Sedna 2012 Xmas event

Sedna Occultation on Christmas Eve.

Note: this page as been updated on Dec. 19 to include a better finder chart (linked) and update the participating observers and their information.

Calling all observers. This Christmas Eve, the Kuiper Belt Object 90377 Sedna should produce a stellar occultation visible from North America! Sedna is one of the most distant known Kuiper Belt objects. Of the large bodies, Sedna certainly has the highest semi-major axis, a=544 AU. Sedna's size is currently unknown. A radiometric size measurement now exists, suggesting it has a diameter D = 995 km.

If you have any questions, don't hesitate to contact me at wesley -dot- fraser -at- nrc -dot- ca

Details of the occultation

The star (coordinates in J2000):

  • RA: 03:33:12.92
  • Dec: +06:46:50.793
  • Magnitude: r'=18.9

The event:

  • Nominal Event Time: 2012 December 25, 03:40:00 ± 2:00 UTC (see below for details)
  • Shadow Velocity (at event centre): 16.4 km/s
  • Maximal Duration: 61 s
  • Instantaneous Shadow Extent: 995 km, 0.016"

There are two predictions made by two independent groups, with independent data, and independent techniques. Both predictions are in agreement that the event will happen somewhere over North America!

The first prediction, by your's truly, uses Megacam data from CFHT and uses the SDSS astrometric catalog to detrend the images and produce a master catalog. A systematic shift in RA of 0.08" has been identified in the SDSS astrometric solution, affecting the most recent CFHT images (taken in middle Nov). This systematic results in two possible event predictions, both shown below:

A few details about the predictions. The nominal body centre path is shown by the black line. The predicted shadow extent (limited to airmass < 4 and assuming D=995 km) is marked by the red lines. Night/day terminator is at the nominal event time. The random uncertainty of the predictions is less than 0.01" in RA and DEC (1-sigma errors). This corresponds to a timing error of about 60 s and a vertical scatter less than the width of the shadow. The systematic errors dominate the solution. But both solutions essentially bracket the possible paths.

In the images, red circles mark 1 minute intervals, with the BIG red circle marking the nominal centre point. Use these to gauge when the event will occur over a particular site.

The second prediction, made by Bruno Sicardy's team is shown below (original image taken from Bruno's webpage).

As the image shows, Bruno's prediction is in agreement with one of my predictions. It is unclear if Bruno's prediction suffers from the same systematic error that affects the SDSS astrometry.

Additional CFHT images and prediction refinement will occur in early December. These refinements will be posted here as soon as they are available.

The Field

The image below shows a local finder chart about 1.2 arcminutes on a side. A larger chart has been produced in fits format (see link at bottom of chart). The target star is circled in red, and three appropriate reference stars are shown with vectors, and the distance to each is shown. The top left has a brightness r'=21.0 and the bright at lower right has r'=18.0. Remember to choose your pointing such that the target star and at least one of the reference stars is in the field of view. If your FOV is large, make sure to include the bright star 47" to the South-West.

Wide Finder Chart.

Basic Telescope and Observing Requirements

One major goal is to determine Sedna's size. This can be constrained both by the geographic extent of the telescopes that detect (or not) occultations, and the shadow cord lengths themselves. It is most desirable to have as fast imaging as possible. But imaging as infrequent as roughly 1/5 Hz (1 image every 5 seconds) is useful. At that cadence a central cord will produce an occultation roughly 10 frames long.

The overall cadence should be chosen so as to not sacrifice the effective signal-to-noise of the occultation. A good goal to shoot for is a SNR>5 and imaging with cadence >1/2 Hz. Given the faintness of the target star, these requirements likely put this occultation out of the reach of telescopes much smaller than a meter in aperture.

Accurate timing of the event is very important. And this can be achieved using proper sync with the NTP internet protocol service. Make sure your computer clock gets synced roughly an hour before the the event observations begin. It is also very important to determine the accuracy of the time record in the images. This is a real challenge with no easy solution at most facilities. The best thing to do is run a few tests to see how consistent the frame output is. For example, take 10 minutes worth of data at the cadence you anticipate for your actual observations, and see if the number of images per second is consistent. Note: the times recorded in the headers of the images are often not accurate enough for our purposes and not necessarily even correct! So it is important to create a log of image creation times. Email me so we can work on acquiring this from your particular computer.

For the actual event, the timing of nominal event centre is uncertain by +-3 minutes (3-sigma errors). It will also take the shadow roughly 3 minutes to cross the entire observable area.

So make sure to be observing the target star at least ten minutes before the event centre time!

Participating Telescopes

Telescope Aperture Operators Location Site Code Longitude (E) Latitude (N)
Plaskett Telescope 1.8 m Wes Fraser Victoria, BC 658 236.583° 48.52°
R. A. Cross Telescope 1.8 m Phil Langill Calgary, AB 661 245.968° 50.78°
LCOGT - Texas 1.0 m Federica Bianco, Tim Lister McDonald Observatory 711 255.978° 30.67°
ARC Consortium 3.5 m Russet McMillan Apache Point 645 254.179° 32.78°
VATT 1.8 m Richard Boyle Mount Graham 290 250.108° 32.70°
WIRO 2.0 m Eliot Young, Leslie Young Jelm Mt, WY N/A 254.023° 41.09°
Brigham University Telescope 0.9 m Michael Joner West Mountain Observatory N/A 248.174° 40.09°
Perkins Telescope 1.8 m Melissa Brucker Lowell Observatory 690 248.335° 35.202°
IOTA TBD Steve Conrad TBD N/A N/A N/A