The qustion is, what was used to 'observe' the photon particles as they passed through the slits?
The experiment is; when firing single photons the wave pattern was still observed.
Nothing about observing them as they pass through the slits, a photon detection device is never mentioned.
You can observe other photons as they hit the one or other of the slits, but these are other photons, not the photons causing the diffraction.
Does anyone on this underachieving backward planet understand particle physics?