The reconstruction of an object needs a set of target and psf images. At least one psf image is needed for each target image. The exact configuration (e.g. one image for each position angle or a summed up image) depends on the algorithm used for the reconstruction. The target preprocessing needs the part of the preprocessed data which contains the target and some additional information (e.g. sky background) as an input. Normally, the following steps are necessary:

  1. Find and define the area of the target in the preprocessed data.
  2. Specify the form and level of the sky background if it was not removed during the calibration step.

In figure 1, an image window shows the target in the center, two psf stars near the corners, and an image probe in the upper left corner. The image probe is used to estimate the sky background in the image. Alternatively, an image probe in a sky image can be used.

Preprocessed data
Figure 1: Preprocessed interferogram with a target object, two psf stars and an image probe.

Target Tool

The purpose of the target tool (see section Tools) is to specifiy the target area in at least the first image of each preprocessed data file. This target area is used to cut out the part of an image later used for reconstruction. The target definition is the major input for the Target Preprocessing step.

Mouse buttonFunction
Left clickCreates a new target object centered at the mouse position.
Middle clickShows a dialog window with the details of the selected target object (figure 2).
Right clickRemoves the target object at the selected position.
Middle dragMoves the selected target object to a different position.
Control left clickEnlarges the selected target object by a factor of 2.
Control right clickShrinks the selected target object by a factor of 2.
Shift left clickZooms into the image, and the click position is used as the new image center (from view tool).
Shift middle clickUses the click position as the new image center (from view tool).
Shift right clickZooms out of the image, and the click position is used as the new image center (from view tool).
Shift middle dragMoves the image (from view tool).
Table 1: Target tool mouse button assignment

In the preprocessed data, exactly one target area must be defined. The graphical representation is shown in figure 2. When selecting the graphic object with the middle mouse button (target tool) or the left mouse button (info tool), a dialog box containing all parameters for that target (figure 2 right panel) appears.

Target objectTarget infos
Figure 2: Graphical representation and dialog window of a target.

The target dialog box contains the following information:

  1. The label (automatically generated).
  2. The position and size of the target area. The position gives the coordinates of the lower left corner. It is possible to recenter the target area using one of the available methods.
  3. The sky background subtracted from each extracted target is specified by one or four parameters.

The sky background must be defined even if it should be kept during the target extraction, because some algorithms require the background level to begin. The overall shape and the respective parameters must be specified in the dialog window.

Target Extraction

The target preprocessing is executed like the calibration or preprocessing step. The algorithm-specific dialog window is presented in figure 3; in figure 4, an extracted target is shown.

Dialog for target preprocessing
Figure 3: The algorithm-specific target creation setup dialog.

The setup dialog box contains the following elements:

  1. Depending on the algorithm used for the reconstruction, different kinds of target images are generated:
  2. Check if the sky background should be removed.
  3. Check if a fourier mask should be used. This mask sets all values outside the transfer function to zero.
  4. Check if the target should be recentered using the fourier phase.


Whether or not you are allowed to subtract the sky background depends on the algorithm used in the reconstruction. For example, in the Richardson-Lucy deconvolution algorithm, the target image must contain the sky background. In this case, the sky background must be kept and subtracted later during the psf preprocessing step.

Target image
Figure 4: The target image created from the target preprocessing step.

Additional information: