Geostationary Satellites

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If you've ever looked at the sky just after sunset, you've probably seen low-orbit satellites (~200 miles up) crossing the sky. At that time of the evening, direct sunlight still hits the satellite, making them easily visible against the darkening sky. However, I never gave much thought about the possibility of seeing geosynchronous satellites because of their much greater (~100x farther) distance from us. This changed several years ago while scanning an astrophoto taken on film for a friend. We found 3 small parallel line segments in a scan of an astrophotograph of Orion. Suspecting scratches in the film, I checked the actual film with a magnifying glass and found that the lines were actually in the image, rather than scratches on the the surface of the film.


Because the length of the lines indicated a very slow motion, I began to suspect geosynchronous satellites, and confirmed that suspicion by calculating where they should be seen in the sky from our position in southern California. Since then, I have often been annoyed by the satellites appearing as streaks in my photos, and have always had the intention of photographing them with a stationary camera.



However, when an article about observing the satellites in a telescope appeared in the October, 2011 issue of Sky & Telescope, I was reminded to try my hand at photographing them, with the result at right (noise-reduced, contrast-enhanced image). Stars are trailed over the 30 seond exposure, while the satellites appear as dots since the camera was stationary.


To make the satellites more obvious, I elected to create a time-lapse movie using frames spanning most of the night. The resulting video, has been uploaded to YouTube. Be sure to view the highest resolution version of the video possible. The original video was output in the form of a Quicktime HD video (1920x1080 pixels)., which is already reduced in resolution by over 2x from the original camera still frames. A sped up (3x) video closeup of the eastern-most satellite pair shows the motion due to the satellites being slightly off the exact position for a geosynchronous orbit.



In the video, vertical arrows mark the positions of the satellites I found. The dark horizontal line parallels the equatorial plane and serves a reference against which any vertical motion of the satellites can be seen for those positioned a bit off the true equatorial plane. The diagonal arrows show the path of a satellite on a greatly inclined orbital plane. The satellite can be seen passing through the image around the time that the Orion Nebula (M42) can be seen too, about 2/3 through the video.


The brightness of the satellites can be seen to change as the sun's illumination angle changes. The background sky color in the video is the unprocessed color of the sky (due mostly to man-made light-pollution) as captured by the camera.


Technical Information


The movie was created by using a Nikon D700 DSLR on a stationary tripod and a zoom lens set at 125mm focal length, f/4.5. The camera was set for ISO 6400. Frames were taken 75 seconds apart using the camera's built-in intervalometer, with 30 seconds used for the actual exposure, and 30 more for an in-camera dark-frame subtraction. This eliminated the "hot" pixels which might have been mistaken for stationary satellites. The frames were taken with the camera level to the horizon, but the movie was made after the frames were rotated and cropped to isolate the equatorial plane to maximize resolution.