Friday, July 31, 2009

Aligning M5 with M2

The first step in this procedure is to set the alignment telescope up so that it's aligned with the optical axis of M2. Reflections from a mirror resting on the back of M2 & from the alignment target in the hole in M2 need to land on the graticule of the alignment telescope. This is achieved by carefully (& tediously!) adjusting the cones on the alignment telescope's mount & the tip-tilt stage on the fold mirror (that allows the telescope to look downwards).

One then has to view both the pinhole & the image of the pinhole as those two points give you M5's optical axis. That was made easier by etching a crosshair onto a piece of glass & attaching it to the top of the pinhole carrier.

The crosshair marks the location of the pinhole, which can then be viewed with the alignment telescope, as can the image of the pinhole (aka "the spot"). The pinhole carrier position is tweaked to get the best image quality for the spot & these two points then land roughly on the alignment telescope's graticule.

At this stage, the only degrees of freedom left belong to M5 & so the mirror itself needs to be moved to get all these points precisely centred on the graticule. M5 movements are effected by placing shims under the spherical bearings where the tie rods attach to the mirror.

That's the story in principle & all the fixturing's in place - now we just have to DO all this to the required tolerances... A task made tougher by the fact that Simon's led IT to quite the comeback on the pool table this past week!

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