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Figure 4.2 The spectral sensitivity of detectors varies considerably. Thinned back-illuminated CCDs offer high quantum efficiency over a wide range of wavelengths, but the front-illuminated detectors used by most amateur astronomers are considerably more rugged and much less expensive.

^ 206265 x J i p _ -eixei [miiiimeters], (Equ. 4.5)

^pixel and then for 7.5-micron pixels and 0.25-arcsecond resolution, you need a focal length of about 6,180 mm, corresponding to a focal ratio of/725 for the 250-mm aperture. At that focal length, the image of Jupiter will be about 160 pixels wide, so you don't need a very large sensor—in fact, a good-quality 640x480 webcam would probably do quite nicely.

4.1.3 Spectral Sensitivity

Spectral sensitivity is the third important factor that determines what digital cameras capture in their images, and the one most often ignored because accurate spectral sensitivity curves have traditionally been reported in engineering units that are difficult to interpret. The information you need is the quantum efficiency of the image sensor as a function of wavelength. This measures what percentage of the photons (also called "quanta") falling on the sensor get converted into useful signal. The highest possible value is 100%, implying that the sensor absorbs every photon and converts it into signal.

Specialized CCDs used by professional astronomers usually have a peak quantum efficiency between 70% and 90% that peaks at the red end of the spectrum (compare this to quantum efficiencies of around 1% for fast photographic films), and drops to around 20% for blue light. Typical amateur CCD cameras are

Figure 4.2 The spectral sensitivity of detectors varies considerably. Thinned back-illuminated CCDs offer high quantum efficiency over a wide range of wavelengths, but the front-illuminated detectors used by most amateur astronomers are considerably more rugged and much less expensive.

Wavelength (nanometers)

Wavelength (nanometers)

based on digital camera sensors designed to have a peak sensitivity of about 60% in the green or yellow-green at around 580 nanometers wavelength.

Any disparity between the red end and blue end of the quantum efficiency curve causes difficulties in color imaging—which is why amateurs prefer CCDs that peak in the visual part of the spectrum. Spectral sensitivity plays a key role not only in making color images—where you need good sensitivity in blue, green, and red—but also in determining the faintest objects that a CCD can image. At the best dark-sky sites, the sky is darker for blue wavelengths than it is for the red and near-infrared, because molecules in the upper atmosphere fluoresce in the red and near-infrared part of the spectrum. For the ultimate in deep-sky penetration from a dark-sky site, you need a CCD camera with a high quantum efficiency in blue light. However, in areas with lots of light pollution, or on nights when the Moon is bright, the sky is brightest in the blue, green, and yellow regions of the spectrum, and darkest in the red and near-infrared. For imaging from suburban sites, a red-sensitive CCD is best.

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