The Hamamatsu R15458-02 PMT's photocathode was scanned with a collimated beam at 777 different positions to determine the relative transit time, TTS and gain. This measurement was performed at 192 different magnetic field orientations with a strength of 54.7 microtesla. The analyzed data is available in a JSON file. To access the results for example in Python, you can import the data to a dictionary, and retrieve the values as follows: The relative transit time is stored in the "RelTransitTime" key, TTS in "TTS," and gain in "Gain." Each of these keys contains a list of 192 lists, with each list containing 777 values. The 192 lists for each key correspond to scans measured at different magnetic fields. The "B_field" key retrieves the corresponding normalized magnetic field direction, which is a list of 192x3 elements. The 777 values in each list correspond to the value at different positions on the photocathode. You can access these positions using the "x" and "y" keys. For further details, you can refer to Chapter 7 of M. Unland thesis "Development, simulation, and characterisation of a novel multi-PMT optical module for IceCube Upgrade with emphasis on detailed understanding of photomultiplier performance parameters" published in 2023.