Low-cost interferometric testing for conical surfaces

Published 14.2.2013

A Two-slit interferometric wavefront error measuring technique combines high sensitivity and low-cost components.

Testing of conical surfaces for mirrors or lenses most often require interferometry and with it, expensive devices to detect wavefront aberrations.

A Two-slit Interferometric wavefront error measuring technique, first deviced by Yrjö Väisälä of Tuorla Observatory, combines high sensitivity (much higher than with traditional Foucault or Ronchi tests) with few layout components most probably already in posession of ATM mirror manufacturers. The principles of this test have been developed into an interferometric Hartmann test.

Test consists of inserting a slitted mask in front of the mirror, at exit pupil. A point source and detector/ocular is positioned at the radius of curvature. Slits are coved so that only two slits side by side are shown at any time. Beams from the slits converge (more or less, depending on the quality of the mirror) at the center of curvature, where they form interference pattern. The difference in positions between the interference pattern and where it should be, determines the wavefront error at the location of the open slits. One slit is then closed and another opened, and another zone of the mirror is examined.

Test measures wavefront aberrations in one radius, so in order to make a 3D map, one must measure along several diameters.

The translated description of the test can be downloaded here (pdf). A spreadsheet calculation sheet for seven slit pair test can be downloaded here (xls).

The translator has used this test for measuring several conical surfaces and gladly answers any questions and queries you might have.