The relative sizes and distances involved are particularly neat for this composite photo of the transit. Saturn is over a million times larger than the space station, but over three million times as far away. The resulting perspective puts them on roughly the same scale, the elongated station stretching the width of the ringed planet as it passes. The result is like seeing a child attempt to hide behind a spindly tree. (Note: Phil Plait points out the composite scale is off—the ISS should appear twice as large as the planet.)
Astrophotographer Julian Wessel put a lot of planning into how to get the best shot. He used the sky calendar CalSky to plot out where he needed to be to get the right view at the right time, then the planetarium software Stellarium to visualize his shot. The weather forecast on the fateful night wasn’t the greatest — clear, but with enough instability to potentially blur his targets. He headed out anyway on January 15, 2016, set up his 10" telescope, and started shooting.
Check out more details on how he set up the shot in his video here:
Update: After vigorous debate about the image’s authenticity in the Astronomy Picture of the Day forums, Wessel wrote:
To make things clear I wanted to say that the APOD picture of Saturn and ISS is a composition of 2 Frames from different capturing session. They’re both overlayed and processed to make the event as detailed as possible.
I’m sorry to all the astronomers feeling betrayed, this was not my intention. I just wanted to top my Jupiter transit and failed by overprocessing this image.
Nevertheless I will prove that it’s possible to make this catch as perfect as shown. It’s all a matter of planning and knowing his equipment. And I know I can do this. I’m one and a half year into astrophotography now and this is a mistake I won’t do again! I’ve learned from it.
So the composite is not two images from the same night, but two images taken at different times and composited together. This accounts for the relative brightnesses of both objects, which could not otherwise both be exposed equally in the same shot. Another amateur astrophotographer, Stephen Ramsden, wrote a beautiful piece about how the line between compositing and forging photographs. The always-skeptical Bad Astronomer Phil Plait has a more technical debunker of the image.
We’re just glad we called it a composite from the start!
Image credit: Julian Wessel