2.5 Filters and Filter Transformations
17
coupled device, or CCD, or more likely a complementary metal-oxide semiconductor, or CMOS, you will see a coloured grid over the surface of the CCD made
of a series of green, red, and blue squares. We will discuss the difference between
CMOS and CCD chips in later chapters. Looking closely, you will see twice as many
green squares as red and blue. Each coloured square covers a pixel and lets only the
light of the colour corresponding to the colour of the square pass through to the pixel.
The process of letting only part of the spectrum pass through is known as filtering,
and the optical device that does this is a filter. The camera’s firmware joins together
four pixels (one blue, one red, and two green) to form one colour pixel. The reason
that there is twice as much green is due to the need to make the picture match what
you perceive. The human eye is much more sensitive to green than to red and blue,
unlike the imaging chip, so twice as many green pixels are used to allow for this.
Looking at the CCDs of most astronomical cameras, we would see no such grid
(some exceptions exist, but those cameras are not considered to be of research quality
and are aimed at amateur astronomers). This is because we wish to be able to change
the range of wavelengths we want to filter out, and we need to control precisely the
characteristics of the filter in use. Hence, rather than filter individual pixels, we filter
the whole array. Typically, multiple filters are held in a filter wheel mounted between
the camera and the telescope. Issuing a computer command causes the wheel to rotate
so that the correct filter is in the optical pathway. Filters come in a range of sizes and
can be either circular or square mounted or unmounted.
Filters are divided into two classes: broadband, which are transparent over a
wide range of wavelengths, and narrow band, which let light from only a specific
spectral line pass, allowing for a small degree of movement and broadening of the
line. Each filter type has a set standard, so that observations at different observatories
using the same filters will produce the same result. Typically, broadband filters will
come in a set, with each member assigned a letter that is known as the band. Hence
the V band (V for visual) covers a region of the spectrum from 511 to 595 nm, and
the R band covers the region from 520 to 796 nm. However, you should be aware
that filter names are an indication of the range they cover and not their profile with
that region. The exact profile is dependent on the photometric system being used.
Also, filter names are case sensitive, so a V-band filter is not the same as a v-band
filter. Some filter names also have a prime symbol associated, so a V-band filter is
not the same as ´
v, and v is not the same as ´
v, although since they should cover the
same region of the spectrum, they will be broadly similar, but for scientific purposes
you cannot use, for example, an R filter when an r filter is needed without employing
a complicated filter transformation calculation.
A filter set in wide use in the astronomy community is the Johnson–Morgan UVB
and its extended form the Johnson–Cousin set of filters UBRVI (and the very similar
Bessel set), which were designed so that a CCD’s colour response is similar to that
of photographic film. The Johnson–Cousin set consists of a blue (B) filter, a red (R)
filter, a green (V) filter, an infrared (I) filter, and an ultraviolet (U) filter. Since almost
all imaging is now done with a CCD, the original purpose of the Johnson–Cousin set
has been eroded, and its drawbacks, for example the fact that some filters overlap, is
making it less attractive for astronomy. However, there is a very large body of work
17
coupled device, or CCD, or more likely a complementary metal-oxide semiconductor, or CMOS, you will see a coloured grid over the surface of the CCD made
of a series of green, red, and blue squares. We will discuss the difference between
CMOS and CCD chips in later chapters. Looking closely, you will see twice as many
green squares as red and blue. Each coloured square covers a pixel and lets only the
light of the colour corresponding to the colour of the square pass through to the pixel.
The process of letting only part of the spectrum pass through is known as filtering,
and the optical device that does this is a filter. The camera’s firmware joins together
four pixels (one blue, one red, and two green) to form one colour pixel. The reason
that there is twice as much green is due to the need to make the picture match what
you perceive. The human eye is much more sensitive to green than to red and blue,
unlike the imaging chip, so twice as many green pixels are used to allow for this.
Looking at the CCDs of most astronomical cameras, we would see no such grid
(some exceptions exist, but those cameras are not considered to be of research quality
and are aimed at amateur astronomers). This is because we wish to be able to change
the range of wavelengths we want to filter out, and we need to control precisely the
characteristics of the filter in use. Hence, rather than filter individual pixels, we filter
the whole array. Typically, multiple filters are held in a filter wheel mounted between
the camera and the telescope. Issuing a computer command causes the wheel to rotate
so that the correct filter is in the optical pathway. Filters come in a range of sizes and
can be either circular or square mounted or unmounted.
Filters are divided into two classes: broadband, which are transparent over a
wide range of wavelengths, and narrow band, which let light from only a specific
spectral line pass, allowing for a small degree of movement and broadening of the
line. Each filter type has a set standard, so that observations at different observatories
using the same filters will produce the same result. Typically, broadband filters will
come in a set, with each member assigned a letter that is known as the band. Hence
the V band (V for visual) covers a region of the spectrum from 511 to 595 nm, and
the R band covers the region from 520 to 796 nm. However, you should be aware
that filter names are an indication of the range they cover and not their profile with
that region. The exact profile is dependent on the photometric system being used.
Also, filter names are case sensitive, so a V-band filter is not the same as a v-band
filter. Some filter names also have a prime symbol associated, so a V-band filter is
not the same as ´
v, and v is not the same as ´
v, although since they should cover the
same region of the spectrum, they will be broadly similar, but for scientific purposes
you cannot use, for example, an R filter when an r filter is needed without employing
a complicated filter transformation calculation.
A filter set in wide use in the astronomy community is the Johnson–Morgan UVB
and its extended form the Johnson–Cousin set of filters UBRVI (and the very similar
Bessel set), which were designed so that a CCD’s colour response is similar to that
of photographic film. The Johnson–Cousin set consists of a blue (B) filter, a red (R)
filter, a green (V) filter, an infrared (I) filter, and an ultraviolet (U) filter. Since almost
all imaging is now done with a CCD, the original purpose of the Johnson–Cousin set
has been eroded, and its drawbacks, for example the fact that some filters overlap, is
making it less attractive for astronomy. However, there is a very large body of work