Visual and Infrared Mapping Spectrometer

(Printer friendly version)

So what exactly is a Visual and Infrared Mapping Spectrometer?

VIMS is, in essence, a color camera mounted on the Cassini spacecraft bound for Saturn. But its a very special camera because of the KIND of color it captures. When the human eye looks at an object, the cones in the retina are able to discern the amount of light that hits them at 3 different wavelengths, which are interpreted as colors. Light with a wavelenth of around 420 nm (nanometers, or billionths of a meter) looks blue, while light at 534 nm looks yellow and 564 nm looks red. Colors other than red, yellow, and blue are the result of the eye receiving different amounts of light at each wavelength at the same time. Cassini VIMS takes pictures in 352 different colors at the same time, with wavelengths between 300 and 5100 nm. Thus the color range of VIMS's vision is greater than that of the human eye (300 - 5100 nm as opposed to 380 - 620 nm) and VIMS is far more accurate in determining the wavelength of the light that strikes it than the eye as well.

Why is all that color information important?

The range of wavelenghts that the human eye can see is called the visible region of the spectrum. VIMS is sensitive beyond this range. In addition to covering the visible spectrum, VIMS extends its capabilities into what is called the infrared part of the spectrum, which has wavelengths longer than the reddest our eyes can see. This range, coupled with the ability to discern different wavelengths (called spectral resolution), allows the VIMS instrument to be able to very accurately quantify the light it detects.

On the many icy moons of Saturn this will mean the identification of the composition and properties of their surfaces. For instance, the moon Iapetus is one of the strangest in the solar system because one half of it is darkly colored, while the other side is bright. Ever since the Voyager encounters discovered this anomaly, planetary scientists have been trying to determine what causes it. Data from VIMS will help us find out. For instance, it has been proposed that the dark half sweeps up dark dust-like material that was thrown off of Phoebe, another moon of Saturn, by micrometeorite impacts (which is much less farfetched now that such impacts have been shown to be the source of some of the tenuous rings of Jupiter from the Galileo spacecraft). A spectral match between the dark side of Iapetus and Phoebe would lend credence to this hypothesis.

Another aspect of the Saturnian system VIMS will shed light on is the rings. The composition and nature of the rings are still debated, and VIMS will be able to not only determine what the rings are made of, but also how large the particles that make them up are. As an example, in the image to the left you'll notice that you can see through parts of the rings. Particles of different sizes scatter light differently. Dust, for instance, transmits more light at longer (redder) wavelengths (this is why sunsets appear red), thus an analysis of the amount of light you can see through the rings at different wavelengths will reveal the fraction of the ring that is made up of dust as opposed to larger objects.

On Saturn itself VIMS will be able to look at many different layers of the atmosphere simultaneously. Clouds have different optical properties at different wavelenths. Taking advantage of this, at the wavelenghts where the upper cloud decks are transparent, pictures of the lower atmosphere can be obtained and at wavelengths where they absorb or reflect the upper atmosphere is seen. By studying precisely the wavelengths where the atmosphere absorbs, reflects, and emits, the composition of the atmosphere can also be inferred.

Saturn's moon Titan, the only moon in the solar system with an apreciable atmosphere, is covered by a layer of smog think enough that the surface is invisible at optical wavelengths. When Voyager 1 flew by Titan in 1980, its pictures revealed a nearly featureless orange ball (left image). However, the Hubble Space Telescope was able to see through the haze by operating near 1000nm, in the infrared. Although extremely rough due to being taken from such an extreme distance, the HST infrared images revealed a large high reflectivity area about the size of Australia on the surface of Titan. What these brightness differences mean will be something VIMS and other Cassini instruments, including the Huygens probe, will investigate.

How does VIMS work?

VIMS is actually 2 cameras in one: one for visible wavelengths and one for the infrared.

The visible channel (VIMS-V) is a 4.5 cm telescope that deflects its beam through slit, and then through a diffraction grating. The slit determines the field of view, allowing in only light along a line. The diffraction grating is a grooved mirror such that light reflecting from each groove interferes with the light coming from other grooves in a way that causes light to be dispersed according to wavelength, like a prism. Devices that disperse light into its component colors for analysis are called spectrometers. The light is finally focused on a CCD (Charge-Coupled Device) detector. The CCD is an array of 256x512 elements that each count the number of photons that they receive. CCDs are the detectors in digital cameras and newer digital camcorders.

For every exposure, each of the elements along the slit in the y-direction is dispersed into a spectrum in the x-direction by the
grating. Thus the result of each frame isn't an image at all, but rather a spectrum in x for each pixel in y. In order to create an image, the slit is stepped along to different positions across the target object (actually its the mirror that moves), and its this movement of the slit along the object that is called "mapping". If each of these separate xyplanes is placed sequentially behind the previous plane, a three dimensional "image cube" results. In the third dimension each xyplane is separated from every other xyplane by both a positional distance along the target and the exposure time of the frame; the third dimension is usually referred to as t. Spectral information is obtained for every point across the target object, and if you were to slice the cube along its ytplane the result would be an image of the target object at the wavelength you chose to slice it.

Normal spectrometers can be operated in mapping mode by moving the telescope to position the entrance slit over different portions of the target object, but on Cassini this would mean moving the entire spacecraft to repoint the instrument (Cassini's scan platform was descoped due to budget constraints). Its VIMS ability to move its primary mirror so that it obtains spectra of different parts of the target object without targetting the entire spacecraft that makes it a "mapping spectrometer".

The infrared part of VIMS, VIMS-IR, is similar but only has a 1-dimensional InSb (Indium Antimonide, a chemical compound) detector. Thus it can only take the spectrum of one point at a time. In the amount of time it takes the optical half to make one complete exposure of the spectrum of a line of points, the IR half has to take the spectrum of 64 different points one at a time. To do this it requires a larger telescope with which to collect light, 23 cm, than did the optical assembly.

The main electronics assembly drives and reads the data from both VIMS-V and VIMS-IR. The data from the two halves of the instrument are fused together, partially reduced, and compressed before being relayed to the main Cassini computer for storage and later transmission to Earth.

VIMS' Wavelength Calibration

VIMS’ IR wavelength calibration has been observed to be drifting toward longer wavelengths over the roughly 10 years since the start of the Cassini orbital tour. The shift amounts to about 9 nm, or about 0.6 channels over the roughly 10.5 years since the start of the Cassini orbital tour. The reasons for the shift are unknown at present. The shift was detected and quantified by observing changes in the peaks of the atmospheric transmission windows in Titan’s reflectance spectrum in the near infrared. Since the wavelength shift was recognized, the VIMS team has initiated wavelength calibration checks using the internal calibration diode built into the IR channel. As of November 2014, the drift toward longer wavelengths in the VIMS IR channel continues.

Going forward from approximately June, 2014, the wavelength calibration of VIMS will be determined solely using its internal calibration diode. The present plan has the VIMS team performing diode calibrations every few days throughout the rest of the Cassini mission, starting approximately June, 2014.

For details of the results, please refer to the Excel spreadsheet posted on this PDS site. Periodic updates to the spreadsheet will be published on the PDS site. The current version of the wavelength shift document (November 2014) is provided here for your convenience.