In-flight Performance and Calibration of the LOng Range Reconnaissance Imager (LORRI) for the New Horizons Mission

Abstract

The LOng Range Reconnaissance Imager (LORRI) is a panchromatic (360–910 nm for the wavelengths where the responsivity falls to 10% of the peak value), narrow-angle (field of view = 0.°29), high spatial resolution (pixel scale = 1.″02) visible light imager used on NASA’s New Horizons (NH) mission for both science observations and optical navigation. Calibration observations began several months after the New Horizons launch on 2006 January 19 and have been repeated approximately annually throughout the course of the mission, which is ongoing. This paper describes the in-flight LORRI calibration measurements, and the results derived from our analysis of the calibration data. LORRI has been remarkably stable over time with no detectable changes (at the ∼1% level) in sensitivity or optical performance since launch. The point-spread function varies over the FOV but is well-characterized and stable, enabling accurate deconvolution to recover the highest possible spatial resolution during observations of resolved targets, especially when multiple, overlapping images are obtained. By employing 4 × 4 re-binning of the CCD pixels during read out, a special spacecraft tracking mode, exposure times of ∼30 s, and co-addition of ∼100 images, LORRI can detect unresolved targets down to V ≈ 22 with a signal-to-noise ratio (S/N) of ∼5. LORRI images have an instantaneous dynamic range of ∼3500, which combined with exposure time control ranging from 0 ms to 64,967 ms in 1 ms steps supports high resolution, high sensitivity imaging of planetary targets spanning heliocentric distances from Jupiter to deep in the Kuiper Belt, enabling a wide variety of scientific investigations. We describe here how to transform LORRI images from raw (engineering) units into scientific (calibrated) units for both resolved and unresolved targets. Assuming that the wavelength variation of LORRI’s sensitivity is accurately described by the ground-based calibration, we estimate that LORRI’s absolute sensitivity is accurate to ∼2% (1σ) for targets with solar-type spectral energy distributions (SEDs). The accuracy of the absolute calibration for targets with other SEDs should be comparably good when employing synthetic photometry techniques, which we do when deriving LORRI’s photometry keywords. We also describe various instrumental artifacts that could affect the interpretation of LORRI images under some observing circumstances.