46P/Wirtanen 2018-Nov-11 Charles Bell

A study of comet 46P/Wirtanen in R, G, B filters with the same exposure:

- The coma was brightest in the Red filtered image even though the apparent diameter was the smallest.
- The coma was nearly circular in the Green (or V) and Blue filtered images which are dominated by gas emission least affected by solar radiation pressure.
- The coma was largest in the Blue filtered image.
- The coma was elongated more in the Red filtered image from the effects of solar radiation pressure on dust.
- Elongation directions were away from the sun but 10 to 20 degrees clockwise from the anti-solar direction (PsANG = 18.9 deg) toward -v (PsAMV = 265.6 deg) direction.
- Measured magnitudes were close to the Minor Planet Center and JPL Horizons ephemeris values.
- The comet was 31 days before perihelion.

A 120-second exposure was used on each filtered image.
Magnitudes were measured with Astrometrica with UCAC4.
Coma dimensions were measured using Vizier Aladin distance measuring tool and contour plot overlays.
It is expected that longer exposures would show larger apparent diameters. Exposures were kept low in a region of linear detector response and to avoid saturation of central pixels in the ABG CCD. The total exposure of 360 seconds was comparable to the sky motion for one-pixel change so that color components could be accurately merged into RGB color composite in PhotoShop after ArcSinH stretching the R, G, and B fits images with FitsLiberator. The four images were loaded as a stack in ImageJ then cropped to equal size and converted from stack to montage. Annotations and comet vectors were added to the montage image in each frame with PhotoShop layering.

Astrometric positions were measured but not submitted to the Minor Planet Center due to large pixel size (>> 2 arcsec/pix).
Ephemeris data is from Minor Planet Center and JPL Horizons.

Comet 46P/Wirtanen
2018-Nov-11 15:12:11 UTC
Q62 iTelescope Observatory, Siding Spring
Observer: Charles Bell
RGB Color Composite
120 secs exposure per filter
Telescope: 106mm Takahashi FSQ ED APO Refractor Astrograph
CCD: FLI Microline 11002 ABG
Pixel Size: 3.50"/pix

2018-Nov-11 15:09:44 UTC
Red Filter Exposure 120 secs
R mag 11.36 +/- 0.14 (UCAC4)
Aperture radius = 33.2 arcsec
Afrho R = 42 cm rho = 5225 km mSun = -27.12
Coma 3.7 arcmin x 3.0 arcmin extended PA 010 to 015 deg

2018-Nov-11 15:12:14 UTC
Green or V Filter Exposure 120 secs
V mag 11.64 +/- 0.14 (UCAC4)
Aperture radius = 33.2 arcsec
Afrho V = 45 cm rho = 5225 km mSun = -26.78
Coma 10.8 arcmin x 10.7 arcmin extended PA 010 to 015 deg

2018-Nov-11 15:14:38 UTC
Blue Filter Exposure 120 secs
B mag 12.40 +/- 0.15 (UCAC4)
Aperture radius = 33.2 arcsec
Afrho B = 40 cm rho = 5225 km mSun = -26.14
Coma 11.4 arcmin x 11.2 arcmin extended PA 010 to 015 deg

Ephemeris data:
Sky Motion 0.56"/min P.A. 019.9 deg
Altitude = 58.9 deg
Airmass = 1.167
t - T = -31.30621 days
Delta = 0.217 AU
r = 1.136 AU
El