If you're using a computer to control your DSLR, the computer can also help you focus. Many software packages for DSLR control include the ability to take short exposures over and over, download them, and display each image immediately together with an analysis of its sharpness. We always focus on stars, which are perfect point sources, and the computer can analyze exactly how compact each image is.
Figure 8.4 shows a typical star image analysis from MaxDSLR, which includes focusing among its many other functions. A sharply focused star is a tall, narrow peak and has a small FWHM ("full-width-half-maximum," the diameter of the portion of the image that is at least 50% as bright as the central peak).
Many other software packages provide the same function. One of the best known and most elaborate is DSLR Focus (www.dslrfocus.com, Figure 8.5), which has gradually evolved into an all-purpose camera-control program.
If the telescope has an electric focuser, the computer can even adjust the focus for you. MaxDSLR, ImagesPlus, and DSLR Focus all support automatic focusing with most computerized telescopes.
The autofocus system inside the DSLR is not useful for astronomy. It usually does not respond to stars at all.
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