by The Design
MAHLI and a target. To the MAHLI team, working distance is measured from the front of
the sapphire window to the target. For rover planners, the zero point is located at the minimum distance they can safely command MAHLI to take a photo, 19 millimeters in front
of the sapphire window. This is usually – but not always – called the “toolframe distance”,
“standoff distance” or “RP [rover planner] distance,” which distinguishes it from MAHLI
instrument “working distance.” Unfortunately, rover planners sometimes refer to this as
the “working distance,” which gets confusing. To convert rover-planner distances into
MAHLI instrument working distances, add 19 millimeters. If you are not certain of the
convention being used, you can unambiguously determine the range to the in-focus parts
of an image using the motor count.
The pixel scale of a MAHLI image can be derived from the working distance (instru-
ment team convention) using the following formula:17
Pixel scale( m
µ / pixel) = 6.9001+ 3.5201 × Working Distance(cm).
The MAHLI team has derived an empirical relationship between motor count and
working distance, based upon measuring objects of known size and distance on both Earth
and Mars:18
−1
w = ( am−1 + b + cm + dm 2 + em 3 )
in which
w = working distance (instrument team convention)
m = motor count
a = 0.576786
b = –11.8479
c = 2.80153 × 10–3
d = –2.266488 × 10–7
e = 6.26666 × 10–12.
16 The MAHLI Principal Investigator’s Notebooks are available for download from Ken Edgett’s page on Researchgate: https://www.researchgate.net/profile/Ken_Edgett/publications
17 Mars Science Laboratory (MSL) Software Interface Specification for Camera & LIBS Experiment Data Record (EDR) and Reduced Data Record (RDR) Data Products version 3.5, August 5, 2014
18 Yingst R A et al (2014) Cameras on Landed Payload Robotic Arms – MAHLI and Mars and Lessons Learned from One Mars Year of Operations. Paper presented to the International Workshop on Instrumentation for Planetary Missions (IPM-2014), 4-7 Nov 2014
262 Curiosity’s Science Cameras
Table 7.8 is a lookup table relating motor count to working distance, pixel scale, and image size. The last three columns provide a guide to the size of sediment grains that can
be resolved in images taken at different distances, according to the classic Wentworth
(1922) sediment classification scheme. It takes at least 2 pixels across an object to be able to detect it, so sand grains are resolvable from MAHLI standoff distances under about 1
meter, while silt grains are only resolvable at the very closest standoff distances, and then only the coarsest silt grains that have good contrast against the background. As a rule of
thumb, if you can see individual grains in a sedimentary rock in a Mastcam image, you’re
looking at a conglomerate; if you can see individual grains in a MAHLI image but not in
a Mastcam image of the same target, it’s a sandstone; and if you can’t detect grains even
with MAHLI, it’s a siltstone or mudstone.
7.4.2.5 Calibration target
MAHLI’s calibration target is attached to the robotic arm shoulder azimuth actuator
(Figure 7.13). A preflight photograph of the calibration target is shown in Figure 7.14. The
calibration target contains red, green, blue, and gray color swatches made from the same
material used in the Mastcam calibration target, leftover materials from the Mars
Exploration Rover Pancam calibration target. There is also a fluorescent chip made of a
material called SpectraFluor Red that glows red (at a wavelength of 626 nanometers)
when illuminated with the MAHLI ultraviolet LEDs (365 nanometers). An opal glass bar
target has a chart modeled on the US Air Force 1951 Resolution Test Chart, designed to
monitor camera focus and resolution performance over time. The calibration target is
mounted vertically on the rover, which was intended to discourage dust settling and keep
it relatively clean. Unfortunately, the calibration target was coated with a thin film of dust thrown up during landing, but the calibration target functions adequately for its primary
purpose of checking that there is no drift in camera focus. The calibration target has been imaged on sols 34, 165, 179, 322, 411, 591, 825, 989, 1091, 1157, 1340, 1519, 1632, and
1696. MAHLI has imaged the Mastcam calibration target on sols 544, 707, and 1028.
MAHLI also images the Mastcam and ChemCam calibration targets in nearly every
self-portrait.
