4 Observing events

The aim of these observations is to provide lightcurves of the satellite Amalthea when it crosses the umbra cone of Io, related to an accurate timing in Universal time. It is very desirable to obtain large enough data samples of data during the immersion and during the emersion from the umbra cone.

When the best CCD observations give 100 mas precision, it is interesting to give us a time base to obtain more precise observations. That is why, to efficiently observe these phenomena, we need to have an instrument powerful enough to obtain images with exposure and readout times with a minimal sampling of time that corresponds to 10 mas of accuracy in the orbit. According to Amalthea's radius (120 km), and calling v its tangential velocity when it enters in the umbra cone, and d the distance of the satellite from the Earth, we calculated the sampling of time e with:

$$e = \frac{d}{v}\cdot$$

For the December, 18, 2002 event, 1.4 s of time would be necessary since this corresponds to 38.4 km (10 mas) in the orbit accuracy, according to a tangential velocity of Amalthea of 26.49 km s^-1. These satellite configurations (Table 1) vary around this value of e and depend if the satellite is near its opposition. This is why we suggest to observers to acquire 1 images per second when Amalthea's flux is high enough. In this case, we have a precision on the position of the satellite, when it crosses the umbra cone of 6.89 mas. Of course, observations with a stronger time base remain interesting compared to CCD observations, and for one image every 7.25 s the precision is 50 mas. The best case is to be able to acquire one image every 0.14 s and the precision will be 1 mas.