There are ongoing development efforts intended to arrive at the minimal systems that will provide the needed information for both detector calibration and atmospheric calibration. It is mechanically awkward to implement. An alternative system has been under investigation. As suggested by Byron Dieterle, it relies on atmospheric scattering of light from a point flash to produce a diffuse light pulse that is seen by all pixels. If the flashbulb is mounted on the roof of the building at the center of circular symmetry, all telescopes should see the scattered light with comparable intensity.
Every pixel then has a measured response to a known light intensity.
Work is progressing on how to produce a suitable light source and the calibrating devices. It is a challenging task. Our objective is to winnow the techniques to a minimal reliable system.