Chapter 7
The Astronomical Detector
Abstract The charge-coupled device (CCD) is the foremost imaging device in
observational astronomy. The CCD has high quantum efficiency and linearity compared to film, but it suffers from several sources of noise. We illustrate how to reduce
noise by backing and applying flat, bias, and dark frames. We show how to determine
the linearity of a CCD and how to recognize and deal with artifacts such as bad pixels,
cosmic rays, and saturation.
7.1 The Detector
Before the introduction of photographic film, the only usable astronomical detector
was the human eye. Even through an astronomical telescope, the vast majority of
astronomical objects are not bright enough to be detected by the human eye, especially
in colour. In fact, the quantum efficiency (QE), the percentage of photons that trigger
a physical response, is less than 5% for optical observations. This figure is for the
light-sensitive rods within the retina; the colour-sensitive cones achieve much lower
QE, and hence colour is seen in a very small number of astronomical objects when
viewed with the eye. The cones are concentrated in the centre of the retina, and the
rods around the periphery. A common technique used by amateur astronomers to see
faint objects is to look at the target out of the corner of the eye in order to involve
the light-sensitive rods. Another drawback to observations using the eye is that the
exposure time of the eye is around 1/30 s, which is often an insufficient time for
enough photons to hit the retina so that an image is transmitted to the brain.
The 1970s saw the introduction of the charge-coupled device (CDD) for professional astronomy, which is now a ubiquitous component of almost all cameras,
astronomical or terrestrial. The QE of modern astronomical CCDs is now approaching 90%, and they are close to being nonlinear. The introduction of the CCD is one
of the greatest driving factors in the major improvements in observation that have
occurred in the last few decades.
CMOS (complementary metal-oxide-semiconductor) and CCD technologies were
both developed at approximately the same time, the late 1960s and early 1970s, at
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_7
75
The Astronomical Detector
Abstract The charge-coupled device (CCD) is the foremost imaging device in
observational astronomy. The CCD has high quantum efficiency and linearity compared to film, but it suffers from several sources of noise. We illustrate how to reduce
noise by backing and applying flat, bias, and dark frames. We show how to determine
the linearity of a CCD and how to recognize and deal with artifacts such as bad pixels,
cosmic rays, and saturation.
7.1 The Detector
Before the introduction of photographic film, the only usable astronomical detector
was the human eye. Even through an astronomical telescope, the vast majority of
astronomical objects are not bright enough to be detected by the human eye, especially
in colour. In fact, the quantum efficiency (QE), the percentage of photons that trigger
a physical response, is less than 5% for optical observations. This figure is for the
light-sensitive rods within the retina; the colour-sensitive cones achieve much lower
QE, and hence colour is seen in a very small number of astronomical objects when
viewed with the eye. The cones are concentrated in the centre of the retina, and the
rods around the periphery. A common technique used by amateur astronomers to see
faint objects is to look at the target out of the corner of the eye in order to involve
the light-sensitive rods. Another drawback to observations using the eye is that the
exposure time of the eye is around 1/30 s, which is often an insufficient time for
enough photons to hit the retina so that an image is transmitted to the brain.
The 1970s saw the introduction of the charge-coupled device (CDD) for professional astronomy, which is now a ubiquitous component of almost all cameras,
astronomical or terrestrial. The QE of modern astronomical CCDs is now approaching 90%, and they are close to being nonlinear. The introduction of the CCD is one
of the greatest driving factors in the major improvements in observation that have
occurred in the last few decades.
CMOS (complementary metal-oxide-semiconductor) and CCD technologies were
both developed at approximately the same time, the late 1960s and early 1970s, at
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_7
75