Saturday, 19 April 2014

Photographing the International Space Station

Bit of a long post I'm afraid, but I wanted to give a blow-by-blow of what it's like when everything comes together!

With the weather improving and a few clear night time skies coming along, I've been able to indulge the side of my photography that I do purely for fun - the night sky. It's not something I often have time for, especially as, without a telescope, I'm pretty much limited to taking shots of star trails which involves several hours' worth of commitment. However, a conversation on Facebook revealed that the International Space Station is very visible at the moment from the UK, so I thought I'd have a crack at that.

I'd never actually seen the ISS before, although (excuse me while I date myself here) I did see Mir in the late 90's. So a reconnaissance mission was in order, and I headed out to a particularly dark field that I know in Lincolnshire. Having found out rough times and directions from the internet, I set the camera up and right on cue the bright dot of the ISS rose out of the west and hurtled across the sky. And I mean hurtled - it's very fast and extremely bright compared to other satellites. It's a proper event and well worth putting aside a few minutes of the night for.

I managed to get off a couple of shots, nothing special but useful for dialling in exposures and composition. Previously, although I knew the raw data, I didn't really know how much sky the station would cover and how quickly. Here's one of the test shots, a 30-second exposure with a high ISO setting:

This was using my 10-22mm lens at it's widest, so you get a real impression of how much ground the station covers in thirty seconds - horizon to horizon usually takes roughly five minutes.

Based on the above, I came to a few conclusions. I figured I could get away with halving the ISO setting to reduce the chances of getting a noisy image. I also decided that the next shot would be a multiple exposure (more on that in a moment), in portrait format, to capture as much of the very high trajectory as possible.

The final piece of the puzzle was the location - I wanted somewhere that would complement the sky and ISS rather than a bland bit of farmland. A road with traffic would be ideal - light streaks from the cars to balance out the station. I live near the A46 and it's orientation is ideal - diagonally to the projected path of the station. Fortunately the Highways Agency had anticipated my needs when they built the new dual carriageway, by heaping up a great big pile of earth to build a bridge. Voila, one high bank overlooking the road.

You've probably gathered by now that the key to astrophotography of any kind is to plan it like a military operation. Photography is always a balance between science and art and this stuff definitely veers sharply towards science.

Anyway, a couple of nights later, I scrambled up the bank and set the camera up with plenty of time before the ISS was due to appear, giving me a chance to take some practice shots, and test my remote shutter release, which is actually an Android app. I started a continuous sequence of thirty-second exposures a minute before the ISS was due to appear. The beauty of using a programmable release is I could just sit back and enjoy the sight of the station flying by.

Why a sequence of thirty second exposures? Simple really - as a digital camera takes a photo, the sensor heats up, and the image becomes noisy. Breaking up the exposure into chunks reduces the chances of this happening or, at worst, puts the noise in a different place in the image. It means more work afterwards but it's worth it from an image quality standpoint.

Here's the raw sequence of the station passing overhead:

The planning really paid off, with the station sailing across the frame perfectly. There's a bit of luck in there as well - the road was nice and busy, and the clouds over Nottingham really add to the composition.

And so to the processing. All the images were tweaked in Lightroom to bring out the stars and the station, and to improve the colour balance, then sent to Photoshop to be composited together. Photoshop is fine for a small number of images like this; for a large batch of images, for example star trails taken over a few hours, I'd use a dedicated stacking program. Having blended the images, all that remained was a bit of cloning to cover the gaps between exposures and a final crop to improve the composition. I have to admit that I'm really proud of the end result, both from a technical point of view and simply because this puny human on the ground can take a photo of a spacecraft, 200 miles up, moving at 17000mph.

If you want to see the ISS for yourself, it's well worth signing up to NASA's email alerts, which tell you when to see the ISS in language that non-astrophysicists can understand. You can do that from their Spot The Station microsite:

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