Robotic Arm Camera (RAC)

Instrument Overview

The Phoenix Robotic Arm Camera (RAC) is a variable-focus color camera mounted to the robotic arm (RA) of the Phoenix Mars lander. It is designed to acquire both close-up images of the Martian surface and microscopic images (down to a scale of 23 micron/px) of material collected in the RA scoop. The mounting position at the end of the robotic arm allows the RAC to be actively positioned for imaging of targets not easily seen by the Stereo Surface Imager (SSI), such as excavated trench walls and targets under the lander structure. Color information is acquired by illuminating the target with red, green and blue light emitting diodes. Digital terrain models (DTM) can be generated from RAC images acquired from different view points. This can e.g. provide high-resolution stereo information about fine details of the trench walls.

Information in this instrument description is taken from The Phoenix Robotic Arm Camera [KELLERETAL2008]. See this paper for more details.

Scientific Objectives

The science objectives of the RAC are the following: (1) Characterize a soil patch prior to digging (together with the Surface Stereo Imager, SSI). This characterization will also support the selection of the digging site - an important tactical step during mission operations. (2) Characterize scoop - soil interactions (e.g. during scraping or digging) in terms of physical properties such as cohesion and soil strength. (3) Characterize trench walls: (a) Search for fine- scale soil/ice layering, (b) monitor possible temporal changes due to sublimation of water ice away from a newly excavated surface. (4) Characterize soil samples in the scoop prior to delivery to the analyzing instruments on the lander deck MECA and TEGA. (5) Characterize the surface below the lander, and in particular estimate the penetration depth of the foot pads with direct implications on the shallow sub-surface environment. (6) Provide a fast way to create a Digital Terrain Model (DTM) of the landing site. The RAC has a larger field of view (FOV) and stereo base than the SSI. In addition the stereo base can have any orientation (from horizontal to vertical). In that sense the RAC complements SSI data in the generation of the DTM. (7) Atmospheric science: (a) monitor the horizon and search for dust devils, (b) provide 'one shot' overview images of the brightness gradient of the Martian sky. The RAC also fulfills important tactical (sol planning) tasks by supporting instrument checkout of MECA and TEGA and documenting sample delivery to these instruments that cannot be done adequately by the SSI.

More general science goals of the RAC include: (a) Search for seasonal and climate records in the sub-surface, (b) search for records of diurnal/seasonal, vertical/lateral transport processes in the soil (such as evaporative pores or salt cementation), (c) constrain long-term weathering processes from shape and texture of rocks or rock fragments.

 

Calibration

The bulk of the RAC calibration and instrument characterization activities occurred in late 1999. In early 2006 the instrument underwent additional testing to verify the 1999 test results and prepare for the Phoenix mission. The stability of the parameters over more than 7 years is remarkable. During nominal operations the raw images must be corrected on board for blemished pixels in order to minimize compression artifacts. The active-area frames (512 x 256 px) are then JPEG compressed by a factor of ~ 4 (depending on resources) and downloaded. All remaining calibration steps (such as dark current correction (insignificant) and flat fielding) will be done on ground.

Operational Considerations

The CCD was qualified for the temperature range 160 - 320 K. However, the stepper motors for cover and lens system need to be heated to above 210 K before they can be operated. This minimum temperature is required by the grease lubricant in the bearings of these stepper motors. All the other moving parts in the RAC are lubricated with MoS2 and need not to be pre-heated, as long as their temperature is above 140 K.

