Long exposures are beyond the reach of basic consumer digital cameras; electronic noise starts to appear in exposures longer than a few seconds. Dennis di Cicco took this noisy 8-second exposure of
the Orion Nebula (M42) with a Nikon Coolpix 990 on a 6-inch f/6 Maksutov-Newtonian telescope with a 30-mm eyepiece.
Taking the Shot
Unlike single-lens reflex (SLR) cameras, digital point-and-shoots have no "mirror slap" that can cause vibration when taking a picture. But you can still cause shaking when pressing the shutter. To prevent this, use the camera’s self-timer or wireless remote release if it has one. Otherwise, you have to press the shutter yourself (very gently!) without shaking the setup. Some people have fashioned homemade brackets to hold the end of an old-fashioned cable release over the shutter button.
Many popular cameras have no manual override for choosing exposure (you cannot set your own shutter speed and aperture), so you have to rely on the autoexposure function. This works best with large, bright, uniformly lit subjects such as close-ups of the Moon. But the camera’s light meter may overexpose or underexpose the crescent Moon or planets, so you need to manually correct using the camera’s exposure-compensation capabilities (usually +2 to —2 stops). Remember to bracket your exposures. Preview the results on the LCD screen, and save the best. Don’t be afraid to experiment with exposure compensation — try one or two stops brighter or darker than normal, and note the settings that come out best.
Alan Adler took this snapshot of the 110-km-wide crater Gassendi on the northern edge of the Moon's Mare Humorum using a
Nikon Coolpix 950 attached to his 8-inch f/6 Newtonian reflector fitted with a 10.5-millimeter eyepiece.
For the planets, trial and error is needed to find the correct exposure. A good starting point for Jupiter and Saturn is ¼ to ½ second for typical Schmidt-Cassegrain telescopes. Keep exposures short to minimize blurring due to atmospheric turbulence. (The best modern planetary imaging, however, is done not by taking single shots with a camera at all, but by stacking hundreds of video frames taken with a digital webcam.)
Wiphu Rujopakarn of Kirdkao Observatory in Kanchanaburi, Thailand, imaged this naked-eye sunspot group on March 25, 2001, with a
Canon IXUS camera through a filtered
Tele Vue Pronto refractor and a 32-mm eyepiece.
Use your camera’s highest image-quality setting when saving the image. This means using the camera’s full resolution and applying the least image compression. Most popular consumer cameras save images using JPEG compression to reduce file size and get more images into the available memory. The more you compress an image, however, the more it begins to show artifacts such as degraded color and, when magnified, squarish blotchy patterns: "jpegging." Use the lowest compression possible, or save the image in an uncompressed format such as RAW or TIFF if your (high-end) camera allows.
Don’t be surprised if you can store more astronomical than daytime images in the same amount of memory. The dark background of a typical astronomical scene compresses far more than conventional images. You can also experiment with the black-and-white mode if your camera offers this feature. Because of the way color CCDs work, images shot in black and white can appear sharper than color images.
