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
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.
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.
Image Coordinate Readout: The "live" cursor in the image window measures image coordinates and values.
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The difference between measurements made by the mouse pointer and image cursor is generally as follows:
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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.
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:
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.
If you extracted more than 1 channel, press
on the new window so the entire
Image set will be measured.
Perform the measurement on the extracted channels. The result for each channel is reported in the same way as for a luminance image.
Report Windows, Command Toolbars, Image Windows, RGB Image Commands, Scatter Plot, Extract Sources command