[AZ-Observing] Re: 109 Felicitas from Tempe
- From: "Tom Polakis" <tpolakis@xxxxxxx>
- To: <az-observing@xxxxxxxxxxxxx>
- Date: Wed, 1 Mar 2017 15:00:40 -0700
Brian Skiff wrote:
Mr Polakis has also been getting very nice results from Tempe, maybe also
only 7 miles from the airport. Despite being a low-numbered asteroid, this is
the first complete lightcurve for this object observed since 1980.
ftp://ftp.lowell.edu/pub/bas/astlc/109_Felicitas-2017feb.jpg
Now I feel compelled to explain how something like this is created.
The asteroid rotates with a period of 13.2 hours, so one needs to capture
pieces of the full light curve by imaging it frequently on enough nights to
get enough phase coverage. Sometimes, the number of nights required to
determine an unambiguous period is in double figures. In this case, we
completed it in six nights, during which my backyard scope took 327 images.
The typical nightly observing program involves toggling between several
asteroids, and throwing some variable stars in the mix. Pointing and imaging
would quickly become a superhuman task, so it is operated as a "roboscope" on a
script. The equipment has become reliable enough to not require any
babysitting, but the observatory computer can be monitored and controlled on a
phone from any place with cellular or wi-fi service, which is pretty much
anywhere these days.
Exposures times are typically between 1 and 3 minutes through a photometric R
filter, although Brian has recently begun to reduce unfiltered data, which has
the benefit of a gain in signal-to-noise ratio. The art of photometry involves
getting adequate signal-to-noise ratio to minimize errors, but not so much
signal as to saturate pixels of targets or comparison stars. Now that I've
veered into data reduction, I should mention that this is the most difficult
part of the job. The asteroid's brightness is measured against four or five
comparison stars in the same frame. Since asteroids are moving targets, the
comparison stars change with each night. The data reduction task has proven to
be above my pay grade, so I upload the files on Dropbox, and Brian takes it
from there to ultimately create a light curve.
This light curve for this particular asteroid has a complex shape, which
indicates that the surface is anything but a smooth sphere. All those
inflections are of interest to very smart people who use them to do shape
modeling. Here's a site that illustrates asteroid shapes and their associated
light curves.
http://isam.astro.amu.edu.pl/
While the full range of brightness for 109 Felicitas is a couple tenths of a
magnitude, real changes in the 10 to 20 millimagnitude range are apparent in
the light curve. That's not bad for an observatory site in which the Cygnus
Milky Way is barely on the edge of visibility!
Tom
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