Saturday, December 12, 2009

Extracting M5!

It's the scary weekend...  M5's coming out of the SAC for a much needed clean!

James designed a nifty removal jig & a dummy M5 - the same 18 kg mass & overall dimensions - was made up out of aluminium to allow us to practise the removal & replacement procedure.

The mirror was covered, the removal jig + lifting frame installed in the SAC & the M5 tie rods undone.


With scaffolding in place on either side of the SAC, the lifters could pick up M5, clear the Faro & shuffle across to the hand-over position where the carriers were waiting.


We then ferried the precious cargo over to the wash trolley waiting nearby.


A gentle touch-down & breathing could resume...  Thanks to substitute-photographer Hannah!


After dismantling the removal jig & taking the covers off, the full horror of the dirt was revealed!


So we're now down to a 2 mirror corrector!  Surely this must make things simpler?


With M5 safely down on the wash trolley, we could turn our attention to trying out the cleaning process on one of the witness samples.  This small specimen has the same coating as the SAC mirrors & has been living underneath the primary mirror truss since the SAC was installed in 2004.  As a result, it's been exposed to much the same conditions as the upward-looking mirrors in the corrector (M5 & M3).


The first step was simply to rinse the sample with de-ionised water & then with distilled water for a few minutes.  The distilled water has a distinctly different character to either ordinary water, or even the de-ionised water - just magic stuff!

The rinsing process removed most of the dirt, but the surface remained somewhat dull.

The next stage was to soak unbleached, lint-free cotton wool in a 10 grams/litre solution of sodium lauryl sulphate & distilled water.  This mild soap's used on horses, & increasingly in dealing with dirty telescope mirrors!  The cotton wool's simply dragged over the surface, no extra pressure's applied other than the weight of the cotton wool.

This really did the trick - the surface looks as good as new!