Astronomy Principles and Practice Fourth Edition.pdf

296 Detectors for optical telescopes
Figure 18.8. The DC amplification technique.
By assuming a quantum efficiency of 10%, a photomultiplier gain of 10 7 and taking the value of the
charge of an electron to be 1·6 × 10 −19 C, the output current is given by
200D 2
× 0·1 × 10 7 × 1·6 × 10 −19 A = D2
64
2 × 10−12 A.
This current is produced when the telescope is directed to a sixth magnitude star. Using Pogson’s
equation (equation (15.10)), the current obtained according to the brightness of the star observed is,
therefore, given by
= D2
2 × 10−12 × 2·512 −(m−6) (18.3)
where m is the magnitude of the star. As an example, consider the observation of an 11th magnitude
star, using a telescope of 500 mm diameter. The output signal is, therefore, given by
= (500)2 × 10 −12 × 10 −2
2
= 1·25 × 10 −9 A.
Because of the wide range in spectral sensitivity and gains for photomultipliers, equation (18.3) should
not be taken as a hard and fast rule.
It may also be noted that the photomultiplier detector simply responds to all the radiation
illuminating its cathode irrespective of the direction of arrival or the presence of any image structure.
Unlike the photographic plate, it can only record the brightness of one field of view at a time and
is essentially a ‘single pixel’ device. Consequently, when the photomultiplier provides the basis of
a photometric instrument, a field stop in the form of a diaphragm is placed in the focal plane of the
telescope so that only one star at a time is measured.
There are two basic principles applied to the output signal for its registration. These are referred to
as DC amplification and pulse-counting or photon-counting photometry. The two schemes are sketched
out below.
18.11.2 DC amplification
In this technique, the current is made to flow through a high-valued resistor and the voltage developed
across it is then amplified. A simplified scheme of the arrangement is depicted in figure 18.8. By
using Ohm’s law, the magnitude of the voltage developed for any desired further amplification can be
estimated. Thus, if the current from the anode is of the order of 10 −12 A and the value of the resistor
is 10 9 , the voltage developed is given by
V = 10 −12 × 10 9
= 10 −3 V.