RE: [ATM] Fringe testing Dall Kirkham secondary

["Scott Milligan" <starzkey@charter.net>]

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Rod Brackenridge
Sent: Monday, November 15, 2004 1:24 AM
To: atm@atmlist.net
Subject: Re: [ATM] Fringe testing Dall Kirkham secondary

Hi Ken,
 
Thanks for responding.  I must admit that I am finding the fringe testing a
little confusing which is embarassing because everything I read tells me it
is simple to understand.
 
I am probably not explaining what I am seeing very clearly so I'll try
again.  When I press the edge, the bulls eye moves towards the opposite edge
and as it does so the number of fringes rapidly increase, particularly at
the edge where the pressure is applied.  If, when looking at the bulls eye
pattern, I press the centre, fringes move in towards the centre but the
number of fringes does not change.  The spacing between them appears equal-
when I am looking at the bulls eye pattern centred on the disks.  
 
I am concluding that the edges are touching and that I need to polish the
secondary's edge more.  Please tell me if I am misunderstanding.  I don't
want to waste time and make the radii diverge further.
 
Rod.


Rod: When you press the edge and the number of fringes increases, that is
telling you that you are pressing on the thin side of the air wedge between
the two pieces.  Now, when you press in the center and the fringes move
inwards, that is telling you that the parts are contacting at the edge, and
that therefore, the convex part has a slightly longer radius than the
(almost) matching concave.  To improve the match, you must therefore steepen
(i.e. make shorter) the convex radius.  

Using the parabolic approximation to represent the sag of a spherical
surface (more than adequate for this purpose), the radius difference, DR,
between the concave test plate and the convex part can be expressed as a
function of the number of fringes observed in the match, N, as follows:

DR = (4R2Nl)/D2, where R = the radius of curvature of the concave plate, D =
the contact diameter (i.e. the diameter over which the fringe pattern
appears in the test), and l = the wavelength of light used in the test.

I should also note that you must observe the test plate match from a
distance of > 20D away in order to avoid introducing an appreciable cosine
error into the interferogram scale factor (the scale factor equates a
deviation of one fringe to a known optical path difference between the two
plates, i.e. one fringe = 1 wave OPD, or 1/2 wave surface deviation in
reflection).

Scott Milligan

 ----- Original Message -----
> 
> > The thing you need to remember is that the number of
> > fringes will increase as the distance between the
> > surfaces decreases. If you have 2 curved surfaces and
> > the bullseye fringes are closer together in the center
> > then the convex surface is closer to the center of the
> > concave surface. Just opposite of what you said above.
> > 
> > If the centers touch, you'll have an lot of very close
> > rings at the center increasing in spacing as you move
> > out. If the edges touch, the rings will be clustered
> > around the edge increasing in spacing as you move in.
> > 
> > Thr rings will move towards the decreasing separation.
> > 
> > Ken Hunter
-- 
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