{"id":57831,"date":"2024-04-25T08:00:30","date_gmt":"2024-04-25T08:00:30","guid":{"rendered":"http:\/\/ae48d587-8c14-45b3-99cb-3b11394ad29b"},"modified":"2024-04-25T09:08:25","modified_gmt":"2024-04-25T09:08:25","slug":"create-a-composite-astrophoto-showing-a-night-sky-object-in-motion","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/create-a-composite-astrophoto-showing-a-night-sky-object-in-motion\/","title":{"rendered":"Create a composite astrophoto showing a night-sky object in motion"},"content":{"rendered":"

Capture the movement of a planet or other object over multiple nights by creating a composite astrophoto that tracks a celestial target across the night sky. <\/p>

By Pete Lawrence\n <\/p>

Published: Thursday, 25 April 2024 at 08:00 AM<\/p>

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How many times have you seen astronomical objects move in the sky? Ignoring artificial satellites, there isn\u2019t much that you can look at directly and see move. Meteors and the aurora fit into this category, but outside Earth\u2019s atmosphere, there\u2019s little else.<\/p>

Rare examples include bright close-pass asteroids or highly energetic coronal mass ejections from the Sun viewed through a hydrogen-alpha filter<\/a>.<\/p>

Every now and then there are examples of celestial objects moving across the night sky that can’t be seen in real time. This could be a conjunction<\/a> of a planet moving near an open star cluster or a minor planet like Vesta moving through a constellation over a period of weeks.<\/p>

A composite image showing the retrograde motion of Mars’s movement across the night sky. Credit: Pete Lawrence<\/figcaption><\/figure>

So, how can you capture photographs of the night sky that will show celestial objects in motion? If you’re starting out, you might like to read our beginner’s guide to astrophotography<\/a> first or our DSLR guide<\/a>.<\/p>

On long exposure shots of the stars, it is sometimes possible to see asteroid movement as a streak in the image. At high image scales, the asteroid drifts in position over the length of the exposure and its star-like dot draws a line on the image.<\/p>

However, at low to medium image scales, the movement is imperceptible and no such drifting is recorded. The movement we\u2019re interested in recording here occurs over days, rather than the length of a typical long exposure photograph, and this requires a different strategy.<\/p>

\"International
The International Space Station over Ribblehead Viaduct, captured by Pete Collins in 2016. This composite image was captured using a Canon EOS 6D DSLR camera and Samyang 14mm f2.8 lens at f4, ISO 200. 6 frames were used to show the movement of the ISS, but recording the motion of a planet, for example, will require capturing images over multiple nights.<\/figcaption><\/figure>

The best way to show movement over several nights is to create a composite image. Combining results from each night, a single image can be created that shows the apparent movement of an object.<\/p>

Although at first this seems like a straightforward task, there are caveats. The weather can play havoc with this type of image presentation, creating unwanted gaps and, where it\u2019s poor, destroying the impression of movement. For help with this, read our guide to predicting the weather for astronomy<\/a>.<\/p>

The Moon changes the brightness of the surrounding sky, which can also create issues with the composite, so it pays to be aware of the phases of the Moon<\/a> around the time you are shooting.<\/p>

Sign up to receive the BBC Sky at Night Magazine e-newsletter<\/a> for weekly Moon phases delivered directly to your email inbox.<\/p>

Use stars as your background reference point<\/strong><\/h2>
\"The
The stars of the Summer Triangle asterism. Use bacground stars as your positional reference point when capturing images for your composite astrophoto. Credit: A. Fujii<\/figcaption><\/figure>

One of the biggest considerations is positional reference. If you\u2019re lucky enough to have a home observatory<\/a> in which you can leave a telescope plus camera setup untouched for the sequence duration, then fine.<\/p>

But if, like most imagers, you need to move things between shots, getting the final images registered to one another can be a challenge. Fortunately, the stars come to the rescue, as they create an effectively fixed background reference.<\/p>

Even here there are potential issues; if the sky conditions vary from night to night, it\u2019s important that the positions of the reference stars are recorded.<\/p>

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Venus in twilight, 25 December 2019, 4:31pm GMT. A lack of background stars would make capturing a motion composite shot at this time of the evening tricky.<\/figcaption><\/figure>

Then there\u2019s the issue of twilight<\/a> shooting, when there may be no stars seen at all. Here, you rely on the foreground horizon for reference, ensuring that each shot is made at the same time from one night to the next.<\/p>

Time this incorrectly for an evening shoot and you may get close to sunset, naturally bringing the sequence to an unwanted and abrupt end.<\/p>

In the step-by-step instructions below you\u2019ll find some more tips for creating a motion composite.<\/p>