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

Photometry of lunar graze of HD 14688 on Mar 4, 2006

Michael Richmond
Mar 29, 2006

Table of contents:

Walt Morgan's video record

Walt Morgan observed this event from a location in Northern California. I leave it to him to describe his equipment and location, since he will report on his results fully in the near future. All I'll say is that he used a telescope, video camera, and KiwiOSD time insertion unit.

Walt has reduced and analyzed his video record already, using the excellent LiMovie software. He kindly sent me a brief, 3-second clip of one small portion of the entire graze so that I could analyze the data independently, using my own software. Our goal is to verify that both pieces of software produce the same results.

The video clip provided by Walt covers just over 3 seconds of time, at standard video rate and in NTSC format.

Reduction of the video file

First, I broke the video clip into individual frames using TMPGEnc. The result was a set of 95 JPEG images, each 720 x 480 pixels in size. The typical size of each frame was about 160,000 bytes, which means that there were on average 4 bits per pixel used to represent the original 8-bit data.

Here's a sample frame, the first one in the series. Note that the time of this frame is UT 02:39:26.704, with an uncertainty of plus-or-minus 17 milliseconds.

My images consist of full frames; that is, each image represents 1/30 second of exposure time, with 2 interlacing "fields" added together. Based on the 95 frames and their inserted times, I find an interval of 0.033723 seconds per frame, or a rate of 29.965 frames per second.

In the image above, the topmost point of light along the limb of the Moon is the target star, HD 14688, just before it disappears. The small point of light below it is a sunlit mountain peak along the lunar limb. I will measure the amount of light from both sources -- star and mountain -- for later analysis.

In order to process the data, I

  1. converted from JPEG to FITS format via ImageMagick tools

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

The only star visible in the frames is HD 14688 itself. Some information on the star, taken from SIMBAD 

     HD 14688 = HIP 11075 = GSC 1215-0971
 
     B=6.93  V=6.97   spectral type A2V

     spectroscopic binary with period 4.32 days

     RA = 02:22:33.20   Dec = +16:52:13.0

The FITS images showed that the peak pixel values in both the star and the nearby mountain were around 150-220 counts. Neither had many pixels at the maximum value of 255 counts. Other sunlit areas on the lunar limb, however, did saturate at this value. The FWHM of the star was around 5 pixels.

For each of the 95 frames, I

  1. placed a large box covering both empty sky and dark lunar face just above the position of the target star, and used it to calculate a "background sky" and skysig value
  2. placed a small box around the region of the star and mountain
  3. convolved the small box with a 2-D Gaussian of FWHM 5 pixels, and searched for the brightest peaks
  4. noted their position(s); if the star did not appear, I pretended it was still at the last detected position
  5. placed a circular aperture of radius 3, 4, 5 pixels (see below) around these position
  6. added up all the light within this aperture
  7. subtracted the contribution of the background "sky" to the light within the aperture

Here are quick views of the results.

The entire light curve, using an aperture radius of 3 pixels:

The entire light curve, using an aperture radius of 4 pixels:

The entire light curve, using an aperture radius of 5 pixels:

A closeup around the time of disappearance, using aperture of radius 5 pixels.

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 star in 3-pixel-radius aperture, 
                     and estimate of uncertainty in that flux
  4,5            ditto 4-pixel aperture
  6,7            ditto 5-pixel aperture

  8,9            flux of mountain in 3-pixel-radius aperture, 
                     and estimate of uncertainty in that flux
 10,11           ditto 4-pixel aperture
 12,13           ditto 5-pixel aperture

For more information

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