Re: [ATM] 290 mm f/7.5 Refractor Design

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

Richard F.L.R. Snashall wrote:

>290  mm f/7.5;  Flat field; Reasonable correction of astigmatism,
>coma and spherical aberration; Reasonable zonal correction; Cheap
>glass;  Short, but probably has the color equivalent to a doublet
>of the same length -- probably lowers Visual Strehl (0.4827).
>
>    http://users.rcn.com/rflrs/ptz03-33x.atm
>    http://users.rcn.com/rflrs/ptz03-33x.len
>    http://users.rcn.com/rflrs/ptz03-33x.txt
>    http://users.rcn.com/rflrs/ptz03-33x.zmx
>
>  
>
 From what I've been able to change, this geometry only seems to work 
for a narrow
range around f/7.5.  However:

Surf     Radius   Thickness    Glass   Diameter

 STO   Infinity   -10.69447                 250
   2    735.865        37.1    N-BK7        260
   3   -747.592      29.765                 260
   4   -690.765        16.6       F2        260
   5   -11334.7      1390.5                 260
   6   -252.114         4.6    N-BK7         60
   7    356.191      227.87                  60
   8    238.379         8.5    N-BK7         75
   9   -695.413    191.5186                  75
  10   Infinity       0.111            42.13492
 IMA   Infinity                        42.11916

at 250 mm f/10 seems to do OK.  The length here is less than an f/8
doublet. The Visual Strehl is 0.544.

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