Re: [ATM] Ronchi Confusion

[buck childers <bchilders@everestkc.net>]

Joseph,
A short follow-up to Dale as I didn't want to assume how much you may
already know about this stuff.  You probably already know radius of
curvature will be twice the focal length.  Suiter's book and the Orion
instructions both refer to Star Testing where the light source is at
infinity.   It's not uncommon to have an eyepiece with only a ronchi
grating just for this purpose when doing a quick check of someone's
scope.  The scope needs to be well collimated but you get a quick test
of the total optical train this way.

For bench testing a mirror in the process of figuring, it's easier to
throw a ronchi grating in front of a light source (typically on a
Foucault tester), and look at the bands just inside and outside radius
of curvature.

-Buck

----- Original Message ----- 
From: "Dale Eason" <atmpob@yahoo.com>
> The reference to curved lines are for when you are
> testing in the shop using a source at the radius of
> curvature.  For star testing a porabloidal mirror the
> lines should be straight as the instructions say.
>
> Dale Eason
>
> --- Joseph Celmer <celstar@frontiernet.net> wrote:
> > Hello ATM,
> > This is my first post to this list.
> >
> > I am confused about the Ronchi test. I have a
> > grating from Orion
> > Telescopes & Binoculars (no longer offered). The
> > instructions read as
> > follows. "Properly figured optics: With the
> > telescope aimed at a star,
> > straight bands indicate a good paraboloidal mirror
> > figure or a properly
> > corrected lens."
> >
> > Almost all other references I find indicate that a
> > spherical mirror
> > gives straight lines, a parabola slightly curved.
> > Except, page 290 of
> > Harold Suiter's
> > authoritative "Star Testing" book seems to agree
> > with Orion instructions.
> >
> > Am I missing something?
> > ____________
> >
> > Clear Skies!
> >
> > Joe Celmer
> > celstar@frontiernet.net
> > http://www.frontiernet.net/~celstar
> >
> >
> >
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>
>
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