Re: [ap-ug] Re: air vs. oil spaced lenses


Dec 30, 2001

 


----------------------------

#8678 Dec 30, 2001

Hi everyone,



this might be covering old ground, but I don't understand what

advantages an oil spaced lens might have over an air spaced lens in

terms of contrast, light transmission and overall visual

performance. Does the oil medium absorb any light and impede light

transmission? Or are the practical differences nil ?



clear skies,

Andy



----------------------------

#8680 Dec 30, 2001

Andy - all I know is that oil can more closely match the refractive

index of glass...it is discussed on page 139-140 of Telescope Optics

with an f/10 Christen triplet. Paraphrasing: small interior surface

irregularities can be compensated for with oil spacing.



The urban legend is that air spacing has better contrast but oil

spacing has better overall correction.



Cheers - Otto

--- In ap-ug@y..., "rothko77200" rothko@m...> wrote:

> Hi everyone,

>

> this might be covering old ground, but I don't understand what

> advantages an oil spaced lens might have over an air spaced lens in

> terms of contrast, light transmission and overall visual

> performance. Does the oil medium absorb any light and impede light

> transmission? Or are the practical differences nil ?

>

> clear skies,

> Andy



----------------------------

#8681 Dec 30, 2001

Andy-



Here is some haphazard guesswork: If the oil has the same refractive

index as one of the lenses, then the reflection and refraction between

the oil and the lens disappears. So, instead of having two interfaces

(glass to air, then air to glass) you have just one ( glass to

Oil+glass). The interface is where the reflection occurs, so you won't

get as much internal reflection in the lens set. My guess is that one

or both air to glass surfaces would need to be anti-reflection coated to

deal with this reflection if not for the oil. Who knows if just one or

both lenses would go without coating when oil is present between them?

(Roland, help!) As to the reduction of refraction between the oil and

both lenses, I can't even guess. Not enough spare neurons up there.



I recently took the kids to the Fernbank Museum of Natural History here

in Atlanta, which has an exploratorium section. There are all sorts of

hands-on demonstrations that show off some facet of physics, perception,

etc. and even though they are too young (4 y.o.) to understand it all,

they love to play with the apparatuses (apparati?), so this is a good

start. Anyway, one of the demonstrations involves a fishtank of oil

into which can be lowered, by cranking separate pullies, a small

positive lens or a bundle of seven glass rods. When you lower the lens

into the oil, it will no longer magnify the image situated behind it.

When you lower the rods, all but one seem to disappear. The explanation

is that the glass of the lens, and that of six of the rods, has the same

refractive index as the oil. Suspended in air, I don't think the rods

look different, but the "ringer" is in the middle, so it's kind of hard

to tell. The oil eliminates the bending of light and the reflection at

the surface of the glass, making it all but invisible.



Hope this helps.



By the way, I gathered from past discussions that there is not much

space in the oil space. Just enough for a thin wet layer, held in place

by capillary adhesion.

> this might be covering old ground, but I don't understand what

> advantages an oil spaced lens might have over an air spaced lens in

> terms of contrast, light transmission and overall visual

> performance. Does the oil medium absorb any light and impede light

> transmission? Or are the practical differences nil ?

>

> clear skies,

> Andy

--

Best regards,

Chuck Hancock DMD



McDonough GA USA



----------------------------

#8682 Dec 30, 2001

The OTA is a little heavy when it is full of oil.



I believe the light transmission (through put) is better with

the oil spaced objective. The contrast seems excellent with

the oil spaced objective. Holding up the lens in the cell, it is

like looking through air. The glass doesn't seem to be there.



Rich







>Hi everyone,

>

>this might be covering old ground, but I don't understand what

>advantages an oil spaced lens might have over an air spaced lens in

>terms of contrast, light transmission and overall visual

>performance. Does the oil medium absorb any light and impede light

>transmission? Or are the practical differences nil ?

>

>clear skies,

>Andy

>

>

>To UNSUBSCRIBE, or for general information on the ap-ug list

>see groups.yahoo.com/group/ap-ug

>

>Your use of Yahoo! Groups is subject to docs.yahoo.com/info/terms/



----------------------------

#8683 Dec 30, 2001

Otto,



For the oil to work properly, the surfaces must match very closely, so I

find the idea that the oil compensates for small interior surface

irregularities a bit far fetched. The proper oil essentially eliminates

light loss at the interface between the two surfaces. An oiled triplet

looks like one piece of glass and I can only detect the front and real

surfaces.



The urban legend makes little sense. Oil spaces lenses have less internal

reflections and should improve contrast. Air spacing allows the designer

more variables to play with in designing the lenses, so I would think an air

spaced lens has the potential for better control of aberrations. Using air

spaces allows the separation between lenses to be varied in the design and

allows for differing radii on adjacent surfaces.



At any rate, properly designed and executed versions of either are superb

lenses that should keep anyone happy.



Clear skies, Alan

> Andy - all I know is that oil can more closely match the refractive

> index of glass...it is discussed on page 139-140 of Telescope Optics

> with an f/10 Christen triplet. Paraphrasing: small interior surface

> irregularities can be compensated for with oil spacing.

>

> The urban legend is that air spacing has better contrast but oil

> spacing has better overall correction.







----------------------------

#8684 Dec 30, 2001

--- In ap-ug@y..., "Rich N." rnapo@z...> wrote: > The OTA is a little heavy when it is full of oil.

>

> I believe the light transmission (through put) is better with

> the oil spaced objective. The contrast seems excellent with

> the oil spaced objective. Holding up the lens in the cell, it is

> like looking through air. The glass doesn't seem to be there.

>

> Rich

>

>

>

>

> >Hi everyone,

> >

> >this might be covering old ground, but I don't understand what

> >advantages an oil spaced lens might have over an air spaced lens

in > >terms of contrast, light transmission and overall visual

> >performance. Does the oil medium absorb any light and impede

light > >transmission? Or are the practical differences nil ?

> >

> >clear skies,

> >Andy

> >

> >

> >To UNSUBSCRIBE, or for general information on the ap-ug list

> >see groups.yahoo.com/group/ap-ug

> >

> >Your use of Yahoo! Groups is subject to

docs.yahoo.com/info/terms/





Rich,

I tend to agree, The tak triplet has a couple of highly reflecting

surfaces while the AP's do not. On the other hand, the FS series also

have that "looking into a black hole" effect.Perhaps due to the good

coatings on the FS lenses, I am guessing.



----------------------------

#8685 Dec 30, 2001

Rich,

>I tend to agree, The tak triplet has a couple of highly reflecting

>surfaces while the AP's do not. On the other hand, the FS series also

>have that "looking into a black hole" effect.Perhaps due to the good

>coatings on the FS lenses, I am guessing.





The FS also have excellent contrast.



Rich



----------------------------

#8686 Dec 30, 2001

Alan - my understanding from reading Rutten & V. is that the

triplet's interior surfaces can never be an exact match, so the oil

spacing completes the magic and is a better design for manufacturing

considerations. Perhaps "irregularities" is too strong a word.

Either types can be fine scopes, and I'm glad to have them in my

garage. :) CS - Otto

--- In ap-ug@y..., "Alan French" adfrench@n...> wrote:

> Otto,

>

> For the oil to work properly, the surfaces must match very closely,

so I

> find the idea that the oil compensates for small interior surface

> irregularities a bit far fetched. The proper oil essentially

eliminates

> light loss at the interface between the two surfaces. An oiled

triplet

> looks like one piece of glass and I can only detect the front and

real

> surfaces.

>

> The urban legend makes little sense. Oil spaces lenses have less

internal

> reflections and should improve contrast. Air spacing allows the

designer

> more variables to play with in designing the lenses, so I would

think an air

> spaced lens has the potential for better control of aberrations.

Using air

> spaces allows the separation between lenses to be varied in the

design and

> allows for differing radii on adjacent surfaces.

>

> At any rate, properly designed and executed versions of either are

superb

> lenses that should keep anyone happy.

>

> Clear skies, Alan

>



----------------------------

#8689 Dec 30, 2001

Oil spaces lenses have less > internal

> > reflections and should improve contrast.



This discussion came up 3-4 years ago on AOL Observers outpost group

with Thomas Back. Toms take on it was that the less air to glass

surfaces you have than the less reflective light surfaces you have.

With an air spaced doublet you have 4 air to glass surfaces that can

reflect light and with an oil spaced triplet than you only have only

2 air to glass surfaces to reflect light. The oiled triplet slightly

increases contrast and truer colors but increases lobor and costs.

FWIW



Regards, Jeff Vickers



----------------------------

#8694 Dec 30, 2001

In a message dated 12/30/01 7:22:59 PM Central Standard Time,

ottocycle@... writes:



< The urban legend is that air spacing has better contrast >>



Balderdash! The opposite is true. I can dig up some old scientific papers to

prove it.



Roland Christen



----------------------------

#8695 Dec 30, 2001

In a message dated 12/30/01 7:43:45 PM Central Standard Time,

adfrench@... writes:



< Air spacing allows the designer

more variables to play with in designing the lenses, so I would think an air

spaced lens has the potential for better control of aberrations. >>



With the right three glasses, an oiled objective can have an aplanic design.

That means it can be corrected for spherical and coma. An airspaced lens can

also be corrected the same way, however, the designer is not limited to just

those particular glass types. In our case, we chose the glass types for high

color correction, and in order to meet the other criteria, I have an aspheric

surface on one element. The amount of asphericity is not large, not like a

parabola of a Newtonian, so it can be done very smoothly. In practice, you

would not be able to tell an airspaced design from an oil spaced design - the

spot diagrams would be nearly identical. However, the airspaced design can be

made without the aspheric, a distinct advantage to the lens maker.



Roland Christen







Contact Us
This Site's Privacy Policy
Google's privacy policies

S
e
n
i
o
r
T
u
b
e
.
o
r
g