There are several quirky elements in the MAHLI calibration target. A cartoon of “Joe
the Martian” is meant as a thank-you to the public for the opportunity to conduct the
MAHLI investigation and as an invitation for children to follow the Curiosity mission. The
Greek letters “γδβγ” are printed within the “0” of the “1.0” text on the bar target. A 1909
United States penny is embedded in the bottom of the calibration target. It is intended as
an homage to field geologists’ practice of placing a coin or other small object on a rock
outcrop to provide scale before taking a photo, and the MAHLI team often includes a
picture of it in public releases of MAHLI images (Figure 7.15). 1909 was the first year that the Lincoln cent was issued, and would have been a century before the year of Curiosity’s
launch; unfortunately, the launch delay to 2011 obscured the significance of the date on the 1909 coin. The coin is 19 millimeters in diameter.
7.4 MAHLI: Mars Hand Lens Imager 263
Table 7.8. MAHLI image dimensions and pixel scale with respect to motor count. Lines in bold represent very common motor counts for MAHLI, near standard target standoff distances of 25, 5, and 2 centimeters.
els)
w (cm)
els)
els)
μm) (pix
els)
f distance (cm)
el image (cm)
, mm)
, mm)
ver closed)
ver open)
pix
ar
μm)
orking distance from MAHLI front windo
el scale (
idth of 1600-
Motor count (co
Motor count (co
w
toolframe or standof
pix
W
Depth of field (near
Depth of field (f
width of 10mm scale bar (pix
Max diameter of a granule (4mm) (pix
Max diameter of a grain of sand (2mm) (pix
Max diameter of a grain of silt (62.5
4475 12600
231
229
819
131
-891
4531
12
4.9
2.4
<1
4375 12700
84.0
82.1
302
48.4
-155
261
33
13
6.6
<1
4275 12800
50.7
48.8
185
29.7
-60.9
83.2
54
22
11
<1
4175 12900
35.9
34.0
133
21.3
-31.7
39.4
75
30
15
<1
&nb
sp; 4075 13000
27.6
25.7
104
16.6
-19.1
22.4
96
38
19
<1
4061 13014
26.7
24.8
101
16.1
-18.0
20.9
99
40
20
<1
3975 13100
22.2
20.3
85.1
13.6
-12.7
14.2
117
47
23
<1
3875 13200
18.5
16.6
71.9
11.5
-8.9
9.7
139
56
28
<1
3775 13300
15.7
13.8
62.1
9.94
-6.6
7.0
161
64
32
1
3675 13400
13.5
11.6
54.6
8.73
-5.1
5.2
183
73
37
1.1
3575 13500
11.9
10.0
48.6
7.78
-4.0
4.0
206
82
41
1.3
3475 13600
10.5
8.6
43.8
7.00
-3.2
3.2
229
91
46
1.4
3375 13700
9.3
7.4
39.7
6.36
-2.7
2.6
252
101
50
1.6
3275 13800
8.4
6.5
36.4
5.82
-2.2
2.2
275
110
55
1.7
3175 13900
7.5
5.6
33.5
5.36
-1.9
1.8
299
119
60
1.9
3077 13998
6.9
5.0
31.0
4.97
-1.6
1.6
322
129
64
2.0
3075 14000
6.8
4.9
31.0
4.96
-1.6
1.6
323
129
65
2.0
2975 14100
6.2
4.3
28.8
4.61
-1.4
1.4
347
139
69
2.2
2875 14200
5.7
3.8
26.9
4.31
-1.2
1.2
371
148
74
2.3
2775 14300
5.2
3.3
25.3
4.04
-1.1
1.1
396
158
79
2.5
2675 14400
4.8
2.9
23.8
3.81
-1.0
1.0
420
168
84
2.6
2575 14500
4.4
2.5
22.5
3.59
-0.9
0.9
445
178
89
2.8
2475 14600
4.1
2.2
21.3
3.41
-0.8
0.8
470
188
94
2.9
2411 14664
3.9
2.0
20.6
3.29
-0.8
0.8
486
194
97
3.0
2375 14700
3.8
1.9
20.2
3.24
-0.7
0.7
495
198
99
3.1
2275 14800
3.5
1.6
19.3
3.08
-0.7
0.7
519
208
104
3.2
2175 14900
3.3
1.4
18.4
2.94
-0.6
0.6
544
218
109
3.4
2075 15000
3.0
1.1
17.6
2.82
-0.6
0.6
568
227
114
3.6
1975 15100
2.8
0.9
16.9
2.70
-0.5
0.6
593
237
119
3.7
1875 15200
2.6
0.7
16.2
2.59
-0.5
0.5
617
247
123
3.9
264 Curiosity’s Science Cameras
Figure 7.13. Photo of the rover taken during assembly at JPL, showing the location of the calibration target. NASA/JPL-Caltech release PIA14289. Insets: two images of the calibration target, taken before departing Earth using a DSLR camera (left) and after landing on Mars, by MAHLI (right).
