Re: ATM chipped mirror

[The Silent Observer <silent1@ix.netcom.com>]

Jan van Gastel wrote:
> 
> Today Steve Swayze emailed me that my 20 inch f/5 mirror is back from the
> coater (Spectrum). But: during the coating proces somewhere, a chip came of
> the edge. It is about 18 mm wide and 13 mm in from the edge, as Steve says
> "like putting a fingernail on the edge".

<snip>

> For me functionailty is more important then aesthetics and I think I will
> keep the mirror, given what Steve says about the quality. But I want to be
> absolutely sure that the performance will not suffer, for instance: will
> the chip give extra visible diffraction effects?  I  would appreciate every
> advise on this subject very much.

If the surface of the mirror is otherwise excellent, take your $300
rebate, blacken the chip, and go.  Sure, a chip is annoying to look at
(the more so if it's your own work and you know you did something you
shouldn't to put it there), but Steve is right that it won't affect your
performance visually.

If blackened, the chip won't scatter enough light to detect, and the
only way you'd see the diffraction effects from a chip that size is if
the rest of the mirror is perfect -- in which case, the diffraction from
that chip will be a fraction of what you're already getting from your
diagonal.

-- 
Some of their knowledge is corrupt, and inaccurate, being gleaned from
all manner of sources.  But like us, they are wizards too.
                                                    -- Jaldis the Blind

Donald Qualls, aka The Silent Observer           NAR # 70141-SR Insured
Rocket Pages                http://silent1.home.netcom.com/launches.htm
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Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.