Re: [ATM] Fringe testing Dall Kirkham secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Rod,

Rod Brackenridge wrote:
> 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.

Rod, that's a good explanation - pressing at the edge moves the 
fringes towards the edge, and they get closer together.  That's 
exactly the information you need.

Fringes indicate distance differences between the pieces.  Widely 
spaced fringes indicate that the space between the two pieces is 
changing across the pieces - it changes by 1/2 wavelength for every 
fringe.  Narrow spacing indicates the airspace is changing more 
quickly for the same distance, so the "slope" of the airspace is 
increased.

If the top piece is the mirror (convex) and the bottom piece is the 
concave test plate, and the mirror has a shorter ROC than the test 
plate, then you could put them together and align them such that they 
would physically touch in the center, but not the edge.  So, you would 
see a bullseye pattern, as you note.

Now imagine pressing on the right edge of the top of the mirror (as 
looking from above the pieces).  The mirror would tilt to the right, 
and the airspace would decrease there.  So, since the mirror is 
"pivoting" on its center contact point, the decrese in the airspace 
means you would see fewer fringes between the center and the right 
side of the test pieces.  You would observe the fringes moving TOWARD 
the point of pressure, spreading out.  This is the case when the 
mirror is move convex than the test plate.

You have the opposite condition - the mirror is not convex enough. 
When you put the pieces together, the edges would contact.  If you put 
tissue paper between them, the edges are closer than the center. 
Press on the right edge and the mirror tilts to the right, compressing 
the tissue paper.  The distance between the mirror and the test plate 
decreases faster at the right edge than the center, so the fringes 
become closer together at the right edge, as you describe.

> 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.

You have 5 fringes of bullseye.  So, the mirror needs to be 2.5 waves 
more convex.  So, you need to do more polishing with the mirror on the 
bottom to make the mirror more convex.  If you polish the edge only, 
you will probably turn it down.

Polish normally with the mirror on the bottom for 10 minutes and check 
your progress.  Creep up on an exact match.

You're not far from being done.  Fully polished and 2.5 waves off is 
not bad!

	Mike Lockwood

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