GET TRACKED-LIKE LONG EXPOSURES BY STACKING MANY SHORTER ONES
We all remember how exciting it was when we captured the Milky Way for the first time. Such is an incredible feeling! But as we evolve in our photography we eventually want to learn those extra steps that help to make our images better.
To bring out more detail and color in the night sky our camera's need to capture more light. This is achieved by using a higher ISO (which adds noise), a longer exposure (which causes star trailing), or a faster lens - and usually all three.
However, longer exposure's will lead to star trailing, and higher ISO will lead to increase noise. Because of this many photographer opt to get a star tracker.
A star tracker, like the iOptron Skyguider Pro, will rotate the camera at the same speed as earth's rotation, allowing longer exposures without star trailing. And since you can use really long exposures you are also able to lower the camera's ISO setting - leading to cleaner overall images. While a lower ISO will reduce noise in the image, longer exposures tend to increase "hot-pixels" caused by the warming of the image sensor during longer exposures. The other down side to a star tracker is the added weight of carrying more gear in your bag, and the need to get proper polar-alignment with Polaris (aka. North Star), which even when Polaris is visible still takes time and patience.
However, another technique, called Image Stacking can be the best solution - especially with focal lengths under 70 mm. Image stacking allows you to "stack" images together from a sequence and then combine those images to create the equivalent of a longer-exposure photograph - without the hot-pixel issues. This allows photographers to shoot the night sky using very short exposure times, knowing that these images will be combined in post to create a single image with a longer combined exposure time.
Example: If you stack 40 photographs captured with a 4-sec. shutter speed, the stacked photo will have the equivalent exposure time of 160 seconds. (40 x 4).
Stacking is also the ultimate form of night-sky noise reduction. It removes noise in the final stacked image by comparing each image and removing what isn't the same from image to image. Since noise is random, most of it ends up getting removed during the stacking process. This allows you to shoot at higher ISO settings during the 4-second exposure without worrying about noise in your final image.
If you look at the image above you'll notice that the two images look very similar, but the 116 second exposure wins out when it comes to enhanced deep space color (blue and reddish cast) as well with actually showing the color of the fainter stars. The single image (left photo above) captures most of the stars, but the exposure isn't long enough to capture the color of the fainter stars or deep space.
As you can see in the final stacked image below, many of the smaller and fainter stars show color. "The color of a star comes from its temperature. The coolest stars appear red, while the hottest stars are blue. And for a star, the only thing that defines the temperature of a star is its mass. Blue stars are stars that have at least 3 times the mass of the Sun and up. Whether a star has 10 times the mass of the Sun or 150 solar masses, it’s going to appear blue to our eyes. " UniverseToday.com
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