Occultation of 6.6-mag. SAO 115966 by (219) Thusnelda on 2007 December 18 - Updated 2007 Dec. 21, 22h UT
Spectacular Grazing Occultation recorded near Runyon, Florida
A poor-quality copy of the video is now posted below
Most observers had no occultation because the path shifted almost a full path-width north
I think at least 20 observers monitored the star from more than two dozen locations, but as far as I know, only two of us actually observed the occultation, from a total of three sites. More about these and other previously-observed occultations is/will be posted on Brad Timerson's N. American asteroidal occ'n results page, IOTA's official Web site for North American asteroidal occultation results. A brief account of my observations is below, followed by a short description and link to a news Web site about T. Fox's observation. Unfortunately, as far as I know, all attempts to observe this event in Europe were clouded out, and I know of no attempts in Mexico. __________________ From: David Dunham [dunham@starpower.net] Sent: Wednesday, December 19, 2007 12:53 AM Subject: Spectacular Thusnelda graze near Runyon, Florida (+22 km) My remote station a km south of the predicted northern limit, a 4" Meade SCT with image intensifier, recorded a spectacular grazing occultation of SAO 115966 by (219) Thusnelda last night; the pre- pointing was perfect, with the target near the center. With all my asteroidal occultations, this was the first time I've recorded more than one occultation of the star. But it's interesting that this graze was on the south side of Thusnelda, rather than on the north side, due to the almost 1 path-width north shift that occurred. Approximate times of my 4 events are: Disappearance 5:17:47.0 UT 2007 Dec. 18 Reappearance 5:17:47.8 Disappearance 5:17:49.3 Reappearance 5:17:49.9 The events were gradual due to Fresnel diffraction effects. [no-see below] I'll be able to determine precise times from the Kiwi time insertion later. The location was in a clearing in sugar cane fields just north of the intersection of US 441 and Sugar House Rd., about a mile east of Bunyan City, Florida (about a few miles n. of Belle Glade). When I get a chance, I'll review the tape also for coordinates. I copied a short segment of my digital video tape with the events with an MDVR to a .asf file; you will probably need to install the proper codec by running this .exe file in order to view the clip. It shows the events well, with the target star near the center, but the original recording shows many more stars with overall brighter images than I can display from this file with my computer. Sometime after Christmas I will post here a much better .avi file of this intereting observation. At my attended station 9 km farther north, the occultation lasted 4.6 seconds; I can't easily get UTC for it since I ran out of GPS time inserters for it, but did record GPS time with that camcorder before and after the event, so with some work, I can accurately calibrate the camcorder clock to determine UTC. It was recorded with a telephoto lens; with the jerky altitude control, I have to rest one leg of the small tripod on a finger to keep the target in view. My southernmost station using the small MDVR recorder did its schedule record at the proper time; this was 36 km south of the predicted center, near a rest stop off of US 27 6 miles north of I-75. I need to review it in detail later to see if the target was recorded; a small telephoto lens was used. I have not had a chance to review the long tape made at the station 15 km south of center, but it's certain that both of these south-of- center stations had no occultation, as so many of the other observers had, including Chris Stephan at Sebring north of the actual path. Unfortunately, Frank Mraz at about 40 km north had to work and couldn't observe. I have not heard yet from Bob Konior at Cape Coral, FL, several km north of Frank Mraz's location. ___________________________ Gradual events due to stellar angular diameter, not diffraction From: Jean Lecacheux jlx@meteores.net Sent: Wednesday, December 19, 2007 6:01 AM To: David Dunham Subject: Re: Spectacular Thusnelda graze near Runyon, Florida (+22 km) David, Congratulations for your multi-stations Thusnelda success in Florida yesterday ! One remark. You wrote: > The events were gradual due to Fresnel diffraction effects. I disagree. This was because of the stellar diameter, as this "naked eye" target (HR 3033, V= 6.5) is not very remote compared to those of mag. 10 - 12 we usually encounter. From Hipparcos we know its distance : 130 +/-15 parsecs. So its absolute magnitude (i.e. at 10 parsecs, interstellar absorption deduced) was Mv= +0.8 . From the Michigan catalogues we also know this is a G-type giant or subgiant. It results that its apparent angular diameter probably was near 0.6 milli-arc second, or ~750 meters at the 1.685 AU distance of Thusnelda. As at such a distance the Fresnel length was Fl= 275 meters in red light (600 nanometers), the star diameter was about 2.7*Fl, far above unity. Thus any diffraction effect was erased. Taking into account the Thusnelda velocity of 10.20 km/s (topocentric) or 8.34 milli-arc second/s, i.e. about 14 stellar diameters per second, you probably will explain the details of your gradual grazing occultation. ___________________________ Another positive observation From: David Dunham [dunham@starpower.net] Sent: Wednesday, December 19, 2007 10:53 AM Subject: Thusnelda occ'n also recorded (at 26,000 fps?!) by T. Fox in Lee County, FL Message to Daniel Fischer in Bonn, Germany, who pointed out this interesting observation that he found on the Web: Daniel, This is extremely interesting, Fox is not one of the observers I know about. [Later, I found out that he was recruited by Paul Maley, a member of the NASA Johnson Space Center Astronomy Club, via the messages that he sent to s. Florida astronomical societies promoting observation of the event]. The article says 26,000 frames per second, but this must be a misunderstanding, the fastest that the MallinCam camera that Fox used is standard video rate. The number might be a shutter speed, as fast as 1/26,000th of a second. At 07:53 AM 12/19/2007, Daniel Fischer, dfischer@astro.uni-bonn.de, wrote: >Interestingly the Thusnelda event made it even into the 'normal' news: >http://www.winknews.com/news/local/12582501.html reports on >observations by T.L. Fox in Florida's Lee County. > >Daniel ___________________________ Pre-event predictions and plans are here. David Dunham, 2007 Dec. 21, 5 pm EST Phones home 301-474-4722; office 240-228-5609; cell 301-526-5590 office e-mail david.dunham@jhuapl.edu with Blackberry for mobile use home e-mail: dunham@starpower.net .