MESSENGER MDIS Wide-angle Calibrated Data Record (CDRWAC)

CDRWAC – Wide-angle Calibrated Data Record

Instrument: MESSENGER Mercury Dual Imaging System

PDS Data Set ID:MESS-E/V/H-MDIS-4-CDR-CALDATA-V1.0DOI: 10.17189/1520236

For more information about MDIS CDR products, see the CDR/RDR Product SIS.

The Mercury Dual Imaging System (MDIS) Wide-angle Calibrated Data Record (CDRWAC) data products include all valid, calibratable images acquired by the Wide-angle camera (WAC) during the cruise phase to Mercury and the Mercury orbital phase, and includes post-launch checkout images, flyby images of Earth, the Moon, Venus, and Mercury, in-flight calibration images, and images taken during Mercury orbit including those targeted at Mercury, or taken during imaging of comets or searches for Mercury satellites or vulcanoid asteroids. The products are the product of conversion of raw data to one of two options: physical units of radiance, or radiance factor of I/F.

The data in one CDR consists of a single reconstructed image frame, with the accompanying header translated into text format in the label, and converted in physical units. The CDRs are generated from Experiment Data Records (EDRs) through a data pipeline that corrects the EDRs for dark counts, flat field effects, non-linearity in response, wavelength-dependent responsivity, and time- and detector temperature-dependent variations in responsivity.

Conversion to Radiance

MDIS measures relative light intensity in engineering units referred to as data number (DN). DNs are generally converted to radiance, L (Wm-2/μm/sr), following the calibration equation:

where:

  • L(x,y,f,MET) is the calibrated radiance in column x, row y, through filter f measured at time MET;

  • DN(x,y,f,T,τ,b,MET) is the raw DN measured by the pixel in column x, row y, through filter f, at CCD temperature T and exposure time τ, for binning mode b and Mission Elapsed Time (MET);

  • Dk(x,y,T,τ,b) is the dark level in a given pixel, derived either from the dark strip or estimated from exposure time and CCD temperature;

  • Sm(x,y,τ,b) is the scene-dependent frame transfer smear for the pixel;

  • Flat(x,y,f,b) is the non-uniformity or “flat-field” correction at this pixel location,

  • Resp(f,t,b) is temperature-dependent responsivity, relating dark-, flat-, and smear-corrected DN per unit exposure time to radiance; and,

  • τ is the exposure time in milliseconds.

Conversion to I/F

To convert from radiance to I/F (also known as radiance factor, the ratio of measured radiance to that which would be measured from a white perfectly Lambertian surface), which is used to populate CDRs, the following expression should be applied:

where:

  • L(x,y,f,MET) is calibrated radiance calculated as described above for some filter f at some time MET,

  • Correct(f,MET) is an empirical correction for time-varying responsivity, described by Denevi et al., 2016,

  • Solar Distance is the value for distance of the target object from the center of the Sun in kilometers,

  • 149597870.691 is the number of kilometers in 1 AU, and

  • F(f) is effective average solar irradiance at 1 AU sampled under the filter bandpass (for Band Center = 747.70 nm and Bandwidth = 52.55 nm, Solar Irradiance = 1278.85 W/m2μm2).

Solar irradiance used to convert radiance to units of I/F:

Filter Number

Band Center, nm

Bandwidth, nm

Solar Irradiance, W/m2μm

1

698.76

5.30

1429.10

2

701.27

196.51

1432.13

3

479.87

10.14

2091.95

4

558.91

5.82

1833.26

5

628.81

5.52

1669.08

6

433.21

18.11

1733.07

7

748.73

5.09

1293.93

8

947.03

6.15

813.27

9

996.23

14.30

741.46

10

898.80

5.08

900.80

11

1012.56

33.33

714.15

12

828.39

5.20

1062.92

WAC Filter Specifications:

Filter Number

Filter Filename letter

Wavelength (flight)

(nm)

Width (flight)

(nm)

Total Thickness (nm)

Focal

length (mm)

Scale change

(%)

1

A

698.8

5.3

6.00

78.218

-0.104

2

B

700

600.0

6.00

78.163

-0.104

3

C

479.9

10.1

6.30

77.987

-0.329

4

D

558.9

5.8

6.30

78.023

-0.283

5

E

628.8

5.5

6.20

78.109

-0.173

6

F

433.2

18.1

6.00

78.075

-0.216

7

G

748.7

5.1

5.90

78.218

-0.033

8

H

947.0

6.2

5.20

78.449

0.262

9

I

996.2

14.3

5.00

78.510

0.340

10

J

898.8

5.1

5.35

78.390

0.186

11

K

1012.6

33.3

4.93

78.535

0.372

12

L

828.4

5.2

5.60

78.308

0.082

MDIS Wide-angle Camera Specifications:

Parameter

Wide Angle

Field of view

10.5° x 10.5°

Pivot range (observational)

-40° (Sunward) to +50° (Planetward)

Exposure time

1 ms to ~10 ms

Frame transfer time

< 4 ms

Image readout time*

1 s

Spectral filters

12 positions

Spectral range

395-1040 nm

Focal length

78 mm

Collecting area

48 mm2

NAC-WAC coalignment knowledge

0.01 deg (179 μrad)

Spacecraft pointing knowledge

0.1 deg (1.75 mrad)

Spacecraft knowledge

0.02 deg (350 μrad)

Detector-TH7888A

CCD 1024 x 1024, 14-μm pixels

IFOV

179 μrad

Pixel FOV

35.8 m at 200-km altitude

Quantization

12-bits per pixel

Compression

Lossless, multi-resolution lossy, 12-to-n bits

MDIS CDRWAC products have the following file names:

CWrnnnnnnnnnf_tt_v.IMG

where:

C = product type = CDR

N = camera = NAC

r = spacecraft-clock-partition-number minus 1 [0, 1] for pre- or post-spacecraft-clock-reset

nnnnnnnnn = Mission Elapsed Time (MET) counter taken from the image header.

NOTE: this is a spacecraft clock seconds counter, and the value in the filename corresponds to the LAST second of the exposure.

M = filter wheel position = A, B, C, D, E, F, G, H, I, J, K, L, U (U if the position is unknown)

tt = data type

RA = radiance

IF = I/F corrected for time variations in responsivity

IU = I/F uncorrected for time variations in responsivity

v = version number

In ODE, CDRWAC products have the following product IDs:

CWRNNNNNNNNNF_TT_V