Successful (85) Io observation from Calgary

Successful (85) Io observation from Calgary

By Andrew Lowe

(This report was originally posted on the International Occultation Timing Association (IOTA) newsgroup).

I successfully observed the (85) Io occultation tonight from my driveway in Calgary (11405'35"W, 5059'26"N, elevation 1081m) with my Meade 8" LX200GPS and PC164C video camera with f/3.3 focal reducer. Conditions were perfect. Perfect, that is, except for WWV reception. I was doing some pretty fancy footwork with the shortwave radio, trying to get good reception (I don't want to guess what the neighbors were thinking of this), and so I wasn't actually looking at the TV screen at the time of the occultation. After recording a few minutes past the event, I played back and thought I could see something of the 0.4 mag. drop.

After packing up, I used a paper and hole punch to isolate on the screen the minor planet and a nearby star of the same brightness, then replayed the tape a number of times. The brightness change was subtle, but the following events seem to be real:

disappearance: 04:08:12.5 UTC  0.4 sec
reappearance : 04:08:22.5 UTC  0.6 sec
duration     : 10.0 sec
Since I was fairly close to the predicted south limit, it appears there was a south shift (similar to last December's (334) Chicago occultation). The predicted mid-time, however, seems accurate to within a few seconds.

UPDATE -- 2016 January 24

While attempting the (283) Emma stellar occultation and the Aldebaren lunar occultation last week, I found the original VHS recording of this event. I decided to reanalyze the data using new software that had been released in the last ten years. Here was my technique:

1. Transfer the VHS recording into the computer using a Honestech VIDBOX and saving with VirtualDub (format is AVI).

2. Play back the AVI file and note the frame numbers of the WWV minute markers on the audio track. Since I could hear the individual second beats, I could anticipate the minute marker with great accuracy (in other words, the reaction time was assumed to be zero).

3. By capturing some regular video on the same tape with VirtualDub, I could determine that there was a 425 ms mismatch between the video and audio tracks. This correction was applied to the final derived times.

4. Analyzing the AVI file with Limovie. This remarkable program measures the brightness of the star and exports, frame by frame, the value to a CSV file for display.

5. Since I had a relationship between time and frame number (from Step 2), identifying which frames were the start and end of the change in brightness allowed me to derive the time of the disappearance and reappearance (after applying the final correction of 425 ms).

My revised results:

disappearance: 04:08:12.22 UTC
reappearance : 04:08:22.23 UTC
duration     : 10.01 sec

Comparing to my original results, it appears I underestimated the reaction time by about 0.3 sec, which doesn't surprise me.

Back in 2003, it was difficult to accurately time the small brightness changes visually on the original tape, but the graph below using the Limovie data shows how easily the event can be seen. Note that the black line is actually a five-point moving average of the original noisy data, while the red lines are the brightness averages during and outside of the occultation.

Predicted occultation path across North America, courtesy of David Herald's winOCCULT software. My observing site at Calgary, Alberta is indicated with a +. Unfortunately, other observers in the U.S. were clouded out.