Imaging with Digital SLR Cameras

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Digital cameras offer an attractive alternative to the astronomical CCD camera. Not only are they far less expensive (on a cost-per-megapixel basis), but they produce color images without the hassle of shooting separate filtered images. Digital SLRs offer the added advantage of being easy to use with a wide range of standard camera lenses, as well with telescopes.

They have excellent technical specifications. Readout noise is typically under 10 electrons r.m.s., and dark current is low enough to allow exposures of four minutes or longer, depending on the ambient temperature. The full-well capacity tends to be shallow, but it is fully utilized when the CCD or CMOS sensors are digitized and stored in a 12-bit "raw" format.

The greatest drawback from the standpoint of astronomical imaging with standard off-the-shelf digital SLRs is the integral color-balancing filter. This filter blocks the infrared and severely attenuates red light—however, special astronomical versions of digital SLRs are available without the filter. Because of the color-balancing and the integral Bayer array filters, pixels on the sensor receive only 10% to 20% of incident light, so to achieve the same signal-to-noise ratio as a cooled and unfiltered astronomical CCD camera, an off-the-shelf digital camera requires 25 times the exposure as the astronomical CCD. This means that to get a

Figure 5.6 Digital single-lens reflex cameras complement traditional astronomical CCD cameras by offering direct color imaging plus interchangeable lenses—and they are great for terrestrial photography, too! Furthermore, new models appear every few months with more and better features.

"solid" image, bright deep-sky objects require at least 5 to 10 minutes exposure, and faint ones need 60 minutes or more total exposure time.

To make effective use of digital SLRs, try these suggestions:

Use a modest ISO setting. In the camera's raw format, an ISO setting of 200 to 400 fully samples the readout noise of the detector. Raising the ISO to high values multiplies both signal and noise—but does not capture additional information. At high ISO settings, output values reach saturation and reduce the dynamic range of the image data captured.

Use the raw format. The camera's raw format captures more information from the detector than its other output options, so for astronomical imaging, its use is mandatory. If the camera offers the option of saving a raw image and a compressed JPEG image simultaneously, save both.

Use fast optics. You can offset the absorption of the white-balance filter and Bayer array by using digital SLRs with fast optical systems. An//2.8 camera lens is almost ideal; the best telescopes will have fast optics.

Use long exposures. Use the longest exposures possible consistent with keeping dark current reasonably low. Your goal is to accumulate enough photo-electrons that Poisson noise is substantially greater than readout noise.

Don't skimp on optical quality. Digital SLRs have small pixels. Optical systems that produce soft images in terrestrial shooting produce bloated star images. Achromatic camera lenses and refractors generally produce star images surrounded with a blue halo of unfocused starlight.

Infinity focus is not infinity. With camera lenses, infinity focus usually does not yield the sharpest star images. It will almost certainly be necessary to make test images to determine the best focus for your camera's normal lenses.

Focusing is critical. Because the camera's focus screen is too small for critically sharp images, you will need to focus by attaching the camera to a computer, taking test images, and examining them until you achieve best focus.

Accurate guiding/tracking is essential. The small pixel size of the digital SLR reveals guiding or tracking errors. For effective long-exposure imaging, you will need either a very accurate drive system or some means of accurate guiding.

Stack multiple images. Reaching an aesthetically acceptable signal-to-noise ratio in the sky background requires collecting lots of photons, and this demands a lengthy total exposure. Take lots of images to register and stack.

Use a "sunlight" white-balance setting. As a general rule, natural sunlight is the default "white" for human vision. Settings such as "tungsten" and "fluorescent" yield unnaturally blue and green images.

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