extremely resistant to shock and vibration and weighs 1.5Kg or 3Ib 4.5oz. Sold for about $650 in the 1970's. The lens has M42-.75 rear flange, which accepts T mount adapters so that the lens can be fitted to any 35mm camera."

I paid £600 then, a lot of money!

Derek

[perfrej]

TEC refractors are oil spaced, as is the Baader CaFl Travel Companion. All the good old Carl Zeiss refractors, like the APQ-100, are oils spaced. The Baader 193mm refractor is also oil spaced, so not necessarily a thing of the past.

All the best,

Per