Testing the M5 coating integrity

Before diving in & trying to clean M5, we first had to check whether the >12 year old coating would hold up.  The edges are the most vulnerable & there was an area that's normally covered by the outer baffle where some glycol spots had hidden & dried after the great glycol haemorrhage of 2012.  The perfect place to explore the worst case scenario!

The ugliest area on the edge of M5, where drops of glycol had dried & eaten the coating

De-ionised (DI) water, then a sodium lauryl sulphate solution & then more DI water was dabbed on a small test patch to see what would happen... As with the primary mirror segments, where the lighter they are in colour, the dirtier they are (the cleanest ones appear black since they look up at the black dome), it was great to see a black patch on M5!

A first pass at cleaning that patch showed just how dirty the mirror is, but also how bad the glycol damage can be

But we could also see how the coating had been destroyed by the glycol spill as cleaning those spots removed many tiny flecks of aluminium.  A sobering, though not unexpected, sight.  Belated Well Done to Keith for managing to dab away most of the spill on M5 at the time - now we can see what much of the coating would've been like if it'd all been left to dry!

The coating had been destroyed where the glycol had dried & cleaning then removed small shiny flecks

Happily though, a test patch on a more normal looking area nearby held up perfectly well - no shiny bits on the cotton-wool pads.  

An adjacent area that was merely caked with dust did clean up without issue

The improvement is really compelling & we've agreed to not be deterred by the fact that already-dead bits of coating (under the dried glycol spots) could come off during the cleaning. Obviously we'll be extremely cautious throughout the process, & stop to re-evaluate if other areas turn out to be problematic, but the potential gain amply justifies the manageable risk.

Note the black patches at 6 o'clock where the test-cleaning was done

We really need to do this - tomorrow will be a stressful day :|