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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17284
Message-Id: <20250714070621.DE38E2880781@server.astronomerstelegram.org>
Date: Mon, 14 Jul 2025 03:06:21 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17284							     ATEL #17284

Title:	Apache Point Observatory Colors of Interstellar Comet 3I/ATLAS
Author:	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)
Queries:	bolin.astro@gmail.com
Posted:	13 Jul 2025; 17:27 UT
Subjects:Optical, Comet, Solar System Object

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: https://static.wixstatic.com/media/184fc4_17a609f457264d07a691dd4166d78592~mv2.png/v1/fill/w_1190,h_1200,al_c,q_90,usm_0.66_1.00_0.01,enc_avif,quality_auto/184fc4_17a609f457264d07a691dd4166d78592~mv2.png
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Password Certification: Bryce Bolin (bolin.astro@gmail.com)
https://www.astronomerstelegram.org/?read=17284
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