Comet C/2017 E1 (Borisov)

Gennadii Vladimirovich Borisov, Nauchnij, Crimea, reports his discovery of a diffuse comet with a coma diameter of 20" on three unfiltered 120-s exposures obtained on Mar. 1.10 UT with a 0.4-m f/2.3 astrograph at the "Mobil Astronomical Robotics Genon" Observatory (MARGO) near Nauchnij (discovery observations tabulated below); the total red magnitude of 17.0 was measured in a circular aperture of size 0'.5. 2017 UT R.A. (2000) Decl. Mag. Observer Mar. 1.10404 19 05 32.18 -10 33 48.1 17.0 Borisov 1.12545 19 05 39.05 -10 33 45.5 17.0 " 1.12847 19 05 40.19 -10 33 44.4 17.0 " After the comet was posted on the Minor Planet Center's PCCP webpage, other CCD astrometrists also commented on the cometary appearance. R. S. McMillan, University of Arizona, reports that three stacked images obtained with the Spacewatch 1.8-m f/2.7 reflector at Kitt Peak on Mar. 1.5 in 2".4 seeing show a short tail about 6" long in p.a. 270 degrees; the red magnitude was 18.5. E. Guido, Castellammare di Stabia, Italy, writes that twenty stacked 30-s unfiltered images obtained on Mar. 1.51 with an iTelescope 0.43-m f/6.8 astrograph near Mayhill, NM, USA, show a diffuse coma nearly 30" in diameter, the red magnitude measured as 16.1-16.3. H. Sato, Tokyo, Japan, notes that ten stacked 60-s exposures taken on Mar. 1.51 with an iTelescope 0.25-m f/3.4 astrograph (+ luminance filter) near Mayhill show a strongly condensed, round outer coma 50" in diameter with no tail; the w-band magnitude was 13.8 as measured within a circular aperture of radius 26".6. M. Mattiazzo, Swan Hill, Victoria, Australia, communicates that five stacked 60-s exposures taken on Mar. 2.49 with an iTelescope 0.50-m astrograph (+ luminance filter) near Mayhill show a diffuse coma about 0'.5 in diameter, with no tail; the nuclear condensation has magnitude about 16.0, while larger-aperture photometry gives total magnitude about 15.0. G. Hug relates that images obtained with a 0.56-m reflector at Scranton, KS, U.S.A., on Mar. 2.5 show that the comet appears somewhat diffuse. K. Kadota, Ageo, Japan, reports that his images taken with a 0.25-m f/5 reflector show the comet to be diffuse with a central condensation and no tail; his images on Mar. 2.82 show a coma of diameter 0'.8 and total mag 14.7, while his images on Mar. 3.82 show a coma of diameter 1'.0 and total mag 14.0. Ten stacked 60-s exposures taken by C. Jacques, E. Pimentel, and J. Barros with a 0.45-m f/2.9 reflector at the SONEAR Observatory (Oliveira, Brazil) on Mar. 3.33 show a circular, diffuse coma 43" in diameter with no tail, and a 10"-diameter central condensation. S. Yoshida, Ibaraki, Japan, writes that he observed this comet at low altitude visually on Mar. 3.81 with a 0.40-m f/4.5 reflector; the comet, which he estimated at total mag 12.2 with coma diameter 1'.4, was located in a dense field of faint stars and was diffuse and near the limit of visibility.