Date: 29 Nov 86 06:36:27 GMT
From: jtk@s1-c.arpa  (Jordan Kare)
Subject: Re: refractor vs reflector
Sender: space-request@s1-b.arpa
To: space@s1-b.arpa

In article <861128-171715-5212@Xerox> Lynn.ES@XEROX.COM writes:
>>"Why would a refractor be better than a reflector?"
>
>Refractors are notoriously better at reaching theoretical resolution
>because the secondary mirror or light detector hanging out in front of a
>mirror diffracts up the image.  However, an extremely small detector or
>secondary has negligible effects, so a reflector can work essentially at
>the limit of resolution under this limitation.
>
>/Don Lynn

Another factor, particularly interesting for LARGE space telescopes,
is that the required surface (figure) accuracy for a long focal-length
refractor can be very low.  For a mirror, essentially independent of
f/number, the entire surface must be within <<wavelength of some
specified position -- if one side of your 100 or 1000 meter mirror
drifts by 1 micron, your image goes to hell.  For a lens, the 
absolute deflection of the light is small (assuming large f/no's)
and errors can be correspondingly larger; in addition, only the
lens thickness counts, not (to first order) its position.  Thus
if one edge of your 100 meter gas-filled lens drifts axially
(not radially) by a few microns, you'll never notice.

	Jordin Kare	jtk@s1-c.ARPA	jtk@mordor.UUCP

(P.S. my thanks to all who leaped to my defense regarding flat mirrors...
and I had forgotten the trick of switching pairs among 3 blanks to
get flats!  But it's still hard...  JTK)