Because Pin ~3W seems quite convenient to measure viewport absorption (as it makes the 2 surfaces visible on the AC signal), we performed again the absorption measurement of the spare viewport.

Figure 1 shows a long Z scan of the translation stage with the 2 surfaces visibles and at the expected position.

Figure 2 and 3 show the absorption of respectively the spare and cleaned viewport.

The spare viewport shows some dusty spots (some visible by eye+ strong green light).

The cleaned viewport shows more dirty spots  (also some visible by eye+ strong green light).

However, it seems that there is no more large stain pattern visible.