Measuring Images

Mira provides many tools for measuring and quantifying images, including distance and angle, statistics, photometry, shape measurements, and others. The Extract Sources command from the MExtract Module performs automated detection and numerous measurements to quantify the properties of point sources and extended objects.

Measurement procedures are operated through buttons, menu commands, and command toolbars. Measurements share a common user interface in terms of how the target object is marked and how the results are displayed in a Report Window.You can also create a scatter plot, including error bars, from two columns in a table of measurements. Other types of plots and point samples are also useful for quantifying images.

Measurements are divided into classes according to whether they use the Image Cursor or the Mouse Pointer to mark locations. All measurements that use a toolbar use the mouse pointer. Conversely, all measurements that use the image cursor operate from a button (or a duplicate entry in a menu). All Command Toolbars all have a similar interface

Measurements that use the Image Cursor

These commands also have menu commands in case the button is not accessible.

Centroid: Reports a high precision position for a peak or trough.

Statistics: Reports statistics inside a rectangular region.

FWHM: Reports the Full Width at Half Maximum for a peak.

Measurements that use the Mouse Pointer and a Toolbar

Distance & Angle: Measures the distance and angle between two points.

Aperture Photometry: Makes photometric measurements of stars and other objects.

Star Removal: Subtracts stars from images by building and applying a template for the star profile.

Measurements that use the Mouse Pointer

Image Coordinate Readout: The "live" cursor in the image window measures image coordinates and values.



The difference between measurements made by the mouse pointer and image cursor is generally as follows:

  • Measurements that use a window toolbar use the mouse pointer to mark or drag markers on the image.

  • Measurements that use menu commands and buttons on the Image window or main toolbar use the Image Cursor as a position reference.

Source Extraction

The Extract Sources command detects objects in an image and extracts their properties such as coordinates, ellipticity, luminance, etc. This command is available in the optional MExtract module. The extraction procedure runs without any interactive marking procedure except when finding sources within the rectangular bounds of the image cursor.

Measuring RGB Images

Making measurements of luminance images is straightforward. However, RGB color images contain a separate image of luminance data for each of the 3 planes. When Mira measures an RGB image, it performs the calculations on an equivalent grayscale image and returns an equivalent measurement for a single "grayscale" channel. To measure any or all of the individual color channels in an RGB image, do the following:

  1. Use the Extract Channel command to separate the color channels. If extracting more than 1 channel, check the Create Image Set option so that all extracted images end up in the same Image Window.

  2. If you extracted more than 1 channel, press on the new window so the entire Image set will be measured.

  3. Perform the measurement on the extracted channels. The result for each channel is reported in the same way as for a luminance image.

Related Topics

Report Windows, Command Toolbars, Image Windows, RGB Image Commands, Scatter Plot, Extract Sources command