Re: ATM 11.5" mystery glass

[Doug Miller <mildg@sk.sympatico.ca>]

Guy and Peter,

I agree with Peter about this making a really nice Maksutov. Just for fun, I cranked
out a design.  It is a 280 mm f/15.  The design was optimized from 390 to 700 nm
over a 50 mm full FOV.  All dimensions are in millimeters

radius           thickness          glass       semi-aperture
-428.672          20.0000      SK4      140.0
-441.264        833.7166      air          140.0
-2124.6229   -808.7166      mirror    150.0
-723.6668     1088.7166     mirror      42.25
-685.2910                           image      25.0

Performance is excellent over the full field of view and spectral range and
indicates the performance that can be achieved.  Distortion is less than 0.025%.  I
should mention that rms wavefront was used for the optimization using  ZEMAX-EE 8.0
and included diffraction effects of obscuration.   This (Rumak-like) design  is very
convenient with the secondary mirror separated from the Maksutov corrector by only
25 mm, 30% obscuration and all spherical surfaces.  Even with the central
obscuration, this Maksutov design will perform better than a refractor that could be
made from the BaF4 and SK4 blanks and would be far more compact.

If  you are intent on making a Newtonian, you should consider trading the glass
blanks for a good pyrex mirror blank.  I am sure that someone on this list would
oblige.

Regards,

Doug Miller


> Peter,
>
> I have no idea of what they are worth. But as far as I could tell, they came
> from a large store of World War 2 - era glass that was being worked on at the
> US National Institute of Standards; there were lots of boxes of tank-turret
> prisms (I think) and other stuff like that, that I was not interested in.
>
> Yeah, these two would make an incredibly heavy refractor, at about 40 pounds
> (would you in Australia call that ~3 stone, or ~18 kg?) each. Maybe they
> intended to saw them in half, I ask rhetorically? I doubt it, it would have
> been cheaper & to make them 1" thick to begin with.
>
> I really do not know what they are worth. I just wanna make a 12" reflector.
>
> Guy Brandenburg
>
> PETER JOHN SMITH wrote:
>
> > Guy,
> >
> > > My mystery glass has a companion piece, as well, same diameter, but a tad
> > thicker. It has the following markings engraved on one of the parallel, clear
> > sides:
> > > Sk4 3979/# Opt. 7703 / Pos. 3/7
> >
> > SK4 is a more usual glass.  A higher refractive index crown. More demand for
> > it too. But probably worth a bit worth more than the BaF4. They will,
> > togeather, make quite a reasonable refractor BUT by no means APO. It would
> > have to be about F:20 to perform reasonably.  Preferably about
> > F:25. Would have to be a folded type for portability. I honestly doubt if
> > they were originally really intended to make a refractor as the thickness is
> > excessive. (about 2 inch ??? each - thats a big heavy lens at 4 inch) It
> > sounds like someone may have paired them with the intention of making a
> > refractor because they were available - even though they were really too
> > thick.  Make a good camera obscura.The SK4 would also make a good MAK front
> > lens. Better than the BaF4.
> > I am interested in them but am in Australia. The freight and awkwardness of
> > packing etc makes it far less attractive. What sort of money would you want.
> > There will probably not be a much interest in the BaF4 by itself but as a
> > pair they should be more interesting to people. All this has probably
> > interrupted your planned start to your scope. Sorry about that. All the best,
> >
> > Peter Smith.