[ATM] Plastic.....

[Bob May]

I'll chime in with my opinion on this one.
Plastic is fine for low power work  For high power work, it is
quite unsatisfactory for several reasons.
First off, the surfaces can't be made well enough in casting to
be true.  This is a basic problem with casting plastics and
nothing rreally can be done with it at this time.  Maybe in the
future with some other plastic can we gett accurate enough
surfaces.
Machining the plastic surfaces does leave the tail ends of the
plastic molecules exposed and they are going to be of dirrering
lengths and really can't be smoothed of to eh necessary meeded
smoothness.
For low power work like a finder or eyeglasses, plastic will work
just fine tho.  They even havve differing plastics so that some
sort of achromatism can be achiebed  but only approximately as
the casting of those plastics will cause differing indexes of
refraction, something that isn't all that important when doing
eyeglasses as it hardly matters if the lens is right at three
diopters or is a tenth of a diopter off.
MirMir4rror surfaces are also a problem when water adsorption is
involved as this will cause distortion of the mirror when there
is water on the backside and not near the frontside of the piece
of plastic.  The problem here is that the water adsorption isn't
equal over the entire large piece of plastic if it is cast due to
the basics of casting and, if done in a single sheet of plastic,
will again have that same problem due to the initial casting of
the sheet.
All in all, if you want a finder scope, hou can get there but if
you want anything of any significant quality then you ain't gonna
get there.
Bob May
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