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

Photometry of the lunar grazing occultation of 85 Ceti on Jul 15, 2001

Michael Richmond
Dec 3, 2005
Dec 4, 2005

On July 15, 2001, Brad Timerson, near Dresden, New York, USA, observed the Moon graze the star 85 Ceti, also known as SAO 93067 = HR 797 = HD 16861. His equipment included

After the event, David Dunham inserted time signals on the videotape. For example, here's the frame I call "frame index number 1001." It has a time stamp indicating 08:16:03.03.

He kindly sent an MPEG version of the digitized video record to me. I processed it in the following manner:

  1. split into 7199 individual frames and converted to JPEG format by mplayer

  2. converted from JPEG to FITS format via ImageMagick tools

  3. analyzed with programs from the XVista astronomical image processing package

The times on first and last frames in my set were

This yields a frame rate of 29.973 frames per second.

The star was not overwhelmingly strong. In the JPEG files I pulled from the movie, each pixel can have a value between 0 and 255 counts. The typical background near the star was around 40 counts. The star itself was not saturated, having a peak value around 120 counts. The FWHM of the stellar image was around 3 pixels. I estimate very roughly from the size of the Moon in the images that each pixel is about 5 arcsec on a side.

For each of the 7199 frames, I

  1. tried to find the star, using a small box around its rough location. To help, I convolved the image in this box with a 2-D circular gaussian with FWHM 4 pixels
  2. if I found the star, I noted its position; if not, I used the most recent detected position
  3. placed a circular aperture of radius 3 pixels (see below) around this position
  4. added up all the light within this aperture
  5. calculated a sky value based on pixels in an annulus of inner radius 8 pixels, outer radius 15 pixels
  6. subtracted the contribution of the background sky to the light within the aperture

There was a complication: in many images, the star itself was invisible, yet my software claimed to find an "object". These were noise peaks, I suppose. It is clear that there are a significant number of such false detections if one looks at the location of the "object" in all frames. In the graph below, I place a red dot at the (row, col) position of the detected "object" in all frames.

As you can see, the field of view drifted slightly over the course of the 4-minute video record. The real detections define a snake-like locus, with a big break where there was a prolonged disappearance. The individual points located far from this locus are false detections. To get rid of many (but not all) of them, I drew two lines, shown in the diagram above, and discarded any detection outside the lines.

I also found that the detections after frame 7100 were noisy, so I discarded them, too. That left me with 5219 measurements, most of which were either real detections or at least measurements at the proper location.

The photometry taken at the positions of these false detections gave me a useful bit of information: it showed the amount of light above background that an aperture placed on a random location would yield.

 aperture radius        max value of flux in empty aperture
    (pixels)                   (counts above background)
       2                             63
       3                            145
       4                            209
       5                            250
       6                            302

In my graphs below, I place a dotted line at a flux of 145 counts above background, to indicate a rough limit below which measurements are dominated by noise.

To gauge the precision of the photometry, I calculated the mean flux and standard deviation from the mean for each star over several intervals:

             interval            mean counts    standard deviation
   1000  <  frame <  1400          385            95
   5600  <  frame <  6000          420            87

In short, the precision of the photometry is pretty low: around 20 percent. Note that the V-band magnitude for 85 Ceti is about V = 6.3; it is marked in SIMBAD as a variable star, however, and I don't know what its magnitude was exactly at the time of the occultation.

Here are quick views of the results. I have placed a dotted green line at the level of false detections, to indicate a reasonable limit for trusting the measurements.

The entire light curve:

A closeup around the first brief disappearance:

A closeup around the second, prolonged disappearance:

A closeup around frames 3300 to 4200:

A closeup around frames 4000 to 5200:

A closeup around frames 5000 to 5800:

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

 col              quantity          
  1              frame index

  2,3            flux of 85 Ceti in 2-pixel aperture, and estimate
                     of uncertainty in that flux
  4,5            ditto 3-pixel aperture
  6,7            ditto 4-pixel aperture
  8,9            ditto 5-pixel aperture
  10,11          ditto 6-pixel aperture

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