Image Calibration (Calibrate Images command)

The Calibrate Images command uses the Image Calibration dialog to perform quantitative image processing operations on CCD images. Most of the functions are related to the process of Luminance Calibration (see Calibrating Images).

Using this versatile command you can perform operations as simple as creating a master bias frame or repairing CCD column traps, or as complex as creating and applying all the standard CCD calibration steps beginning with a folder of raw images. When used to its fullest potential, this command can automatically "pipeline" the many detailed calibration steps required to calibrate CCD images.

The Calibrate Images command works through the Image Calibration dialog shown below. This dialog organizes the layers of a complex image calibration process into a procedure that applies detailed processing methods to displayed images or source files. For less complex procedures, the Express Calibration command can also be used to apply the basic calibration steps to a set of displayed images.

Note that all images processed through one run of the pipeline must have the same dimensions (rows x columns). Therefore, you should cull images of different dimensions into separate folders before running the pipeline on them.

Highlights of Image Calibration


The Image Calibration dialog organizes a complex hierarchy of operations used to calibrate images: At the top level is the procedure that defines which operations need to be applied. The procedure then uses various methods to perform needed steps of the procedure. For example, applying bias and dark calibrations is a procedure. Creating a bias frame and a dark frame and applying them to the data are the methods that achieve the steps of the procedure. The procedure is applied to images that may reside in a display window or in a folder; when in a folder the particular source files must be selected from among many files of mixed types. These three aspects of the calibration process are organized under 3 tabs in the Image Calibration dialog, these being Procedure , Methods , and Source Files.

How Auto-Calibration Works

There are both simplistic ways and sophisticated ways to make master bias, dark, and flat field frames and then apply them to images. Especially if you are starting with only raw CCD images, to do all this correctly guarantees a huge amount of bookkeeping and processing steps ahead. The image calibration interface in Mira was designed to meet such complex requirements in a way as straightforward as possible to the user, yet maintain maximum flexibility to meet different needs for different types of data.

To understand how the calibration dialog works, consider the following need: "I need to correct my images for Dark Current." To do this makes a requirement: You must have a dark calibration frame. But this seemingly simple procedure may unfold into a long sequence of operations that must be performed. For example:

To recapitulate: To understand how the auto-calibration logic works in Mira , just follow your own thought process: How would you do it manually? The questions you need to answer are 1) Which steps do I need to perform? 2) Which methods do I use? and 3) Which files or images should be used?

Organization of Calibration Tasks

The Image Calibration dialog organizes calibration tasks into 3 sections:

Data Frames vs Calibration Frames

Images from array detectors like CCD's are usually referred to as "frames". This has an historical basis in the use of television technology used both for acquiring and displaying 2-D images prior to the invention of the CCD. Raw image data to be quantitatively calibrated will contain calibration frames for correcting for the detector bias, dark, and flat field effects. Mira , or any other auto-calibration software, must be able to distinguish these different types of images in order to apply them in the proper way. Images can auto-identify themselves using the Image Type concept. The image type, or other method for identification, is entered on the Source Files tab of the Image Calibration dialog.

When calibrating images in Mira, the data frames may be opened from a display window or from files inside a folder. Master calibration images are usually opened from files, but pre-existing master bias, dark, and flat frames may also be selected from displayed images. When multiple master darks or master flats are used to accommodate different exposures or filters, a single calibration frame does not suffice, so you must specify the master files using a file name template (for example, 'master dark*.fts') or by using the resulting master frames that were created in the same procedure.


If the procedure opens a master calibration frame rather than creating it, then the master frame must be prepared to the proper degree. For example, a master flat opened from a file should be bias subtracted, dark corrected, normalized to 1.0, and stored as a 32-bit real type image.

Saving Processing Parameters as a Profile

The calibration pipeline uses almost 500 settings to control all its options. Managing this configuration could be a nightmare without good organization and an easy way to save and retrieve settings. That is why this dialog uses a Profile Control to do the work for you. When you get all the preferences set as you want them, save them as a profile. You can have as many profiles as you want. Just select the profile from the list and all of the preferences are loaded for you.

Processing Images or Files

The target of the image calibration procedure can be images displayed in an Image Window or files in a folder. Specify your choice by clicking the appropriate bullet on the Image Calibration dialog:

Logging of Processing Messages

If the Verbose option is checked in the Processing Preferences dialog, Miralogs processing messages in a Text Editor window. The messages appear after the calibration task finishes. The log entry also lists the clock time at which each image was processed (see the sample below). If a calibration error occurs, Mira stops the procedure and lists the error message in the log. Since the calibration log uses a Mira Text Editor, you can save, edit, or print the messages in the normal way.

Compare the detailed messages above with the more streamlined messages of the Express Calibration command.

Related Topics

Procedure tab, Methods tab, Source Files tab, Processing Preferences, Image Types