Cause of the LORRI post–power-on background decay

Determine the physical mechanism responsible for the additive background signal that appears in New Horizons LORRI images immediately after instrument power-on and decays exponentially over a timescale of approximately 295 seconds, even after a four-minute delay. Establish whether the origin lies in the spacecraft environment or the LORRI instrument (electronics/optics) and develop a mitigation or calibration strategy to eliminate or accurately model this effect in faint-sky measurements of the cosmic optical background.

Background

The authors report that shortly after LORRI is powered on, images display elevated background levels that decay over time, a phenomenon first identified in their earlier NH21 analysis. In the present, richer data set, they find that this decay persists even after the four-minute delay previously used to avoid the anomaly and that it behaves as an additive effect independent of exposure level.

They model the behavior with an exponential decay (amplitude 0.315 DN, timescale 295 s, and a constant term of −0.075 DN), apply corrections to each image, and include a systematic uncertainty from this correction in the error budget. Despite these practical accommodations, the physical cause remains unidentified, leaving a key instrument-level uncertainty unresolved.