The ef wavefront reduction package. -- 2.4 Loading the extra-focal images.

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2.4 Loading the extra-focal images.

The aim of this step should be to load 2 extra-focal images, one from each side of focus. The images should be stored in buffers 1 and 2. By convention the pre-focus (before focus) image should be stored in buffer 1 and the post focus image in buffer 2.

The file open dialog box, used to select image files for the dark and flat field buffers, as well as the two extra-focal image buffers.

To invoke the sub-window for loading images: from the bottom toolbar, hit the open button, or select the File :: Open menu option. This will display a file selection dialog.

This dialog box does not perform a focus grab so may be left displayed if desired. You use this box to load both extra-focal images, and optionally a flat field and dark frame. The upper listbox in the left side is used to select a directory, the lower listbox is used to select a file. To change directories, single click on the desired directory name in the upper listbox . In order to load the example images you should change directory to the example subdirectory of the ef documentation, either by using the directory listbox, or by typing directly into the directory entry. You should see two files pre.fits and post.fits displayed in the lower listbox, when you reach the example sub-directory. Use the radio-buttons on the right hand side of the dialog box to select the destination for the image, then double click on the name of the file you wish to load in the lower listbox. For the example images, you do not need a flat field or a dark image, so simply select destination Image 1 then double-click on pre.fits, similarly select Image 2 and double click on post.fits.

After doing this you should see annular images in each of the two display windows .

After loading two extra-focal images, you should see one in each display window.

2.4.1 Setting the image focus distance.

The program needs to determine the focus distance of each extra-focal image in order to run the wavefront generation algorithm. The example images have the focus value contained in their FITS headers, and the program reads this automatically. To verify that this has happened correctly select the Edit :: Image Parameters menu item. You should see an Image parameters form on your screen. In this case the focus parameters should be -1 and 1 respectively, if for some reason these values were incorrect they can be changed at this point. For convenience, it is recommended that you arrange for the focus value to be recorded in the FITS header. If you need to reduce many sets of data without an appropriate FITS header entry, you may lock a focus value in the Image edit form by clicking on the padlock icon next to the focus value.

The image parameter editor allows one to change the focus distance, and image center coordinates associated with an image. The focus value for an image may be locked across image loads by using the lock buttons in the focus edit column.

By default the program tries to automatically find the center of each extra-focal image. Provided the image is a simple extra-focal image, this centroiding should be successful. Failure of the centroiding, often indicates that there is a problem with the data which would interfere with obtaining accurate wavefronts. If the expected size of the extra-focal image can be computed when the image is loaded, the program tends to do a better job of image centering. If you wish to over-ride these automatic centroid values you may do so by explicitly setting the x and y center in Image Parameters form. The program also reports the estimated radius of the extra-focal images, this number is provided for user feedback only and is not used in the wavefront computation.

Laplacian Optics Inc. Email: laplace@laplacian.com