Version 1.4 rev 23 Oct 2017
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When visually observing, larger telescopes are better than smaller because big apertures collect and focus more
light at the eyepiece. For CCD imaging, however, focal ratio has a greater impact because the telescope’s focal ratio
determines how much light is accumulated by the CCD chip during the exposure. For example, telescopes with shorter
focal ratios will capture faint extended subjects, like nebulae and galaxies, faster than longer focal ratio instruments.
Incredibly, a camera with a typical 200mm lens, and an f-ratio of 2.3, will capture more of the Andromeda galaxy than the
200-inch Hale Telescope, operating at f3.3, during the same exposure time.
However, even though a modest 200mm lens out speeds one of the world’s most famous telescopes, the scale of the
two images will be vastly different- the Hale image will be about 85 times greater than the picture produced with a
simple camera lens because the Hale focal length is approximately 17,000mm.
So, image scale is a function of local length while imaging speed is predicated on focal ratio. The focal length of a 4-inch
f/8 refractor is the same as an 8-inch reflector operating at f/4 thus they will both produce images at the same scale.
However, the larger instrument will be faster than the smaller by a factor of 4.
Since the image scale of a longer focal length telescope is greater, it will capture a smaller area of the sky than a scope
with a smaller focal length when using the same CCD sensor size. As a result, long focal length telescopes are typically
better for imaging small scale subjects such as planetary nebulae, galaxies and planets. Small focal length telescopes,
particularly if they also have a short focal ratio, tend to excel at capturing distended object such as nebulae.
Of course, the field of view for any telescope can be expanded if a CCD imaging sensor with a larger physical surface
area is used.
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There is a wealth of information and technical notes covering all aspects of CCD, imaging and spectroscopy technology
at Andor.com.
