Essentially you need to do a particle analysis. Before you do that, change settings using **Set Measurements** so that you have either the desired boxes (the coordinate system which defines the particles' size and position) and from the *results table*, you should be able to determine the centre positions. You shouldn't need to fit a circle to do this if the images are all this good. A very short custom macro could be used to streamline the process.

Hello! 

I am very new to imagej. I have an image in which I basically have to fit two circles on an image and calculate the distance between their centers.  How would I do this in imageJ and are there any plugins to make it streamlined? Here is a link to my image: https://imgur.com/KpS5QcU . 

Thanks! 

Measuring Concentricity