From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


The simplest setup would be an illuminated pinhole instead of a slit, and
an eyepiece instead of a knife edge or ronchi screen.  By making very small
pinholes and examinining them with the eyepiece, you will see the same
diffraction effects as a test in the scope on a star.  The sphere on the
bench tested at the center of curvature is a null test, all rays focus at a
point when the surface is correct.  The paraboloid in the scope tested at
focus is a null, also.

I don't recommend the bench star test for paraboloids.  It is not
quantitative, and I don't know how to make it so.  My use of the star test
is to make reference spheres for testing flats, and for flex mirror scopes.

using a laser pointer for the source will give sharper diffraction images
because of the single color.  If the voltage is too low, it will not lase,
but will still provide the pinpoint red source.

Colin

-----Original Message-----
From: Peter [mailto:peter{at}kitgear.com]
Sent: Tuesday, March 18, 2003 6:37 PM To: CSC
Subject: Re: ATM Making & Testing a Perfect Sphere


Hi Colin,

Where could I learn about the star test you and Ray are discussing? I have
only heard about star tests being done outside and his photos look like an
inside shot.


Peter

----- Original Message -----
From: CSC 
To: Atm 
Sent: Sunday, March 16, 2003 10:46 PM Subject: RE: ATM Making & Testing
a Perfect Sphere


>
> Great test setup!  I am also fascinated by this laser star test on
spheres.
> Here's my 2cents.
>
> You're not at sphere yet, as shown by Foucault.
>
> Looks like KE from the right, and inside focus, as you say.
>
> You're undercorrected, with the center and edge high, and a low 70% zone,
> classic oblate spheroid, the inverse of a paraboloid or prolate spheroid
> (ellipsoid).  This is likely from the edge of the lap stopping and
changing
> direction in this region, causing greater friction and wear there. Needs
> longer strokes, etc. or whatever your favorite method is for parabolizing.
>
> The edge looks good, based on the diffraction ring that shows in the dark
> half of the mirror.
>
> No astigmatism showing in the star test.
>
> Fairly smooth, as there are no major "ray like" streaks or
lumps in the
> rings.
>
> The star test is most useful comparing inside and outside focus images. I
> suspect that the extrafocal image shows minima (lines) that are not as
> crisp. Ideally the contrast in the rings is equal on both sides.
>
> Try a 30% obstruction on the mirror, like a 2.5" paper disk, taped on the
> mirror and observe when the shadow appears in the diffraction image, both
> inside and outside.  The distances should be equal.  If it appears sooner
> inside, then it's undercorrected.
>
> What is your magnification?  Go as high as you can, and as you get closer
to
> the sphere, look at the star test images closer to focus. The test gets
> extremely sensitive when there are only 2-3 rings in the diffraction
image.
> I use about 700x.
>
> Read Suiter's book on star testing and keep in touch.  I had briefly tried
> to get star test images of this process, but   The testing you're doing
will
> get you to 1/40 wave.
>
> Congratulations,
>
> Colin
>
>
>
>
>
>
> -----Original Message-----
> From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Ray J
> Sent: Monday, March 17, 2003 12:14 AM
> To: Peter
> Cc: atm{at}shore.net
> Subject: Re: ATM Making & Testing a Perfect Sphere
>
>
>
> Peter,
>
> I am working on a the same thing, testing a sphere, except I just want to
> get
> the hang of testing.  I thought I had a good sphere but what happened is
> that I
> got better at testing and the mirror got worse and worse.  I put some pics
> at
> http://www.j-engineering.com/ATM
>
> Some rings in the star test are different from the others.  I have not
> figured
> out what they mean other than that something is not quite right.  For me,
> enclosing the light path was a must.  Quality knife edges and an
3000-7000+
> mcl
> LED's also helped.
>
> To answer your question, I used Sixtests with b fixed to 0.0.  FigureXP
does
> the
> same thing with Desired Deformation set to 0.0  My knife edge positions
were
> close together but different with practice.  For me, the range was 0.008",
> but I
> will test again with more zones.
>
> Knife edge with masks, software, the knife edge straightness near null,
then
> just looking for shadows at null, is the general testing route I'm taking.
>
> If someone could tell me what the star test is telling me, give me a
shout.
>
> 8" F/6.3, Ray
>
> Peter wrote:
>
> > Hi,
> >
> > After sending my 1st email I realized that all I need is a method to
> > evaluate the Foucault results. I'm struggling with the math involved to
> test
> > the sphere.
> >
> > I've used the Foucault test in combination with Tex for porabolized
> mirrors.
> > Is there a program like Tex which will allow me to test spherical
accuracy
> > Vs parapolic accuracy?
> >
> > If not would someone please give me a reference to the applicable
formula.
> >
> > Thanks,
> >
> > Peter
>
>
>
>

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