  We present a new technique for sensing optical cavity mode mismatch and
alignment by using a cylindrical lens mode converting telescope,
radio-frequency quadrant photodiodes, and a heterodyne detection scheme. The
telescope allows the conversion of the Laguerre-Gauss bullseye mode (LG01) into
the 45deg rotated Hermite-Gauss ("pringle") mode (HG11), which can be easily
measured with quadrant photodiodes. We show that we can convert to the HG basis
optically, measure mode mismatched and alignment signals using widely produced
radio-frequency quadrant photodiodes, and obtain a feedback error signal with
heterodyne detection.
