Field of view

Astronomers measure apparent distances in the sky in degrees (Figure 5.10); a degree is divided into 60 arc-minutes (60'), and an arc-minute is divided into 60 arc-seconds (60'').

The exact formula for field of view is:

2 x Focal length

Figure 5.9. The star clusters M35 and NGC 2158 (smaller), imaged at the prime focus of a 14-cm (5.5-inch) f /7 TEC apochromatic refractor. Stack of three 5-minute exposures at ISO 800 with a filter-modified Nikon D50 through a Baader UV/IR-blocking filter, using a Losmandy equatorial mount and an autoguider on a separate guidescope. (William J. Shaheen.)

Moon

Observer

Moon

Observer

Figure 5.10. The apparent size of objects in the sky is measured as an angle. (From

Astro-photography for the Amateur.)

Figure 5.10. The apparent size of objects in the sky is measured as an angle. (From

Astro-photography for the Amateur.)

For focal lengths much longer than the sensor size, such as telescopes, a much simpler formula gives almost exactly the same result:

Sensor width (or height, etc.) Focal length

Figure 5.11 shows the field of view of a Canon or Nikon APS-C sensor (Digital Rebel family, Nikon D70 family, and the like) with various focal lengths, superimposed on an image of the Pleiades star cluster. For the same concept applied to telephoto lenses, see p. 71, and for more about field of view, see Astrophotography for the Amateur (1999), pp. 73-75.

A

• •

*

m

<

I

• •

• 2000 mm

1250 mm

Figure 5.11. Field of view of an APS-C-size sensor with various focal lengths, relative to the Pleiades star cluster. Compare Figure 7.1 on p. 71.

800 mm

Figure 5.11. Field of view of an APS-C-size sensor with various focal lengths, relative to the Pleiades star cluster. Compare Figure 7.1 on p. 71.

Note however that the focal length may not be what you think it is. Schmidt-Cassegrains and similar telescopes change focal length appreciably when you focus them by changing the separation of the mirrors. Even apart from this, the focal length of a telescope or camera lens often differs by a few percent from the advertised value. To determine focal length precisely, use TheSky, Starry Night, or another computerized star atlas to plot the field of view of your camera and telescope, then compare the calculated field to the actual field.

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