Re: [ATM] F10 Achromat

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

aneves@snowsoft.biz wrote:

> Hi,
>
> I am in the process of building a 7" Achromat (Bk7, F2). Would a focal 
> ratio of F10, give too much cromatic aberation? I am trying to decided 
> between an F10 or F12. I would like to try the F10, but if performance 
> is to be too poor, I will make it F12.
> Can anyone give me some advice.
>
> Regards,
>
> Alfredo

If you consider two such 178 mm CA achromats:

         Radius      Thickness    Glass      Diameter

        1046.25       22.35419    N-BK7           187
        -658.88          4.485                    187
        -658.88       12.89333       F2           187
       -2800.55       1752.214                    187
      -645.3607

and:

         Radius      Thickness    Glass      Diameter

        1253.37       20.55949    N-BK7           187
       -787.951           5.61                    187
       -787.951       13.22962       F2           187
        -3371.5       2106.565                    187
      -772.2004

the first being f/10 and the second f/12, the visual Strehl
ration goes from about 0.4 in the first to about 0.5 in the
second.  Will the extra 0.1 be worth it?

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