Re: [ATM] stalking the wild achromat: what price broadbandcorrection?

["Everett Cairns" <ecairns@mcmaster.ca>]

>Just a thought, cut your glass costs and build an achromat from one type of 
>glass (plate glass qualifies). Since it can't have positive power without 
>one mirror thrown in, you end up with a full aperture spherical corrector 
>lens followed by a spherical mangin mirror returning to a sub aperture 
>spherical corrector lens.
    The polychromatic performance of an 8inch f4 designed for the range of 
400-1000 nm is just diffraction limited. Allowing for the diffraction 
effects of a 4" diam diagonal mirror to bring the focus out to the side, for 
a 2.5 degree full field (1.4 inch diameter  FLAT field) 80 % of the image 
energy is within a 9.6 micrometer spot.
    If you place a small diameter ccd, such as the Starlight expess 
design,on the axis and dispence with the diagonal, 80% of the energy is 
within an 8 micrometer spot if the detector is smaller than the 2.6 inch 
diameter corrector lens.
    For the record,were the instrument unobstructed, 80% of the energy would 
fall within a 4.4 micrometer circle. This includes a 0.4 micrometer 
contribution due to residual aberrations.(Consideral incentive here to 
design an off axis variation)
        So there you have it, with considerably lower glass costs you can 
build an instrument with all the construction difficulties of a refractor 
and a reflector combined!

        Everett 

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