Capturing an ISS Lunar Transit
One of the brightest objects in the night’s sky is the International Space Station or ISS. This is humanity’s orbital platform, where people have lived and worked continuously since November 2000 - over twenty years. During that time it has grown, as new components are launched and attached, and now it’s size is around 73m x 100m.
The orbit of the ISS is well known, published using the Two Line Element (TLE) format which can be used to predict where it will be in the future. This means it’s possible to know in advance when it will be visible. It’s quite a sight, a bright star moving from west to east.
There are times when it appears to cross the face of the Moon, what is called a Lunar transit. This is a spectacular sight, and London was lucky enough to see one last week. I used this site to identify where and when it would occur: it turned out there was a suitable location (dark and on the track) within a short bike ride, so I stuffed my cameras in my rucksack and set out into the night.
This is what it looked like, as seen by my camera:
I stacked all the frames together to make this single image of the pass:
The image at the top is an extract of one of the frames, showing the ISS over the lunar surface.
At the same time with a different camera, I took a set of wide angle photos of the track of the ISS across the night’s sky and then stacked them like this:
It was very exciting, watching the ISS approach the moon, then see it on the screen at the back!
Finally, a few details on camera settings for the photo geeks out there. To capture details of the ISS needs really high magnification plus a lunar transit is very quick - it took about 0.5 seconds to cross the moon. I was using a Sony A6500 with 70-300mm lens at the maximum zoom. The A6500 uses an APSC sensor, so the full frame equivalent zoom length was 450mm, but even that wasn’t really enough. The other problem was that the maximum frame rate for still images is around 10 fps, which would result in only a handful of images as the ISS crossed the lunar surface.
The solution was to switch to 4K video, in particular 30p, as this would give 30 images every second, so about 15 frames as the ISS crossed the moon. To get the 4k/30p option required re-configuring the camera to use NTSC mode rather than PAL, which required a reboot. The next problem was the high data rate 4k, i.e. 100 Mbps 4k/30 requires a very high speed memory card, so I had to get one of those, which then had to be formatted in NTSC mode.
An advantage (in this instance) of the A6500’s 4k/30 mode is that there is a slight crop - around 1.23x according to this review. The crop here is a good thing, as it effectively leads to a zoom of that factor.
The final tweak was to switch from standard RAW + JPEG mode to JPEG only. This allows a not well documented Sony camera feature called Clear Zoom to be enabled. This provides up to 2x digital zoom, but in part this is actually a physical zoom, as the 4k frame is much less than the 24 megapixels of the camera’s sensor. Hence it is possible to sample a 4k frame (3840 x 2160 pixels) within the 24 megapixel sensor (6000 x 4000 pixels) without any loss of resolution.
I think I’ve pushed this tech to it’s limit - but have been very pleased with the result!
