Posted By John Biretta
advertising precisely what the standard is
-- is it Wavefront? Surface? Peak-to-Valley?
RMS? What area -- 1/4 inch? Entire mirror?
Normal incidence? 45 degrees? Tested before
or after they coated it? Do they test each one,
or only 1 in 10? 1 in 100? How is the mirror
mounted? What is the substrate -- Coke bottles?
I think the only way to know what you've got, is
to put it in a scope and try it out. Do some
star tests at high power with and without it.
If you don't like it, send it back.
If you want a stronger test, you can put an
extension tube between the diagonal and
eyepiece -- effectively using a larger beam at
the mirror surface. That will make the errors
easier to see (though you probably won't
actually observe in that mode).
I've tested about 8 of these from AP, TV, etc.
The ones from AP and TV were pretty good -- there
are only minor issues visible in high power
star testing by looking at the images inside and
outside focus. Most had slight astigmatism or
rarely coma, but probably better than 1/10 wave
on the wavefront. Most folks would probably never
notice these small errors in real use.
The best one was something I cobbled together myself
from a TV NEAF "blem" mirror element (with no blem)
and a beat-up old 2" diagonal shell I got off AM.
Ugly housing, but the wavefront is perfect.
If you want a perfect one, you'll to need to
test them yourself.
The ones from Vernonscope are probably worth a look
-- quartz, protected silver, and interferogram
included -- if you are trying to find the best
wavefront. Not tried them myself yet.
The advertising hype will tell you that silver
mirrors rapidly degrade, are unstable, etc., but
I've got 30+ year old ones that are good as new.
The coating is thin (vs thick dielectric) and
will not warp the substrate.