title: Adaptive astigmatism-correcting device for eyepieces.
abstract: OBJECTIVE
  We describe how to construct a low-cost and robust variable astigmatism-correcting device manufactured using a pair of identical commercial ophthalmic lenses.
  
  
  METHODS
  Variable astigmatism power can be obtained by relative rotation of two cross cylinders with the same prescription (for this work: S = +2.00 D, C = -4.00 D was used). The rotation of the whole ensemble allows the user to choose the astigmatism axis.
  
  
  RESULTS
  The lens system presented here is able to correct astigmatism for eyepieces, compensating eyeglasses prescriptions from -8.00 to 0.00 D without generation of higher order aberrations and always with mean sphere power close to 0. Theoretical description of the piece, calibration, measurement of aberrations, and examples of use within a microscope are included.
  
  
  CONCLUSIONS
  An inexpensive and robust variable-astigmatism device for eyepieces can be manufactured using two equal ophthalmic lenses. The system is robust in the sense that small misalignments only minimally affect mean sphere power and do not affect higher order aberrations. This unique device can be adapted to many users.
authors: Area de Óptica, Departamento de Física Aplicada, Facultad de Física, Universidad de Santiago de Compostela, Spain.
authors: Justo
authors: J
authors: Arines
authors: Eva
authors: E
authors: Acosta
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology*; D001251:Astigmatism / Q000503:physiopathology / Q000534:rehabilitation*; D004867:Equipment Design; D005139:Eyeglasses; D006801:Humans; D007909:Lenses; D008962:Models, Theoretical; D012029:Refraction, Ocular; D014792:Visual Acuity