Comparisons of Image Analyst and Digimizer

Theodore Lunar Observatory Australia

Comparison

A very difficult image to measure was that from the partial lunar eclipse of 2005 October 17, as the Moon only just entered the umbra due to the very low magnitude for this eclipse of only 0.0678.

 Image Analyst produced a poor fit to the umbra edge and gave a semi-diameter for the umbra of only 0.2710° as shown in Figure 1.

Figure 1: Image Analyst measures for 2005 October 17

Figure 2 2005 October 17 at 10h 07m10s

Figure 2 illustrates the same image as shown in Figure 1 above, but measured with Digimizer which produced a meaningful result for the umbra of 0.734°. The expected semi-diameter of the umbra was 0.7334°. The distance measured between the centers of the Moon and umbra was 0.901°.

Another Comparison

A more appropriate comparison of the two measuring engines is from images captured from the total lunar eclipse of 2004 May 4 where the magnitude of this eclipse was 1.3093.

Figure 3: Image Analyst Measures

Image Analyst managed a very good fit to the umbra edge profile shown in green, giving a calibrated semi-diameter of 0.7607° for the umbra at a contour level of 58 as shown on the histogram. A profile plot of the Moon's image is marked at the position of the best fit circle in blue.

Figure 4: Digimizer Measurements

Digimizer gave a calibrated measurement for the semi-diameter of the umbra as 0.763°, although the gray scale profile for the edge of the umbra shown in red, appears to be in a slightly different position on the imaged Moon than that shown in Figure 3 for the Image Analyst measures. The expected semi-diameter of the umbra was 0.7627°.

There is also a Digimizer measurement of Figure 3 here - another interesting comparison.

Conclusions

The above comparisons illustrate that the end result is very similar for total eclipses and improved measurements can be obtained with Digimizer for the more difficult partial eclipses.

Both measuring engines are very useful for the determination of the size of the umbra by measurements of best fit circles to the selected umbra profile on the Moon. However, as Image Analyst is no longer available and can only be executed on my Mac IIvx, I have decided to use Digimizer for all future lunar eclipse image measurement with my Dell Inspiron 600m laptop.

Produced by Byron Soulsby 2006 July 24