42
3 The Telescope
places a limit on photometric observations. Additionally, the film has to be developed,
which generates a considerable lag between the observation of the target and seeing
the image produced. It was not uncommon to find a night’s observation ruined by
poorly handled film.
In the 1970s, professional astronomers started to use light-sensitive detectors using
charged-coupled devices (CCD). We discuss CCDs (as well as the similar technology of the complementary metal-oxide semiconductor (CMOS) in later chapters.
At their core, digital light detectors capture electrons displaced by being struck by
photons in cells called pixels, and these pixels are turned into an image. Modern
astronomical imagers make use of long exposure times, have exceptionally good
quantum efficiency, and respond linearly to light throughout most of their range.
Perhaps, however, their main advantage over photographic film is that they can be
digitally measured and manipulated. It is easy to measure star brightness and position.
Commercial digital cameras for terrestrial use employ both CCD and CMOS chips,
although the move now is mostly towards CMOS, as they are cheaper to produce and
have lower power consumption while providing performance approaching that of
CCDs. A very good digital single lens reflex (DLSR) body can cost several thousand
dollars, whilst a top-end amateur astronomical imaging camera can cost ten times as
much while offering what appears to be less functionality. What you might find more
surprising is that the imaging chips in both camera types might well have come out
of the same factory and even the same production run. So why is the astronomical
camera so much more expensive? The first reason is quality control. Every imaging
chip is tested. With millions of pixels, likely a few will not work or underperform.
For normal use, this is not a problem, but for use in astronomy, it is, and hence only
the very best imaging chips go into astronomical cameras. An additional problem is
the noise generated by the circuitry attached to the imaging chip. Although for nonastronomical use, low-noise electronics are desirable, they is very costly, and there is
a cost–benefit tradeoff. In astronomical imagers, low-noise readout of imaging chips
is vital, and custom-designed electronics with carefully selected and tested components are used to achieve this goal, driving up cost. Typical research-grade images
have read noise an order of magnitude less than that of a typical non-research camera.
Lastly, there are benefits of large-scale production to be considered. Companies such
a Nikon, Sony, and Canon may produce hundreds of thousands of exemplars of any
one model, whilst an astronomical camera manufacturer may produce only a few
hundred.
When you start taking images, it will most likely be with an astronomical camera.
You might also be using an adapted DLSR, however, and it is important to keep in
mind that DLSRs, although they can produce amazing astronomical images, are not
suitable for producing images from which scientific measurements can be made, as
they have inbuilt nonstandard filters and perform much of the calibration internally
without human intervention.
In recent years, astronomical webcams have become popular in the amateur community and have been moving slowly into professional use. Initially, these where
domestic webcams hacked to be used for astronomy by the removal of their infrared
filter, the attachment of a tube to allow fitting to a telescope, and the use of special
3 The Telescope
places a limit on photometric observations. Additionally, the film has to be developed,
which generates a considerable lag between the observation of the target and seeing
the image produced. It was not uncommon to find a night’s observation ruined by
poorly handled film.
In the 1970s, professional astronomers started to use light-sensitive detectors using
charged-coupled devices (CCD). We discuss CCDs (as well as the similar technology of the complementary metal-oxide semiconductor (CMOS) in later chapters.
At their core, digital light detectors capture electrons displaced by being struck by
photons in cells called pixels, and these pixels are turned into an image. Modern
astronomical imagers make use of long exposure times, have exceptionally good
quantum efficiency, and respond linearly to light throughout most of their range.
Perhaps, however, their main advantage over photographic film is that they can be
digitally measured and manipulated. It is easy to measure star brightness and position.
Commercial digital cameras for terrestrial use employ both CCD and CMOS chips,
although the move now is mostly towards CMOS, as they are cheaper to produce and
have lower power consumption while providing performance approaching that of
CCDs. A very good digital single lens reflex (DLSR) body can cost several thousand
dollars, whilst a top-end amateur astronomical imaging camera can cost ten times as
much while offering what appears to be less functionality. What you might find more
surprising is that the imaging chips in both camera types might well have come out
of the same factory and even the same production run. So why is the astronomical
camera so much more expensive? The first reason is quality control. Every imaging
chip is tested. With millions of pixels, likely a few will not work or underperform.
For normal use, this is not a problem, but for use in astronomy, it is, and hence only
the very best imaging chips go into astronomical cameras. An additional problem is
the noise generated by the circuitry attached to the imaging chip. Although for nonastronomical use, low-noise electronics are desirable, they is very costly, and there is
a cost–benefit tradeoff. In astronomical imagers, low-noise readout of imaging chips
is vital, and custom-designed electronics with carefully selected and tested components are used to achieve this goal, driving up cost. Typical research-grade images
have read noise an order of magnitude less than that of a typical non-research camera.
Lastly, there are benefits of large-scale production to be considered. Companies such
a Nikon, Sony, and Canon may produce hundreds of thousands of exemplars of any
one model, whilst an astronomical camera manufacturer may produce only a few
hundred.
When you start taking images, it will most likely be with an astronomical camera.
You might also be using an adapted DLSR, however, and it is important to keep in
mind that DLSRs, although they can produce amazing astronomical images, are not
suitable for producing images from which scientific measurements can be made, as
they have inbuilt nonstandard filters and perform much of the calibration internally
without human intervention.
In recent years, astronomical webcams have become popular in the amateur community and have been moving slowly into professional use. Initially, these where
domestic webcams hacked to be used for astronomy by the removal of their infrared
filter, the attachment of a tube to allow fitting to a telescope, and the use of special