A new alignment procedure for the South African Astronomical Observatory's 74-inch telescope

Considerable effort has gone into improving the performance and reliability of the SAAO’s 74-inch telescope. This included replacing the telescope encoders, refining the pointing model and increasing the telescope throughput. The latter involved re-aluminising the primary and formulating a procedure to ensure optimal alignment of the telescope mirrors. To this end, we developed the necessary hardware and techniques to ensure that such alignment is achieved and maintained, particularly following re-aluminising of the mirrors. In essence, the procedure involves: placing a Taylor Hobson Alignment Telescope on the mechanical rotation axis of the 74-inch (which we define to be the optical axis, since the Cassegrain instruments attach to the associated turntable), then adjusting the tip/tilt of the secondary mirror to get it onto that axis and, lastly, adjusting the tip/tilt of the primary mirror to eliminate coma. An eyepiece (or wavefront camera) is installed at the Cassegrain port for this final step since comatic star images indicate the need to tip/tilt the primary mirror to align it to the secondary. Tuning out any brightness gradients seen in an out-of-focus image of a bright star may also be used for feedback when adjusting the tip/tilt of the primary mirror to null coma.