by The Design
taken after landing, from an elevation 66 centimeters above the ground, are slightly out of focus, and MARDI’s ability to resolve ground features since landing is the same as it was
from a height of 2 meters. On the ground, the MARDI field of view covers an area about 92
centimeters across the driving direction and 66 centimeters along the driving direction. The fixed MARDI view includes part of the left front wheel and the area immediately behind it
and next to it, underneath the rover. Further MARDI facts are summarized in Table 7.4.
Figure 7.10 shows examples of MARDI images taken under different conditions. They have been projected to correct for the distortion caused by its wide-angle lens.
9 The MARDI instrument is described in Malin et al. (2009) and Malin et al. (2017)
7.3 MARDI: Mars Descent Imager 253
Figure 7.9. Location of the MARDI instrument on the rover. Inset photo taken at Malin Space Science Systems; large photo taken during final assembly at Kennedy Space Center, showing MARDI’s point of view beneath the rover in descent configuration. NASA/JPL- Caltech/MSSS.
Table 7.4. MARDI Facts.
2 km elevation
2 m elevation
66 cm elevation
depth of field
2m - infinity
angular resolution
0.76 mrad/pixel
field of view (FOV) 70° x 55°
spatial resolution
1.5 m/pixel
1.5 mm/pixel
0.5 mm/pixel at image center (out of focus)
7.3.2 Using MARDI
Landing video. On landing day, MARDI switched on about 6 seconds before heat shield
separation and took 1504 images at an average 3.88 frames per second with exposures of 0.9
milliseconds. The first 25 were black, seeing the inside of the heat shield; the next 622
254 Curiosity’s Science Cameras
Figure 7.10. Examples of MARDI images. Top: View of the heat shield during descent. Middle: sol 45 image under daylight conditions, showing reduced contrast. Bottom: sol 738 image under twilight conditions, which improves contrast. Images 0000MD0000000000100035C00,
0045MD0000300000101520E01, and 0738MD0003120000102267E01. NASA/JPL-Caltech/
MSSS.
7.3 MARDI: Mars Descent Imager 255
chronicled the final 2.5 minutes of landing, from heat shield separation to touchdown; and
the final 857 were taken on the surface. The MARDI descent images not only captured the
dynamics of the spacecraft’s descent, they also documented large gravel being propelled
toward the rover as the rockets impinged on the surface, quickly providing an explanation of how the rover deck came to be salted with gravel. 10 Unfortunately, the front element of the MARDI lens was coated with dust during the landing. The dust coating scatters sunlight,
which has the effect of reducing the contrast of MARDI images taken since landing.
Clast surveys. At parking spots between traverses, MARDI shoots a photo to document
the sizes and shapes of rock fragments on the surface. Since sol 310, MARDI clast survey
images have mostly been taken at twilight (around 18:30 local true solar time), which
reduces the light scattering off of the dusty front window, producing much higher quality
images (see Figure 7.10). 11 Beginning on sol 488, MARDI has also sometimes been used before and after short “bumps” to obtain 4 or 5 overlapping images for analysis of the
three-dimensional shapes of rock clasts. 12
Sidewalk mode. MARDI can take movies during drives, acquiring mosaics along drive
paths. 13 In sidewalk mode, MARDI takes an image every 3 seconds, but only saves the image if onboard software determines that the new image is significantly different from
the previous one. The saved images have more than 75% overlap. Returning every third
image to Earth allows the construction of a mosaic, but if all images are returned, the team can generate a digital elevation model in addition to the mosaic. Sidewalk mode was tested
on sol 651 and has since been used to document terrain thought to be hazardous to rover
wheels as well as science stops like Pahrump Hills. A complete list of sidewalk mode
observations to sol 1647 is in Table 7.5.
Table 7.5. MARDI Sidewalk Mode sequences to sol 1800.
Sol
Description
651
Test of sidewalk mode
691
Characterize terrain thought to be hazardous to rover wheels
739
Characterize terrain thought to be hazardous to rover wheels
780
Stratigraphy of Pahrump Hills outcrop
785
Stratigraphy approaching Book Cliffs
787
Stratigraphy between Book Cliffs and Gilbert Peak
790
Stratigraphy to Alexander Hills
792
Stratigraphy to Chinle outcrop
794
Stratigraphy to Whale Rock
797
Stratigraphy between Whale Rock and scuff test site
1181
Document sediment interaction with the wheels at Bagnold Dunes
1281
Document drive across knobbly Stimson contact
10 Schieber et al. (2013)
11 Garvin et al. (2014)
12 Garvin et al. (2015)
13 Minitti et al. (2015)
256 Curiosity’s Science Cameras
7.4 MAHLI: MARS HAND LENS IMAGER
7.4.1 Introduction
The Mars Hand Lens Imager (MAHLI, pronounced “Molly”) functions as it is named: it
works like the hand lens that any field geologist carries in order to examine the grain size and structure of rocks. But it’s capable of many other tricks. Located on the turret on the end of the robotic arm, MAHLI can work very close to targets, taking photos with microscopic detail, like its predecessor, the Microscopic Imager (MI) on the Mars Exploration
Rover mission. Unlike MI, MAHLI is focusable, from 2.04 centimeters to infinity. When
held at its minimum working distance, MAHLI can take images with resolutions of 13.9
microns per pixel, enough to resolve the finest grains of sand. But its wide (~35°) field of view makes it a useful tool for imaging large and distant objects, too. It can acquire detailed mosaics of interesting outcrops with arm motions, and obtains 3D information from stereo
pairs or its focal depth.14 Figure 7.11 shows parts of the MAHLI camera, and Table 7.6
summarizes MAHLI facts.
Its position on the end of the robotic arm allows MAHLI to examine Curiosity itself, so
the mission often commands it to document the condition of the wheels, remote sensing
mast, and instruments. And it’s taken some of the most iconic photos of the whole mission:
self-portraits of the rover at drill sites, such as the one on the cover of this book. MAHLI was built and is operated by Malin Space Science Systems, San Diego, California. The
principal investigator is Ken Edgett of Malin Space Science Systems.
7.4.2 How MAHLI works
MAHLI components include a camera head mounted on the turret at the end of the rover
arm, an electronics assembly located inside the body of the rover, and a calibration target mounted on the “shoulder” of the rover arm, its azimuth actuator. The camera head detector and other electronics and internal electronics assembly are identical to those of
Mastcam (see section 7.2.1). Its optics, calibration target, and usage are different.
7.4.2.1 Camera head and electronics
MAHLI’s optics include a sapphire window in front of a group of stationary elements
(refractive lenses). Behind the sapphire window and stationary elements is a movable
group of three elements, operated by a single motor. 15
Because the sapphire window has a long wave cutoff filter and the lens elements filter out ultraviolet light, only light with wavelengths ranging from 394 to 670 nanometers reaches the detector.
Various design elements help MAHLI survive extreme temperatures, exposure to dust,
and sampling-related vibration. MAHLI was designed to operate between –40°C and +40°C,
14 The paper of record for MAHLI is Edgett et al. (2012); two other valuable resources are Edgett et al. (2015) and Yingst et al. (2016)
15 Ghaemi (2009)
7.4 MAHLI: Mars Hand Lens Imager 257
Figure 7.11. Upper left: MAHLI camera head with 89-millimeter-long pocket knife for scale.
Upper right: MAHLI camera head with dust cover open. From Edgett et al, 2012 . Bottom:
MAHLI on Mars as viewed from the right Mastcam, Curiosity sol 128. Image
0128ML0006510000103943E01. NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
258 Curiosity’s Science Cameras
Table 7.6. MAHLI Facts.
Target located 25 cm away from front window
Target at infinity
depth of field
1 mm
–
field of view (FOV, diagonal)
34°
38.5°
FOV (horizontal 1600 pixels)
26.8°
31.1°
FOV (vertical 1200 pixels)
20.1°
23.3°
instantaneous field of view (IFOV) 402 μrad
346 μrad
focal ratio
f/9.8
f/8.5
effective focal length
18.4 mm
21.4 mm
but because it is outside the rover it must be able to survive far more extreme temperatures when it is powered off. It has a movable lens cover to keep the dust out. The dust cover
includes a transparent Lexan lens window in order to permit imaging with the cover closed.
Unfortunately, as with MARDI, a thin film of dust covered the window during landing, and
photos taken with the cover closed have very low contrast and an orange- colored cast. A
contact sensor assembly with two probes protects the camera against contact with the sur-
face, causing the forward motion of the arm to stop when it has brought MAHLI’s sapphire
window within 17 millimeters of a hard surface. A vibration isolation platform, which con-
nects MAHLI to the turret through a set of three springy wire rope assemblies, isolates
MAHLI somewhat from the intense vibration of the drill and CHIMRA.
MAHLI carries six light-emitting diodes (LEDs) on a ring around the outside of the
front lens element in order to illuminate science targets from different directions and with ultraviolet light. Windows on the dust cover allow light from the LEDs to be visible even
with the cover closed. Four of the LEDs emit white light, positioned in two pairs on either side of the lens. The white-light LEDs can be commanded to operate together or independently (with either one side or the other sides or both sides lit), to simulate the way a
geologist tilts a rock while examining it with a hand lens to catch glints of sunlight off of crystal facets. Nighttime white-light LED imaging also allows scientists to directly compare the colors of rock targets at different locations without having to account for differences in solar illumination.
Two ultraviolet LEDs that emit light at a wavelength of 365 nanometers are intended
for identifying minerals that are fluorescent or phosphorescent, and are usually used over-
night. They do leak some light in short visible wavelengths, so white surfaces appear blue
in MAHLI nighttime ultraviolet images. So far there has been no unambiguous detection
of fluorescent or phosphorescent minerals. It would have helped to equip MAHLI with
shorter-wavelength LEDs, but no such LEDs were available when MAHLI was being
designed (no flight-qualified ones, anyway).
