First of all, consider how we see light. Light is simply electromagnetic energy. This electromagnetic energy exists in a wide range; from radio waves at the low frequency range to gamma rays at the high frequency range. Our eyes only see a narrow segment of this electromagnetic energy range as light. Frequencies lower than our eyes can see include radio, microwave, and infrared. Frequencies higher than our eyes can see include ultraviolet, X-ray, and gamma ray. Scientists have instruments that, unlike our eyes, can see the entire spectrum of electromagnetic energy. This is where you might hear the term radio telescope, or infrared telescope. Most amateur astrophotographers, including myself, are restricted by our equipment to recording, generally, the visible spectrum of light. (The other kind is very expensive)
When engaging in deep sky astrophotography, there are generally two types of targets; narrow band and broadband. Many nebulae are composed of clouds of gas that are being radiated by a nearby star. The radiation from the star affects the molecular makeup of the gas, causing it to emit light. Because of the nature of the molecular change in the gas cloud (it becomes ionized), the gas begins to emit light. The light is at very specific wavelengths. Some of these wavelengths are in the visible light spectrum.
We generally know the frequencies emitted by these ionized clouds of gas. Therefore, we can use filters that will filter out all light except for the frequencies these objects emit. By allowing to pass only the very narrow frequency that we know the object emits, we can filter out light that comes to our telescope from other sources. Thus, by using a narrow band filter, we catch the light from the object, but do not record the extraneous light. Thus, we can filter out moonlight, street lights, car lights, kitchen lights, glow from the computer screen, and any other light pollution. So, we can get a much sharper and cleaner image. Also, since we are not pulling in all this extraneous light, we can take longer exposures. (Longer exposures capture more detail).
Other objects, such as stars, galaxies, kitchen lights and moonlight emit electromagnetic radiation across the entire visible spectrum. Some objects, such as the moon and nebulae that have not been bombarded with ionizing radiation, don't emit their own light. Rather, these objects reflect the light from nearby stars that are shining on them. Since we see these nebulae because of reflected starlight, they are called reflection nebulae. This reflected starlight covers the entire visible spectrum. Therefore to see a star, a galaxy, a moon, or a reflection nebula, we cannot use narrow band filters that filter out all but specific wavelengths of light. Instead, we must capture the entire range of the visible spectrum. This is called broadband photography. Of course since we need to capture all of the visible spectrum in order to see all of our object, we are now susceptible to light pollution and light coming from sources other than our object. Also, since we are capturing all the light that comes to our telescope, We are limited to shorter exposures. If you expose an unfiltered (broadband) camera sensor at the same time interval you expose a sensor that has been filtered, then the image will be much brighter and washed out.
Unless you are photographing the moon, bright moonlight becomes a problem. It washes out your broadband images and introduces light pollution. So, it is better to shoot your broadband objects when there is little to no moonlight. If you must shoot when there is a lot of moonlight pollution, then this is a good time to shoot your narrow band targets (Emission Nebulae).
You would need to shoot the following picture in broadband.
You would shoot the following emission nebula in narrow band.
Sometimes an astrophotographer will shoot an emission nebula in narrowband to get the nebula details and then shoot the same object in broadband to capture the star colors. The two images are then blended to get all the details of the nebula and all the rich colors from the surrounding stars.
Now, the next time you hear someone talking about shooting in broadband or narrow band, you will know what they are talking about and why they are doing it.
