By Brian M. Sherrod
Here is a method that is within the reach of the Amateur to automatically compare two digital images of the same star field for Novae or Supernovae. Images MUST be the same scale in order for this to work with some reliability. If an equipment standard is used for all imaging, your scale should always be the same unless you resize the image after the photo is taken. The exact center of the field will likely vary between the two images, but that can be adjusted for by aligning the two photos together using at least three stars so that one image exactly covers the other.
In Adobe Photo Deluxe (and other graphic programs) we use the "Layers" functions to easily determine our comparisons. In the following example, we will consider Image 1 as the first or previous image taken say 5 days before image 2, which was just obtained.
Image 1
Image 2
In Photo Deluxe, the entire area of Image 2 is selected and copied to the clipboard. It is then pasted into Image 1. This will create two Layers, layer0 and layer1. Layer1 is the last image we took. Now we want the program to compare the two fields for difference. In the "Layers" menu, double click on layer1 and change the "Blend" function to "Difference. Here is what we get:
Since the Nova was in one of the images and not the other, the difference function subtracted all the stars that were the same and left this lone suspect.
Results will vary depending on several factors. Make certain exposure time does not vary between your comparison images or you might get unpredictable results.
We would like to hear about any programs out there that automate this process!
Copyright Arkansas Sky Observatory © 2001 [A.S.O.] All rights reserved. Revised: December 16, 2001
