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:
Bias Frames. These are zero length exposures used to remove a "fixed" electronic offset from images.
Dark Frames. These are images that record the thermal signal (or dark current) across the surface of the imaging device. A raw dark frame usually contains bias.
Flat Frames. These are images that record the sensitivity variations across the surface of the image. These irregularities may be caused by pixel to pixel variations in sensitivity as well as vignetting and non-uniform illumination of the field of view onto the imager. Some of these effects are dependent upon the spectrum of light, so the flat field calibration is color filter dependent. A raw flat frame usually includes the effects of bias and dark current.
Data Frames. Data frames may be opened from displayed images or files.
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 and Mira modifies the IMAGETYP keyword by setting it to 'Master Bias', 'Master Dark', and 'Master Flat'. The image type is summarized in the table below.
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Image Type Definitions |
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Image Type |
IMAGETYP Keyword Value |
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Raw data frame |
'data', 'normal', and 'light' are commonly used values. |
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Raw bias frame |
'bias' |
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Raw dark frame |
'dark' |
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Raw flat field frame |
'flat' |
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Master Bias Frame |
'Master Bias' |
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Master Dark Frame |
'Master Dark' |
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Master Flat Field Frame |
'Master Flat' |