In Bath interferometer camera must be focused on the mirror under test. Usually this is done by placing some well illuminated printed text on the mirror under test and camera is focused until the text is sharp.
Focusing can be done also in other way where separate illumination is not required and laser from interferometer is used for this purpose.
The beam splitter cube is removed so that there is no reference beam. Test beam diverged by the lens is illuminating the mirror and reflected light is directed to the camera via flat mirror. If an object (mask) is placed on the mirror under test, its outline can be used for focusing. First picture shows the outline of such object (mask) in interferogram
and second picture how it looks with proposed setup.
If the beam splitter cube can not be easily removed, the same method can be used if reference spot on the mirror under test is blocked by the same object (mask) used for focusing (in other words, reference beam spot should fall on the mask and thus can not be reflected). This picture shows the resulting image.
