MECA Microscopy Sample Stage Configuration Sols 0-90


Overview

Figure 1 shows a model of the MECA microscope. On the left is the fixed-focus, 6x microscope and on the right is the sample stage, with the Atomic Force Microscope in the center. The MECA sample stage rotates to select substrates for imaging, and translates to focus the image. A translation step is 0.25 µm (compared to a depth-of-field of over 50 µm), and a rotation step is 15 µm (just under 4 pixels). The microscope image is 2 mm high by 1 mm wide, with 512x256 pixels.

Micro 01
Figure 1: The MECA microscope system, with optical microscope on the left, Atomic Force Microscope in the center, and Sample Wheel and Translation Stage (SWTS) on the right. At the front of the microscope are LEDs for illumination. A dark enclosure covers the instrument.

Substrate arrangement

Figure 2 shows the arrangement of the substrates around the wheel. The 69 substrates, each 3 mm in diameter, are spaced on 5 mm centers around the wheel. A sample delivery coats one set of 6 substrates with particles. The wheel holds ten such sets of 6, with an additional 9 calibration targets or tools.

Micro 02
Figure 2: The sample wheel layout
Each of the ten sets includes:
- A sticky silicone target.
- A textured substrate, micro-machined from silicon with posts and pits, designed to hold particles of a particular size (sometimes called "nanobuckets").
- A weak magnet.
- A strong magnet.
- Two "microbuckets," cups more than 2 mm deep, to capture larger particles.

Comparing Before and After

Some contamination from Earth is unavoidable on this scale, so meaningful conclusions about specific particles can only be drawn by comparing images acquired before and after the sample is delivered. For example, Figure 3 shows a sliver, possibly metal wire, in both the "before" and "after" images, indicating that it hitch-hiked from Earth. Particles can sometimes move as a result of the stage motion (as in Figure 3), so it is not sufficient just to subtract the two images. In at least one case, the substrate itself rotated between images.

Micro 03
Figure 3: These images show a comparison of the weak magnet OM7 from the Optical Microscope on NASA's Phoenix Mars Lander before (left) and after (right) soil deposition.

The microscope took the left image during Phoenix's Sol 15 (June 10, 2008) and the right image during Sol 21 (Jun 16, 2008).

Image Credit: NASA/JPL-Caltech/University of Arizona/Imperial College London

Lighting and Color

While the camera itself is black-and-white, color images can be simulated by use of multi-color lighting, provided by 12 light-emitting diodes (LEDs). We typically use two each of red, blue, or green, then combine the three images with a weighting of 0.8 (red), 0.9 (green), and 1.0 (blue). This type of coloring can lead to artifacts when a particular LED reflects off a shiny surface, giving the false impression of a pure red, green, or blue feature.

UV fluorescence imaging

Three of the LEDs are ultraviolet with a wavelength of 375 nm (filtered to remove any visible light). The camera also has a filter to keep UV light from the image, so only features that fluoresce (like a T-shirt under "black light") will be seen in the UV image. Since these are very long exposures and some stray light leaks into the enclosure, it is necessary to subtract a "dark" image from the UV image to see the fluorescence. This will also remove most of the small bright spots brought out by the long exposure due to "hot pixels" in the CCD. After the subtraction, before and after images must still be compared. A "ghost image" remains due to leakage of red light (~700 nm) from the UV LED. Tiny light or dark streaks are often due to cosmic rays hitting the CCD.

How to Use the Table

The ten sample sets and the calibration set are listed by their designation and their nominal rotation and translation positions for an in-focus image at the center of the substrate. By comparing actual translation and rotation positions in the image captions, it can be determined which substrate was used and where the image was taken relative to the center and relative to the focus. Since the wheel sometimes slips a bit, these values are only approximate. Finally, the columns indicate on which sols images were acquired and on which sols samples were delivered from the landing (L), the scoop (S), or from the air (A). Sol 0-30 Table  Sol 31-60 Table  Sol 61-90 Table  Sol 91-120 Table  Sol 121-150 Table