A set of new papers in *Astrophysics Letters* describes on-orbit calibration methods for NASA’s Carruthers Geocorona Observatory (launched September 2025). The mission studies Earth’s exosphere from a halo orbit around the Earth–Sun L1 Lagrange point. Its primary payload, the GeoCoronal Imager, uses two co-aligned ultraviolet photometric imagers to measure hydrogen Lyman-alpha emission radiance simultaneously in wide and narrow fields of view.

One paper focuses on removing instrument effects introduced by microchannel plate intensified CMOS detectors. It details algorithms that correct raw telemetry for factors including detector voltage bias, thermal dark current, particle radiation, flat-field response, and optical distortion, and then describes a processing pipeline that outputs instrument-effect corrected images.

A second paper addresses radiometric sensitivity by constraining the instrument’s wavelength-dependent responsivity and final passband. It proposes observing stars, selecting an optimal star subset using an objective ranking criterion based on a refined UV spectral library tied to CALSPEC standards, and inverting observed fluxes to recover the passband. Synthetic validation indicates passband error rates below 7% for the main Lyman-alpha channels.

A third paper presents algorithms for separating the in-band exospheric H signal from other photon sources, including interplanetary hydrogen background and out-of-band oxygen emissions near Earth’s limb. Validation with a synthetic image generator indicates about a 3% expected error for exospheric radiance measurements, and it outlines the pipeline from L1B to absolutely calibrated physical-unit outputs (L1C).