Combine Files

The Combine Files command combines any number of images residing in files. The files to combine are specified using a file list. In comparison, the Combine Image Set command combines images in an Image Set that have been displayed. The Combine Files command is that it does not require all images to be loaded into memory; instead, this command can combine any number of images. Several statistical methods are available for combining the images and for adjusting ("normalizing") them to a common reference value (see Statistical Estimators for Image Combining). Normalization is used to adjust the images to a common statistical value when using any combining method that rejects bad data such as the mean, rank, and alpha methods. Statistical methods for computing the normalization values are discussed in Region Statistics Estimators.

The Combine Files command opens from the button on the Image Calibration toolbar and the global Calibration menu using the button on the Main Toolbar.

Combine Files Properties
Profile [\|>]	Selects a parameter collection for display in the dialog.
File List	The name of the Mira File List containing the names of files to combine. All images must be in FITS format.
Method	This list box selects the type of processing used to combine the image values. These are described in Statistical Estimators for Image Combining. If the combining method has parameters, such as high and low sign clipping limits, they are shown in fields to the right of the Method selection.
Normalization	This control group specifies whether the images are adjusted (normalized) to a common statistical value before being combined. If normalization is not set to "none", then the button opens the Region Statistics Estimators dialog. In this dialog you specify how the normalization is to be computed, including the target statistic and the region over which it is calculated. See the discussion below.
Pixel Type	Select the Pixel Type so that it can accommodate the increased signal or dynamic range of the combined images. For example, if combining a large number of 16-bit integer type images it is usually prudent to set the Pixel Type of the output image to 32-bit real.
Adjust Exposure time	Check this box to update the exposure time in the output image so that it is characteristic of the exposure time of an equivalent single image. This can be used to maintain the image flux, or the value of signal per unit time. This option also updates the mid-time of the exposure based on the weighted combination of the mid-times of all the combined images. The mid-time is saved in the keywords DATE-MID and TIME-MID, which can be used by the photometry tools.
Scale Result to 1.0	Check this box to normalize the combined image to a reference statistical value of 1.0. The Normalization settings are used to compute and apply the scaling. Use this setting, for example, when creating a flat field correction frame manually, rather than by using the Create Master Flat command.
Verbose listing of results	Check this box to list detailed messages about the combining process.

Normalization

Normalization is used to adjust the images to a common statistical value when using any combining method that rejects bad data. This is essential when using a combining method that performs rejection of bad data, such as the Sigma Clipping, Min/Max or Median methods.

Choose the normalization operation, Scale, Offset, or None, according to the nature of the difference between the images being combined.

If the pixel values differ because of an additive offset, like variations in the CCD bias offset, select Offset.
If the images differ because of a scale factor, as caused by differences in illumination or exposure time, select Scale.
The None setting disables normalization.

Results

The Mira Text Editor below shows the results for normalizing the 5 BL-CAM*.fts images supplied with Mira. This example used the "Offset" normalization method and the settings shown in the Region Statistics dialog below. For the present function, the Region Statistics dialog offers only a Relative Region option for computing the statistic. This happens because the images sizes are not generally know before running the command. In comparison, the general Region Statistics dialog allows both Relative and Absolute region options.

Normalization

Results

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