Aligning a Computerized Telescope for Imaging
Note: For specific instructions on aligning various models of computerized telescopes, see the Aligning a Computerized Telescope section of theTelescope Basics site. Below are general recommendations for aligning for CCD imaging.
Most computerized or goto telescopes need to be aligned after the CCD camera is in place. This is due to the fact that a telescope must berebalanced when the camera is added and this cannot be done without disturbing the alignment of the scope. (A rare exception to this is Celestron's Ultima 2000 telescope which can be moved by hand without affecting the alignment.) The alignment procedure is basically the same as it would be for visual use. One difference if you are using a goto Schmidt-Cassegrain telescope is that the scope will be mounted equatorially (using a wedge) for CCD imaging instead of the standard altitude-azimuth (up-down/left-right) arrangement. This requires a polar alignment, and changing the tracking mode in the telescope's computer to "Equatorial" instead of "Alt-Az". Otherwise, a standard two-star alignment is all that is necessary.
Note: Some polar-aligned telescopes only require one alignment star to synchronize the computer to the sky. However, some telescopes still benefit from a two-star alignment to increase pointing accuracy. Pointing accuracy is critical with a CCD camera since the field of view is usually fairly small.
Except for the fact that you cannot look through an eyepiece, the alignment procedure with a CCD is the same as it is for visual observing. Proceed as you normally would, centering the first alignment star in the finderscope.
Use the CCD control software's focus mode to display an image of the alignment star. Make sure the focus mode is set to display continuous images and that the update time is as short as possible. Since you will be aligning on a fairly bright star, exposure times can be very short - usually less than half a second is sufficient. Setting the CCD to low-resolution mode also helps (some software does this automatically in focus mode). Low resolution mode bins the pixels to create a smaller image which is downloaded much faster than a high-resolution image, thus shortening the waiting time between successive exposures.
Above: A short exposure showing the alignment star off center.
Use the telescope's hand controller (set to the same speed you would use to center stars visually) to move the alignment star into the center of the image. The orientation will be different with a CCD camera than with an eyepiece, so the buttons may not correspond to the directions you are used to. A little trial and error will likely be necessary.
Above: The alignment star properly centered.
Since most CCDs provide small fields of view, approximating the center of the field by eye is sufficient. A large CCD chip on a short focal length scope provides a wider field of view, making the centering a bit less precise, but there is also a lot more room for error when pointing to a target with such a big field.