Apache Point Observatory Colors of Interstellar Comet 3I/ATLAS

ATel #17284; Bryce T. Bolin (Eureka Scientific), Matthew Belyakov (Caltech), Candace L. Gray (Apache Point Observatory), Carl Ingebretsen (Johns Hopkins University), Gracyn Jewett (University of Oklahoma), Mukremin Kilic (University of Oklahoma), Carey M. Lisse (Applied Physics Laboratory)
on 13 Jul 2025; 17:27 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We report color measurements of interstellar comet 3I/ATLAS from g, r, i, and z band photometry taken with the Astrophysical Research Consortium 3.5-m telescope (ARC 3.5-m) at Apache Point Observatory (observatory code 705). We used the ARC Telescope Imaging Camera (ARCTIC) to observe 3I/ATLAS on 2025 July 06 03:23 UTC. The comet appears to be extended in the ARCTIC images, with a coma having a FWHM between 1.8-2.3 arcsec in the g, r, i, and z images compared to the FWHM measured for nearby background stars of ~1.5 arcsec (see attached g, r, i, and z-band band image composite), confirming the results of recent ATels: ATel #17263 (Jewitt & Luu), ATel#17264 (Alacron et al.), ATel #17275 (Minev et al.), and ATel #17276 (Belyakov et al.). Using our multi-band images, we measured the following color indices using an aperture with a 4.1 arcsec, equivalent to 10,000 km at the 3.36 au geocentric distance of the comet: g-r = 0.81 +/- 0.06, r-i = 0.18+/-0.04, i-z = -0.02+/-0.07, and g-i = 0.99+/-0.08, equivalent to a spectral slope of 15.5+/-2.6 % / 100 nm. The r-i measurement made with the ARC 3.5-m/ARCTIC appears to be equivalent to the r-i measurement from photometry and spectroscopy described by Belyakov et al. in ATel #17276 of r-i = 0.16+/-0.02. However, our g-r measurement is significantly more red (g-r = 0.81 +/- 0.06 vs. g-r = 0.43 +/- 0.02). The g-r and overall color slope measured with the ARC 3.5/ARCTIC appear to be in line with the spectral slope measurements for the comet reported in other recent results (e.g., ATel #17283 by Champagne et al.). Based on observations obtained with the Apache Point Observatory 3.5-meter telescope, which is owned and operated by the Astrophysical Research Consortium.

Images of 3I/ATLAS taken in g, r, i, and z bands on 2025 July 6 UTC from the ARC 3.5 m/ARCTIC