Re: ATM Lens sources.

[Anthony Stillman <atmer@flash.net>]

>...I'd like to look into
>building a 6" or 7", f12 or so, refractor.  Anyone know of good sources of
>information and catalogs for lenses or blanks?



Tom,

As I've toyed with the idea of making refractors for some time.  I figure I
through my 2 cents in.

First, as to glass.  I've only checked a few sources for glass.  Most were
surplus houses and they suffered from not having any predictable glass.  I
also called and got a few quotes from Newport.  EXPENSIVE!!!  This has led
me to learn how to measure indicies at various wavelengths and to keep a
sharp eye open for flint.

Second, as to design.  Unfortunetly, it isn't possible to make an achromat
6" or 7" in diameter that is diffraction limited at f/12.  OK, it is
possible but it requires at least one very strange glass (read florite)
that is very very expensive.  Now, if you don't mind the purple halo you
can get by with f/12, otherwise you will need to go out to f/24.  Oviously
this is going to be a rather long instrument.  To shorten it, you can fold
it or you can go with more layers of glass.  A triplet at this diameter can
be made apomatic down to f/10 or so with fairly ordinary glasses but the
glass indicies need to be known to better than four places, and perferably
better than five.  This also tends to drive the cost of the glass up.
Further the surface figures, radii and CTs for a triplet need to be held to
tollerances that most amateurs arn't capable of doing.

It is possible to oil space an achromat and with care even use the oil as a
refracting element.  I saw a nice folded refractor at Riverside a couple of
years ago made this way.  Of course there are problems with this too.  Oil
leaks.  Oil indicies tend to be a function of temperature and to a small
amount pressure and are definitly a function of age.  So such an instrument
will require some maintence.  Also you should buy a couple of life times
worth of oil  as buying more with the exact same properties down the road
will likely be impossible.

Lastly, there is the matter of optical abberations.  Any amount of study
into achromatic design will quickly show you that its a juggling act with
issues like field curvature, chromatic abberation, sphero-chromatic
abberation, distortion, coma, spherical abberation, transverse abberation,
the list goes on,  fix one, screw up the others.  So why bother?  Well with
a 16" under your belt you already know the answer, cause.

As for me, I'm starting small.  I've got the glass and have made the tools
for a 90mm Fraunhofer refractor.  Currently the design is about f/18, but
I'm still working on it.  After that I'll try my had at a 110mm f/somthing
triplet.  With luck, some day I'll make a big multi-element instrument, but
this will likely be after the first U.S female President finishes her
second term.




Anthony