Re: ATM 2nd post - first scope : ) 4.25" off axis sphere

["David Harbour" <scarab@peakonline.com>]

Donovan-
For a pretty good explanation of the difference between tilted component
reflectors (i.e., "off axis" unobstructed reflectors), and those unobstrcted
instruments whose mirror represents a section of an eccentrically placed
entrance pupil (NOT off axis, but unobstructed), see my article "Stopping
Down" at my website. It is short, and has some interesting diagrams that
make some of the concepts clearer and easier to understand-
Dave
www.greatplainsinstruments.com

----- Original Message -----
From: "donovan" <donovan@bravoz.com>
To: "Sidor . Kurt" <KSidor@drc.com>
Cc: <atm@shore.net>
Sent: Thursday, July 12, 2001 1:29 PM
Subject: RE: ATM 2nd post - first scope : ) 4.25" off axis sphere



Thanks for the response. It makes sense, but I need clarification on one
thing. When you say "tilted, and off axis" what's the difference between
those two?

Donovan

> -----Original Message-----
> From: Sidor . Kurt [mailto:KSidor@drc.com]
> Sent: Thursday, July 12, 2001 6:44 AM
> To: donovan
> Cc: 'atm@shore.net'
> Subject: ATM 2nd post - first scope : ) 4.25" off axis sphere
>
>
> Donovan,
>
> If you use a 4.25" F #10 spherical mirror tilted, and off
> axis, I do not
> think you will get very good performance.  If your 4.25" mirror was a
> cored-out off axis section of a larger parent mirror, the
> full size mirror
> would have been say about a 10" diameter mirror of 10 times
> 4.25" or about
> 43" focal length.  That would make the full size (on axis
> mirror) a 10" F
> #4.3 mirror.  If this parent mirror were left spherical and
> not parabolized
> it would have about 6 waves of surface deviation from the
> ideal paraboloid.
> Your off axis portion would have about 5 waves of that error at least.
> Using your 4.25" F #10 mirror as an on-axis Newtonian your
> mirror would only
> deviate from the ideal paraboloid by about a 1/5 of a wave (@510 nM).
> Despite the central obstruction this would produce a much
> better performing
> instrument.
>
> Good Luck!
>
> Kurt Sidor
> Mechanical Engineer
> Dynamics Research Corp.
> Encoder Division
>
>