2026 May Keck/LRIS imaging of interstellar comet 3I/ATLAS

ATel #18098; Bryce Bolin (Eureka Scientific), Ian Wong (STScI), Leo Alcorn (W. M. Keck Observatory), Matthew Belyakov (California Institute of Technology), Percy Gomez (W. M. Keck Observatory), Heather Hershley (W. M. Keck Observatory), Michael Lundquist (W. M. Keck Observatory)
on 9 Oct 2026; 03:55 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We observed 3I/ATLAS with the Low Resolution Imaging Spectrometer (LRIS) on the 10-m Keck I telescope atop Maunakea on 2026 May 15, 05:56:01-06:41:40 UTC (N041; PI I. Wong), at airmass 1.97-2.90, tracking at 0.14 arcsec/min. SDSS g (473.1 nm), Johnson-Cousins R (641.7 nm), and SDSS i (759.9 nm) images were aligned on the comet and sum-stacked before photometry: 11/6/4 exposures totaling 1380 s/900 s/720 s, respectively. R-band FWHM was ~2.2 arcsec for the comet versus 1.4 arcsec for stars of similar brightness. We adopt the LRIS aperture-photometry technique and Pan-STARRS1 calibration method of Bolin et al. (2025a, AJ, 169, 303).

At heliocentric/observer distances of 7.118 au / 7.761 au and phase angle 6.02 degrees, 1.8-arcsec-radius apertures (~10,100 km), with masked sky fits at 6-10 arcsec and no point-source aperture correction for the comet, give g = 22.83 +/- 0.14, R = 21.73 +/- 0.12 and i = 21.67 +/- 0.15. Pan-STARRS1 DR2 stars were transformed to SDSS g/i and Cousins R (Tonry et al. 2012, ApJ, 750, 99), with fitted color terms. Stellar apertures of radius 3.6 arcsec were corrected to 5 arcsec. One-sigma errors include blank-aperture noise, sky/mask sensitivity, and calibration.

Our SDSS color is g-i = 1.16 +/- 0.21 mag; mixed-system colors are g-R = 1.10 +/- 0.19 and R-i = 0.06 +/- 0.18. Earlier g-i values were 1.00 +/- 0.05 in July 2025 (Bolin et al. 2025b, MNRAS, 542, L139-L143), ~0.80 in 2025 November (Bolin et al. 2025c, ATel #17561), and ~0.92 in 2025 December (Bolin et al. 2026, ATel #17719). Each differs from May by less than twice our May uncertainty; no significant color evolution is detected.

Assuming dust-dominated flux and solar magnitudes in matching systems, the Halley-Marcus composite phase function (Schleicher et al. 1998; Marcus 2007) gives zero-phase A(0)f rho = 65 +/- 8 cm, 97 +/- 10 cm and 111 +/- 15 cm in g, R, and i. One-sigma errors exclude passband, solar-calibration, and phase-function systematics; gas and nucleus contributions are unseparated.

These are below July 2025 values of 188 +/- 4 cm, 281 +/- 3 cm, and 288 +/- 5 cm in g, r, and i (Bolin et al. 2025b) at a similar ~10,000-km radius, although passband/calibration differences limit the comparison. The cutouts in the link below span 15 x 15 arcsec.

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The raw Keck Observatory data presented herein, once public, can be accessed from the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

2026 May 15 UT Keck/LRIS g, R, i images of 3I/ATLAS