Image Types


The Image Calibration process involves using 4 basic types of image data in the "raw", or uncalibrated state. The image type is usually stored in the IMAGETYP keyword in the FITS image header. Mira uses the image type when collecting and assembling raw source files into the master calibration frames.

The standard source file types are defined as follows:

How do you instruct Mira which source file types to use for each of the methods in a procedure? On the Image Calibration dialog, the Source Files tab has 4 tab pages for each of the kinds of frames that Mira may need to use. On each page you give Mira instructions on how to select which files to use. Although the IMAGETYP keyword is normally used to specify the image type, Mira allows you to enter any keyword. In particular, OBSTYPE was once the keyword used to identify image types but was replaced to the currently favored IMAGETYP name. In older image data you may find the OBSTYPE keyword instead of IMAGETYP.

The IMAGETYP keyword uses commonly accepted values for the raw images (or "frames") produced by the camera. These values are used to identify the images being used to create and apply calibrations to the data. After Mira creates the master calibration frames—master bias, master dark, and master flat—the images are no longer in their raw state, so Mira provides an option for the image type to be prefixed with the word Master. This is controlled by the Change Master Frame Image Type option is in the The Processing Properties dialog (open using the [Setup] button on the Image Calibration dialog). If this option is checked, Mira modifies the IMAGETYP keyword by setting it to 'Master xxxx', where "xxxx" is whatever IMAGETYP value was used for the raw frame used in making the master frame. For example, if the raw image had IMAGETYP = 'dark' then the master frame would be changed to IMAGETYP = 'Master dark'.

The normal IMAGETYP values are summarized in the table below. However, your images can use other values of the IMAGETYP or even a keyword other than IMAGETYP to specify the Image Type. The settings are configured on the pages under the Source Files tab.

Image Type Properties

Image Type

Typical IMAGETYP Keyword Values

Raw data frame

'data', 'normal', and 'light' are commonly used.

Raw Bias frame

'Bias' and 'zero' are commonly used.

Raw dark frame

'dark'

Raw flat field frame

'flat'

Master Bias Frame

'Master Bias', 'Bias', 'Master zero', etc.

Master Flat Frame

'Master dark' or 'dark'

Master Flat Field Frame

'Master flat' or 'flat'

Related Topics

Image Calibration, Source Files tab