Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Photometry of (no) occultation by (31) Euphrosyne on UT Oct 7, 2006

Michael Richmond
Oct 9, 2006

Executive summary: I didn't detect any occultation. Video compression with the Cinepak codec offers a nice decrease in size (by factor of about 6) with only a small increase in noise (by a factor of about 20 percent). Video compression with the DivX codec decreases the accuracy of photometry a lot.

Table of contents:

Was there an occultation? No.

I observed the asteroid (31) Metis occult the star TYC 2979-00969-1 on UT Oct 7, 2006, from the RIT Observatory:

  Latitude +43.0758 degrees North = +43:04:33
  Longitude 77.6647 degrees West of Greenwich = 77:39:53 W
  Altitude 168 meters

  Meade LX200GPS 12-inch f/10 Schmidt-Cassegrain telescope
  PC164C CCD video camera
  Kiwi OSD GPS video time inserter
  Garmin 16 GPS
  random old VCR

The nice Aladin tool shows that the target star, "A", has a close companion, "C", about 16 arcseconds to the northwest. The brighter star "B" has magnitude Vt = 10.1, about half a magnitude brighter than the target star.

I videotaped the event under decent conditions. The star was very low in the sky, only about 19 degrees above the very bright north-eastern horizon, and the nearly full Moon made the sky even brighter. Nonetheless, I could see all three stars on the TV screen in the dome. I did not detect any dip in brightness at the predicted time, around UT 05:27:05, but brought the tape home for further analysis.

I digitized one minute of the tape using the Canopus ADVC 110. In its raw digitized form, the clip occupies about 240 MBytes -- yikes!

When I analyzed the tape (for the details, read the next section) I found no evidence for any dip in the target star's apparent brightness.

So, I conclude that there was no occultation at my location.

Tests of different video compression algorithms

In this section, I will describe very simple tests of three different methods for turning the raw digital video clip into individual frames for analysis. In each case, once I had a set of individual frames, I followed this procedure:

  1. convert JPEG frames into 16-bit FITS images via ImageMagick and XVista
  2. find the position of the bright star "B" on each frame
  3. use fixed offsets from "B" to determine the position of target star "A" and faint star "C"
  4. add up the light for each star using a circular aperture of radius 6 pixels
  5. subtract light in an annulus of radii 12 to 18 pixels around each star

I tried three methods for turning the video clip into individual JPEG frames (1754 in each case):

   Method          movie file size       individual JPEG     total JPEG
                     (MBytes)             size (Kbytes)       size (MBytes)
   Direct              ---                  217                 383

   Cinepak              37                  155                 272

   DivX                 10                   12                  22

Below are samples of one frame from each method. I zoom in to the target star and its nearby faint companion. The contrast is set in each case so that zero counts shows as pure black, and 100 counts as pure white. The direct frame is first (on the left), then the Cinepak frame, and finally the heavily compressed DivX frame.

Let's look at the light curves for three objects -- target "A", bright star "B", faint star "C" -- derived from each set of images. First, the frames converted directly from raw video.

Next, the frames derived from a Cinepak video.

Finally, the frames derived from the heavily compressed DivX video.

The gross characteristics are the same in all cases. However, we can see some differences if we zoom in a bit. Let's look at a set of 30 frames.

The direct and Cinepak frames track each other pretty well. The DivX frames show choppier behavior.

If we compare the Cinepak to the direct frames, we see pretty good agreement. The green symbols on the plot below are the differences (shifted by -500 counts) between the target star's brightness in Cinepak and the target star's brightness in direct frames.

It is very instructive to look at the sky values derived from each set of frames. In theory, the background sky value should remain constant, with slow changes due to changing altitude, moonshine, thin clouds, and so forth. The DivX sky values, however, occasionally jump by large amounts:

For a quantitative measure, we can look at the statistics of each star's brightness over the entire video clip, or some small section. In theory, since no occultation occurred, each star should have a constant value. There will be random variations from frame to frame, and those variations should be larger for faint objects.

  star   method         entire clip  scatter        frames 900-930
                        mean   sigma   %           mean   sigma   %
  A      direct         1667    458   27           1411    392    28
         Cinepak        1274    438   34           1006    351    35
         DivX            892    499   56            677    409    60

  B      direct         2215    617   28           2201    467    21
         Cinepak        1812    596   33           1781    481    27
         DivX           1462    682   47           1392    608    44

  C      direct          695    185   27            708    180    25
         Cinepak         383    164   43            421    159    38
         DivX             83    177  213             80    166   208

The direct method yields photometry with the smallest scatter in all cases. The Cinepak compression increases the scatter by a small amount; it also decreases the mean level of each star. Hmmm. The DivX compression increases the scatter greatly, especially in the case of the faint star "C"; in a number of frames, light from that star is not detected.

You can grab the data in multi-column ASCII text files below. The columns are

 col              quantity          
  1              frame index

  2,3            flux of target star "A" in 4-pixel-radius aperture, 
                     and estimate of uncertainty in that flux
  4,5            ditto 6-pixel aperture
  6,7            ditto 8-pixel aperture

  8,9            flux of bright star "B" in 4-pixel-radius aperture, 
                     and estimate of uncertainty in that flux
 10,11           ditto 6-pixel aperture
 12,13           ditto 8-pixel aperture

 14,15           flux of faint star "C" in 4-pixel-radius aperture, 
                     and estimate of uncertainty in that flux
 16,17           ditto 6-pixel aperture
 18,19           ditto 8-pixel aperture

For more information

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.