Thursday, October 15, 2009

Shimming & twiddling!

Some violent SAC interventions today! To get M5's optical axis aligned with that of M2, we had to break the epoxy locking the M5 tie rods - 1 size 10 spanner to hold the rod & another to torque the nut at each end & break the glue...

The tie rods (which attach at 1, 5 & 9 o'clock) allow adjustment of M5 in decentre. The 9 o'clock rod moves M5 in Y only, while the 1 & 5 o'clock rods move M5 in X only - provided you rotate them equal amounts in opposite directions.

There is a spherical bearing where each of the tie rods attach to M5 & this is where shims get inserted to adjust the tip & tilt of the mirror.

Combinations of shims & tie rod rotations enabled us to get the 2 points defining M5's optical axis to coincide & then these had to be made to line up with the centre of the alignment target that represents the optical axis of M2. The next 2 images show the positions of the pinhole itself & the pinhole image (both marked with a red cross-hair) with respect to the centre of the M2 target (indicated by the blue circle).

It then became an iterative process to optimise the image quality by twiddling the tie rods (using the wavefront camera's live aberration breakdown for feedback) & moving the pinhole image ever closer to the target by shimming accordingly.

Inserting & removing the shims involves lifting M5 with our soon to be patented "shimmin' tool" & slipping the required feeler-gauge in or out from under the particular spherical bearing.

By the end of a very long day we had the 3 spots lined up & M5 delivering an excellent wavefront - with that, M5 aligned with respect to M2! A fantastic way to end an epic 10 day run - off to Cape Town tomorrow for a much needed long weekend :)

Tune in next week when we hope to reinstall M4 with its improved mounting scheme!

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