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

Photometry of occultation of HD 121323 by (52) Europa on UT Dec 3, 2005

Michael Richmond
Dec 8, 2005

On Dec 3, 2005, Ed Morana watched the asteroid (52) Europa cover the star HD 121323, also known as SAO 139619 = TYC 4974-00584-1. His location for observing this event was near San Luis Obispo, California, USA. You can see his report at http://pictures.ed-morana.com/AsteroidOccultations/

His equipment included

Each video frame has a time stamped on it, according to the KIWI OSD protocol. Below is an example, frame number 2 in the sequence. The video file contains frames 320x240 pixels in size, in WMV format.

Note that each frame in the movie file contains a single field of video data (1/60 of a second); only every other field is present in the movie file I was given. That means that the times stamped onto each frame jump discontinuously: frame number 2 runs from 13:07:28.129 to 13:07:28.146, frame 3 from 13:07:28.163 to 13:07:28.179, frame 4 from 13:07:28.196 to 13:07:28.213. In short, half the video fields are unavailable.

I also noticed that the first frame in the movie is duplicated in the second frame.

I processed this movie file in the following manner:

  1. split into 298 individual frames (of which number 1 duplicates number 2) 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

I will skip frame 1, since it duplicates number 2. The times on first good and last frames in my set were

This yields a frame rate of 29.969 frames per second.

There are two stars visible in the field:

The fainter star near the center is the target star, HD 121323, which has Tycho magnitude Vt = 9.01. The brighter star in the lower-right corner is (I think) HD 121324, aka HIP 67949 = SAO 139617. It is brighter in the V-band, with Vt = 7.99, and certainly looks brighter on the video. Neither star is apparently saturated in the FITS images I created from the video: the brighter reference star had peak values around 160 counts, out of a maximum of 255 counts; the target star had peak values around 100 counts. The FWHM of the stellar image was around 2 pixels.

For each of the 297 good frames (ignoring frame 1), I

  1. found the reference star, using a small box around its rough location.
  2. tried to find the target star, using a small box around its rough location.
  3. if I found the target star, I noted its position; if not, I used a fixed offset from the reference star's position
  4. placed a circular aperture of radius 3 pixels (see below) around each position
  5. added up all the light within this aperture
  6. calculated a local sky value for each star based on pixels in an annulus of inner radius 5 pixels, outer radius 12 pixels
  7. subtracted the contribution of the background sky to the light within the aperture

To gauge the precision of the photometry, I calculated the mean flux and standard deviation from the mean for each star over the first 70 frames. 



      star                       mean counts    standard deviation
-------------------------------------------------------------------
   reference star                 1124           226
   
   target star                     302            82
--------------------------------------------------------------------

In short, the precision of the photometry is around 20-25 percent.

Here are quick views of the results.

The entire light curve:

A closeup around the disappearance:

A closeup around the reappearance:

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

 col              quantity          
------------------------------
  1              frame index

  5,6            flux of ref star HD 121324 in 2-pixel aperture, 
                     and estimate of uncertainty in that flux
  7,8            ditto 3-pixel aperture
  9,10           ditto 4-pixel aperture

  11,12          flux of target star HD 121323 in 2-pixel aperture, 
                     and estimate of uncertainty in that flux
  13,14
  15,16

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