I viewed a solar transit of the International Space Station (ISS) for the first time ever! As a bonus, there was an active region on the Sun, something of a rarity during the current solar minimum.
If you want to observe a solar transit, do remember to put safety first!
The Space Station
The International Space Station (ISS) is a habitable artificial satellite in low Earth orbit. It is a joint project between five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA (Canada).
The ISS maintains a nearly circular orbit around the Earth around an altitude of between 330 km and 410 km. It travels at an average speed of 27,724 kilometres per hour and completes an orbit in 93 minutes. This high speed explains the challenge of catching a solar transit of the ISS – the transit took a grand total of 0.57s!
Capturing the Image
Planning
The easiest way to see the ISS is to view it at dawn or dusk as it zips by overhead. Illuminated by the sun, it will appear as a bright, fast-moving point of light. With a good telescope mount that can track satellites, you can even image it with a long focal length telescope. This will be my next ISS challenge!
The other way to see the ISS is to observe it as it passes in front of the Moon or Sun. Please note that these transit events take less than 1s!
The first step is knowing when the ISS will pass by. I used the ISS Transit Finder to identity a good opportunity. It can calculate the path of the ISS about 2 weeks from the current date. This limitation is because the ISS’ orbit is erratic due to factors like air resistance or orbital maneuvers. Thus, the transit events can only be reliably calculated a short time into the future.
Indeed, I had to re-plan my initial observation location as the centerline of the transit path shifted 500m from the west side of Kranji Dam to the East.
Capturing
Next will be capturing the image. I set up my portable solar imaging rig (Celestron Evolution mount, 80mm Lunt Hydrogen Alpha telescope, and ASI183MM camera) about half an hour before the transit. I aligned the mount, focused the scope, tuned the etalon, and tested the recording.
Then my camera connection hung! I reconnected it but it was running as a USB 2.0 device, causing the frame rate to drop from 30fps to 4fps. It was too late to try to reboot the computer, so I had to make do.
It was a partially cloudy day with a forecast for thunderstorms. The night before, my son and I had prayed for clear skies for the transit. 1 minute before the transit, we could not see the Sun. And then it started clearing up 30s before the transit! Once again, we see answers to prayers and are reminded that God is sovereign over all creation!
The Shot
At the measly 4fps frame rate, we captured 3 frames with the ISS in it. The image above was the one with the ISS nearest the middle of the Sun’s disk. While slightly blurred and with light clouds, you can still see the main modules of the ISS and the solar panels fully extended to face the Sun.
Active Region AR2759
Since we had set up for solar observing, I also imaged the active region. By this time, the active region was already decreasing in size. You can see some filaments on the right and lower-right of the image.
The image was processed from a stack of 500 monochrome frames, sharpened in ImPPG, and then colored in Photoshop.
It was a nice way to conclude an exciting expedition to image a solar transit of the ISS!