MAHLI’s camera head is connected to the electronics through an astonishing 12.7
meters of cable harness. As with Mastcam, MAHLI images are usually stored onboard the
instrument in raw 8-bit form without compression or Bayer interpolation. Full MAHLI
frames can be acquired at a maximum rate of about 1 frame per second, slower than the
maximum rate for Mastcam. MAHLI has video capability because of its shared electronic
design with Mastcam and MARDI, but it has rarely used this ability. Table 7.7 lists the few video observations with MAHLI. A rare, spectacular MAHLI video observation happened
7.4 MAHLI: Mars Hand Lens Imager 259
Table 7.7. Summary of all
Date or Sol
Target Name
Day/Night
MAHLI video mode
observations to sol 1800.
30 Nov 2010 rover deck
ATLO
“ATLO” = Assembly, test,
2 Dec 2010
MAHLI cal target
ATLO
and launch operations, i.e.
3 Feb 2011
infinity focus position ATLO
images taken before launch.
26 May 2011 ATLO
ATLO
165
Sayunei
Night
166
Sayunei
Day
282
CheMin inlet
Night
687
Nova
Day
on sol 687, when MAHLI took video of the target Nova as ChemCam lasered it. MAHLI
was able to see the flash of the plasma excited by ChemCam, and saw motion of dust as
ChemCam blasted the target.
7.4.2.2 Focusing MAHLI
MAHLI’s focusing ability means it can point at targets from working distances as close as
2.04 centimeters from the sapphire window, and also focus at infinity. MAHLI can focus
on targets with the cover in either the open or closed position. MAHLI focuses using the
same motor that opens and closes the dust cover. It is a stepper motor with up to 16100 step positions (Figure 7.12). From steps 0 to 5100, the cover is closed. Then the motor engages the dust cover. Beyond a motor count of 12000, the cover is fully open. From motor steps
0 to 1480, MAHLI is in focus at its minimum working distance of 2.04 centimeters with
the cover closed. With the cover still closed, it is in focus at infinity at a motor count of about 4523.
To work with the cover open, the MAHLI team commands it to fully open to a motor
count of 15504, then steps to the desired focus position. MAHLI is in focus at infinity at
a motor count of 12552, and in focus at its minimum distance of 2.04 centimeters at a
motor count of 15600. The stepper motor moves at 150 motor steps per second, so it takes
less than 2 minutes to open the cover and focus the camera. The dust cover sweeps
through a space about 5 centimeters beyond the camera. To be sure of ample space to
operate the dust cover safely, it is never articulated with the camera head less than 10
centimeters from a target.
Like the Mastcams, MAHLI can either be manually focused (commanded to a spe-
cific motor count position) or autofocused. The MAHLI team manually focuses images
when the scene will contain a wide depth range, such as during rover wheel imaging or
wide views of outcrops. Autofocus is more commonly used for hand-lens imaging
where it is crucial because of MAHLI’s shallow depth of field. The depth of field ranges
from about a millimeter at a working distance of 2 centimeters, to about 8 millimeters
at a working distance of 12 centimeters, and continues increasing toward infinity. In
fact, MAHLI focus is so sensitive to distance from the target that M
AHLI autofocus
distance is a tactically useful measure of the distance between a commanded turret
position and a target. Ordinarily, rover planners nudge a target with the APXS contact
260 Curiosity’s Science Cameras
Figure 7.12. Relationship between MAHLI motor count, behavior of the lens cover, and working distance at which the target is in focus. By Emily Lakdawalla after Edgett et al.
(2015 ).
sensor to precisely determine its range from the rover (see section 9.3). But when the
APXS contact sensor isn’t available for ranging (when a surface is especially rough or
when it is composed of loose materials), MAHLI autofocus distance works as a mea-
sure of the distance to the target.
7.4.2.3 Z-stacks
MAHLI can obtain “z-stack” images in order to compensate for its shallow depth of field.
To obtain in-focus images over a greater distance range, MAHLI can be commanded to
acquire several (typically 8 or 16) images at different motor count positions. Later, using a different set of commands, onboard software will locate the best-focus parts of each
image, and merge them into a single data product, called a focus-merge image product or
a z-stack. As a byproduct of the focus merge process, MAHLI also creates another data
product called a range map – essentially, a digital elevation model of the target. Returning a single, color-interpolated z-stack and its associated grayscale range map requires less
data volume than returning the 8 images used to produce them, so it functions as a kind of
data compression.
MAHLI z-stacks are usually not made until later in the sol, or even one or many sols
after the original images were taken. The resulting focus-merge data products have file
names and time stamps that reflect the date and time that they were produced, not the date
and time of the original data, which can make them difficult to track down. Recognizing
this issue, the MAHLI team has released an open-source “Principal Investigator’s
7.4 MAHLI: Mars Hand Lens Imager 261
Notebook” for each of the public releases of MAHLI data, and information about which
sol’s data contributed to z-stack images is included in metadata delivered to the PDS.16
7.4.2.4 Figuring out MAHLI image scale
Both the field of view and the pixel scale depend upon the working distance between