I've been working on my little Wright telescope lately, trying to
improve
spherical correction as new generation sensors get smaller and smaller
(300D
pixels are 7.1 microns square). After refiguring the corrector I
realized that
old Coulter 2.14" secondary isn't that flat - it started showing
astigmatism that was not noticable before. Time for a new secondary. I
figured I don't need a premium secondary - after all this is supposed
to be a photographic telescope. Discovery has reasonable reputation
and promises "Manufactured and tested in our own optical facility" and
"Guaranteed 1/10 wave surface accuracy" and prices aren't that bad
either. I've seen the cheap Chinese sourced "flats" and I can only say
stay well away. (that includes big "M" and "C").
Dissapointment #1 - their ruller must be of a small variety.
Supposedly 2.14" secondary measured only 2.045" across the small axis.
Well, OK, not that critical.
But much more important, when I placed the secondary in front of my
homemade 60mm f/10 refractor at 45 deg, it showed significant
astigmatism on star images. Not as bad as Coulter flat (which cost
whopping $14.50 in early 80's), but a far cry from "guaranteted" 1/10
wave. Best guess was between 1/4 and 1/3 of a wave convex. Perhaps
just passable for an astrograph, but considering it cost me A$ 165,
and my freshly autocollimation refigured Wright optics is now
definitely better than that, I have to say I'm rather dissapointed.
From now on all my secondaries will be sourced from Protostar,
regardless of the application.
If you are shopping for a secondary, I suggest you do the same.