Re: ATM Loveday Secondary, Seidel Coefficients

[Aplanatic@aol.com]

Bratislav wrote:

> This is very interesting. I alwas thought that confocal paraboloidal
>  Mersenne is free of coma. The tertiary in this case (as in any Mersenne)
>  doesn't really amplify anything, it simply adds its own aberrations (as
>  it is presented with a parallel beam, just the same as if it was no
>  additional optics). So this can also be checked by comparing off axis
>  images in a Mersenne to stand-alone parabolid of the same focal length
>  and f-ratio as the tertiarry. As we are talking f/25 or so, ANY
>  discernible coma in a final image means that "beam de-spander" adds its
>  own. In a f/25 parabola, dominant aberration ought to be astigmatism.
>  If anything can be called "dominant"; at 5 to 10mm field it was
>  discussed here, surely it has to be well within a fraction of Airy 
>  disc.

Yes, out to 10mm diameter field, the geometric spot size is tiny compared to
the Airy disk diameter in the original f/25 case that I analyzed.

When we speak of coma and astigmatism we mean something very specific, i.e., a
particular coefficient in the Schwartzchild expansion (Seidel coefficient)
which is not the same as looking at a ray trace and estimating the coma or
astigmatism from that.  I have so far been unable to figure out the Seidel
coefficient calculations given in "Telescope Optics" or "Advanced Telescope
Making Techniques."  In each case there are some inconsistencies and poorly
defined variables.  Does anyone on this list have or no of a decent
mathematical prescription for such a calculation?

Dave Rowe
Torrance, CA