Capturing your images
A little planning will ensure you capture the best images to animate
Jupiter is at its best when the atmosphere it’s viewed through is stable. Atmospheric stability is affected by the weather, altitude of the object and the presence of the jetstream. You can determine when the most promising conditions are due from meteorological forecasts. Active fronts and deep low-pressure systems tend to cause instability, while high pressure tends to produce the most stable conditions. Weather dominated by high pressure with an absence of an overhead jetstream typically produces the best conditions, and websites such as Netweather provide jetstream forecasts for this: www.netweather.tv/charts-and-data/jetstream.
It is when Jupiter is positioned due south that it’s highest in the sky and above the thickest, most turbulent part of the atmosphere. If you work out how long you want to image for, you can time your session so the planet is due south at the mid-point. Jupiter moves a fair bit over a long session, so make sure it won’t disappear behind any foreground objects.
‘Still imaging’ – taking a single frame of a target – tends to produce poor results with planets. A stills camera, such as a DSLR, captures a snapshot, frozen in time, which is typically heavily distorted by the atmosphere. ‘Lucky imaging’ is a better solution: taking many images in quick succession, harvesting the best and combining them into a single image. Often resulting in thousands of images, the results are analysed and organised by quality, aligned and averaged to reduce noise. Known as registrationstacking, this laborious process is largely automated by freeware programs such as RegiStax (www. astronomie.be/registax) and AutoStakkert! (www. autostakkert.com). The final image these generate should be robust enough for post-capture tweaking.
Low-altitude bright targets suffer from chromatic dispersion, creating colour fringes around their north and south edges. With a colour camera, an atmospheric dispersion corrector can reduce this, and a filtered mono camera can diminish the problem too. Capturing individual red (R), green (G) and blue (B) frames is an excellent way to get high-quality results. Taking multiple filter sequences means you have a choice of frames to combine. For example, by taking two RGB sequences you can create an image with the RGB from the first set, but also an image with R from the second set, and G and B from the first, and so on. An infrared-sensitive camera with an IR-pass filter will produce high-contrast results that will animate well, and wavelengths at the red end of the spectrum tend to be less affected by atmospheric instability.
Jupiter’s four largest, Galilean moons, Io, Europa, Ganymede and Callisto, are bright enough to be recorded with a conventional stills camera. With a large enough image scale, they can be shown clearly as separate entities to the planet and, over time, their movement around Jupiter can be recorded.
There are several ways to accomplish this. One is to ignore the planet’s disc entirely and simply adjust the exposure so the moons record as bright points of light. It pays to maintain a reasonably short exposure time here: too long an exposure can produce issues such as each moon ‘dot’ ‘wiggling’ at high frequency, due to instabilities in our atmosphere. If the telescope’s mount isn’t accurately polar-aligned, extended exposures will also result in the moons appearing as short lines rather than points.
Lucky imaging can help overcome many of the vagaries introduced by our unstable atmosphere. If imaging simply to record a moon as a bright dot, you may still end up with a moon’s image looking larger than it should, but it’ll be tighter than it would be using a stills camera with a long exposure.
The Galilean moons orbit Jupiter at different periods according to their distance from the planet. Innermost Io takes 1.8 Earth days, Europa 3.6 days, Ganymede 7.2 days and Callisto 16.7 days. Consequently, the fastest movement across the sky will be shown by Io.
To create an animation sequence, determine how you want to image the moons: bright, or properly exposed. If you’re just starting out, go for the bright option, adjusting your camera settings to record an over-exposed point of light. Then decide on the frequency of shots. If you’re a novice, try to record a shot or frame sequence every 15 minutes, shortening the gap once you’ve become more accustomed to building animations. For high-frame-rate captures, try to keep your individual capture lengths to no more than 60 seconds.
You’ll also need a point of reference for the final animation build, Jupiter being the obvious choice. If you intend to over-expose Jupiter, try to limit the over-exposure so you can still determine where the planet’s edge is in each frame. This isn’t an issue if you intend to capture the moon frames against a properly exposed planet.
As you get more comfortable creating moon animations, you can adjust the period between captures, or change the image scale to create interesting compositions.