The steel coat hanger (see the May 4 post), used to mark the optical axis of the corrector for the CGH tests, has now been replaced with an invar one. This niftier looking model ought to be more stable due to invar's much lower thermal expansion coefficient.
Once the SAC's back up on the telescope we'll need an accessible reference to check its overall alignment. The SAC reference mirror is set up to locate repeatably on the back of M4 & mark the optical axis.
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