[groos]

Excellent. I might have a go at this myself. Was there a particular AliExpress seller that you got the mirror from?

[chantepierre]

Hi, I could send you the link but as outlined in the post, this specific mirror was very out of tolerance..

[groos]

I looked but AliExpress makes it impossible to locate even specific sellers, like BohrOptics, who I found to have consistently good quality (bought a beam splitter and fused silica ball lenses from them).