Deep-Sky Imaging with Digital Cameras
Consumer point-and-shoot digital cameras costing under $300 are revolutionizing the way amateur astronomers take images of the sky. What makes these cameras so attractive is their versatility, ease of use, capability of recording color images in a single shot, their high resolution, and the elimination of film and processing expenses.
What's more, digital cameras can also be used for regular daytime photography. This is not the case with expensive astronomical CCD cameras, which are really single-purpose units.
Practically anyone can now capture close-up views of the Moon and bright planets, often achieving better results than with a 35-millimeter film camera. All you have to do is hold the camera up to the telescope eyepiece or attach it directly to the eyepiece with a homemade or commercial bracket, aim, focus, and shoot. You get instant results and can just delete the bloopers.
For basic, wide-field shots of constellations and planet conjunctions, you don’t even need a telescope — just a camera capable of making exposures up to about 8 to 15 seconds and a means to hold it steady, such as a tripod. Since all the controls are on the camera itself, you don’t need a computer in the field. Your home computer is used later to sort, process, and archive the images.
In another article, I covered the basics of how to image the Sun, Moon, and planets bright targets through a telescope with a digital camera. This article looks at ways advanced amateurs can record dim star clusters, nebulae, and galaxies.
Overcoming Electronic Noise
Because of their inherent dimness, deep-sky objects require exposures of up to several minutes. For photographic films and astronomical CCD cameras, this is not a problem. But in a digital camera designed mainly for daytime use, the maximum useful exposure time is just a few seconds before electronic noise sets in, degrading the image. Unlike astronomical CCD cameras, digital cameras don’t have a built-in active cooling system. Cooling is essential for minimizing the noise generated in the camera’s electronics. You need a way to keep a digital camera relatively cool in order to increase its exposure time and therefore, enhance its sensitivity to faint objects and its overall image quality.
The frigid ambient air of winter cools digital cameras naturally. During warm summer nights, however, images are often riddled with "hot pixels" and noise artifacts, so image processing at the computer is a must for long exposures.
"At ambient temperatures above 32° Fahrenheit, or 0° Celsius, noise builds up quickly," notes Arpad Kovacsy, who has imaged with a Coolpix 995 and 6-inch Astro-Physics refractor from his light-polluted home in suburban Washington, D.C. "One simple way to partly compensate for this effect is to turn off the camera between exposures to allow it to cool down. My dark-frame sequence [below] demonstrates how rapidly noise builds up in consecutive shots, and how turning off the camera mitigates this problem somewhat."
Kovacsy's first image, taken at an ambient temperature of 66° F, is nearly free of noise. The 10th consecutive image is the worst. Turning off the camera for 10 minutes produced an image as clean as the first. "Note how even the 10th image taken at 40° F has less noise than the second image taken at 66° F," he adds. "Based on my trials, temperatures of 32° F or below work best when taking minute-long exposures with the Coolpix 995."