Calibrating Images
Images often need to be calibrated to convert observed values to standard values that correctly represent measurements in the real world. Calibration may be done in two realms:
Intensity (or "luminance") Calibration: This calibration converts the pixel values to a proportional or exact representation of the true signal. The calibration may also be in terms of physical units. Aperture photometry is an addition type of intensity calibration.
Spatial Calibration: This calibration converts positions on the image from the native pixel coordinates of (column,row) to a world coordinate system, such as the celestial coordinate system of Right Ascension and Declination.
After calibration, image values and positional measurements may be made using a calibrated system. For example, after spatial calibration is performed, image positions, distances, and angles are measured and reported in the calibrated units. FITS format supports these types of calibration and is designed to hold information in the image header which is necessary for accomplishing calibration. For this reason, many Mira calibration commands work only on FITS format images.
Most calibration procedures require information from the image header, including the exposure time, filter name, and others. Before calibrating, you may need to add or correct header keyword information to standardize your images. Mira provides a tool for this which adds, replaces, or rename keyword information. The Modify Keywords command is designed to accomplish this task efficiently.
Mira provides a number of tools to accomplish intensity calibration and spatial calibration:
Commands for intensity calibration are exposed in the Calibration menu and others as well as the Calibration Toolbar of Mira's main window:
Commands on the Calibration Toolbar are described in the table below.
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Commands for Intensity Calibration |
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Apply bias, dark, flat, and cosmetic corrections to images. |
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Create a master bias frame from raw images. |
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Create a master dark frame from raw images. |
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Create a master flat frame from raw images. |
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Create a master illumination flat from raw images. |
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Create master pixel response flat(s) from raw images. |
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Combines the images of a displayed image set to create a master image. |
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Use this command to combine files for making high signal-to-noise ratio master images without first opening the images. This is useful for calibration and other purposes. |
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Create a pixel mask file by scanning an image showing point defects, such as a dark frame. |
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Create or edit a list containing descriptions of isolated point defects. This is not available from the Calibration Toolbar. |
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Apples a pixel mask to correct point artefacts in an image. |
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Create or edit a list containing descriptions of defects in points, lines, and regions. This is not available from the Calibration Toolbar. |
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Apples a blemish mask to correct extended artefacts in an image. |
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Opens a toolbar with commands for interactvely removing pointdefects from images. |
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Removes artifacts from an image set by comparing statistical properties of the images. |
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Changes the values of image header keywords, adds new keywords and values, or renames keywords already present in the image header. |
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Crops an image using the region description stored in a trim section type keyword in the image header. |
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In addition, aperture photometry can be used to calibrate the photometric zero point of an image but is not strictly a tool for intensity calibration.
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Calibrate the image brightness to a photometric magnitude system using standard stars. |
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Calibrate the spatial coordinates as a high-precision equatorial system using astrometric standard stars. |
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Calibrate the spatial coordinates in units of arcseconds. |
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Calibrate the spatial coordinates as a low-precision equatorial system by specifying the field center and scale. |
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Calibrate the spatial coordinates in linear units such as mm, microns, or km, with equal scale in both dimensions. |
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Calibrate the spatial coordinates in linear units such as mm, microns, or km, with unequal scale in both dimensions. Also see the Set X-Axis Linear Format and Set Y-Axis Linear Format commands. |
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Assign coordinate units to different linear coordinates used in each axis. For example, the X axis may use hertz, and the Y axis may use km. This command is often used to change units in combination with the Set Independent Linear Scales command. |
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Assign quantitative units for the pixel value, such as ergs/s/cm2 or jansky. This is not available from the Calibration Toolbar. |
The Create Synthetic Image command may be used, with or without artificial stars, to evaluate calibration techniques and to explore the propagation of noise through a calibration or processing strategy.
Contents, Processing Images, Pixel Coordinate Definition, Subpixel Coordinate Definition, Tutorial: Introduction to Astrometric Calibration