(106) Dione – 15th Feb 2011

This was my first +ve asteroid occultation. It featured a faint star, a low drop in magnitude (dM) and a very close Moon (just 14° away).

  • Predicted event time was 23:47:42 UTC for my observing position.
  • Predicted maximum duration was 35.0 seconds with a 0.5 magnitude drop.
  • Position was 13 km from predicted central line in N direction between centre line and path edge with a probability of 73.6%.
(106) Dione map prediction (centre line green, predicted shadow between blue lines, telescope is my location)
(106) Dione map prediction

I recorded 1280ms integrations using a Watec 120N video camera on a 200mm F/4 Newtonian telescope. Video was captured via a USB2 capture device on a laptop with VirtualDub software. I used a timestamp from a NTP service but I had to frame count to get the correct times.

This is the light curve for the event from video analysis in Tangra v1.1 software. The blue line is the occulted star, the yellow line is a nearby comparison star. The vertical red line marks the beginning of the occultation. Frame count on the x-axis and star intensity on the y-axis. The downward spikes are dropped frames.

Light Curve
Light Curve for (106) Dione
Start 23:48:04.13 ±0.65s
End 23:48:27.17 ±0.65s
Duration 23.04s ±1.30s


(106) Dione before occultation
(106) Dione before occultation

(106) Dione during occultation
(106) Dione during occultation


There was one other +ve result for this event and the preliminary chords below give an asteroid diameter of at least 102km.

Preliminary chords
Preliminary chords