Dear aneves,               

many, many thanks for your posting!

With the Bath interferometer you can test optics till f/4 correctly, but a f/3 spherical mirror
you probably can get problems in the matter you describe them. With a f/4 mirror there
are no problems.

This interferometer works with two beams and not exactly on optical axis. These two
beams distance should be 4-5 mm, no more, best 4 mm and parallel till 3-4 meters. After
fixing the laser diode I fix the cube alignable, the two beams distance about 4 mm, parallel
and at last the bikonvex lense with 7-10 mm fokus in the first beam

Important is, that the second beam is concentrated in the first light conus after passing the
bikonvex lense, it sould be 5 mm in diameter and about 2 mm thick. The cube is 15x15x15 mm till 20 mm.

This is the way, how the Bath interferometer works. You can test optics with straight
fringes and circles for rotation symetry.

How big is your hole? The reference beam #2 should be near your hole. If you send me
one of your fringes pattern, I can put it on my server for you. You just write in your posting

Can you let us see, how your Bath interferometer
looks like? The upper picture is the one I use. Feel free to ask us more questions for
that.