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.
Originally published 14.2.2013. You can read more about it here.