Photometry of occultation by (976) Benjamina on UT 2007 Apr 10 (Ellington tape)

Michael Richmond
Apr 13, 2007

On UT 2007 Apr 10, asteroid (976) Benjamina occulted the faint star UCAC2 36634523. Chad Ellington made a video record of the event from his location in the Pacific Northwest.

Telescope: alt-azi mounted 10" (25 cm) LX200R with a f/3.3 focal reducer,
Camera:    PC-164 and audio 10 MHz WWV audio
Location:  Kent, Washington, USA  
Longitude: 122d9.5m west longitude
Latitude:  47d21.5m north latitude 
Altitude:  124 m

You can read messages from Chad about this event on the IOTA Occultations web site.

Chad sent me a digitized copy of his video record, so that I might do an independent analysis of the event. The target star is very faint, so I co-added all 309 frames to show the field more clearly.

Here's a chart from SIMBAD showing the same field:

The comparison stars "A" and "B" are in Tycho-2.

  star    Tycho-2 ID            Bt                   Vt
   A      777-1014-1       12.694 +/- 0.242    10.484 +/- 0.048

   B      777-1119-1       12.798 +/- 0.260    11.236 +/- 0.089

The target star was very faintly detected in the original images, so I co-added sets of 10 consecutive frames to improve the signal-to-noise. That means the original 309 frames turn into 30 coadded frames:

     original frames      turn into      coadded frame index
       1 -  10                                  1
      11 -  20                                  2
      21 -  30                                  3
        ...                                   ...
     290 - 300                                 30
     301 - 309   were discarded

Even after adding 10 frames together, the target is still very faint. Below are radial profiles of the bright star A and the target in coadded frame number 1.

The FWHM of stellar images was between 3 and 4 pixels in the coadded frames.

I followed my usual procedure:

  1. broke the AVI movie file into individual frames using the MPlayer program
  2. converted the JPEG frames into 16-bit integer FITS files
  3. analyzed the FITS images with programs from the XVista astronomical image processing package

I decided that the 4-pixel radius yielded the best results, but I'll show the light curves using each of the three radii below. To start, I'll use a linear scale in integrated light intensity.

First, using a 2-pixel aperture,

Next, using a 3-pixel aperture,

Finally, using a 4-pixel aperture,

Now let's look at a closeup of the target star. I'll continue to use a 4-pixel aperture, and zoom in on low light levels.

It seems pretty clear that the target star's light decreases to (effectively) zero from around frame 6 to around frame 24 or 25.

I think one sees the event more clearly if one converts the integrated light values to a magnitude scale. I adopted an instrumental magnitude scale

  instr. "mag"  =  25.0 -   2.5 * log10( integrated counts above sky)
In two of the 30 measurements, the target star's integrated light was slightly negative; I replaced the negative values by positive 1.0 to allow me to compute a "mag". Here are the light curves in magnitude form:

Again, I see strong evidence for an occultation lasting roughly 19 co-added frames, or 190 original frames. At a standard video rate of 30 frames per second, that corresponds to a duration between 6 and 6.7 seconds. I'll leave it to Chad and others to decide how best to derive a more precise duration.

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