The L'Ralph Instrument

Mar 31, 2021

L’ Ralph is actually made up of two instruments — MVIC (Multispectral Visible Imaging Camera), a color visible imager, and LEISA (Linear Etalon Imaging Spectral Array), an infrared spectrometer. A beamsplitter inside sends the infrared light to LEISA and reflects the visible light to MVIC.

L'Ralph SPECIFICATIONS

Mass: 31.0 kg
Power: 25.1 W
Apeture: 75 mm
Focal Length: 450 mm

MVIC

Instataneous field of view: 29 μrad/pixel
Spectral Range: 0.38-0.92 μm
Spectral Resolution: 47-550 nm

LEISA

Instataneous field of view: 80 μrad/pixel
Spectral Range: 1.0-3.6 μm
Spectral Resolution: 10 nm

L’ Ralph is actually made up of two instruments — MVIC (Multispectral Visible Imaging Camera), a color visible imager, and LEISA (Linear Etalon Imaging Spectral Array), an infrared spectrometer. A beamsplitter inside sends the infrared light to LEISA and reflects the visible light to MVIC. While color images can be produced with any three color bands, MVIC has five color bands that cover the complete spectrum. Each band was specifically chosen to help the scientists identify a different compositional unit on the surfaces of the Trojan asteroids. For example, the red band is sensitive to phyllosilicates, a kind of hydrated mineral scientists expect to find on the Trojans’ surfaces, while the violet band will help determine whether the asteroids’ surfaces contain troilite (FeS). The violet band is also sensitive to CN absorption, which if detected around an asteroid would indicate activity. MVIC also has six CCDs (the light-sensitive devices used in digital cameras in place of film), each with up to 64 rows of 5000 pixels each.

LEISA contains an etalon, a pair of thin reflective surfaces that separate light into a spectrum, like a prism. LEISA can then analyze how different substances on the asteroid’s surface behave when absorbing and emitting the different wavelengths, allowing it to identify the different kinds of rocks, ices, and organic compounds that could be present. LEISA’s detector has 1400 rows of 1000 pixels each, each row designed to detect a different wavelength of infrared light. From 1000 km away, LEISA will be able to resolve craters on the order of 500 m wide.

Because the photographs L’Ralph will take are massive, it also has 256 gigabits of onboard memory. L’Ralph will search the Trojans for organics, ices, and hydrated minerals, and its images will help us determine the Trojans’ surface compositions. A team at Goddard Space Flight Center is building L’Ralph, under the leadership of Dennis Reuter (Instrument Principal Investigator) and Amy Simon (Instrument Deputy Principal Investigator). L’Ralph is based on the New Horizons instrument Ralph (named after Ralph Kramden from The Honeymooners, since the instrument worked closely with an ultraviolet spectrometer called Alice) as well as OSIRIS-REx’s OVIRS (OSIRIS-REx Visible and InfraRed Spectrometer). Ralph is responsible for New Horizons’ incredible color images of Pluto, Charon, and Arrokoth. Lucy’s new iteration of Ralph has a multitude of changes from its New Horizons predecessor (such as the addition of onboard memory), resulting in an instrument with roughly three times the mass and four times the power requirement. Despite this, L’Ralph still runs on less power than the average ceiling fan.

It was important to limit the number of moving parts on Lucy’s instruments, since increasing the number of moving parts increases the risk associated with part failure. Therefore, like L’LORRI, L’Ralph does not have a focusing mechanism. One might expect extreme temperature differences in space to cause a device like this to defocus (since the optical system would be expanding and contracting as the temperature changes), but most of L’Ralph is made from a single block of aluminum. Using one material throughout means that if a part expands or contracts, the other parts will expand or contract at the same rate, helping to keep L’Ralph in focus. Even the mirrors are made of aluminum, and diamonds were used to turn the metal into finely polished surfaces.

Color Composit and LEISA image of Arrokoth
Images of Arrokoth from New Horizons. On the left is MVIC color data added to a detailed LORRI image, and on the right is an image taken by LEISA. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute