The key to successful deconvolution is obtaining a good point-spread function (PSF). The PSF is a model of how the image was blurred. Deconvolution uses this information to remove the blur from the image. Given the importance of obtaining a good PSF, a number of tools are supplied for this purpose.
Three types of PSF are available. Gaussian is commonly used for removing blur caused by atmospheric seeing. Exponential is often used to model blur from optical systems. From Image allows you to extract a point-like object (such as a star) from the image to use directly as the PSF; this is especially useful when the blur is not symmetrical. All three types of PSF are represented internally as a small image of 64x64 pixels, and the From File option provides for loading such a PSF from a file.
The Function Type must be set up first. From Image can be used when a clean star or other point source is available in the image, and is recommended if there is trailing (resulting from poor guiding) in the image. Otherwise, the Gaussian or Exponential models should be used.
If From Image is selected, a star must be extracted from the image (see below). This extracted star image is used directly as the point-spread function. If the background behind the star is very noisy, the Clean Up function can be used to remove this noise. Use the Clean Up option only when the background around the PSF is very noisy, as it can lead to "donuts" around stars.
When using Gaussian or Exponential point-spread function models, the PSF Radius (i.e. the standard deviation of the distribution) must be determined. A point-like object (star) can be extracted from the image to initialize the model. The selected distribution is fitted to the object in order to calculate the best-fit radius. If no suitable object is available to extract, you can estimate the required PSF Radius by inspecting the image.
Whether initializing either the Gaussian or Exponential modes, or extracting a point-like object to use directly as the PSF, a means of extracting such an object from the image is required. The Automatic Select button runs an algorithm which attempts to locate a point-like object having a high signal-to-noise ratio, and when found marks it with a red circle; this is particularly suitable for astronomical images containing stars. Alternatively, the Select from Image button allows you to select a specific object. When this button is pressed, use the mouse to click on a suitable point source in the image and mark it as the source of the PSF. Both methods display the coordinates in the X and Y field, which you can change directly if desired. The Select from Image button pops up automatically when you select a new PSF.
You can obtain the Point Spread Function from any open image, although normally you will want to take it from the image you are about to deconvolve. To capture it from a different one, click on the title bar or image tab of the desired image before clicking Automatic Select or Select from Image.
When the From File option is selected, you can click the Select File button to bring up the Open dialog. MaxIm DL will take the brightest pixel in the file as the center of the PSF. The resulting PSF is cropped or expanded to 64x64 pixels if necessary.
Once a PSF has been created, you can view it in its own window by clicking on the Show PSF button. An image buffer will be created containing the PSF image, which you can save to a file for later reuse with the From File option. The window is a copy of the PSF and not the PSF itself; that is, if you filter or process the image in any way, or close its window, you will not affect the current PSF. Show PSF is a latching button and remains depressed as long as the PSF window exists. Clicking the Show PSF button again (causing it to pop back up to the raised state) causes the PSF image window to be closed.
Point-spread functions can be sampled from color images, but are always themselves monochrome. When conversion is necessary, it is done automatically.