Washing M4 this evening was quick & painless, compared to yesterday's slog of doing M5. Not only is M4 tiny by comparison (only about 120 mm in diameter), but being downward-facing it also wasn't particularly dirty.
|Smudges around the edge of M4|
That said, being so small but still having to reflect all of the light gathered by the primary mirror means that the marks on M4 roughly equated to segment-size blemishes on the primary. So it too really needed a clean!
|Harsher illumination to show the marks|
M4 got the same treatment as M5, with alternating doses of DI water & sodium lauryl sulphate.
|Wet with DI water & ready to lather up with soap|
Tried using isopropyl alcohol again to dry the mirror & this time there were no colourful streaks. This confirmed suspicions that the trouble yesterday was due to residual dirt on the mirror that would've taken more intensive washing (which would've been too risky given the state of the coating) to remove. Since M4 was extremely clean after its wash, the alcohol worked well.
|Clean & dry|
All of the marks were successfully removed :)
|Still looks good, even when subject to a brutal camera flash|
M4 also has no pinholes to speak of, so the coating's in really good condition...
|Peering up at the dome, shutter, tracker & tracker access platform|
We'll pop the M4 "steering wheel" back in its kinematic mounting assembly above M5 tomorrow & happily check the SAC cleaning task off the shut-down to-do list.