I already have a some knowledge about sigma 1 (sigma 1 is known) and since I made multiple measurements on the target, I have the experimental variance s².

Is it correct to say that, since s² = s1²+s2², then s2 = sqrt(s²-s1²) and is a good estimate of sigma 2?

If yes, what to do when the contribution from sigma 2 is very small, which leads me sometimes with a s2² being negative...

Kinda lost, would really appreciate any help