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:

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:

Intensity 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.

Commands for Intensity Calibration

Express Image Calibration

Apply bias, dark, flat, and cosmetic corrections to images.

Create Master Bias

Create a master bias frame from raw images.

Create Master Dark

Create a master dark frame from raw images.

Create Master Flat

Create a master flat frame from raw images.

Create Illumination Flat

Create a master illumination flat from raw images.

Create Pixel Flat

Create master pixel response flat(s) from raw images.

Combine Image Set

Combines the images of a displayed image set to create a master image.

Combine Files

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.

 

 

Create Pixel Mask

Create a pixel mask file by scanning an image showing point defects, such as a dark frame.

Edit Pixel Mask

Create or edit a list containing descriptions of isolated point defects. This is not available from the Calibration Toolbar.

Apply Pixel Mask

Apples a pixel mask to correct point artefacts in an image.

 

 

Edit Blemish Mask

Create or edit a list containing descriptions of defects in points, lines, and regions. This is not available from the Calibration Toolbar.

Apply Blemish Mask

Apples a blemish mask to correct extended artefacts in an image.

 

 

Interactive Repair

Opens a toolbar with commands for interactvely removing pointdefects from images.

Clean Image Set

Removes artifacts from an image set by comparing statistical properties of the images.

 

 

Modify Keywords

Changes the values of image header keywords, adds new keywords and values, or renames keywords already present in the image header.

Trim Image Section

Crops an image using the region description stored in a trim section type keyword in the image header.

Photometric Zero Point Calibration

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.

Aperture Photometry

Calibrate the image brightness to a photometric magnitude system using standard stars.

Spatial Calibration

Astrometric Calibration

Calibrate the spatial coordinates as a high-precision equatorial system using astrometric standard stars.

Set Arcsecond Scale

Calibrate the spatial coordinates in units of arcseconds.

Set Equatorial Scale

Calibrate the spatial coordinates as a low-precision equatorial system by specifying the field center and scale.

Set Linear Scale

Calibrate the spatial coordinates in linear units such as mm, microns, or km, with equal scale in both dimensions.

Set Independent Linear Scales

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.

Set Linear Coordinate Units

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.

Set Brightness Units

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.

Related Topics

Contents, Processing Images, Pixel Coordinate Definition, Subpixel Coordinate Definition, Tutorial: Introduction to Astrometric Calibration