Image Quality in Telescopes with Image Motion Compensation by Secondary Mirror Control

Image motion compensation by servocontrolled tilt of the secondary mirror is applied in a 40-cm diam f/7.5 Cassegrainian balloon-borne telescope. A tilted-aplanatic configuration is used, i.e., the mirrors are aspherized to render the stabilized image free of third-order coma. A spatial resolution of 2 sec of arc is maintained in the presence of pointing errors up to 10 min of arc. The most important remaining aberrations are third-order astigmatism and fifth-order spherical aberration. For the latter, a general expression is presented that can also be applied to other two-mirror telescopes.