The minor planet Anneliese (910) will occult the star HIP 74600 early on the morning of Saturday 8th May 2010, just after midnight (c. 00:22 local time).  The star, also named 26 Librae, is of sixth magnitude and is in the middle of the constellation Libra.  The asteroid is about 47km diameter and, at a distance of 1.5 AU, it shines at about magnitude13.4.  You do not need to see it. Few will be able to. It is its effect on the star you will see.

The expected path runs across the south of Victoria from Mallacoota, through Melbourne to Glenelg River near the South Australian border.  ALL OBSERVATIONS WITHIN RED AND BLUE LINES ARE IMPORTANT! It is almost only visible in Victoria, or at sea.   There may well be an error in the prediction and observations by members outside this path, especially to the north, will be especially valuable.

THAT MAKES IT OUR OCCULTATION!
And to see and record it, you may well not even need to stray from home!
If we don’t record it, who will?

Click to Enlarge Path of Anneliese Occultation

The constellation Libra lying just west of Scorpius, is not cluttered with numerous stars, and the target star is quite easy to find.  About half way between Antares and Spica you can find the flat triangle comprising the Libra stars Brachium, Zubenelgenubi and Zubeneschamali.  One third of the way along the long base from Brachium lie Iota and 25 Librae, at the corners of an elongated triangle about half a degree in size. 

About half a degree in the direction the triangle points, there is a neat isosceles triangle, also about half a degree in size. The target star is the one farthest from Iota Librae.

Click to Enlarge Star Chart showing HIP74600		Click to Enlarge Star Chart showing HIP74600

I want to hear from you, something like this.

• I found the star and watched at the right time but saw nothing
• I found the star and saw it blink for about 1 second
• I found the star and recorded the duration of occultation but not its start time
• I timed the start (and end) of occultation using watch (or clock) and tape recorder
• I took a webcam picture of the occultation and marked the image with timed flashes
• I took a video film with time embedded and included a clip of clock before and afterwards
• It seemed to get darker before it went out

EVERY ONE OF THESE AND SIMILAR RESULTS ARE ACTUALLY VALUABLE.

There are two MUSTS for recording occultations:

1. PLACE : This should be an easy one for many. You know where you live.
2. TIME : This can be made very accurate, surprisingly easily.

If you use a tape or video to record time marks and the event IT MUST BE KEPT RUNNING BEFORE DURING AND AFTER THE EVENT. The watch or clock is then compared with an accurate clock and a little maths is done. Some time recording examples include:

A few minutes before the event, start the tape recorder. A couple of times announce in advance a mark time and say “NOW” on the mark. Await the event, recorder still running. Say “GONE” when the star disappears and “BACK” when it reappears. Add another couple of time marks on the tape. Extract the times by playing the tape back on the same recorder. Remember that events may occur a few minutes early or late. Be prepared for them.

Do the same with a web or video camera and watch or clock, adding pre-arranged time marks by flashing a torch on the telescope lens. You may need a tape recorder to remind you of times or time changes. Relative event times can then be obtained from the video and the watch. Camera must be running from start to finish. If using a camcorder, take a clip of the clock just before and after the event, keeping recorder on all the time.

You can measure time error on your watch/clock with a GPS (in hand or car or telescope) or with a computer  synchronised to a standard using BeeperSync. DO NOT ADJUST THE WATCH OR CLOCK UNTIL AFTER ALL TIMES ARE EXTRACTED.

Most Important:

• Know where the star is WELL BEFORE THE EVENT
• Know your equipment works WELL BEFORE THE EVENT
• Know how you will correct clock and watch times WELL BEFORE THE EVENT
• Check clock and watch times before and after event BUT DO NOT ALTER THEM TOO EARLY
• Did the star darken (when? … for how long? …) before it blinked out? The star may be a double!

Some Software to Help

• BeeperSync
• Occult Watcher

Let me Know in Advance

Whether this is your first occultation or not, please let me know where you are and how you will make the observation, and send me a copy of your results. Then at least one person knows what happened.  If all the results are combined we should get a GOOD shape of the asteroid. Comparing ALL results also provides a check on errors. If the response is good (and it should be) we may run a round-table at Parer Street, or even at a country site if required, before and/or after the occultation..

Remember!

The event is interesting. If records are collected and formally collated THEY ARE VALUABLE!

Article by Ron Garrett. Discuss this article on the IceInSpace Forum.