LOCALadk Magazine

38 Fall 2020 LOCALadk Magazine LOCALadk Getting a close-in shot of a comet, or any object in the night sky, is complicated due to the rotation of the Earth relative to the stars and the lack of light at night. Long exposures are necessar y to gather enough light, but exposures need to be short enough to not have the stars turn into star trails. What we're talking about here is astrophotography. A basic rule in astrophotography is the 500 rule. The 500 rule says that the exposure time, in seconds, needs to be less than 500 divided by the focal of the lens used. The close-in shot of comet NEOWISE seen here was taken at an 80 mm focal length. Based on the 500 rule, this meant that the exposure time needed to be 6 seconds or less. For modern high-resolution cameras this time is even shorter. To get around such a short exposure time star tracking mounts and photo stacking tools are used. By using an in- expensive tracking mount I was able to take 30 second long photos of the comet. I took 20 of these photos. The star tracker works by rotating at the same rate as the Earth, so the stars appear to stay in place. Noise, the photography version of static, is another prob- lem to be overcome in astrophotography. While the star tracker helps, it's not enough. Stacking photos is the solu- tion to this. For photo stacking in astrophotography, you need to get several types of photos (frames) to eliminate noise. The first is what are called light frames, which are what you are tr ying to take a picture of. Then three types of calibration frames are needed: darks, flats and bias. Dark frames are taken with the same settings as light frames, but with the lens cap left on the lens. Flat frames are taken against an evenly lit white surface. Bias frames are taken with the cap on and the fast- est possible exposure. Plan on taking at least 20 of each type of frame. These calibration frames, together with the light frames, are then all combined using stacking software, such as DeepSkyStacker, to give a single photo of what you were tr ying to photograph, minus the unwanted noise. The last problem in astrophotography is light pollution. Find a place where the sky is dark enough to see the stars. The lights of towns and cities drown out our ability to see the stars in their full beauty. We are blessed in the Adirondacks, because most of the park has skies dark enough to appreci- ate the night sky and capture for others to see. William Adamczak

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