Digital single lens reflex cameras (DSLRs) are more capable and versatile than their compact cousins, generally offering higher resolution images, capable of taking time-exposure photographs, and offering a greater range of fine adjustments. The DSLR camera body is attached to the telescope using a T-mount adapter, effectively transforming the telescope into a large telephoto lens (Figure 2.38). Using prime focus, a telescope with a focal length of under 2,000 mm will project the entire half-degree-wide lunar disc onto the camera's CCD chip. Some DSLRs, such as the Olympus E-300 (of which this author has considerable experience), don't offer a live preview on their LCD view screens, so focusing needs to be performed by squinting through the camera's viewfinder. This may be easy enough for the Moon, but not so easy for dimmer objects. Here's where a full-aperture focusing mask comes in handy, using it to merge the image of a bright star to produce optimum focus.
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