Star occultation by asteroid Iris

On April 29, 2011, a star of magnitude 10.3 in the constellation Cancer was predicted to be occulted by a 253 km (157 mi) diameter asteroid known as “(7) Iris”. The eclipse path did not include the Tinyblue Observatory, but due to the uncertainty in the orbit of the asteroid, there was a possibility that the eclipse would be observed here. The sky was cloudy throughout the day, but the clouds started breaking up around 8:00 pm.

Upon Cynthia’s advice, I hastily set up the telescope and turned on the CCD camera, then opened the observatory dome and began to calculate exactly when I needed to start the exposure, when I needed to stop the mount tracking (so that the stars, including the target star, would leave trails on the image frame during the predicted eclipse event), and how long the exposure should be, to take into account the uncertainty of the event but without over-exposing the frame.

Due to some residual moisture inside my CCD camera, soon after the cooling mechanism was turned on, a layer of ice formed on the CCD image chip. This would have made imaging impossible. However, if one waits long enough, frost gradually clears from the chip on its own. Unfortunately, I only had about 20 minutes before the forecast eclipse, and I still had to focus the telescope.

At 9:05, I was able to focus the scope using a mag 1.93 star nearby, then slewed the scope to the target star, TYC 0808-00566-1.

At 9:28:00 pm (according to my GPS timing device), I began a 130s exposure. Then at 9:28:30 pm. I stopped the mount tracking. The exposure ended at 9:30:10 pm.

The image below shows the star trails, with the target star indicated with tick marks. Note that there is no gap in the trail of the target star, indicating that our location was not within the eclipse path. While not as critical as a “positive” observation in which a short blockage of starlight is observed, my observation, a “negative” observation, also has some value as it narrows the range of uncertainty in the path of the eclipse. Compare this with an example of a positive observation.

In the image below, note that the large oval region is the area that has become frost-free in the short time since the frame was completely covered with frost.

Star occultation by asteroid Anahita

Around midnight on October 26 2008, a 51km asteroid known as (270) Anahita occulted a 10.5 magnitude star in the constellation Gemini for observers along a path across western Canada and Northwest USA, blocking starlight for a little over 10 seconds. This image, taken using a “drift scan” technique, shows a gap in the star trail of the target star (indicated by tick marks). The star’s identification is TYC 1880-01064-1.

Drift scan image of star occultation by asteroid Anahita