George Roberts (Boston)
Another way to find focus point is almost identical to how you do it with Foucault tester:
First remove all tilt - in other words try to get a "bulls eye" pattern visible. Then push gently on the bath left or right. Watch which way the "bulls eye" moves when you push left or right. If the bulls eye moves the same direction as you are pushing then I call this "same". If they move opposite directions I call this "different". When you pass through the focus point it will switch to the other .
Now I need to know if you are looking at the igram through a camera or on paper and how your bath is setup.
If your bath is like the left picture where the laser points towards the mirror under test then
a) if you are projecting onto paper then "same" means inside focus and "different" means outside focus
b) if you are looking at the igram directly with your eye or with a camera then "same" means outside focus
If your bath is like the right picture where the laser points 90 degrees from the mirror then
c) if you are projecting onto paper then "same" means outside focus and "different" means inside focus
d) if you are looking at the igram directly with your eye or with a camera then "same" means inside focus
"inside" means the bath is closer to the mirror than "at focus". "outside" means the bath is farther from the mirror.
For me, "d" is most common and the one I memorized. I memorized this "for me, 'same' is inside".
Note also that if you are setup like the left photo you need to check the box in the dftfringe mirror configuration that says "flip interferogram left to right on loading". If you don't do that you may be rotating the images the wrong way or polishing your mirror in the wrong spot.