There are over 500,000 known asteroids that orbit the Sun.  Most of these objects are too small to resolve in Earth-based telescopes, so our knowledge of their size and shape is very crude.  As they move across the sky, many asteroids pass in front of (occult) stars as seen from Earth.  Photometry of these occultations provides a simple way to measure the diameter and shape of asteroids.  On April 16, 2009, I observed an occultation of 10.3 magnitude TYCHO 0819-00852-1 by the asteroid 336 Lacadiera.  This page presents my observations of this occultation.


Observations

The IOTA issues predictions every month for dozens of asteroid occultations.  Most occultations can be seen only from a narrow path due to the small size of most asteroids.  The prediction for the 336 Lacadiera occultation is shown below (courtesy of Steve Preston).  My observing location is shown by the red dot.  Click on the image for a larger version.

occultation prediction

The field around TYCHO 0819-00852-1 was recorded with a low-light video camera from roughly 5:20 to 5:37 UT on April 16, 2009.  Video frames were acquired at a rate of 29.97 fps.  Every video frame was time-stamped using a KIWI GPS time inserter and recorded to a portable VCR.  This method of time-stamping is accurate to +/- 0.001s with respect to UTC.  Transparency was good but the seeing was fair during the observation time.

Equipment Used

  • Telescope: 20.0cm f/5 Newtonian Reflector
  • Camera: PC-164C
  • Filter: none
  • Exposure: 0.03s
  • Time Source: GPS

Location

  • Richmond Heights, Ohio, USA
  • Latitude: 41d 33' 50.5" N
  • Longitude: 81d 30' 03.0" W
  • Elevation: 257m
  • Datum: WGS84

Observer: Robert J. Modic (IOTA)


Below is a frame from the video of the occultation.  TYCHO 0819-00852-1 is the marked star.  North is up and the width of the frame is about 16 arc-minutes.  Click on the image for a larger version.

Lacadiera Occultation vidcap

A video of the occultation can be downloaded here.  This video file is 6.7 MB in size and uses the XVID video codec.


Reduction

The occultation video was digitized and transferred to a computer for measurement.  Aperture photometry was performed on the digitized video using LiMovie, a program created by Kazuhisa Miyashita specifically for the measurement of occultation videos.  LiMovie has the ability to measure the two fields that make up each frame of interlaced video, allowing a time resolution of 0.017s to be obtained.  The first plot below shows the measurements of each field (0.5 frame).  TYCHO 0819-01259-1 was used as the comparison star.

336 Lacadiera plot 1

The plot above shows the intensity of the sky subtracted fluxes of the occulted star (blue) and comparison star (red) plotted vs. frame number.  An aperture of 6 pixels diameter was used for this plot.  Click on the image for a larger version.

336 Lacadiera plot 2

The plot above is the same as the first except that each point is a 3 field (1.5 frame) average to help reduce noise.  Click on the image for a larger version.

As shown by the plots above, an obvious occultation occurred close to the predicted time.  According to my measurements, the star disappeared at 5:31:04.271 +/- 0.017 UT and reappeared at 5:31:13.296 +/- 0.017 UT.  Each event was instantaneous.


Discussion

My observation of the 336 Lacadiera occultation represents one chord across the profile of the asteroid.  By combining my chord with that of observers at other locations, a better idea of the asteroid's size and shape can be obtained.  There were four other observers who saw the occultation.  A plot of the observed chords from all five observers is shown below (courtesy of Brad Timerson).  Click on the image for a larger version.

336 Lacadiera profile

This profile shows 336 Lacadiera to be 65 +/- 4 km in diameter with an approximately round shape.