MID reports an observation that is particular to the operation at the currently high photon energy of 23 keV

MID inserted the XTD6-MID-preabsorber and assumed that it completely stops the beam.
They acquired AGIPD detector images as dark frames but were surprised to still find some weak background signal (not dark).
The explanation by PRC and XRO is that the purpose of the pre-absorber is to protect the shutters against potential
damage and not to entirely stop the beam like a radiation protection shutter itself.
The pre-absorbers contain diamond and B4C but no tungsten. The calculated transmission of the pre-absorber at 25 keV is 4%.
3rd harmonic of the FEL (here at 69 keV) is on the order of 1%, and MID uses the mirror coating
stripes for high photon energies, thus transmitting well these high photon energies.

As another result, the apparent beam size observed on the downstream imagers can be more complex to understand:
MID actually also reports that the beam size appeared larger in the afternoon than in the morning,
depending on the insertion status of the pre-absorber and the lenses.
The scintillation efficiency of the YAG screens is known to be high also for hard x-rays beyond 30 keV.
Additionally, the CRLs focus the various harmonics differently. When CRL1 is set to focus the beam,
it will likely defocus higher harmonics yielding a larger spot size on a downstream imager.

PRC informed XRO to follow up together.