3.4 Problems
41
the Schmidt–Cassegrain, are not so straightforward to collimate. So if you suspect
you have a collimation problem, please see your observatory support staff.
The telescope you will be using is looking at the sky, which is considered a
curved surface. Likewise, except for some systems that might have a flat secondary,
our optical surfaces are also curved, yet our imaging device is flat. This mismatch
of a curved surface projected onto a flat surface leads to field curvature. This leads
to the distortion of separation distances between objects within the field as well as
nonuniform illumination of the imaging surface. This is a particular problem for
low F-number fast telescopes that have a large field of view. Field curvature can be
addressed with additional optics, or in very rare cases, such as with the Kepler Space
Telescope, by curving the detector surface. As we will see later, in the chapter on
CCD, certain processing techniques can also partly resolve field curvature.
3.5 Introduction to Astronomical Detectors
For most of human civilisation, the best optical imaging system was the eye. The
human eye has a lens and adjustable aperture and a light-sensitive film, the retina. In
humans, the retina is a thin film on the back of the eye covered in light-sensitive cells.
The rods are sensitive to all light, and the cones are colour sensitive. There are three
types of cones, sensitive to red, green, and blue light only. Near the centre of the eye,
at the focal point of the lens, we find the macula. Measuring only 5 mm across, it is
where most of the cones are located. The inner part of the macula contains the fovea,
where most detailed vision is found. If you are reading this book, you are doing
so using your fovea. Outside the macula, the density of light-sensitive cells drops
off, cones especially. Hence, humans have better light sensitivity in their peripheral
vision. This leads to an observation technique whereby observers look at a faint target
indirectly to get the light to impinge more fully on their rods than on their cones.
Cones are much less sensitive than rods, hence the reasoning behind this technique.
This is also the reason that you lose your colour vision in dark conditions; there isn’t
enough light for the cones.
As a detector, the human eye isn’t perfect. It has a limited aperture size of about
7 mm, a fixed focal length, poor quantum efficiency, with only about two percent of
photons generating a signal, and a fixed exposure time of about 30 s.
In 1840, the first image was taken of an astronomical object, the Moon, and the
science/art of astrophotography was born. As improvements in sensitivity were made,
scientists became able to image fainter objects than could be observed with the naked
eye. The combination of long exposure times and greater sensitivity (photographic
film is five times more efficient than the human eye) allowed a much deeper image
to be taken than could be observed with just the eye. Film also allowed a permanent
record of the observation that is not so strongly subject to human interpretation as
hand-drawn images. It also enabled more accurate measurements of position and
brightness. However, photographic film response to light is highly nonlinear, with
most of the image being formed in the first few seconds of the observation, which
41
the Schmidt–Cassegrain, are not so straightforward to collimate. So if you suspect
you have a collimation problem, please see your observatory support staff.
The telescope you will be using is looking at the sky, which is considered a
curved surface. Likewise, except for some systems that might have a flat secondary,
our optical surfaces are also curved, yet our imaging device is flat. This mismatch
of a curved surface projected onto a flat surface leads to field curvature. This leads
to the distortion of separation distances between objects within the field as well as
nonuniform illumination of the imaging surface. This is a particular problem for
low F-number fast telescopes that have a large field of view. Field curvature can be
addressed with additional optics, or in very rare cases, such as with the Kepler Space
Telescope, by curving the detector surface. As we will see later, in the chapter on
CCD, certain processing techniques can also partly resolve field curvature.
3.5 Introduction to Astronomical Detectors
For most of human civilisation, the best optical imaging system was the eye. The
human eye has a lens and adjustable aperture and a light-sensitive film, the retina. In
humans, the retina is a thin film on the back of the eye covered in light-sensitive cells.
The rods are sensitive to all light, and the cones are colour sensitive. There are three
types of cones, sensitive to red, green, and blue light only. Near the centre of the eye,
at the focal point of the lens, we find the macula. Measuring only 5 mm across, it is
where most of the cones are located. The inner part of the macula contains the fovea,
where most detailed vision is found. If you are reading this book, you are doing
so using your fovea. Outside the macula, the density of light-sensitive cells drops
off, cones especially. Hence, humans have better light sensitivity in their peripheral
vision. This leads to an observation technique whereby observers look at a faint target
indirectly to get the light to impinge more fully on their rods than on their cones.
Cones are much less sensitive than rods, hence the reasoning behind this technique.
This is also the reason that you lose your colour vision in dark conditions; there isn’t
enough light for the cones.
As a detector, the human eye isn’t perfect. It has a limited aperture size of about
7 mm, a fixed focal length, poor quantum efficiency, with only about two percent of
photons generating a signal, and a fixed exposure time of about 30 s.
In 1840, the first image was taken of an astronomical object, the Moon, and the
science/art of astrophotography was born. As improvements in sensitivity were made,
scientists became able to image fainter objects than could be observed with the naked
eye. The combination of long exposure times and greater sensitivity (photographic
film is five times more efficient than the human eye) allowed a much deeper image
to be taken than could be observed with just the eye. Film also allowed a permanent
record of the observation that is not so strongly subject to human interpretation as
hand-drawn images. It also enabled more accurate measurements of position and
brightness. However, photographic film response to light is highly nonlinear, with
most of the image being formed in the first few seconds of the observation, which