Re: [ATM] Removing a chrome(?) coating layer

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

Lenses are made from different types of glasses.  The problem is
that whenever you run light through a piece of glass, the colors
of the light get seperated into a spectrum just like you get from
a prism.  As a result, you need a glass which does the focusing
of the light and a second (or more) lenses that are negative in
their focusing that bring the light back towards the point where
thedey all focus at the same point in space.
  Thus, you have the first lens which is generally of a crown
type glass which does the focusing towards a point and it has a
relatively low refraction index and then run the light through
another flint type glass which has a much higher index of
refraction which tends to bring the light to a common focus.  The
system works because the dispersion of thge flint type glass
(dispersion being the amount of spread of the light into its
color is a lot higher in a flint glass) than the dispersion of
the crown glass and thus the flint doesn't need to be as powerful
as the crown is to bring the light to the cxorrect focus point.
The only problems here is that the dispersion isn't constant per
change of the frequency of the light but reather is of a curve so
the two curves don't exactly match eacy other.  As a result, you
really get a good focus only at one color and all of teh other
colors are somewheere near this baluce, depending upon the glass
type seleced and the curgves put on those glasses as well as the
thicknesses and any air spacing that may be used.
There are programs for doing all of this ray tracing from simple
ones like the one that Rutten and VenRooij have as a suppplement
to their book (highly recommended as a first book on optics, by
the way) to professional programs like Zemax, and other such
programs.  OSLO-EDU (the educational version of the professional
OSLO program) is freeware and can be downloaded from the net as
well as Dave Rowe's TRACE program and you c can have a lot of fun
trying to design refractors, following Snashall's and others
playing around trying for cheap good refractor designs.
Glass, of course, for rrefractors isn't going to be anywhere near
a s cheap as a piece of plate glass or pyrex for a mirror but the
exercise in making a refractor is always fun in many ways.
Finally, I'll note that Newport Glass Works does have several
kits for refractors that have been available on their website.
The refractors made from such kits are of typical normal glass
designs as there are no exoctic glasses used in the designs and
will produce telescopes of the types normally done more than
fifty years ago.
Get the Telescope Optics book of Rutten and VenRooij from
Willmann-Bell (they publish it) for the twenty five dollars and
get the software (runs on a PC in DOS mode) for the additional
twenty five dollars (last prices that I'vve seen) and you will
gain a huuuge pile of info on telescope optics of all kinds and
will advancce your knowledge of optical theory and telescope
design  to a much higher plane than many  You'll also understand
why some designs work well while others jus quickly fall by the
wayside.
While you're shopping at Willmann-Bell, you may also want to pick
up the three book set of Amateur Telescope Making by Ingalls.
Willmann0Bell rearranged the bookset so that the articles are
grouped togethger into subjectts rather than the original set
which were more done chronologically as they were originally
published in the Scientific American.
Hope this gets you started down the right roads to making a
refractor.  I found that my six inch F8 to be an interesting
exewrcise of making something other than another silly Newtonian
on a DOB mount.
Bob May
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