1.6. Measurements in Astro ImageJ

1.6.1. Selecting a region

In the AstroImageJ toolbar are a number of selection tools. They include the line selection tool, a rectangle selection tool, and several others.

  • Start by selecting a rectangular region in one of your bias images.
  • Now try selecting exactly the same region in a different bias image. Can you? Don’t waste a lot of time trying please!

To take consistent measurements across a series of images it would be helpful to be able to tell AstroImageJ explicitly what region you want. You can do that by:

  • Click on the toolbar so it is on top.
  • Select Specify under the Selection menu under the Edit (i.e. Edit -> Selection -> Specify)
  • You can set the size and position of the selection.


  1. Where is the origin used for the x and y position? In other words, where in the image is the point (0,0)?
  2. What is the position of the center of the image?

1.6.2. Taking a measurement

Once you have selected a region you can measure a number of the properties of the pixels in that region. You can control which things are measured by going to one of two menus:

  • Go to Analyze -> Set Measurements... if the AstroImageJ toolbar is on top, or...
  • Go to Edit -> Measurement Settings... if an image window is on top.

To make a measurement, type Cmd-M, or select Analyze -> Measure.


Be sure that you are set up to measure at least the bounding rectangle, mean, median and standard deviation of the selected region.


  1. Measure the mean pixel value and the standard deviation of the pixel value for one of the images you have open.