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Visible light and the JANUS camera on JUICE

JANUS is one of the instruments on JUICE. As a camera, its main purpose is to take pictures. Before we look in detail at what JANUS can do, let’s explore the concept of the electromagnetic spectrum and visible light, because light is essential for good pictures.

The visible range of wavelengths refers to the portion of the electromagnetic spectrum that is visible to the human eye. This is what we generally call visible light. It spans from approximately 400 nanometers (nm, one billionth of a meter) to 700 nm. Within this range, different colours are perceived by the eye, with shorter wavelengths appearing as violet and longer wavelengths appearing as red. The visible spectrum includes all the colours of the rainbow: violet, blue, green, yellow, orange, and red. When combined, these colours create white light. The ability to perceive the visible spectrum allows humans to see and distinguish objects, shapes, and colours in their surroundings.

JANUS is an advanced camera system specially designed to capture stunning images of Jupiter’s moon Ganymede and the majestic planet Jupiter itself. Its remarkable capabilities allow it to acquire images slightly beyond the wavelengths of visible light, starting at the near-ultraviolet (UV) at 350 nm, and extending into the near-infrared range (IR) at 1050 nm. This broad spectrum enables scientists to study the different colours and chemical elements present on the moons and planets, providing valuable insights into their compositions and characteristics. With a wide field of view of 1.3 degrees, JANUS can even resolve a tennis ball from a distance of one kilometre. Its spatial resolution is exceptional, reaching up to 2.4 meters per pixel when in close orbit around Ganymede, and about 10 kilometres per pixel for Jupiter.

The JANUS camera system. Credit: LDO, DLR

One of the unique features of JANUS is its innovative filter wheel equipped with 13 filters in various colours that can be rotated in front of the camera’s focal plane module (FPM). This versatility allows JANUS to detect different concentrations of chemical elements, offering valuable data for scientific research. For example, the camera can use red for detecting methane and yellow for identifying sodium, while the near-infrared filter helps analyse rock-forming minerals and salts on the moons. Moreover, the JANUS camera boasts exceptional stability, ensuring its optics remain rigid and precisely aligned despite the challenges of vibrations during launch and sudden temperature changes in space. Engineers have developed a special mechanical and thermal design to minimise deformations, which are limited to less than a tenth of a human hair’s thickness.

The JANUS camera system consists of three distinct units: the optical head, including the telescope and the filter wheel, and the FPM. Additionally, the camera incorporates proximity and main electronics responsible for camera control, data management, image data compression, and a power supply unit. The optical system employed in JANUS is called the catadioptric telescope, combining refractive lenses and reflective mirrors, resulting in exceptional optical qualities. This telescope is coupled with a framing detector using a sensor with 2000 by 1504 pixels, strategically located within the FPM. The combined efforts of these components enable JANUS to deliver high-quality, detailed images, enriching our understanding of Jupiter and its captivating moon, Ganymede.

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