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Plot Coordinate Systems
Mira plots image data using and (x,y) system of
either the native array coordinate units of pixels and
intensity or
world coordinate system units of
arcseconds and intensity. 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.
For example,
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A
Line Profile plot in World Coordinates has
Distance in arcseconds or other physical units on the x axis
and intensity on the y axis.
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A
Line Profile plot in Pixel Coordinates has
Distance in Pixels on the x axis and intensity on the
y axis.
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The
Column and
Row profile plots use the same strategy for pixel and
world coordinates.
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In both cases, Intensity refers to pixel
value at each position, including whatever units may be attached to
it, such as "ADU", "ergs/sec/cm^2", or whatever.
A plot in WCS shows position or distance as an
offset relative to the WCS coordinate at the center of the
data.
When to use WCS Units
World Coordinate System (WCS) units can be used
under the following circumstances:
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The image was loaded from a FITS format file and
also has WCS information in the image header.
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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.
Selecting WCS or Pixel Coordinates
The choice of plot coordinate system is made on the
Plot Properties Page of the General
Properties. Make your selection from the Plot Coordinate
System list box and be sure to click [OK], [Apply], or
[Save] 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
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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
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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 Scale
command. After doing this, do not save the image unless you want
the WCS system made a permanent part of the image.
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Related Topics
Image Coordinate Display
World Coordinate System
Plot Properties Page
Plotting in World Coordinates
Mira Pro x64 User's Guide, Copyright Ⓒ 2023 Mirametrics, Inc. All
Rights Reserved.
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