THE STAR TEST

For most purposes, the Abbe test plate has been superceded by the simpler and more sensitive star test. This compares the out-of-focus appearances of a small bright point in the image above and below the level at which the Airy disc has its minimum diameter.These tiny bright spots can be created by examining a cover slip, the under surface of which has been silvered, leaving small perforations. For the testing of immersion objectives, the top of the slide itself is silvered. The star test depends on the fact that if the objective is free from spherical aberration, the distribution of light in the discs seen above and below the point of focus ought to be identical.If the central focus of the objective is shorter than its marginal focus - i.e. if it is over-corrected- the light will form a bright spot below the true focal plane, and a ring above it, owing to the uneven distribution of light rays. The reverse effect occurs with an undercorrected lens. The correction collar is then used to get an as symmetrical effect as possible, i.e. the same appearance above- and below the focal point. As an alternative, adjustments can be made to the length of the draw tube (see diagram). In principle, this test can also be performed when focusing on a small dark speck in the slide to be examined against a bright background, and examining the out of focus images above and below the focal plane, but this requires a bit more practice.