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. Deep sky astrophotography is now more tangible and at a more affordable price point for amateurs to enjoy than ever before!
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.
Basics of Deep Sky Astrophotography
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 as part of getting started in deep sky astrophotography.
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."
Pennsylvania astro imager Gary Honis uses a different approach to image deep-sky objects with his Olympus C-2000 Z and C-2020 Z cameras and 20-inch Starmaster Dobsonian reflector, which is equipped with a tracking system. Although his cameras are limited to maximum exposure times of 32 and 16 seconds, respectively, his telescope’s large aperture can gather a lot of photons during such exposures before noise effects become a problem.
Imaging relatively bright deep-sky objects such as globular clusters is a piece of cake for Honis. "In fact, a single 16-second exposure is sometimes acceptable," he says. "For dim clusters, stacking several images can help improve the image. For dimmer galaxies or nebulae, the exposure limit becomes a problem, even with a large aperture. I need to stack multiple exposures to improve the results."
To assist in the camera’s cooling, Honis attached a small computer fan to his C-2000 Z camera. "This involved taking the camera housing apart, removing the tripod-mount socket to create an opening, and mounting a 12-volt DC fan over the opening to force air into the camera body," he explains. "The results are dramatic. Heat generated by the electronics is quickly pumped out of the camera. On freezing nights I can do practically noise-free, single-shot deep-sky imaging with a 2-megapixel $300 point-and-shoot digital camera in full color!"
Honis describes a complete, step-by-step procedure on how to disassemble a C-2000 Z on his Web site. He and other astroimagers are experimenting with adding thermoelectric cooling to the digital camera's CCD detector. Thermoelectric cooling systems are currently used in many astronomical CCD cameras.
Bear in mind that modifying your camera will void its warranty, so proceed at your own risk. Failure to take proper precautions can result in permanent damage to the camera’s sensitive electronics and/or optics. "After disassembly and reassembly my camera functioned properly, but your camera may not," warns Honis.
How deep can such an air-cooled camera setup go? "By stacking multiple exposures, the bright galaxies are not a problem," he says, "and I can crudely image very faint objects such as the Horsehead Nebula."
Imaging Tips and Techniques
Focusing in deep sky astrophotography can be a challenge, since deep-sky objects are too dim to show on a camera’s built-in LCD viewfinder. Moreover, the typical viewfinder is too small for reliable, accurate focusing. To achieve precise focus many astro imagers point the telescope to a nearby bright star and focus on it with the camera’s focus fixed at infinity and its digital zoom set at maximum. They also set the camera’s ISO rating to a high value (say, 800) to increase its sensitivity so they can focus on much dimmer stars.
Alternatively, if your camera has a video output, you can attach an external monitor or TV to increase your focusing accuracy. After focusing, the ISO and zoom settings are switched back to desired values (but watch out — on some low-end cameras the zoom changes the focus slightly). Then the telescope is slewed to the target object. To do this you may need to remove the camera, center the target visually in the eyepiece without touching the focus, reattach the camera, and shoot. Some imagers take a series of test shots for centering and composing the picture.
Many of today’s types of telescope drives are accurate enough for you to make exposures up to a minute long without guiding. Generally, fast instruments with short focal lengths produce the best results and are more forgiving of errors in tracking and polar alignment.
The camera’s LCD viewfinder should be used only for focusing and composing; it should remain off the rest of the time to prevent heavy drain on the batteries, which adds to the heat generated within the camera.
Subtracting dark frames is very helpful for removing "hot pixels" and other artifacts. Nikon's Coolpix 995, Canon’s PowerShot G2, and others have built-in noise-reduction functions that automatically take a second exposure — the dark frame — with the shutter closed and use it for processing the original image. Some astro imagers use this feature, while others prefer to disable it and take separate images with the telescope covered, then manually subtract the dark frames from the originals with programs such as BlackFrame.
For most digital cameras, the way to increase exposure time to the lengths deep-sky objects need is to stack several images. This increases brightness and contrast and smooths out noise. Astro imagers use a variety of programs for this task, including Adobe PhotoShop, Maxim DL, AstroStack, and ImagesPlus.
Austrian astro imager Johannes Schedler does his stacking in Photoshop by aligning up to five raw images as separate layers. The first raw image is assigned to be the background, the second is layer No. 1, third is No. 2, and so forth. The background is set for normal (100 percent) opacity, while the succeeding layers are set at 50, 33, 25, and 20 percent opacity. The layers are then flattened (combined) to form a single image. "If I have 15 raws, I combine three groups of five images, then combine the resulting three images into one," he says.
Schedler further processes the final image in Photoshop by adjusting its brightness and contrast levels and using unsharp masking. "Usually, the colors need only slight correction, since the camera gives true-color rendition," he adds.
Photoshop's "Curves" function is especially valuable for deep-sky images. It allows you to adjust the brightness and contrast within different brightness ranges separately. In other words, it lets you custom-allocate all parts of the available dynamic range (that is, from 100 percent white to 100 percent black in your final version) so you can reveal everything that's captured in your image to best effect.
Before doing any such processing, of course, be sure to save an unprocessed copy of your image that you can revert to if you mess up. Remember, when touching up your photos, go easy. You are striving for realism, not artificiality. As soon as the photo starts to get the slightest "processed look" to an experienced eye, you have gone too far. Back up, or start over.
For deep-sky use, today’s consumer digital cameras are still limited by noise and short exposure times compared to true astronomical CCD cameras (though the gap is narrowing). But digital cameras do offer most people the easiest and simplest way to venture into the joys of astro imaging.
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A digital-camera convert, Sky & Telescope associate editor Edwin Aguirre has taken more than 4,500 images of astronomical and terrestrial subjects with his trusty old Nikon Coolpix 990.