Space Shuttle Discovery can be seen docked to the ISS (on the left) in this stereo image from 2011. Image Credit: Thierry Legault
If you own a telescope and a camera, you may have already taken detailed pictures of the Moon and the planets.
But have you ever imagined chasing the International Space Station and taking pictures of it that reveal its shape and its main structures?
There is a kind of magic to the silent and majestic movement of this huge structure as you watch it sail across the sky.
At the incredible speed of almost 8km/s, the ISS accomplishes one orbit around Earth in only 90 minutes.
It is often visible over the UK at dawn or dusk.
Hardware – Telescope and mount, planetary camera or DSLR, laptop.
Software – Programs to extract your video frames as stills and edit them afterwards.
Ideally you are looking for a passage with a culmination of 30° above the horizon as the ISS will appear larger and will be less disturbed by atmospheric turbulence.
Given its brightness and its apparent size – the latter is comparable to Jupiter – the ISS would be an easy target for a planetary imager if it remained still in the sky.
The downside of its low altitude (400km) – the very reason it is so bright – is that it has an apparent speed of more than 1° per second when overhead.
Even equipped with motors and computerised Go-To systems, most commercial mounts do not possess the timing precision, reactivity or fine range of speeds needed to follow the ISS by programmed tracking: you’ll have to do it visually through your finderscope.
You can use any kind of mount – Dobsonian, altazimuth or equatorial – so long as it offers smooth and well-balanced manual movements.
One trick that can be employed is to deliberately not polar align an equatorial mount as usual.
Instead, align it with the trajectory of the ISS during its targeted passage.
To do this, aim the polar axis of the mount opposite the direction the ISS culminates in: for example, if the ISS culminates in the south-southwest, align the polar axis north-northeast.
Adjust the latitude of the mount to 90° minus the culmination altitude of the ISS: for example, if the ISS culminates at 60°, set the latitude to 30°. This will make tracking much easier.
The movement in declination will be (almost) nothing, and the tracking in right ascension will be the slowest possible.
Any type of telescope can be used. As in planetary imaging, long focal lengths may lead to more detailed pictures – or to more visible tracking difficulties and errors.
Use a reliable and stable crosshair finderscope with precise alignment screws.
A right-angled erecting model may be more convenient, especially if the ISS passes close to the zenith.
The most capable cameras for ISS imaging are the ones used in planetary, lunar and solar imaging.
A large sensor may help to keep the ISS within its field of view.
You can also try a DSLR in video mode, although the ISS will look smaller and will therefore need a longer focal length.
Another option, with a DSLR, is the continuous shooting mode (RAW or JPEG).
There is no need to wait for the next passage of the ISS to test and improve your tracking ability: practise on aeroplanes, day or night.
At high altitude, their apparent speed is similar to the space station’s.
If you can, image the ISS with a friend: one of you can track it, the other can check the position of the ISS on the camera or computer screen and adjust the shooting parameters.
Begin with your shortest possible focal length; the exposure time will be shorter and keeping the ISS within the sensor will be easier.
Increase your focal length with a Barlow lens only once you have refined your tracking skills.
As in planetary imaging, the best results will be obtained in favourable seeing conditions: perseverance an important part of successful ISS imaging.
Once your telescope is set up and aligned, aim it at a celestial target: a bright star, a planet or the Moon.
Centre it precisely in the field of the camera and then carefully align your finderscope with the optical axis so the views coincide accurately.
Focus the camera on the same target.
Fortunately all celestial objects, including the ISS and other satellites, are so far away that all can be considered to be at infinity and need the same focus position.
If your focuser features a locking system, use it.
You’ll need to adjust the camera parameters by trial and error.
For your first attempt, set the exposure to 1/1,000s in ‘daylight’ colour balance mode; with a DSLR set the ISO to 400-1600, with an astronomical video camera set the gain to a medium value.
Once you’re ready to shoot, check that the mount is able to follow the whole trajectory of the ISS in the sky.
A few instants before the predicted time, aim the telescope in the direction it will appear above the western horizon.
Start recording video.
Spot the ISS with your eyes first, then loosen the mount axis brakes as much as necessary, centre the ISS in the finderscope and track it as smoothly as possible, holding the optical tube, the mount or the counterweight bar.
At the end of tracking, stop recording.
If you succeeded in keeping the ISS within the field for a few seconds or more, use an application such as VirtualDub to extract all video frames in JPEG, TIFF or BMP format.
Tweak the sharpest ones in editing software.
Thierry Legault is a world-renowned astrophotographer whose pictures have been published and broadcast all over the world.