Electronic Telegram No. 5468 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network COMET C/2023 A3 (TSUCHINSHAN-ATLAS) K. Yoshimoto, Yamaguchi, Japan, provides the following G-channel total magnitudes, coma diameters, and tail lengths for comet C/2023 A3 as obtained with a Canon EOS 6D digital camera with a 135-mm-f.l. f/2.0 lens (an asterisk indicates correction for atmospheric extinction with respect to comparison stars): Oct. 12.40 UT, 0.0*, 6'.8, 4-dag tail in p.a. 65 deg; Oct. 13.41, 1.2*, 8'.1, 8-dag tail in p.a. 68 deg; Oct. 16.42, 2.9, 7'.8, tail longer than 9 degrees in p.a. 73 degrees). On Oct. 16 in an image taken with a 35-mm-f.l. f/2.0 lens, the comet's tail extends over 20 degrees. Total CCD magnitudes measured by K. Kadota at Ageo, Japan, with a 0.25-m f/5.0 reflector (comparison-star magnitudes from the Tycho-2 catalogue): Oct. 13.38 UT, -0.1; 15.37, 1.1; 17.38, 2.4. Selected additional visual total-magnitude and coma-diameter estimates (cf. CBET 5454) contributed to the Central Bureau and the ICQ (with an asterisk indicating that correction for atmospheric extinction with respect to comparison stars was made using ICQ tables): Oct. 3.18 UT, 1.6, 3' (V. Gonano, Udine, Italy, 20x80 binoculars); 4.19, 1.3, 3' (Gonano); 5.35, 1.3*, 2' (M. Goiato, Aracatuba, Brazil, 7x50 binoculars); 6.23, 1.1, 3' (Gonano); 9.15, [-3:, -- (A. Novichonok, Simeiz, Russia, 10x50 binoculars and DSLR camera + 140-mm-f.l. lens); 9.74, [-3.0, -- (J. E. Bortle, Stormville, NY, USA, 15x70 binoculars); 9.81, [-3.0, -- (C. S. Morris, Fillmore, CA, USA, 16x70 binoculars); 11.74, -1.5, 7' (Gonano); 12.40, 0.2:*, -- (K. Yoshimoto, Yamaguchi, Japan, 8x42 binoculars; visible to naked eye); 12.76, 0.6, 3' (C. Labordena, Castellon, Spain, 11x80 binoculars); 13.38, 1.1*, 8' (Y. Nagai, Saitama, Japan, naked eye; 10-deg tail in p.a. 70 deg); 13.75, 1.4, 8' (Gonano); 14.38, 1.2, 6' (Nagai, 7x35 binoculars); 14.77, 2.0, 3' (Labordena); 15.38, 1.8, 6' (Nagai, 8x40 binoculars; 6-deg tail in p.a. 70 deg); 15.69, 2.2, 8' (S. E. Shurpakov, Baran, Belarus, 8x30 binoculars); 16.41, 3.2*, -- (Yoshimoto; 6-degree tail in p.a. 72 degrees); 17.89, 3.4, 3' (J. G. de S. Aguiar, Campinas, Brazil, 11x80 binoculars); 18.73, 3.3, 5' (Gonano); 19.11, 3.3, -- (Morris, 10x50 binoculars; tail about 10 degrees long); 19.70, 3.7, 8' (Shurpakov); 20.12, 3.5, -- (Morris, naked eye); 20.80, 4.3, 3' (J. J. Gonzalez, Asturias, Spain, naked eye); 21.10, 4.1, -- (Morris, 10x50 binoculars; tail about 10 deg long); 22.11, 4.4, -- (Morris; tail about 10 degrees long); 22.97, 4.0, -- (S. J. O'Meara, Concord, MA, USA, naked eye; 7-degree tail); 23.70, 4.3, 8' (Shurpakov); 23.80, 4.6, 3' (Gonzalez, Leon, Spain, naked eye); 24.98, 4.5, -- (D. W. E. Green, Salisbury, NH, USA, naked eye; 7-degree tail); 25.39, 4.5, 6' (Nagai, 8x40 binoculars; 5-deg tail in p.a. 75 deg); 25.79, 5.0, 5' (Gonano); 25.92, 5.2, 3' (W. Souza, Sao Paulo, Brazil, 10x50 binoculars); 26.73, 5.5, 5' (Gonano); 27.72, 5.7, 5' (Gonano); 28.79, 5.8, 4' (Gonzalez, Asturias, Spain, 10x50 binoculars); 29.13, 5.8, -- (Morris; tail about 7 deg long); 29.73, 6.0, 5' (Gonano). Following J. N. Marcus' prediction (CBET 5445) that comet C/2023 A3 might exhibit some significant forward-scattering brightness enhancement as seen from the earth when the comet was at very small solar elongations in the first part of October, several experienced observers worldwide tried visually to observe the comet when the comet was within 7 degrees of the sun during Oct. 8-11, without success. (A couple of unverified visual binocular sightings with limited information that were posted elsewhere have not been sent to the Central Bureau or the ICQ.) The negative reports suggest that the comet was unlikely to have been much brighter than total visual mag -2 at peak, given that visual observers would only have seen perhaps the inner coma with diameter perhaps 2'-3'. However, significant brightening was seen in SOHO LASCO C3 coronagraph images when the comet was in the field-of-view, supporting Marcus' calculations generally. The Central Bureau asked the SOHO-satellite comet specialists for their assessment of the comet's peak brightness in the LASCO C3 coronagraph images. Q.-c. Zhang (Lowell Observatory) responded with measurements of total mag -2.9 in a 5' aperture and -3.8 within a 15' aperture centered on the comet's nuclear condensation on 2024 Oct. 9.40 UT, calibrated to Johnson V with comparison star 39 Tau. As interpreted by C. S. Morris and the undersigned, the diagram supplied by Zhang with circular apertures superimposed on the comet's head and inner tail suggests that roughly 7'-10' would be the size of the coma for a traditional total magnitude of the comet, yielding perhaps total mag -3.1 to -3.2 for a peak brightness. This would be consistent with the visual observers not finding the comet, with a brightness of perhaps -2 for the inner 2'-3' of coma that would have been detected visually. Zhang measured the brightness for the 1' diameter centered on the nuclear condensation on Oct. 9.40 to be -1.3 (V band, calibrated against Vega). Note that brightness measurements of SOHO images larger than 10' would include a large part of the tail that is not conventionally taken as part of the total magnitude. C. Schur (Payson, AZ, USA) reported on the "ICQ Comet Observations" Facebook forum that under ideal conditions on 2024 Oct. 10.71 UT, he was barely able to image the comet at 6.5 degrees from the sun with a 25.4-cm f/3.9 telescope at elevation 1680 meters above sea level, adding that "the comet was not visible in the recorded video directly and only became evident once the vignette and solar sky gradient was removed ... in my opinion, anyone that claims to see this [via] naked eye or in binoculars have a pretty active imagination". M. Jaeger posted a stacked image of C/2023 A3 on the same forum that he took from Noechling, Austria, on Oct. 11.72 (comet 11.1 degrees from the sun) with a 280-mm-f.l. f/4 Leico-Apo-Telyt lens and a Nikon Z6 camera just after sunset that shows the inner coma to be much brighter than the faint, short tail -- as expected in a day-lit sky. The following revised orbital elements by S. Nakano (Central Bureau) are from 5470 observations spanning 2022 Apr. 9-2024 Oct. 28 (mean residual 0".5), with corresponding "original" and "future" values of 1/a being -0.000020 and -0.000062 AU**-1, respectively (the mean error is less than +/- 0.000001). The comet passed 0.47 AU from the earth on 2024 Oct. 12.6 UT. Nakano notes that the observations are well-fitted with a gravitational-only solution. Epoch = 2024 Oct. 17.0 TT T = 2024 Sept.27.74206 TT Peri. = 308.49209 e = 1.0000982 Node = 21.55928 2000.0 q = 0.3914292 AU Incl. = 139.11073 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 October 29 (CBET 5468) Daniel W. E. Green