This section summarizes techniques that lead to outstanding deep-sky images with a digital camera. Because these techniques are based on the fundamental properties of photon counting, sensor noise, and image statistics, they are valid with any and every digital camera.
Shoot under Dark Skies. The darker your sky, the better your pictures will be. In the sensor of your digital camera, sky background photons increase the photon event count without adding useful image information, while adding to the statistical uncertainty of the photons from your deep-sky target object.
Shoot with a Fast Optical System. Since noise sources like dark current increase with longer exposure times, using a fast optical system means delivering more photons to each pixel, and therefore less time required to get a fully exposed image. In the context of digital cameras, "fast" is a relative term. With a camera lens, f/2.8 is not particularly fast, but with a telescope, such a low focal ratio is virtually unheard of—so a better way to approach the issue of focal ratio would be to say, "Avoid slow focal ratios." Use the fastest optics you can consistent with the type of optical system you're using, good image quality, and low vignetting.
Avoid Optics with Vignetting. Optical systems that vignette deliver less light to the edges of your images than they do to the center. The result is that you get images with dark corners. Although flat-fielding corrects vignetting, the less work your flat-frames have to do, the better your results will be. Fast optical systems tend to have worse vignetting than slower optics—not always, but generally. Try to find a good compromise between focal ratio and vignetting.
Stack your Images. Nothing beats stacking for collecting lots of target-object photons. The reason is that you can stack and stack and keep on stacking images even if your telescope has a mediocre drive system and you cannot afford a fancy autoguider. Shooting 50 exposures that are each 2 minutes long typically yields something in the neighborhood of 40 images that are well tracked and free of airplane trails, and 10 images with defects that you can discard.
Technically, stacking makes efficient use of the limited full-well capacity of your digital camera's sensor and limited number of brightness levels captured in 12-bit images. It's the smart way to capture lots of photons with the sensor types used in digital cameras.
Store Images in your Camera's "Raw" Format. The standard 8-bit JPEG format of digital cameras yield excellent daytime images; for serious astronomical imaging, use your digital camera's 12-bit "raw" format. Not only does a 12-bit raw mode store the dynamic range suitable for astronomical images, but it bypasses in-camera processing that is almost certainly inappropriate for astronomical images. If your camera supports a "raw plus JPEG" mode, use it so that you can easily and quickly check the images before stacking them.
Shoot Using a Low ISO Speed. The high ISO settings in digital cameras multiply pixel values and noise from the sensor by the same amount; and, since noise is adequately sampled at low ISO ratings, when you shoot using the "raw" format, you gain nothing by using a high ISO setting. However, if you shoot compressed JPEG images, the dynamic range of the captured data is greatly reduced, and it will be necessary to use a high ISO speed setting.
Make Dark Frames and Flat Frames. If you don't shoot dark frames and flat frames when making images, you won't be able to remove dark current, fixed-pattern noise, and vignetting from your images. Treat these auxiliary images as a necessary part of your imaging routine and you'll be better able to produce exciting results with your digital camera.
Mix Long and Short Exposures. In your digital camera's sensor, object photons compete with dark current, readout noise, amplifier glow, fixed pattern noise, and a myriad other unwanted signals and noise. As a general rule, the longer your individual exposures, the better the ratio between object photons and everything that competes with them.
However, it is also necessary to balance the gain from long exposures against overexposing the stars and the brightest parts of your target object. In digital imaging, once a pixel saturates, its signal says nothing but "overcooked."
As a hedge against overexposure, plan your image sessions to include a few short exposures (10 seconds or so), a few somewhat longer (20 seconds), a few mid-length exposures (60 seconds), and as many long ones (120 to 300 seconds) as you can stand making. When you stack the images, the short exposures will fill in the "overcooked" portions of the long ones, and you will capture the full range of brightness in your subject.
Dither Tracking. Although we didn't dwell on this when we discussed im age artifacts, most of the artifacts in digital camera images always occur in the same pixels. Dark current, fixed pattern noise, hot pixels, and even JPEG artifacts do not move in the image. If you make a stack with perfectly tracked images, the artifacts can add together just as well as features in the image do.
So—instead of tracking with great precision over many images, track each well, but change the telescope pointing slightly—by a dozen or so pixels—from one image to the next. When you stack your images, the processing software will register them so that image features add—but artifacts will be spread around and averaged out.
The sequence of images shown in Figures 21.8 and 21.9 demonstrates a progression in quality from a single short exposure to a stack of fifteen relatively lengthy exposures—the last image made with 60 times as many photons as the first. The bottom line to making a great image: Expose deep to collect lots of photons and calibrate the images to overcome dark current and vignetting.
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