Combine Images

The Combine Images command combines the images in an Image Set into a single new image. Applications include stacking images after registration, creating master calibration frames, and creating high signal-to-noise ratio images. This command is not used to merge color channels into an RGB image.

Combine Images uses an Image Set displayed in an Image Window. The result is an output image created in a new window. Several statistical methods are available for combining the images and for adjusting ("normalizing") them to a common reference value (see Image Combining Methods and Region Statistics Methods). The settings are saved as a Profile.

Preferences

These settings appear when the dialog is in the "dropped", or open position.

Method

This list box selects the type of processing used to combine the image values. These are described in Image Combining Methods. Depending upon the method selected, extra fields may appear for method-specific parameters, like High sigma.

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 Stat Preferences 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.

Data Type

Select the Data 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 Data type of the output image to 32 bit real.

Other Options

These are miscellaneous settings grouped together to conserve screen space:

  • Adjust Exptime updates 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.

  • Verbose controls whether detailed messages are listed during the processing.

  • Scale Result to 1.0 normalizes the combined image to a reference value of 1.0. The Normalization settings are used to compute and apply the scaling. Use this setting when using the command to create a flat field correction frame. See the Apply Flat Field method.

Normalizationn

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.

Related Topics

Math Commands, Image Set, Image Combining Methods, Region Statistics Methods, Region Stat Preferences, Register Images, Drop Dialogs, Profiles