Re: [ATM] Reuse

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

Richard F.L.R. Snashall wrote:
> Note the radii of curvature on the Littrow doublet.  Provided
> these are made before the larger lens is AR coated, the curved
> surfaces can then be figured against mating surfaces of that
> larger doublet.
This Petzval design with the specific Littrow restriction shifts
the minimum focus quite a bit.  However, with other glass
combinations, the shift is different.  With the primary glass
combination of N-BK7/N-F2 (the cheapest ECO crown/flint combo
I could find), the Clark design has, according to one measure,
the highest Visual Strehl when the C-F crossover is at the
80.60% zone, with the minimum paraxial focus is at 549.596 nm.  I
checked some glass combinations for the specific Littrow doublet
secondary.  The glass combination N-BK7/N-BaSF64 appears to shift
the C-F crossover only a few percent (of the semi-aperture).

When that particular glass combination is used, the highest Visual
Strehl occurs when the C-F crossover is at the 90.52% zone, with
the minimum paraxial focus at 548.504 nm.

Given that I was using the GSUM tool to design the primary doublets,
I struck a mean color correction between these two.

The doublet (175 mm f/12):

    http://users.rcn.com/rflrs/dbl02-09x.atm    ATMOS input file
    http://users.rcn.com/rflrs/dbl02-09x.len    OSLO input file
    http://users.rcn.com/rflrs/dbl02-09x.txt    TEXT prescription
    http://users.rcn.com/rflrs/dbl02-09x.zmx    ZEMAX input file

The Petzval (175 mm f/8.91):

    http://users.rcn.com/rflrs/Ptz08-74x.atm    ATMOS input file
    http://users.rcn.com/rflrs/Ptz08-74x.len    OSLO input file
    http://users.rcn.com/rflrs/Ptz08-74x.txt    TEXT prescription
    http://users.rcn.com/rflrs/Ptz08-74x.zmx    ZEMAX input file

The C-F crossover for the doublet is at the 88.51% zone, with the
minimum paraxial focus at 548.511 nm.  The C-F crossover for the
Petzval is at the 83.68% zone, with the minimum paraxial focus at
549.451 nm.  Both of these indicate a shift in the minimum paraxial
focus of about 1 nm.

So how much is the loss?  The measure of Visual Strehl is 0.7127
for the best doublet color correction and 0.6903 for the best
Petzval color correction.  The corresponding values for the two
above are 0.7125 and 0.6901, respectively -- a loss of 0.0002 for
both systems.
_______________________________________________
ATM mailing list http://www.atmlist.net/