  We have characterized the molecular tellurium (Te$_2$) spectrum in the
vicinity of the 423nm $^1S_0-^1P_1$ and the 431nm $^3P_1-^3P_0$ transitions in
neutral calcium. These transitions are relevant to optical clocks for
atomic-beam characterization and cooling (423nm) and enhanced detection
(431nm). The use of a Te$_2$ vapor cell as a frequency reference has many
advantages over other laser stabilization techniques, and we discuss an
application to measuring the instability due to the second-order Doppler shift
in a calcium beam clock.
