Re: [ATM] Alternative Petzval Design

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

Richard F.L.R. Snashall wrote:
> Richard F.L.R. Snashall wrote:
>
> An all ECO version:
>
> 288 mm f/7.5 ECO Petzval
OK, so maybe that size is a bit scary.

Here's another design for a 200 mm f/9 refractor (field diameter
42 mm or just over 1.25 degrees).  It has some nice features.
It is achromatically corrected both longitudinally and laterally.
It is centrally corrected for spherical aberration, coma and
astigmatism.

The net instrument length ends up being a bit shorter than the
focal length, while RoC of the field is over three times the focal
length.  Two of the surfaces are flat; the first large doublet
actually is an air-spaced Littrow design.  The remaining surfaces
are not strong (for refractors -- more than eight times the
semi-aperture) and have only two different RoCs; within that,
they could be matched interferometrically.

The design uses quite common glass (N-BK7, N-F2) for all but the
smallest lens.  That glass (N-SSK2) is bit more expensive, but
not outlandishly; it is also quite robust.

While the last surface of the Littrow doublet could be figured,
the performance improvement would be minimal, as the system is
already better than lambda/20 P-V at the e-line.

    http://users.rcn.com/rflrs/Ptz06-14x.atm
    http://users.rcn.com/rflrs/Ptz06-14x.len
    http://users.rcn.com/rflrs/Ptz06-14x.txt
    http://users.rcn.com/rflrs/Ptz06-14x.zmx

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