In a Nutshell: Narrowband Imaging
Capturing the Universe under City Light
Note: There is a wealth of information online regarding narrowband imaging, and as such, this page is only intended as a short summary and high-level overview of what it is all about.
A large proportion of people around the world live in cities today, a number that is projected to continue increasing for many decades ahead. For astronomers, living in a city is a major drawback, as the combined power of the city lights create a powerful glow that lights up the night sky. For those not in the hobby, the bright city sky may not be obvious because the human eye is adaptive. However, the vast majority of objects in the night sky are rendered invisible going from a rural sky to the city.
Enter narrowband imaging. For those of us who prefer to look through the telescope with a silicon chip, this seems like a nearly divine gift. However, one must consider that as with many things, narrowband imaging has both pros and cons. In terms of drawbacks, narrowband imaging is expensive (specialised equipment is necessary), complicated, and limited (only certain classes of astronomical objects can be photographed this way). With this in mind, let's dive into the technique.
In terms of equipment, narrowband imaging requires a monochrome camera and narrowband filters, aside from a typical mount and telescope used for astrophotography. For my case, these are the equipment I use:
Monochrome Astronomy Camera (TEC cooled)
Filter Wheel with Narrowband Filters
Telescope and Equatorial Mount
Notice the labels 'HA', 'OIII' and 'SII' on the image of the filter wheel. These are the 3 filters that are used to isolate light emitted from various elements (Hydrogen, Oxygen and Sulfur respectively). Since the camera can't pick up colour, the final full colour image is created by assigning these filter channels into Green, Blue and Red light respectively.
Creating the Image
To create a complete narrowband project, we need to shoot exposures through each of these filters individually. For each filter, we also need to shoot multiple long exposures of 10-30 minutes each and combine them together in software (stacking). A typical narrowband project usually requires between 5 to 20 hours of total exposure.
To demonstrate how this works, here is a set of images from each narrowband filter for the object, NGC 6188 (Fighting Dragons of Ara):
Hydrogen Alpha (HA)
Oxygen III (OIII)
Sulfur II (SII)
Notice the difference in strength of the 3 filter channels. HA is typically very strong, as hydrogen is the most abundant element in the Universe. OIII and SII are typically much weaker than HA, and in this case, SII is also considerably weaker than OIII.
These three channels are processed separately first, and then combined into a colour image using the aforementioned colour mapping scheme. The colour image is then further processed to balance the relative strengths of the various channels. From this, we get our final image:
The Science Behind
So, how does it all work? The basic principle behind narrowband imaging lies in emission line spectra. Essentially, powerful stars energise gases in space, causing them to ionise and emit light. The light emitted from various gases are always of a very specific wavelength (colour). Each gas has a unique "fingerprint", and from a scientific standpoint, this allows the identification of the chemicals from light years away.
As such, narrowband filters filter out all other wavelengths of light except a very narrow region, which allows only a very specific wavelength range to pass through. The narrower the range of allowable wavelengths, the narrower the bandpass, and the greater its ability to reject unwanted light.
With the exception of old mercury-sodium vapour lamps, all other forms of lighting used today are broadband emitters. This means that the light from florescent bulbs or LEDs are emitted across the entire visible spectrum. This effectively spreads the light energy across the entire visible spectrum, and the intensity of light at each individual wavelength is therefore very small. Since narrowband filters only allow a very narrow wavelength range to pass, this effectively allows the rejection of the vast majority of visible light energy whilst allowing most of the light from nebulae emission through.
For this reason, narrowband filters enable imaging from even under the heaviest levels of light pollution. In fact, all my narrowband images are photographed through my home window in the city-state of Singapore, which was "crowned" by the IAU as the most light polluted country in the world. Although our view of the stars has been shrouded by modernity, the heavens are still there for us to seek.