September 14, 2026: During summer shutdown 2026 restart, it was discovered that the PEPSI SX PFU side was suffering from severe vignetting despite attempts to realign with the M3 on-sky.  The cause was tracked down to index-matching oil in between some of the optical elements leaking into an airgap, causing elevation-dependent vignetting.  This was first noticed in Feb. 2026 when it wasn’t as severe, after which a realignment in June 2026 seemed to have solved the issue.  A fix is currently being planned to happen very soon, after which it will take appx. 1 week to complete the repairs.  These repairs will involve removing, disassembling, cleaning, reassembling, and realigning the PFU entrance optics.  We appreciate your patience as we work to resolve this issue.

September 21, 2026: After a valiant effort by our staff, we are comfortable bringing back SX PFU in shared risk mode.  Significant telescope M1, M2, and M3 range balance was recovered after backing out corrections made in June 2026, when the vignetting was thought to be caused by M3 misalignment.  With this, the required time for manual on-sky collimation on SX side was deemed acceptable for science use.  We believe the impact to science data is flux loss between approximately 15-25% depending on the target elevation (this includes some loss due to imperfect collimation), with worse throughput at lower elevations.  It is recommended to account for this by extending exposure times, and to plan PEPSI observations to be at as high of an elevation as possible.  Due to staff scheduling, a repair mission is anticipated for late Oct. or early Nov.

October 2, 2026: We’ve noticed recently that the vignetting can be up to almost 40% of the beam as seen on the WFS, at low elevation.  Therefore we recommend closely monitoring the WFS image to assess flux loss, and adjust exposure times accordingly.  The repair mission is currently scheduled to begin on Oct. 22 in the morning, when SX PFU will be taken offline.  The downtime is expected to last just over a week until beginning of Nov., with most of that time needed for the glue to completely cure.