Telescope Basics 29
subjects photographically you need to set your shutter speed accordingly. Most 35mm single lens reflex
(SLR) cameras offer through-the-lens metering which lets you know if your picture is under or overexposed.
This is more of a consideration when doing terrestrial photography, where exposure times are measured in
fractions of a second. This also applies to lunar photography and filtered solar photography. In deep-sky
astrophotography, the exposures are much longer, requiring that you use the ‘B’ setting on your camera.
The actual exposure time is determined by how long you keep the shutter open.
To reduce vibration when tripping the shutter, use a cable release. Releasing the shutter manually can cause
vibration, which produces blurred photos. A cable release allows you to keep your hands clear of the
camera and telescope, thus reducing the possibility of shaking the telescope. Mechanical cable releases can
be used, though air type releases are best.
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