Rather than the afocal method through the telescope, a wider field can be realized with a digital camera by using a telephoto conversion lens. These lenses screw into the filter adapters of many digital camera models. The camera is then mounted piggyback on the telescope as shown in Figure 5.3.
Most of these lenses increase the focal length of the fixed camera lens by a factor of two. A typical digital camera lens has a focal length of around 24 mm when zoomed to 3x. A telephoto conversion lens will produce an effective focal length of 48 mm. With a 7.2 x 5.3 mm detector the resulting field width is 8.6°. This is equivalent to a 232-mm telephoto lens on a 35-mm film camera. The limiting photographic magnitude for a 30-second exposure with these conversion lenses
is about eight. They are good for taking wide field photographs of constellations and producing a personal photographic star atlas.
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