Radiation Telecon Summary (Mar 25, 2025): ----------------------------------------- At the 2025:084:02:15 (10:15 PM Mar 24 Mon) comm SCS-107 was observed to have run at 2025:083:21:34:08 in response to an ACIS/TXings high radiation flag set at 2025:083:21:31:42. The cause is still under investigation, but is believed to be benign and to originate from fluctuations in the bright X-ray source being observed at the time (ObsID 30869: Cygnus X-2) rather than from a solar radiation event. The transition was nominal, with science loads halted and the spacecraft and instruments in a safe configuration. Pre-approved CAP 893B to disable SCS 29 was run as a clean-up action. The comm pass at 085:02:15 (Mar 25 Tue at 10:15PM local) is being targeted for uplink of replan loads, which are expected out for review tomorrow. Any updates will follow the 9AM tagup tomorrow (Tuesday). ACIS Radiation Monitor Trip Update (Mar 28, 2025): -------------------------------------------------- After some sleuthing, ACIS has determined the cause of Monday's unintended radiation trip of the ACIS radiation monitor, txings. Txings includes a check for monotonically increasing threshold-crossing events in each active chip. The bottom line is that this is a result of the newer (as of 2022) longer-period, larger amplitude dither having moved the bright source and its dispersed spectrum off of and back onto the subarray, leading to a large-amplitude oscillation of threshold crossing events. Although dozens such observations have been carried out in the past, the Cyg X-2 observations which resulted in Monday’s shutdown were the first to use the larger dither, with the associated longer period that can lead to a txings trigger. The observation in question used a 134-row subarray ACIS-S/HETG observation of a 0.5 Crab-bright source. The 0th order image and dispersed emission dithered off of and then onto the S3-subarray with the 2000s z-axis dither period making it relatively likely for a txings trip from the monotonically-increasing threshold-crossing rates during the increasing portion of the sinusoidal dither variation. As you likely recall, the present baseline rule is to use reduced dither (in this case 8" semi-amplitude and 1000s period along the z-axis) only for <=128-row subarrays. If ACA finds this agreeable, ACIS requests that all future observations like this (SI-mode TE_00C82, with its 134-row subarray) make use of the reduced dither-amplitude in the z-direction. I.e., as if they were 128-row subarrays instead of 134. Furthermore, we want to make sure that Norbert and Herman as HETG USINT leads are aware of this issue. (Also noting that it appears that over the last few years, the MIT/HETG group are the primary observers for this kind of study.) While the 134-row subarray is likely to be the most problematic case for this, any HETG observation with the full z-direction dither moving a bright source off of the chip or subarray could lead to this kind of unintended txings trigger. ACIS has other workarounds available if similar observations with different SI-modes fit this mold, but those workarounds will require SARs and so should be planned in coordination with ACIS.