Our team performed follow-up observations of comet C/2012 S1 (ISON) on 2012, Sept. 28.6, remotely through the 2m, f/10 Ritchey-Chretien + CCD of Faulkes Telescope North (Haleakala) under good seeing conditions, and a scale of 0.3"/px. After stacking 13 R-filtered exposures, 120-seconds each, comet ISON appears as a pale blob of light, slightly elongated toward the south-west (this is particularly obvious looking at the azimuthal median subtraction rendition). Click on the image below to see a bigger version.
The Afrho (proxy of dust abundance within the coma) calculation we performed on this dataset, using a few Tycho reference stars having colour indexes close to that of the Sun, provided rather puzzling results: in short, we found a significant variation of the Afrho amount, according to the dimension of the measurement window (something pretty different from the steady state coma model).
We measured an Afrho maximum value of 1365 +/- 200 cm, for a corresponding aperture of nearly 16,000 km in diameter at the comet distance. After this peak, the Afrho amount decreases, reaching a minimum of about 900 +/- 200cm for apertures of 50,000 to 60,000 km. Similar afrho values has been found analyzing CCD images obtained with the 1.5-m f/8 reflector at the Majdanak observatory on Sept. 21.99 (courtesy of A. Novichonok, scale 0.42"/px).
We measured an Afrho maximum value of 1365 +/- 200 cm, for a corresponding aperture of nearly 16,000 km in diameter at the comet distance. After this peak, the Afrho amount decreases, reaching a minimum of about 900 +/- 200cm for apertures of 50,000 to 60,000 km. Similar afrho values has been found analyzing CCD images obtained with the 1.5-m f/8 reflector at the Majdanak observatory on Sept. 21.99 (courtesy of A. Novichonok, scale 0.42"/px).
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| Afrho variation along the coma of C/2012 S1 |
This behaviour seems to indicate a steep photometric variation along the coma's profile, with a significant increase of the afrho values close to the central condensation (i.e. within about 6" to 8" of the nucleus), which would not be detected using average amateur telescope, for reasons of resolution and seeing conditions.
Indeed, our previous Afrho measurements, performed on Sept. 22.5 through the 0-25, f/3.4 reflector + CCD (scale 1.7"/px), provided an Afrho amount of about 600 +/- 150 cm, for a corresponding aperture of nearly 46,000 km at the current comet distance (taking into account the respective error bars, this compares rather well with the values found through the bigger scopes at similar distances from the nucleus).
From this data it seems that, at the current distance of comet ISON from the Sun, small amateur telescopes will have difficulty accounting for its near-nucleus activity, due to their unfavourable scale factor, and seeing conditions (however they still can play a role monitoring the evolution and general trend of this comet).
The punch line : Comet C/2012 S1 appears to be even more active than we first thought...watch this space!
Giovanni Sostero, Nick Howes and Ernesto Guido
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