Re: [ATM] fringe testing three flats

[Mike Lockwood <melockwo@uiuc.edu>]

Thomas,

Thomas Janstrom wrote:
> So to take an example, when I have B on C I get fringes that look 
> kind a like this "]", and when I push on the center the fringes 
> move towards the theoretical center of the arc segments, however 
> the spacing between them doesn't change so long as I push on the 
> center, if I push of center then they change in spacing. I'm 
> assuming that the high spot (see below in the original post) is 
> towards the "center" of the arcs that I see, is this right?

OK, so the fringes move to the left when you press in the center. 
That means the left sides of the disks are farther apart.

So, if the fringes curve to the left, that means the glass surfaces 
must get closer together in order to maintain the spacing that makes 
the fringe.  So, the outer part of the disks are closer together than 
the inner part.  (If flat B is perfectly flat, that means C has a 
raised edge, for example.)

Does that help?

> BTW any suggestions on how to deal with a turned down edge 1/2 
> fringe by sub 1/4" wide....

You need to figure out if this is a result of involuntarily putting 
pressure on the edge of the disk when polishing, or if it is a result 
of something else.

I recommend that you figure out which flat is flattest, and work it to 
completion.  If it has a turned edge, to correct I favor working MOT, 
and placing the high zone over the edge of the lap.  Then simply wear 
away the high places until it is flat.  This means starting with the 
pieces centered, with pressure on one edge of the mirror.  As you do 
short strokes, move the mirror off the lap and reduce the pressure. 
This removes more material near the edge, and gradually less towards 
the center.  (This is similar to a parabolizing stroke, but with 
accented pressure when you are COC.)  I believe Texereau illustrates 
this technique in his book.

You will learn a LOT about figuring by watching the fringes change - 
you get direct feedback very quickly, and you can tell exactly what 
you did.  It is a great learning experiences.

Jeff Baldwin wrote:
> I use small triangular pieces of cellophane, like a CD wrapper cut 
> up, placed between the glasses at the edges with the points towards
> the center. As you pull on one slowly you will alter the gap at
> that corner and you can do that without pushing down on the glass,
> which changes the shape and can cause misjudgment of the figure.

This is a nice technique, but the only problem with it is that the air 
gap will be larger than if there are no spacers.  This means that the 
viewing distance will need to be greater to avoid parallax.  I think a 
typical air gap is only a few wavelengths when there are no spacers. 
How thick is the CD wrapper?

> Also, the further back you go the more accurate the fringe image
> will be.  Go all the way to the other side of the room and use a
> telephoto lens to image the fringes.

I do the same thing, and a 2-second exposure is required to get good 
fringe contrast with my digital camera, sitting on a tripod.

	Mike Lockwood

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