Observatory 1.5.3 has been released to the Mac App Store.
The Calibrate adjustment now tolerates small differences in ISO speed, pedestal, gain and exposure duration for its calibration frame validation. This solves the issue where the calibration frame selections sometimes didn‘t list the expected master calibration frames.
The adjustment also gained the new option to skip most of the validation altogether. So if some of the metadata is incorrect, or you want to calibrate your image with a flat taken at a different sensor temperature, now you can without having to manually edit the metadata.