Introduction

Before extracting a psf from the preprocessed images, it is necessary to mark at least one psf star 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 which contains 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.

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

PSF Star Tool

The purpose of the psf star tool (see section Tools) is the specification of 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 which is later used for the reconstruction. The psf star definition is the major input for the Psf Preprocessing step.

Mouse buttonFunction
Left clickCreates a new psf star centered at the mouse position.
Middle clickShows a dialog window with the details of the selected psf star (figure 2).
Right clickRemoves the psf star at the selected position.
Middle dragMoves the selected psf star to a different position.
Control left clickEnlarges the selected psf star by a factor of 2.
Control right clickShrinks the selected psf star 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: Psf star tool mouse button assignment

In the preprocessed data, at least one psf star has to be defined. If more stars are used as psf stars, 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 is shown which contains all parameters for that psf star (figure 2, right panel)

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

The psf star dialog box contains the following elements:

  1. The label is 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 which is subtracted from each extracted psf is 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 should be kept during the psf extraction, because some algorithms must know the background level. The overall shape and the respective parameters must be specified in the dialog window.

Halo Tool

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

Mouse buttonFunction
Left clickCreates a new target halo centered at the mouse position.
Middle clickShows a dialog window with the details of the selected target halo (figure 3).
Right clickRemoves the target halo at the selected position.
Middle dragMoves the selected target halo to a different position.
Control left clickEnlarges the selected target halo by a factor of 2.
Control right clickShrinks the selected target halo 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 2: Halo tool mouse button assignment

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

Target halo objectTarget halo infos
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 which is 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 which is 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. Therefore, the sky background at the halo position must be defined. Recentering the halo uses an approach where only the pixels between the two dashed circles are used to calculate the intensity mean.

Psf Extraction

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.

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

Additional information: