[ATM] Re: ATM Digest, Vol 17, Issue 10

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

> I also toured the Hobby-Eberle.  Here's a puzzle (one the guide couldn't
> answer).  Its primary is an 11 meter f/1.5 sphere.  How much difference in
> sagitta would there be if it were a paraboloid? (answer withheld as an
> exercise for the reader, and also because I haven't figured it out yet)

Equivalent formalae (equal paraxial ROC = R) would be:
sphere:     x^2 + (y-R)^2 = R^2, or y = R - sqrt(R^2 - x^2)
parabola:  y = (x^2)/2R

so for x = radius = 5.5m and R = 33, the sag would be
sphere:    929.8 mm
parabola: 458.3 mm

This deduction may, of course, be wrong.

Cheers
/Arjan

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