Astronomy Principles and Practice Fourth Edition.pdf

284 Detectors for optical telescopes
Figure 18.1. The relative spectral sensitivity of an average human eye; the dotted curve illustrates the Purkinje
effect, showing how the sensitivity curve is modified when the eye is dark-adapted.
where h is Planck’s constant, equal to 6·63 × 10 −34 Jsandν is the frequency of the electromagnetic
wave. It is easy, therefore, to perform the conversion between the amount of energy in a beam, given
in watts for example, to the number of photons s −1 which are travelling with the beam—as done, for
example, in section 16.3.1.
From the size of response of a detector to a beam of light whose energy is known in terms of
photons s −1 , it is possible to evaluate the fraction of the photons which are effectively used to provide
the response. The ratio of the number of photons present in the beam to the number which contribute
to the detector’s response defines the quantum efficiency of the detector.
18.4 The eye as a detector
Very briefly, the main elements of the eye consist of a pupil which controls the amount of light which
enters it, a lens and a photosensitive surface called the retina on to which the lens focuses the images.
The brain converts the sensations of the retina to give the observer the impression of there being objects
set out in space.
There is wide variation in the different properties of the eye according to the individual and the
discussion here is related to that of an average observer. The daytime spectral response or visibility
curve of an average eye is illustrated in figure 18.1. It can be seen that the eye is capable of covering
the optical range from 4000 to 7000 Å and this range is known as the visible spectrum. Theeyeis
unable to store or make a permanent record of any image. The image it produces can be considered to
be instantaneous. If the light-level is below a certain threshold, then the eye does not respond. When
the eye has been working in conditions of normal illumination and is then removed to a darkened
environment, such as might be expected for night observations, its sensitivity undergoes changes,