Re: (ATM) off-axis paraboloid mirrors
Chuck Grant (grant@aretha.llnl.gov)
Wed, 23 Aug 1995 15:23:00 -0600
> From: cds@peanut.sarnoff.com (Clay Spence x3039)
>
> "G. Pearson" <gpearson@blue.weeg.uiowa.edu> wrote:
>
> > Subject: (ATM) off-axis paraboloid mirrors
> > Can anyone give me any solid information about how good/bad these
>things are
> > for building an unobstructed scope? My gut feeling is that they'd have
>more
> > abberation -- probably coma -- than a symmetrical paraboloidal mirror
>of the
> > same diameter and focal length, but how much more, and what kind?
>
> (I'm responding because I haven't seen anyone else respond.) I think
> you're correct, since it's a peripheral piece of a large short f/#
> paraboloid. Of course, since it's a small piece its aberrations aren't
> as bad as the large paraboloid. I can do a reasonable approximate
> calculation, or a more tedious but exact third-order calculation. I'll
> let you know what I find.
>
> Clay Spence
> cspence@sarnoff.com
>
It is fairly easy to ray trace an off axis section of a paraboloid
to get an idea of what is happening. Yes they have more coma than
an on axis paraboloid the same diameter, and less coma than a full on axis
paraboloid with a diameter large enough that your off axis section can
be cut from it.
The spot diagrams (showing the off axis abberations) are shaped differently
than the usual spot diagrams from centered systems, so comparing them is
not as straightforeward as it might be.
Chuck