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.
- Where is the origin used for the x and y position? In other words, where in the image is the point (0,0)?
- 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.
- Measure the mean pixel value and the standard deviation of the pixel value for one of the images you have open.