18
2 The Nature of Light
that uses UBVRI. Observatories are now moving to Sloan filters, which are known
as ú, ´
g, ´
r, í. Sloan filters are an improvement over UBRVI, since they do not overlap
and have flatter transmission curves. However, you should be aware that there are
multiple “standards,” over 200 at last count, with many telescopes, both groundbased and space-based, using variations of, for example, the Johnson–Cousin set.
Within this book, this difference should not be a problem, since you should be using
the same filter set all the time. It may, however, cause problems if you are using
data from elsewhere. A number of good websites show the transmission curves for
individual filters, and it is possible to convert from one system to another, especially
if the spectral class of the target is known. However, this can be time-consuming and
may introduce additional uncertainties into your results.
You might also encounter RGB filters, which shouldn’t be confused with Johnson–
Cousin R and B filters, since RGB are imaging filters and are non-photometric. In
many ways they are the same filters that we see on colour CCDs. For scientific purposes, they should be avoided, since there is no RGB standard, and your results will
be difficult to reproduce. The RGB sets are the imaging filters often used by amateur
astronomers to produce colour images, although similar results can be achieved using
the Johnson–Cousin RVB filters. However, as you might expect, an RGB filter set is
considerably less expensive to purchase than an RVB set.
Narrow-band filters tend to have a transmission window only a few nanometers
wide, and in some cases, such as a solar H-α, which is much narrower than an
astronomical H-α, they may be less than a tenth of a nanometre wide.
1 Many narrowband filters are centred on a forbidden line, which should be notated by putting the
emission line in square brackets, whence OIII should be notated as [OIII], and SII
as [SII]. However, H-α is not a forbidden line, although it is a narrow-band filter.
As mentioned above, in astrophysics, forbidden lines are emission lines that cannot
be produced in the laboratory. The mechanism needed to produce the excited state
has a low probability of occurrence and tends to be collisionally suppressed in all
but the very lowest densities, which although common in deep space, are all but
impossible to achieve in a terrestrial setting. When the [OIII] line was first detected
in planetary nebulae, it was thought to be an emission line from a new element,
dubbed Nebulium. Forbidden line emission is an important component of cooling of
low-density astronomical environments such as planetary nebulae; hence emission
line filters are an important tool in observing such objects (Fig. 2.1).
Filters form part of the optical pathway, so changing the filter will change the
optical characteristics of that pathway. To avoid refocusing every time an image is
taken, a set of filters should have the same refractive index, so that within a set,
the same type of glass is used, but it is differently doped to achieve the correct
transmission characteristics. A filter set may include a clear, i.e., transparent, filter
with the same characteristics as the coloured filters. If a blank space on the filter
wheel was used instead of a clear filter, the telescope will have to be refocused in
changing to the clear filter and then again on moving away. Thus a physical clear
filter is highly desirable.
1 Use only specially designed solar H-α filters to observe the Sun. Standard H-α filters are unsafe.
2 The Nature of Light
that uses UBVRI. Observatories are now moving to Sloan filters, which are known
as ú, ´
g, ´
r, í. Sloan filters are an improvement over UBRVI, since they do not overlap
and have flatter transmission curves. However, you should be aware that there are
multiple “standards,” over 200 at last count, with many telescopes, both groundbased and space-based, using variations of, for example, the Johnson–Cousin set.
Within this book, this difference should not be a problem, since you should be using
the same filter set all the time. It may, however, cause problems if you are using
data from elsewhere. A number of good websites show the transmission curves for
individual filters, and it is possible to convert from one system to another, especially
if the spectral class of the target is known. However, this can be time-consuming and
may introduce additional uncertainties into your results.
You might also encounter RGB filters, which shouldn’t be confused with Johnson–
Cousin R and B filters, since RGB are imaging filters and are non-photometric. In
many ways they are the same filters that we see on colour CCDs. For scientific purposes, they should be avoided, since there is no RGB standard, and your results will
be difficult to reproduce. The RGB sets are the imaging filters often used by amateur
astronomers to produce colour images, although similar results can be achieved using
the Johnson–Cousin RVB filters. However, as you might expect, an RGB filter set is
considerably less expensive to purchase than an RVB set.
Narrow-band filters tend to have a transmission window only a few nanometers
wide, and in some cases, such as a solar H-α, which is much narrower than an
astronomical H-α, they may be less than a tenth of a nanometre wide.
1 Many narrowband filters are centred on a forbidden line, which should be notated by putting the
emission line in square brackets, whence OIII should be notated as [OIII], and SII
as [SII]. However, H-α is not a forbidden line, although it is a narrow-band filter.
As mentioned above, in astrophysics, forbidden lines are emission lines that cannot
be produced in the laboratory. The mechanism needed to produce the excited state
has a low probability of occurrence and tends to be collisionally suppressed in all
but the very lowest densities, which although common in deep space, are all but
impossible to achieve in a terrestrial setting. When the [OIII] line was first detected
in planetary nebulae, it was thought to be an emission line from a new element,
dubbed Nebulium. Forbidden line emission is an important component of cooling of
low-density astronomical environments such as planetary nebulae; hence emission
line filters are an important tool in observing such objects (Fig. 2.1).
Filters form part of the optical pathway, so changing the filter will change the
optical characteristics of that pathway. To avoid refocusing every time an image is
taken, a set of filters should have the same refractive index, so that within a set,
the same type of glass is used, but it is differently doped to achieve the correct
transmission characteristics. A filter set may include a clear, i.e., transparent, filter
with the same characteristics as the coloured filters. If a blank space on the filter
wheel was used instead of a clear filter, the telescope will have to be refocused in
changing to the clear filter and then again on moving away. Thus a physical clear
filter is highly desirable.
1 Use only specially designed solar H-α filters to observe the Sun. Standard H-α filters are unsafe.