Plot Coordinate Systems
Mira plots image data using and (x,y) system of either the native array coordinate units of pixels and luminance or world coordinate system units of arcseconds and luminance. The only difference between the two systems is what gets used for the distance or position axis of the plot. Typically, the distance or position is plotted on the x-axis.
A plot in World Coordinates has Distance in microns, mm, arcseconds, or other physical units on the x-axis and luminance on the y-axis.
A plot in Pixel Coordinates has Distance in Pixels on the x-axis and luminance on the y-axis.
The Column and Row profile plots and the Radial Profile Plot all use the same interface for selecting pixel or world coordinates: the coordinate system is selected as a menu item in the drop-button next to the plotting button on the main plotting toolbar. An image must be the top-most window for these buttons and menus to be available.
The term Luminance refers to pixel value at each position, using whatever "brightness" units may be attached to the image, such as "ADU", "ergs/sec/cm^2", or whatever.
A plot in WCS units shows position or distance as an offset relative to the WCS coordinate at a reference point at the center of the data.
World Coordinate System (WCS) units can be used under the following circumstances:
The image was loaded from a FITS format file and also has WCS information in the image header.
An image of any format was calibrated in world coordinates after it was opened. When such an image is closed, the WCS calibration will be retained permanently only if the image is saved to a file in FITS format.
The FITS format requirement exists because the WCS definition is part of the FITS format standard.
The choice of plot coordinate system is made on the Plot Preferences Page of the Mira Preferences Dialog. Make your selection from the Plot Coordinate System list box and be sure to click [OK], [Apply], or [Default] as appropriate.. If you select World Coordinates and no WCS information exists in the image header, Mira automatically defaults to Pixel Coordinates for the plot. The difference is shown in the Comparison of Plot Coordinate Systems.
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Note |
The choice of plot coordinate system must be made before the plot is made. This is required because the image data are projected into WCS units before being sent to the Plot Window. |
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tip |
If the image does not have WCS information but you want a plot against arcseconds on the x-axis, you can perform a "quick & dirty" WCS calibration using the Set Equatorial WCS command. After doing this, do not save the image unless you want the WCS system made a permanent part of the image. |
Plotting commands, Image Coordinate Display, World Coordinate System, Plot Preferences Page, Plotting in World Coordinates