Although the primary mission of the Centennial Observatory is to provide visitors with opportunities to experience the universe visually through telescopes, the facility is also used for astronomical research. The main research focus is the
determination of asteroids' sizes and the refinement of their orbits via stellar occultation.
The method is straightforward: A precise measurement of how long a distant star's light is occulted (blocked) by a passing asteroid, together with the asteroid's previously-determined distance and speed, allows the asteroid's diameter to
be calculated (by distance = rate x time). This measurement is accomplished via video photometry: video from a high-sensitivity camera attached to the telescope passes through a GPS-based device which "stamps" each video frame with the time (down
to millisecond precision). The video is analyzed with software that precisely measures the intensity of the target star in each video frame and assigns it a numerical relative brightness value. The light curve (a graph of brightness vs.
time—see fig. 3, below) reveals the duration of the occultation.
Fig. 1: Predicted shadow path of asteroid (105) Artemis, as cast on Earth by HIP 62736, a mag. 8.7 star in Virgo, on 11 April 2017. The northward-moving shadow was expected to pass
east of the Centennial Observatory, but uncertainty in the asteroid's precise orbital path allowed for a 26.3% chance that the shadow would pass over Twin Falls (which it did).
With several astronomers observing the same event from different location, the size and shape of the asteroid's shadow (identical to that of the asteroid itself, since stars are so distant that their light reaches Earth on essentially-parallel paths)
may be mapped out.
Fig. 2: Profile of asteroid (105) Artemis as determined by its occultation of HIP 62736 on 11 April 2017. Diagonal lines represent the star's apparent path, relative to the asteroid, as seen by observers at different locations. The
widths of the gaps ("chords") are determined by the duration of HIP 62736's disappearance as recorded by each observer. The Centennial Observatory's chord is cyan, at far right. The asteroid passed a bit to the southwest of
its predicted path (dotted line). (Note the 100 km scale bar at bottom.)
Since asteroids' exact orbits (like all measured quantities) are imperfectly known, their shadows (cast by starlight) take paths across the Earth whose precise location is uncertain. While many asteroid occultation observers utilize small,
portable instruments which allow them to travel to locations where a given asteroid's shadow is most likely to pass, this option is not available for the Centennial Observatory's permanently-mounted 24" (0.6m) telescope. Therefore we observe
many occultations, with probabilities ranging from near-certainty (weather notwithstanding) to less than 1-in-2000, to increase the frequency of success. To date, roughly one out of every nine occultation observations conducted at the Centennial
Observatory (not including those which were clouded out) has resulted in seeing the target star temporarily vanish as the asteroid obscures it. It should be noted that close misses can also be scientifically useful, as they may also help constrain the
asteroid's shape and path (e.g. the red chord at far left in fig. 2).
Fig. 3: Light curve of the 11 April 2017 (105) Artemis occultation, as derived by photometric video analysis. The jagged shape of the curve is due to a combination of signal noise and atmospheric distortion. For just over four seconds,
the light of HIP 62736 was blocked by (105) Artemis, causing the signal to drop by around 85% (from the combined light of the star and asteroid, to the light of the much fainter asteroid alone plus background sky glow).
The Centennial Observatory's first asteroid occultation observation was conducted on 24 August 2012, when asteroid (1585) Union cast its shadow from the star TYC 5777-010444-1 onto the Earth. No occultation was seen (i.e. the shadow missed
Twin Falls). All subsequent "positives" (timings performed when the Centennial Observatory was in an asteroid's stellar shadow) are listed below, in reverse chronological order.
Click on the date for a map of the asteroid's predicted shadow path. Click on the asteroid name for a profile of the asteroid showing all observers' chords (star tracks relative to the asteroid as seen from different locations). Click on
the star name for a graph of the Centennial Observatory's photometric data. Click on the observers' names for a map of their locations.
Positive Asteroid Occultations Recorded at theCentennial Observatory
*At the Centennial Observatory. Observers in other locations not listed.
The table below includes all the negative observations (misses) recorded at the Centennial Observatory for which at least one other observer recorded a positive, and no other negatives
lay between the asteroid's shadow and us. Such misses may also help constrain the asteroid's size and shape (depending on distance from the shadow path, best seen in the profile).
Significant Asteroid Occultation Misses Recorded at the Centennial Observatory
*The 1-σ zone (delineated in red on the "Observers" maps) has a 68.27% chance of containing at least some of the asteroid's shadow ("one
standard deviation" from the predicted shadow path). The wider 2-σ zone (two standard deviations from the shadow) has a 95.45% chance of containing at least
some of the asteroid's shadow.
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