Home Page    Directory    Specials    My Account    My Wishlist    My Basket  

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


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.


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...

Next Page

Copyright ©2000-2014 Starizona
Adventures In Astronomy & Nature, All rights reserved
5757 N. Oracle Rd., Suite 103 · Tucson, Arizona 85704 · Call Us: (520) 292-5010
Map & Directions -  Return Policy