title: Bath interferometer (common path)
text: # Bath interferometer (common path)
  
  Karl-Ludwig Bath patented 5 designs of common path interferometers in 1973. Bath interferometers can be used to test telescope mirrors of any size.
  
  A Common path interferometer has the test and reference beams traveling over effectively the same path which has the advantage that you can use an inexpensive semiconducting laser pointer with low coherence versus other interferometers which need a high coherence laser (typically a He Ne laser).
  
  Bath also published an article about his favorite variation in June of 1973.
  
  Before the patent there was a functionally identical Right Angle Bath interferometer described and published in the journal Optical Engineering (the article was received by the journal on July 23, 1973).
  
  ## Citations
  
  * ↑ DE patent 2300958, Karl-Ludwig Bath, "Ein Interferometer mit gemeinsamem Weg für Referenzstrahl und messtrahl (common path interferometer)", published 1974-07-01
  * ↑ Bath, Karl-Ludwig (1973-06-01). "Ein einfaches Interferometer zur Prüfung astronomischer Optik". Sterne und Weltraum: 177–180.
  * ↑ Augustyn Jr., Walter H (July 23, 1973). "Common Path Interferometery". Optical Engineering. 12 (5): 180–181.
  