Before extracting a psf from the preprocessed images, at least one psf star must be marked in the first image of each file. In the provided example, two psf stars are already marked and ready to use. The psf preprocessing is executed like the target preprocessing step. The psf preprocessing needs the part of the preprocessed data containing at least one psf star and some additional information (e.g. sky background) as an input. Normally, the following steps are necessary:

1. Find and define at least one psf star in the preprocessed data.
2. Specify the form and level of the sky background if it was not removed during the calibration step.
3. If a psf star is influenced by an extended target, a target halo must be specified.

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.

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

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

In the preprocessed data, at least one psf star must be defined. If more stars are used as psf stars, either the quality of the extracted psf will increase or a space variant psf can be used for deconvolution. When selecting the graphic object with the middle mouse button (psf star tool) or the left mouse button (info tool), a dialog box appears which contains all parameters for that psf star (figure 2, right panel)

Figure 2: Graphical representation and dialog window of a psf star.

The psf star dialog box contains the following elements:

1. The label (automatically generated).
2. The position and size of the psf star. The position gives the center of the psf. It is possible to recenter the psf star using one of the available methods.
3. The sky background subtracted from each extracted psf (specified by one or four parameters).
4. If the psf star is influenced by the target halo, the type of compensation is specified here.

The sky background must be defined even if it will be kept during the psf extraction, because some algorithms need the background level. The overall shape and the respective parameters must be specified in the dialog window.

Sometimes a bright or extended target shows an extended halo, which can influence one or more psf stars. The halo must be removed from these psfs. The basis for this compensation is the target halo. It defines an area of the preprocessed image where the target halo is smooth and symmetric and can therefore be removed. The target halo tool (see section Tools) allows the creation and modification of such a target halo.

In the preprocessed data, only one target halo can be defined and used. The graphical representation is shown in figure 3. When the graphic object is selected with the middle mouse button (halo tool) or the left mouse button (info tool), a dialog box containing all parameters for that target appears (figure 3, right panel).

Figure 3: Graphical representation and dialog window of a target halo.

The halo dialog box contains the following values:

1. The label is automatically generated.
2. The position of the target halo. The position gives the center of the halo and not the center of the target. It is possible to recenter the halo.
3. The sky background subtracted from the halo is specified by one or four parameters.
4. The inner and outer radii of the ring define the part of the halo used for recentering.
5. The inner and outer limits define the part of the image where the halo is sufficiently symmetric and smooth.
6. The angle defines a sector of the halo where no background stars influence the halo smoothness.

Because the sky background at the halo position can differ from the background at the psf star position, it is only necessary to compensate for the halo component. The sky background at the halo position must therefore be defined. Recentering the halo uses an approach in which only the pixels between the two dashed circles are used to calculate the intensity mean.

The psf preprocessing is carried out like the target preprocessing step. The algorithm-specific dialog window is presented in figure 4; an extracted psf is shown in figure 5.

Figure 4: The algorithm-specific psf creation setup dialog.

The setup dialog box contains the following elements:

1. Depending in the algorithm used for the reconstruction, different kinds of psf images are generated:
   • One: All psf images of all files and all position angles are summed up into exactly one psf image.
   • File: For each preprocessed file, a psf image is generated.
   • Image: For each image in each preprocessed file, a psf image is generated.
   • Object: For each psf star in each image in each file, a psf image is generated. This allows a reconstruction with a space-variant psf.
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 psf should be recentered using the fourier phase.

Figure 5: The psf image created from the psf preprocessing step.