Re: ATM refractor question

["Barlow Pepin" <atmjedit@conceptsnet.com>]

Just a note on your Q.

There are generic differences between air-spaced and oil-spaced objectives,
but as to the performance of any particular design there are literally
dozens of additional factors to consider. Some of the best-rated objectives
ever produced have been, and are, oil-spaced, but as far as I know these are
all triplets or quads, not doublets.

Generically, with a doublet:

Air-spacing a doublet gives you four air/glass surfaces, thus light loss is
potentially greater than in an oil-spaced doublet system, which would have
two.

Air-spacing opens the door to reflection between face-to-face interior lens
surfaces, leading to light scattering, thus coating may be more critical to
image quality than in an oil-spaced doublet which has no facing air-glass
surfaces.

In some oil-spaced designs the interior facing surfaces don't necessarily
need to be polished as perfectly along the oil-glass interface as with an
air-spaced, since the oil mitigates the scattering effects of roughness,
digs, scratches, etc. This may be the "easier" factor and has anecdotally
even led to oil-spacing being considered as 'cheating' by certain devotees
of the classic air-spaced designs like the Fraunhofer doublet.

Oil-spacing a doublet (especially with normal glasses) limits design freedom
since it generally means identical curves between the interior surfaces of
the doublet -- which also happens to be a characteristic of certain
less-highly-corrected designs that would normally be simply cemented
together in smaller apertures (up to 70 mm or so).  Although some workers
have cemented large objectives (with mixed success) six inches is generally
considered too large to cement due to different coefficients of expansion
between glass types under temperature change, which would strain the
elements, the cement bond, or both. One answer would be to oil-space such an
objective, and the components might be more cheaply produced in this case.

-----Original Message-----
From: Mike Coslo <mjc5@psu.edu>
To: atm@shore.net <atm@shore.net>
Date: Saturday, July 10, 1999 8:23 AM
Subject: ATM refractor question


>
>I am pondering building a refractor and considering the purchase of a
>newport lens kit.
>
> I notice they sell both air-spaced and oil spaced kits with a
>significant price difference, the air spaced being more expensive. (100
>dollars more expensive for a 6-inch lens)
>
> As a newbie to refractor optics, I am curious why.
>
> Are there performance differences between oil and air-spaced acromats?
>Difficulties in fabrication?
>
> - Mike -

-----Original Message-----
From: Mike Coslo <mjc5@psu.edu>
To: atm@shore.net <atm@shore.net>
Date: Saturday, July 10, 1999 8:23 AM
Subject: ATM refractor question


>
>I am pondering building a refractor and considering the purchase of a
>newport lens kit.
>
> I notice they sell both air-spaced and oil spaced kits with a
>significant price difference, the air spaced being more expensive. (100
>dollars more expensive for a 6-inch lens)
>
> As a newbie to refractor optics, I am curious why.
>
> Are there performance differences between oil and air-spaced acromats?
>Difficulties in fabrication?
>
> - Mike -