Re: [ATM] Surplus Shed 90mm F10 objective

["Bob May" <rmay@nethere.com>]

The general rule on doublets is that the crown is first and the
flint is second.  This is true for all but the Stenheill style
doublets which hae the flint first and they can be identified
usually by the stron curvature of the first surface on the flint
in the convex direction, I.E. a meniscuis style enns.  The normal
lenns for medium sized lllenses like a 90mm lens is to have them
airspaced and the double convex lense usually has a strongly
curved surace and a much, usually about half, curved surface.
The flint is usually a concave surface with a very weeakly, if at
all, cuvvved rear surface.  With this lens, the two stronly
curved surfaces go together which leaves the weaker surface on
the double convex lens to the front, being the first surface that
the light hits and then exits through the flat rear surface of
the flint.  The two inner surfaces, the stronly curved conveex on
the crown and the one on the flint, are often not exactly the
same radius but are usually very close in curvature and usually
don't need much of a space in which to work properly.  This air
gap can be used to fix, to some degree, color errors in the lens
set.
Common problems of these lenses is that they are often assembled
wrongly with the second surface of the crown lens being the first
that light hits and thus, the focal length of the vqrious colors
is way off.  In addition, all kinds of odd effects are noted on
off-axis images as the light is being improperly refracted to the
focus point.  Putting a lens totally in backwards will also make
for bad images as the basic curvature of the lens is backwqrds
for best focus and may be the problem with the lens earilier in
this thread.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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