Amicitia is typical main belt asteroid. It orbits the Sun during 3 years and 112 days in average distance of 2.22 astronomical units (about 340 mln km). It was discovered in 1893 and its apparent magnitude is around 12.5 mag. Amicitia is is good photometry target for amateurs due to fast rotation.
I wrote “around” for a reason, because its brightness varies much due to its rotation. Amplitude of these changes may exceed 0.7 mag, and the rotation period is little bit over 5 hours. So it is possible to monitor theses changes even visually using some telescope. I gathered Amicitia lightcurve at one of January nights, when asteroid was moving away from M44 cluster to west.
Series contained 140 images taken with 120s exposure time with Meade ACF 10″ telescope and QHY163M camera through Baade L filter. Transparency was poor, and seeing was mediocre. Brightness changes amplitude of this object is so large, that it is even visible in stacked image of all 140 frames.
Lightcurve was generated in Muniwin software. And below there are selected comparison and check stars plus calculated lightcurve.
Despite the conditions it was possible to achieve large asteroid SNR (over 1200) and that gave pretty smooth lightcurve and good accuracy. As you can see at the lightcurve above it is bimodal. Next and last picture presents calculated shape of Amicitia asteroid (source: wikipedia). Diameter of this object is 19.13km.
Clear skies!
