Monitoring optical system for scanning optical device

A scanning optical device comprising:

• a laser source for emitting a laser beam;
• a deflector for deflecting said laser beam, the deflected laser beam being separated from the incident laser beam in a auxiliary scanning direction;
• a first anamorphic optical system located between said laser source and said deflector;
• a second anamorphic optical system for converging said deflected laser beam onto a drawing surface, said second anamorphic optical system including a correcting optical system for correcting an effect of skew of said laser beam;
• a monitoring anamorphic lens for converging a monitor beam that in separated from said deflected laser beam at a separation point between said deflector and said correcting optical system;
• and
• a monitoring sensor for detecting said monitor beam converged by said monitoring anamorphic lens, said monitoring sensor generating synchronizing signal when it detects said monitor beam;
• wherein a main meridian that indicates a direction of main refractive power of said monitoring anamorphic lens is inclined with respect to a z-direction on a plan perpendicular to said monitor beam so that wavefront aberration on said monitoring sensor is minimized, said z-direction being perpendicular to a scanning direction of said monitor beam on said plane.