Narrowband Imaging I: Motivation

(First of a series)

Disclaimer: I do not presume to be anything other than a beginner at narrowband imaging.  All I've done so far is spend one summer and fall getting started. So what I present here should be taken as one person's opinions based upon reading and limited experience.

Why get into CCD narrowband imaging (NI)?

• You've been DSLR or one-shot color or broadband CCD imaging, but have become dissatisfied with the limits it imposes
• You live under light-polluted skies and don't want to confine your imaging to trips to dark sky sites.
• You want a challenge.
• You have some disposable income.

DSLR cameras are relatively easy to use and provide high-resolution images. An amazing amount of hardware and freeware exists for using them in astrophotography. When modified they can provide fair sensitivity to the wavelengths of light emitted by many nebulae.

The central problem with DSLRs is that they are not temperature-regulated.  The optical sensor within a DSLR operates at a temperature near the ambient.  The warmer the temperature, the more noise generated by the sensor.  This noise shows up in images as a random graininess that grows brighter with exposure time.  This accumulated noise can be somewhat compensated for by shooting dark frames (exposures where the shutter remains closed) and subtracting them from the light images (the pictures of what you want to image). This compensation is limited because it's hard to match the temperature of the dark frames to the temperatures at which the light frames were exposed.  Ambient temperature usually changes during the course of an evening, so you're forced to create dark frames at a temperature only approximating the temperature at which the light frames were taken. If the dark frames are cooler than the light frames you get undercompensation; if too warm, overcompensation results; in either case you lose some of the detail in your image.

One-shot color CCDs are a common alternative to DSLRs. When equipped with a regulated cooling system they can hold the CCD sensor at a fixed temperature, allowing better use of dark frames. In fact, the regulation allows you to preshoot dark frames so that you are spared the need to shoot them during each imaging session. The light sensitivity of color CCDs is also better than that of DSLRs.

Broadband CCD imaging employs a monochrome sensor that is very sensitive to wavelengths of visible and near-infrared wavelengths. As its name implies, it produces gray-shaded images.  Exposures through red, green and blue filters are combined to produce a color image.

A serious shortcoming of DSLR, one-shot and broadband CCD imaging is how poorly they cope with light pollution. Light pollution is generally a broadband phenomenon, filling the entire visible spectrum with noise (i.e., undesirable brightness). Under reasonably dark skies, all  three methods work well. In a suburban or urban location, light pollution produces a serious decrease in signal-to-noise ratio as it overwhelms the photons you're trying to gather from your target object.

NI addresses this problem by employing special filters that pass light only at the wavelengths at which emission nebulae radiate. Light pollution is largely filtered out, whether it comes from urban glow, neighbor's yard lights, or even the Moon. For nebular targets the urban imager can be just as successful as one at a dark site.

There are prices to be paid for this ability.  Narrowband filters can be expensive. Exposures must be very long, requiring a good mount and guiding.  Image acquisition and processing are more difficult. Non-nebular targets such as star clusters and galaxies generally don't turn out as well. But the fact is that NI offers the urban or suburban imager a wide range of targets that would otherwise be impossible.

Next time: A basic NI setup