Digimizer and Image Analyst

Theodore Lunar Observatory

I have used Digimizer to measure one Image Analyst image of the 2004 May 4 lunar eclipse to compare grey scale profiles and measurements, as shown below:

The Moon has the same calibration with its semi-diameter at 0.2755°, but the grey scale profile is somewhat different in the red Digimizer selection. This gives rise to a different umbra best fit circle in green with its centre measured at 0.865° (the measurement box is hidden) from the Moon's centre. The umbra semi-diameter was slightly larger at 0.763° when compared with 0.7607° for Image Analyst.

The true distance of the Moon from the umbra center at the time of this image (19h04m58s UT) has been found from the Besselian elements of this lunar eclipse to be 0.8648°, which is very near the Digimizer measurement of 0.865°. A lower value of 0.811° was found from the Image Analyst contour in green and its best fit circle shown in blue.

Prepared by Byron Soulsby 2006 July 18