Re: [ATM] Petzval Design

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Richard F.L.R. Snashall wrote:
> Of the "well-known" glasses, Schott N-K5 and Schott F2 have the
> same CTE (to 0.1 ppm/deg.C).  That is important if you want to
> mate them in a larger design (290 mm f/7.5):
However, this cannot be done with an arbitrary glass selection.
Choosing some random glass types (and, if you believe that, Bob,
I have this bridge in the Manhattan suburbs I'd like to show you),
and scaling to 200 mm f/7.5:

    http://users.rcn.com/rflrs/Ptz09-51x.atm   ATMOS input file
    http://users.rcn.com/rflrs/Ptz09-51x.len   OSLO input file
    http://users.rcn.com/rflrs/Ptz09-51x.txt   TEXT input file
    http://users.rcn.com/rflrs/Ptz09-51x.zmx   ZEMAX input file

The first glass is one considered obsolete, BSC2; I entered this
glass into the ZEMAX User catalog with the Cauchy coefficients
{2.271101, -0.009951082, 0.01039149, 0.000287308, -0.000020685,
1.2169e-6} and entered the refractive indices directly in ATMOS.

The geometry here may be more difficult to make precisely.  The
main objective spacing was chosen, however, to force the trailing
surfaces of the secondary lenses to be precisely equal -- but not
mated, unfortunately.

Even though the zonal longitudinal error is about 11.4 microns,
the wavefront error for the e-line is about lambda/35 P-V, still
quite respectable.

In spite of being only about 1% longer than the corresponding
N-K5/F2 design, it has about 37% less secondary color, measured
as the distance from the F-C intersection to the e-line focus.

In addition, it has less than 20% of the chromatic variation in
coma and 80% of the lateral color.  Off axis performance is
better.
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