Re: ATM Effects of an inferior secondary

["Richard" <richas@earthlink.net>]

One more question about secondaries, while we are at it...

When you get a secondary with an enhanced coating, is that coating identical
with the coating on your primary, or is it re-engineered for the 45 degree
angle of incidence?

Clausing, are you out there?  I prefer to here it from the guy who does it,
not the rumor mill.

. . . Richard


----- Original Message -----
From: "Michael Koch" <astro.electronic@t-online.de>
To: <atm@shore.net>
Sent: Thursday, July 19, 2001 11:31 PM
Subject: Re: ATM Effects of an inferior secondary


>
> Hi Bill,
>
> > Speaking of secondaries, who can characterize the effects of a secondary
> > that is less good than the primary?  What sort of defects does one see
with
> > a crummy secondary, say on a star test?
>
> The most common defect of a bad secondary is spherical aberation
(secondary is
> not flat), which becomes visible in the star test as astigmatism because
the
> mirror is tilted 45 degrees.
>
> > Is it possible to sort out
> > secondary defects versus primary defects at the eyepiece?
>
> Yes, in case of astimatism you can rotate the main mirror. If the
astigmatism
> doesn't rotate, it must be in the secondary or in the eyepiece. Rotate the
> eyepiece to make sure that it comes from the secondary.
> In case of other secondary defects it's difficult to see in the eyepiece
> where the defect comes from.
>
> > Must primary and secondary PV ratings be matched for optimal
performance?
>
> No. Both PV values should be independently as small as possible. It's very
> unlikely (but not impossible) that the PV error of the main mirror is
> compensated by an opposite error of same size in the secondary. For error
> compensation, the surface errors of the two mirrors must match over the
full
> surface. This is _VERY_ unlikely.
>
> > Can the secondary have a somewhat poorer PV rating than the mirror, ...
>
> Yes, the secondary can be poorer by a factor sqrt(2) = 1.414 because it is
> tilted 45 degrees. In this case the main mirror and the secondary produce
same
> amounts of image degradation.
>
> > ... without image degradation?
>
> No, every surface error causes image degradation.
>
> Michael
>
>
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