Photometry of stars in the field of Palma occultation, Jan 26, 2007

Michael Richmond
Feb 8, 2007

On UT 2007 January 26, the asteroid (372) Palma occulted the bright star 36 Lyncis for observers across the US.

Derek Breit observed this event from Anderson, California.

Derek asked me if I could use his videotape to determine the magnitude of the asteroid relative to a pair of comparison stars in the field. I did my best, but (as you will see below), the results aren't very precise.

Derek placed a digitized version of his video record on his web site. The movie was in MPEG-PS format, with frames 720x480 pixels in size. I chopped the movie into individual frames using mplayer. Each frame became a JPEG image, 720x480 pixels in size, in 24-bit color. The actual pixel values, however, represent an 8-bit greyscale palette.

For example, here is frame number 1 of the video. The field of view is roughly 20 arcminutes wide (North-South) by 13 arcminutes high (East-West). I've flipped the orientation so that the timestamps are upside down.

The three objects are

 label      object             TASS V                TASS I
   A   Palma + 36 Lyncis        ----                 -----
   B   TYC 2489-934-1           10.10 +/- 0.07      9.33 +/- 0.08

   C   TYC 2489-841-1           10.47     0.07      9.39     0.09


I show the TASS Mark IV magnitudes for the two stars. Neither one shows any evidence for variability on long time scales.

Now, the image of 36-Lyncis-plus-Palma is very saturated. When the asteroid covers the star, however, the asteroid's image alone is not saturated, but roughly equal in brightness to the two stars; all have peak values of about 130 counts per pixel, which is far from the limit of 255 counts per pixel imposed by the 8-bit movie.

In order to see more details, I co-added 50 frames. A sample coadded image is shown below. Note the strong change in the background properties across the field:

The central portion of the frame has a slightly higher background, and a much high scatter in background levels, as this histogram of pixel values shows:

I fear that any comparison of the target asteroid -- in the central region -- to the comparison stars -- in the outer region -- will incur a significant, unknown systematic error.

So, keep that in mind as I continue.

I coadded sets of 10 consecutive frames to create a set of deeper images. I started with frame 100 of the movie, adding frames 100-109 to create deep image number "10", then adding frames 110-119 to create deep image number "11", and so forth, up to adding frames 190-199 to create deep image number "19". In all of these frames, the asteroid covered the star completely, so that only the asteroid's light appears.

I then used the XVista image processing package to measure the instrumental magnitude of the three objects: comp star B, comp star C, and the asteroid Palma. I subtracted a local sky background determined from an annulus around each object. The FWHM of the images was about 3.5-4.0 pixels, and I found that a small aperture of radius 3 pixels yielded the most consistent results.

However, the "most consistent" results were not very consistent. Below are light curves of the three objects over the span of 100 video frames = 10 coadded frames. Note the large variation in the relative brightness of each.

The statistics of these results yield differential magnitudes of

  object         instr. mag diff (B - obj)         
                   mean     stdev
  star C            -0.28     0.08

  asteroid Palma    -0.18     0.10

In other words, the coadded frames indicate that star B was 0.28 mag brighter than star C, but with quite a large uncertainty.

Comparing the measurements of the two comparison stars to the TASS magnitudes, we see that the instrumental system lies in between the V-band and I-band differences, but a bit closer to V-band. One might speculate that it might be reasonable to compare the video record to V-band.

We might therefore conclude that the asteroid Palma was 0.18 magnitudes fainter than star B in the V-band:

      Palma magnitude  "V" = 10.28  +/- 0.30     ???

Remember the unknown systematic error due to the variation in the background/sensitivity of the images.

For comparison, the JPL Horizons system provides a "magnitude" of 10.57 for the asteroid during this time.