Picture of the week

September 13, 2015

International space station

International Space Station from Earth

Super telephoto setupThis image is a capture of the International Space Station orbiting above during one of its closer passes to my location.  It was photographed from my backyard using a Canon 500mm f/4 IS (mark I) lens with a Canon mark II 2x tele-converter as paired on a Canon EOS 7D mark II body and mounted on a Wimberley gimbal head for easier tracking (see an image of the setup to the right).  The captured photograph was then cropped to 100% (1 image pixel for each camera pixel) for the resultant image shown here.

ISS pathIt takes about 4-5 minutes for the ISS to move across the visible area of the night sky, and comes into a reasonable distance for such photography about once or twice a week.  Using this website I was able to use the prediction information to be ready with my equipment, searching with binoculars, and then switching to my camera setup after spotting it come into view over the horizon.  The ideal conditions will be when the ISS has a brightness magnitude of at least -3.  Lower magnitude numbers imply it is farther away, and your subsequent details will be diminished. Given it is moving rather quickly, I established the camera focus ahead of time on the planet Saturn, while also setting my desired exposure, and then switched to manual settings during the actual shoot.   The ISS is rather bright in comparison to a planet like Saturn, so I ended up reducing the exposure by a stop after my first trial attempt. 

The speed of the ISS’s movement also required that I set the shutter speed to 1/1000th of a second in order to get a reasonable number of keepers, despite trying to pan with a more fluid and relative motion to the ISS.  Since the use of a tele-converter tends to degrade the IQ (especially the 2x version), I reduced the aperture from the minimum f/8 (you lose 2 stops with the 2x TC) to f/11.  Given these settings, it was necessary to increase the camera ISO sensitivity to 1600 in order to keep the exposure sufficiently centered. Ideally if not constrained by these other factors it would be preferred to utilize an even faster shutter speed due to some motion blur in some of my captures.  Also, even though the distance between the ISS and Saturn is quite far, the depth-of-field at even the most open aperture are sufficient for both to stay in focus.  For example, at 1000mm with an aperture of f/8, the hyperfocal distance is 21527 feet, while the ISS is 1.267 million feet away.

ISS side profileAfter about three different trial shootings and with different viewing angles and camera settings, I learned that the altitude angle of the ISS in the sky to the observer on the ground is also important.  Even though it may be closest when directly overhead, this angle tends to be more difficult to track and also yields too oblique of a perspective for showing more of the station (see the image to the right).  An altitude between 45 and 60 degrees seems to work best. 

What seems interesting to me is that it was even possible to capture this level of detail with my wildlife photography setup while the ISS is about 240~250 miles away and moving through space at 17,000 mph.  I am also surprised that the atmospheric pollution from my location near sea level in the central valley of California (Sacramento) did not further obscure the image. In the future I would like to try some captures of the ISS at a higher up and more remote location not as affected by atmospheric pollution.