On the night of June 26 UT when moving the 2.4″ longslit into the focal plane, the mask failed to insert into the focal plane despite the software thinking so.  When the instrument was opened, the grabber was found to be in the FPU but the mask was still in the cassette.  Further tests showed the same issue at the same telescope/instrument orientation (EL=40 deg., instrument rotation=238 deg.).  A temporary solution was found by removing all of the custom masks in slots 13-24 and placing the 2.4″ long slit mask into position 24, after which it was inserting into the FPU without issue.

Update 8/28/26: During summer shutdown 2026 all of the MOS mask mechanisms were checked for all 24 slots in MODS2, and slots 13-24 (i.e. custom mask slots) in MODS1.  In MODS2 several screws were found to be loose that affected the alignment.  After tightening them the MODS2 cassette was extensively tested, and found to perform similarly to MODS1.  As a result all MODS2 cassette slots are now operational.