On Asteroid Dembowska:
According to the scientific literature (Majaess et al. 2008): “located just prior to the prominent 7:3 resonance with Jupiter, 349 Dembowska is among the larger asteroids in the main belt with an estimated diameter of ~140 km (Tedesco 1989) and is classified as an R-type asteroid from the presence of strong absorption bands of olivine and pyroxene with little or no metals (Gaffey et al. 1993). ” From the same source, 346 Dembowska has a “synodic rotational period of 4.7029 ± 0.0054 h”.
Here I provide 2 sets of observations, the second set being the more complete, with data covering over 1 period.
Asteroid photometry and rotational period determination through light curve reduction is something I had wanted to do for some time, but there was always more exciting targets taking precedence. On December 1, 2022 evening, with a 60% waxing gibbous moon and a 15% sky cover, imaging being unattractive, I decided to give asteroid photometry a go. Asteroid 349 Dembowska was at opposition that day, well positioned in the sky and about 90 degrees away from the moon. Further Dembowska’s rotational period is short, a little under 5 hours, so I could run an observation for almost one period, then switch to some imaging target around 1:30 am, after moonset.
The asteroid was bright, around Mag 9.5, so I had to reduce the camera gain all the way to zero and the exposure down to 20s to avoid target saturation, even with the red filter on (I used red to be less sensitive to seeing issues and also decrease luminosity of bright targets). In the end I recorded 400 frames over 4 hours. The frames are distributed registered, aligned, solved and cropped. I usually distribute raw data, but here, with 400 frames I figured I had to keep the size of the dataset commensurate to its relative interest, which translated to cropping. Since I cropped and the data was dithered, it made sense to align beforehand, so all the dataset would be usable to the edges, and of course one should registers before aligning.
The result is a bit noisy due to the very short exposure and low gain, but the oversampling in time and space is so high the noise can be easily filtered out. I reduced the data in Tycho (but many other software or libraries will do it) and ended up with a calculated rotation period of about 4.5 hours (4.7 in the literature), for a light curve looking very similar to the one published in Majaess 2008.
Bottom line: this is a fun dataset, well suited for photometry (light curve), astrometry and synthetic tracking.
I returned to Observing Dembowska, but this time over more than a full period, to get a complete light curve very suitable for precise period determination. I ended up with a 4.67hours period measured, to be compared with the f 4.7029 ± 0.0054 h from Majaess, so a 99.3% accordance with the peer reviewed results. The observations also allow to compare both side of the phase curve, an initial step toward asteroid shape inversion.
Another asteroid, namely 55871, is also present in the frame not to far from Dembowska, but much fainter. So the same dataset can be used for a “double whammy”, to extract curve information from yet another asteroid, as shown below. I calculate a rotational period of 5.66 hours for asteroid 55871. But in the absence of any published results I could find, this number would need more observations to be confirmed.