Detectors

The size of the RAC was driven by the available charge-coupled device (CCD) detector package and Sensor Head Board (SHB) that were originally designed for the Descent Imager/Spectral Radiometer (DISR) camera on board the Huygens Probe of the Cassini mission. The RAC has a double Gauss lens system and a frame-transfer CCD detector. The CCD is a front side illuminated frame transfer device employing buried channel technology with 2 phase Multi- Pinned-Phase (MPP) clocking. The pixel spacing is 23 micron in both directions, however, 6 microns in line direction of each pixel are covered by an anti-blooming structure to remove excess charge in case of overexposure. The CCD has no anti-reflection coating. It consists of a (512 active + 16) columns by 256 lines imaging area, and a (512 active + 16) columns by 256 lines storage area covered by a metal mask. Each line from the serial readout register contains 4 null pixels (the 'null strip') providing system noise information, 8 dark pixels (the 'dark strip') measuring dark current, 512 active pixels, and 4 null pixels again. These dark current strips are used to scale dark current corrections on board using the line- by-line ratios. After exposure the photogenerated electrons are moved from the active area to the storage area within 1 ms. The readout of the storage area takes 2 s. The CCD output signal is first amplified by the sensor head board (SHB), and then transmitted via long (up to 4 m) electrical lines to the CCD Readout Board (CRB) located inside the central electronics box of the lander. The CRB accommodates the analog signal chains with correlated double sampling, a sample and hold amplifier, 12 bit A/D converter, clock driver, power converter, and a digital control unit with a parallel interface to the experiment processor.

Electronics

After exposure the photogenerated electrons are moved from the active area to the storage area within 1 ms. The readout of the storage area takes 2 s. The CCD output signal is first amplified by the SHB, and then transmitted via long (up to 4 m) electrical lines to the CCD Readout Board (CRB) located inside the central electronics box of the lander. The volume between front bulkhead and centre accommodates all optical and mechanical subsystems while the volume between centre bulkhead and rear houses the SHB electronics.

During nominal operations the raw images must be corrected on board for blemished pixels in order to minimize compression artifacts. The active-area frames (512 x 256 px) are then JPEG compressed by a factor of ~ 4 (depending on resources) and downloaded. All remaining calibration steps (such as dark current correction (insignificant) and flat fielding) will be done on ground. The flight software allows for subframing. However, in general mostly full frames will be downloaded.

Filters

The RAC front window is a 2 mm thick, blue-green glass filter (BG40, Schott Inc.) that blocks all red and near-infrared light with wavelengths longer than 700 nm. The color capability of the RAC arises from composing images taken with illumination provided by red, green, and blue light emitting diodes (LED). Since the lighting system operates in the visible part of the spectrum, the camera's color capability is greatly enhanced by cutting off the near-infrared part of the incident radiation. A movable transparent sapphire cover (referred to as dust cover) driven by a stepper motor protects the front window of the camera against atmospheric dust and flying debris that may be kicked up during digging operations. The cover will be open during normal image acquisition.

Optics

The optical bench consists of three walls or bulkheads [front, centre, and rear] which are braced against each other by two side frames [left and right]. The volume between front bulkhead and centre accommodates all optical and mechanical subsystems while the volume between centre bulkhead and rear houses the SHB electronics, with two connectors mounted to the rear bulkhead.

Location

For ground-based data, coordinates of the observatory

Operational Modes

The RAC employs a movable variable focus objective ranging from macro 1:1 mode to infinity. The lens in its cell is mounted to a translation stage which allows its position to be changed along the optical axis. The optical distortion was measured by imaging a target of equally spaced holes manufactured out of photo-etched chrome on glass. The RAC flight model turned out to have a positive distortion that reaches 0.28% at the corners of the image, implying that an image point at that location is by 0.80 pixels further out than it ideally should be.

Subsystems

The RAC consists of the following subsystems: (1) optical bench / frame, (2) Sensor Head Board with CCD detector, (3) double Gauss lens with lens cell, (4) lens focusing mechanism with stepper motor and reference switch, (5) protective dust cover with mechanism, stepper motor, and reference switch, (6) upper and lower lamp assemblies, (7) two temperature sensors, and (8) a protective shell.

Measured Parameters

The mounting position at the end of the robotic arm allows the RAC to be actively positioned for imaging of targets not easily seen by the SSI, such as excavated trench walls and targets under the lander structure. Color information is acquired by illuminating the target with red, green and blue light emitting diodes. Digital terrain models (DTM) can be generated from RAC images acquired from different view points.