Welcome to the world of Ha....
The chip size of the ASI 120 is 3.6 x 4.8mm - just too small to give full disk coverage with the 400mm fl PST.
Due to the limited backfocus (The PST was designed as a visual instrument) it's well neigh impossible to accommodate a reducer.
Unfortunately the only viable option is to use a camera with a larger chip, or practise preparing mosaics......
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