VIMS PREVIEW IMAGES =================== Mark Showalter, PDS Rings Node, January 2011 VIMS preview images are designed to provide a quick view of the the contents of each spectral image cube. Because each cube contains up to 352 spectral channels and a color image can only consist of red, green and blue (RGB) intensities, these images will only highlight a tiny subset of the information contained in the data files. The images shown are customized to the instrument's target or mode so as to maximize the amount of useful information visible to the eye. The type of image shown is indicated by the color of the frame that surrounds it: Frame Image Type ----- ---------- WHITE: Default visual RGB. CYAN (blue-green): Enhanced visual. RED: Default IR. BLUE: IR enhanced for water ice. GREEN: IR enhanced for methane and Titan's surface. ORANGE: IR enhanced for Titan's cloud features. GRAY: IR for occultation profiles. Note that the images are derived from un-calibrated VIMS data, which means that some tints and biases appear in the coloring due solely to the responsivity of the instrument. These instrument-specific traits are suppressed when possible. Up to four image tiles are generated for each VIMS cube. The selection is as follows: For occultations: GRAY For Saturn images: WHITE, CYAN, RED, GREEN For Titan images: WHITE, RED, GREEN, ORANGE For other images: WHITE, CYAN, RED, BLUE Individual tiles are omitted if the corresponding channel (visual or IR) is off for that particular observation. The tiles are arranged to fit optimally inside a single square image; the arrangement chosen depends on the number and shape of the tiles. VIMS TILE DETAILS ================= WHITE: Default visual RGB This is a rough representation of what the human eye would see. Red, green and blue channels correspond roughly to the color response of the human eye. Because the VIMS files are un-calibrated, be aware that the images often have a strong yellow tint. CYAN: Enhanced visual These images are designed to show more of the wavelength range of the VIMS visual channel. Blue corresponds to ~ 0.35 microns; green to ~ 0.55 microns, and red to 0.89 microns. Each color is normalized to the same intensity so a typical image will appear gray. However, image regions showing a bluish tint indicate that the near UV spectral channels are relatively brighter, and regions with a reddish tint indicate that the IR channels are relatively brighter. Note that the 0.89 micron wavelength (mapped to red) corresponds to an absorption band of methane, so changes in methane content within the field of view will produce reddish variations. Different sizes of water ice grains, as well as different types of contaminants, tend to change the relative slopes of the ice spectrum below and above 0.55 microns; these can also produce color variations in the images. RED: Default IR These images show key aspects of the target's IR spectrum. Blue maps to ~ 1.8 microns, green to ~ 3.0 microns, and red to ~ 5 microns. Water ice is bright in blue and very dark in green; methane is dark in blue and brighter in green. Thermal emission dominates in red; this is also a band where Titan's clouds are transparent so surface features become visible. BLUE: IR enhanced for water ice These images are designed for icy targets. Blue maps to the mean of the range 0.9-1.3 microns, where ice is bright. Green maps to the ratio of the 2.2 micron peak to the 0.9-1.3 band. Red maps to the shape of ice's 2-micron absorption band, with darker red values corresponding to a deeper band. Images are normalized to a uniform gray, so reddish tints indicate a shallower absorption band and green tints indicate a higher 2.2 micron peak. GREEN: IR enhanced for methane and for Titan's surface These images show the land forms on Titan clearly, and they also show variations in the methane in Saturn's atmosphere. Blue maps to a mean of two methane peaks near 1 and 1.3 microns (which are also holes in the absorption of Titan's atmosphere). Green maps to ~ 2.01 microns and blue to ~ 2.8 microns. Colors are normalized independently, so that the overlaid red, green and blue image channels each contains the full range of contrast from nearly black to nearly white. ORANGE: IR enhanced for Titan's cloud features These images map red, green and blue to bands where the atmosphere of Titan is relatively opaque: red is the mean of 1.2 and 1.5 microns; green is ~ 2.1 microns and blue is 4-4.8 microns. Colors are normalized independently, so that the overlaid red, green and blue image channels each contains the full range of contrast from nearly black to nearly white. GRAY: Occultation profiles Occultation mode data are stored as cubes, with time increasing first in the "sample" direction (rightward in the images) and then in the "line" direction (downward). This matches the ordering of words on a page. The blue and red intensities of each pixel are proportional the to the mean signal in the ranges 0.9-2.9 microns and 3.4-5 microns, respectively. The green intensity is based on the depth of the ice band at 2.9-3.1 microns. All three bands are normalize to the same value, so the image should appear to be a uniform gray except for subtle tints related to any wavelength-dependent variations in the occulted signal. In addition, the mean intensity in each line is marked by a red, green or blue pixel, with values of zero at left and values of full intensity at right. In this way, the image also shows a crude, "sideways" profile derived from the occultation, with time increasing downward and intensity increasing toward the right.