Magnitudes and colors of comet 220P/McNaught during outburst
ATel #17956; A. Kostov (IA and NAO, BAS, Bulgaria), T. Bonev (IA and NAO, BAS, Bulgaria)
on 10 Aug 2026; 19:15 UT
Credential Certification: Andon Kostov (akostov@astro.bas.bg)
Following the CBET 5722 we observe the comet 220P/McNaught in BVRI broadband filters, between 01:01 ÷ 02:01 UT on 2026-Aug-08 with the 50/70cm Schmidt Telescope of National Astronomical Observatory Rozhen, Bulgaria. The following magnitudes were calculated using the transformation equations described in Kostov and Bonev (2018). The measurements were done with three different aperture radii: 6, 11 and 22 px, which correspond to 5000 km, 10000 km and 20000 km at the comet geocentric distance.
Filter 5000 (km) 10000 (km) 20000 (km)
B 11.66 ± 0.04 10.80 ± 0.04 9.92 ± 0.04
V 11.04 ± 0.01 10.19 ± 0.01 9.33 ± 0.01
R 10.72 ± 0.01 9.94 ± 0.01 9.17 ± 0.01
I 10.31 ± 0.01 9.53 ± 0.01 8.77 ± 0.01
and the corresponding colors:
Color 5000 (km) 10000 (km) 20000 (km) Sun
B-V 0.62 ± 0.02 0.61 ± 0.02 0.59 ± 0.02 0.653
V-R 0.32 ± 0.01 0.25 ± 0.01 0.16 ± 0.01 0.352
V-I 0.73 ± 0.01 0.66 ± 0.01 0.56 ± 0.01 0.702
where in the last column are the colors of the Sun by
Ramírez et al. (2012). The close color values to those of the Sun are an indication that the comet's coma is dominated by dust.
The parameter Afρo, a proxy for the dust production, was calculated for filter R images. We use the Composite Dust Phase Function for Comets by
Schleicher (2010) to correct our measurements for the phase angle. Final estimations for the Afρo are: 12245 ± 90; 13671 ± 99; 14566 ± 104 for 5000 km, 10000 km, 20000 km aperture respectively.
Additionally isophote brightness maps of the comet in all filters can be found at the links below:
B filter contour map
V filter contour map
R filter contour map
I filter contour map
The outermost contour (value shown in the map) is 3 sigma above the night sky, and the step of the contours is 0.25mag. For each of the maps the corresponding heliocentric (r) and geocentric (Δ) distances are shown, along with the anti-solar direction (PsAng), negative velocity vector (PsAMV), and the angle between Sun-comet-observer (Phase angle). The dashed lines trough the optocenter of the comet represent the anti-solar direction and the negative velocity vector direction. Note that in the map for filter B, there is a satellite trail, traversing trough the image, and passing very near to the optocenter of the comet. We measure that the coma of the comet is extended to, at least, 300 000 km at the corresponding comet geocentric distance. The coma is relatively symmetrical, round shaped, slightly elongated in the anti-solar direction.
The research that led to these results was partially carried out with the help of infrastructure purchased under the National Roadmap for Research Infrastructure, financially coordinated by the Ministry of Education and Science of Republic of Bulgaria. It is also partially supported by Bulgarian Scientific Research Fund of the Ministry of Education and Science under the grant KΠ-06-H88/5 "Physical properties and chemical composition of asteroids and comets - a key to increasing our knowledge of the Solar System origin and evolution."
