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What is narrowband imaging?
In normal color imaging, three filters (red, green, and blue) are used to
separate the primary colors of the visual spectrum. Red, green, and blue (RGB)
filters are designed to approximate the color sensitivity of the human eye, so
that the resulting image is true color.
Each of the RGB filters covers approximately one third of the visual spectrum
and the filters overlap slightly so that the whole spectrum is detected by the
CCD. (There is sometimes a gap between the green and red filters to block
a prominent light pollution emission line, as in the diagram below.)
Narrowband filters instead capture only a very small part of the spectrum.
They are said to have a narrow bandpass. The bandpass is simply how much
of the spectrum the filter allows to pass. This is usually measured in
nanometers. The entire visual spectrum runs, approximately, from a
wavelength of 400nm (blue) to 700nm (red). Therefore, a typical RGB filter
might have a bandpass of 100nm. In contrast, a typical narrowband filter
has a bandpass of just 3-5nm (see following pages for specifics).
Above: Typical set of RGB filters
Above: Some of the more common narrowband filters, with RGB
filters in the background for comparison
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Bandpass and Focal Ratio
An interesting effect of narrowband filters is that the bandpass is a
function of incident light angle. In other words, a steep light cone
entering a narrowband filter can actually change the bandpass wavelength.
An example of this effect is the need for solar H-alpha filters to have a
roughly parallel beam of entering light. With the older DayStar filters,
the primary filter was located at the back of the scope and required the
telescope to operate at a very slow focal ratio (greater than f/30) to have an
approximately parallel beam of light entering the filter. The newer
Coronado filters place the primary filter on the front of the scope.
Therefore the filter is receiving a parallel beam of light (directly from the
Sun) and the telescope itself can then operate at any focal ratio.
The same effect is seen with narrowband filters for CCD imaging. At a
certain focal ratio (faster than f/4), the bandpass of the filter has shifted so
much that the peak wavelength now sits off the main transmission portion of the
filter and the effect is significantly reduced filter efficiency. For
telescopes operating at focal ratios below f/4, wider bandpass filters (10nm)
are recommended to keep the peak wavelength within the highest transmission part
of the filter. For slower scopes, a narrower filter is preferable as it
enhances the effect of the filter.
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Emission Lines
Narrowband filters are designed to capture specific wavelengths of light.
There is a large class of celestial objects known as emission nebulae, and their
name arises from the fact that they are actually emitting their own light (as
opposed to reflection nebulae, which shine by reflected starlight). The
Orion Nebula, Lagoon Nebula, and Swan Nebula are three common examples of
emission nebula. Planetary nebulae are normally considered a separate class
of objects than emission nebulae, since they represent a very different
phenomenon (star death instead of star birth), but for CCD imaging purposes,
they can also be considered emission nebulae as they are emitting their own
light. Supernova remnants also fall into this category, so objects like
the Ring Nebula, Dumbell Nebula, Veil Nebula, and Crab Nebula are all potential
targets for narrowband imaging as well. (The blue nebulosity surrounding
the Pleiades is a classic example of a reflection nebula, and would be something
that is not well suited for narrowband imaging.)
What all these emission nebulae have in common is that they are composed of
gases, and these gases are emitting light. The atoms within the gas are
being excited by energy from nearby stars (either the stars forming within the
nebula, as in the Orion Nebula, or by the remnant of the dead central star in a
planetary nebula like the Ring). The energy imparted by the starlight
causes electrons within the gas atoms to jump up to a higher atomic orbit.
Electrons are lazy by nature and prefer to be in the lowest energy state
possible. The electrons will re-emit their excess energy and drop back
down to a lower orbit. They give off the extra energy in the form of a
photon of light. And since electrons always make jumps in discrete steps
(they go from high energy to low and there is nothing in between), an electron
going from one orbit to a lower one always gives off the same amount of energy
and therefore the same wavelength of light. Thus, each atom has a distinct emission line or color of light associated with it. Also, each atom
contains different orbits, so there can be multiple wavelengths of light from a
single element, such as hydrogen.
Above: An electron drops back to its ground state from a
higher orbit and gives
off a photon of light in the process.
Common Filters
The two most common elements contributing to emission lines in nebulae are
hydrogen and oxygen. Other elements such as sulfur and nitrogen also
create prominent lines. Listed below are the common emission lines and
filter types used in narrowband imaging.
Hydrogen-Alpha - 656.3nm
The most dominant emission line in a star-forming region such as the Orion
Nebula is called hydrogen-alpha, or H-alpha. This light is created by
atomic hydrogen, the primary constituent of the Universe and the basis of the
nuclear fusion that powers stars. H-alpha is in the red part of the
spectrum and contributes to the overwhelming red color of most nebulae as seen
in normal RGB images.
Hydrogen-Beta - 486.1nm
Hydrogen gives of light at several wavelengths. The second most common,
after H-alpha, is the H-beta line in the blue part of the spectrum. Since
the dark-adapted human eye is sensitive to blue and green but not red, H-beta
filters are sometimes used for visual observations of certain nebulae.
Oxygen-III - 500.7nm
This line is given off by doubly-ionized oxygen atoms, meaning the electrons
are dropping two energy levels. This line is in the blue-green portion of
the spectrum. It corresponds, by happy coincidence, to the peak
sensitivity of the dark-adapted human eye, so OIII filters are common visual
accessories. The OIII line is the dominant emission from planetary
nebulae. (By the way, OI is non-ionized oxygen, and OII is singly-ionized
oxygen. Hence doubly ionized gets the designation oxygen-III.)
Sulfur-II - 672.4nm
Singly ionized sulfur emits light in the deep red part of the spectrum,
beyond H-alpha. It is a weaker emission than H-alpha and OIII, but it is
the most common filter used after these two.
Nitrogen-II - 658.4nm
Singly ionized nitrogen, like H-alpha and SII, also gives off light in the
red part of the spectrum. NII is a less commonly used filter, but its use
is seen often in famous Hubble Space Telescope pictures and it is occasionally
used by amateur imagers as well.
Advantages of Narrowband Imaging
The primary advantages of narrowband imaging are the ability to detect more
detail and the ability to image from a light-polluted area since the filters do
not pass the light emitted by most types of street lights (or moonlight, for
that matter). Also,
narrowband images isolate the light given off by specific kinds of gas, so the
images are also scientifically interested in and can tell a lot about what is
going on inside a nebula. Another advantage is for users of non-antiblooming
CCD cameras. Since the filters let through less starlight (but still pass
most of the nebula's light), you can take a much longer, and hence more
detailed, exposure without blooming the brighter stars in the picture.
Next, Combining Colors in Narrowband Images...
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