Image 0034MH0000460010100041E01. NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
7.4.2.6 Bad pixels and blemishes
MAHLI has a few dozen dark specks in all images. These have been constant throughout the
mission, and are caused by microscopic particles on the detector. The MAHLI team is more
concerned about the possibility that operating the camera close to the surface will invite dust to settle on the optics when the cover is open, especially when operating near freshly drilled rocks and their piles of fine drill tailings. They try to take flat-field images of the sky about once every 180 days, in part to watch for new dust particles affecting their images. One dust particle was detected on the sapphire window in a Mastcam image of MAHLI taken on sol 617.
Like the Mastcams, MAHLI is susceptible to shutter smear (see section 7.2.1.5). In fact, MAHLI experiences shutter smear at longer exposures than Mastcam and MARDI
because it takes longer for MAHLI to read out its images. However, it doesn’t affect sci-
ence data and is mostly only noticeable in self-portrait photos, when white surfaces of the rover can appear smeared. Hot pixels often cause streaks running down images due to
shutter smear. MAHLI landed with a number of hot pixels, and new ones have appeared
during the course of the mission; some heal, but others have persisted. While these are
cosmetically annoying, they don’t affect the quality of the data for science.
7.4.3 Using MAHLI
Because using MAHLI almost always requires using the arm, there are fewer opportuni-
ties for MAHLI scien
ce than for Mastcam science. A substantial portion of the MAHLI
data set is engineering support imaging and rover self-portraits. Various types of MAHLI
7.4 MAHLI: Mars Hand Lens Imager 265
Figure 7.14. MAHLI calibration target, taken during spacecraft assembly. Inset images are from Mars, images 0034MH0000440010100031C00 and 0034MH0000450010100C00.
NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
266 Curiosity’s Science Cameras
Figure 7.15. A mosaic of nine MAHLI images on a conglomerate target taken at the Darwin waypoint, sol 400. The penny is 19 millimeters in diameter. NASA/JPL-Caltech/MSSS image release PIA17362.
imaging also provide context and support for drilling, scooping, sample dumping, and
APXS activities, helping to build a multi-instrument data set. When MAHLI can reach
them, it’s often pointed at ChemCam shot points. Occasionally, MAHLI captures large
mosaics, allowing detailed study of the sizes, shapes, colors, and distributions of grains
within a rock for sedimentology studies.
7.4 MAHLI: Mars Hand Lens Imager 267
7.4.3.1 MAHLI nested target imaging
A major use of MAHLI is to capture sets of nested images of targets that are usually also
APXS targets, including drill sites. Usually, MAHLI takes a single context image from a
standoff distance of 25 centimeters, achieving a scale of about 100 microns per pixel. Then it moves to a standoff distance of 5 centimeters and takes a z-stack at about 31 microns per pixel, often repeating the observation from a slightly different position for stereo imaging purposes. This image scale is the same as that of MI images taken by the Mars Exploration
Rovers, making it simple to compare close-up data from Spirit or Opportunity APXS and
MI with Curiosity APXS and MAHLI. For some observations, MAHLI acquires an obser-
vation from a standoff distance of only 1 or 2 centimeters, with a “best” resolution of 16 to 21 microns per pixel (Figure 7.16). To capture these, the MAHLI team asks the rover planners to get MAHLI as close as possible to the target, which varies depending upon the tar-
get’s topography and reachability. Sometimes MAHLI will do these observations at night,
so that the LEDs can illuminate the target with light of a well-known intensity, allowing the team to directly compare the color of one target to another imaged at another location.
Sometimes MAHLI will take nested images of targets both before and after brushing.
7.4.3.2 Mosaics
It takes close cooperation between the rover planners and MAHLI team to produce a MAHLI
mosaic, so there have not been many, but they are scientifically productive on sedimentary
targets with varying grain size (e.g. Figure 7.16). It’s also fun and educational to use MAHLI to image a vertical rock face from a low angle not accessible by Mastcam; these “dog’s eye”
views are often expanded into mosaics. A list of MAHLI mosaics is in Table 7.9.
7.4.3.3 MAHLI Landscape Imaging
When the zoom capability was descoped from the Mastcams, the rover lost its ability to
capture wide views of the Martian landscape in color using a single frame. MAHLI is now
the widest-angle color camera on the rover that can do landscape imaging, so soon after