OMEGA GROUND CALIBRATION

B. Gondet,Saint Louis, 21/05/2008

Optical test bench

• OMEGA was mounted on a two axis platform 0.1 mrad in both site and azimuth (0.1 IFOV) Cold sink for regulating the operating temperatures

• light sources: - monochromator (spectraPro 275) the grating can be set as a mirror - tungsten ribbon lamp (Russia) brightness temperature from 2500 K to 2850 K five filters with different transmission profiles pinhole point source (steps of 0.05 IFOV) sample holder - black body, from 325 K to 1400 K (Galaï – 1200 BB) circular aperture covering 4 IFOV’s - grid of 4 x 4 IFOV-sized black bodies (Galileo) - cold plate, from 150 K to 293 K

OMEGA calibration plan

• spectral calibration • photometric calibration influence of detector temperature and spectrometer temperature linearity • geometric calibration co-alignment, registration stray light

• reference targets

OMEGA IN THE CALIBRATION TANK

THE OPTICAL BENCH

1. Spectral calibration (lambda.dat)

Tspectro T det (IR) Position in the FOV

-90°C 20°C 5

-90°C

20°C 1 (center)

-80°C -195°C 5

-80°C -195°C 1 (center)

-70°C -195°C 3

-95°C 20°C 3

VNIR (100 &200) msTspectro Tdet (IR) Position in the FOV

-90°C -195°C 3

-80°C -195°C center

-70°C -195°C 3

-90°C -191°C center

-90°C -198°C center

-90°C -200°C center

-90°C -197°C center

-90°C -193°C center

-90°C -188°C center

SWIR (2,5 ms & 5ms)

List of spectral calibration performed using the monochromator

Procedure and examples of results for SWIR

Tspectro = -90°C Tdet = -195°CTspectro = -70°C Tdet = -195°C

Spectel n° 60 (C)

Nanomètres

Dig

its

14nm

Spectel N°188 (L)

Nanomètres

Dig

its 23nm

Spectel number

Nan

omet

e rs

SWIR-L spectral calibration as a function of position in the FOV

right of FOV

left of FOV

Spectral match between OMEGA Earth spectrum, and "MODTRAN"  simulated

SWIR-C SWIR-L

2. radiometric calibration

mtfYYMM_25.dat (2.5 msec, release of year YY, month MM)mtfYYMM_50.dat ( 50 msec, release of year YY, month MM)

Operational parameters

Hot Black body source temperature, varying from 9°C to 1175°CCold BB temperature from 10°C down -120°CIR detector temperature from –201°C to –188°C (-195°C nominal)Integration time = 2,5 ms, 5 ms & 10ms for SWIR = 100 ms & 200 ms for VNIRTest at several positions in the FOV

197°C

295°C

368°C523°C

saturation

low S/N

Signal in DN divided by the corresponding black body flux

DETERMINATION OF THE PHOTOMETRIC FUNCTION (C CHANNEL)

Photometric response of OMEGA

VIS

SWIR C

SWIR L

Bright region

Dark region

S/N ratio for observations of MarsPhase = 0°, surface temperature = 250 K

Wavelength (µm)

Wavelength (µm)

INFLUENCE OF THE TEMPERATURE OF THE SPECTROMETER

For OMEGA, photons remove electrons from pre-charged wells The available dynamics decreases rapidly with the temperature of the spectrometerbeyond 4.5 µm, in particular for long integration times (5 msec)

This is essential for the detection of sulfates

200 K 190 K175 K

Solid line: 5 msecDashed lines: 2.5 msec

Wavelength (µm)

3. geometric calibration

"AZIMUTH" = pitch: spacecraft motion

"SITE" = yaw: SCANNING MIRROR motion

SLIT

Point Source(ribbon lamp, grid Officine Galileo)

Geometric calibration

"Site", scanning mirror

Azimuth, spacecraft motion

1. IFOV for each spectel, for each of the 128 spatial directions

2. perpendicular to the slit ("azimuth") direction

- for each channel, relative variation of the viewing direction along the spectrum, for each IFOV within the entire 128 px FOV

- spectral registration for the 3 channels, and each of the 128 positions

3. along the slit ("site") direction

- relation between the IR swath position and the VNIR column number- spectral registration for the 3 channels, and each of the 128 directions

Geometric calibration files

"VIS" SPECTELS (0.35 – 1.1 µm)

Arc

min

utes

IR SPECTELS (1.0 – 2.7 µm)

Arc

min

ute

s

IFOV = f(wavelength), at the center of the FOV

12 pixels

l l l l.45 µm 1. µm 2.5 µm 5 µm

(Y,λ) maps of a point source scanned in azimuth

VNIR

SWIR-LSWIR-C

IRC IRL VIS

(i,j) (i,j-/+n) (i-/+m,j-/+n)

REGISTRATION : creation of false feature

in case the viewing direction of # spectels would be as follows…

…we would end up with a misinterpretated spectral feature

PIX

ELS

IR spectels (1 to 2.7 µm)

# positions along the slit64/12832/12896/128

5/128125/128

3/128

2/128

1/128

122/128

121/128

120/128

59/128

60/128

9, 124, 62 /128

Pix

els

VIS and IR spectels (0.35 to 2.7 µm)

# positions (pixels) along the slit

IR spectels (1 to 4.5 µm)

Pix

els

Center of the field

REGISTRATION EFFECTS along the azimuth

4. Stray light

Measurements made:± 30° ("site")± 40° ("azimuth")

Source: ribbon lamp, full power

No stray light detected

5. Reference samples

Samples used:

"ISM " samples"NASA" samples"Lecce" samples

Source: Ribbon lamp

Integration times:2.5 ms, 5 ms, 10 ms (SWIR)100 ms, 200 ms (VNIR)

Number of files acquired : 140

HYPERSTENE

μm

GYPSUM

μm Nanomètres

20% calcite 80% Palagonite5% calcite 95% Palagonite1% calcite 99% Palagonite

μm

Calcite10% Calcite + 90% Montmorillonite4% Calcite + 96% Montmorillonite

CalciteMontmorilloniteGypsumDiopsiteFayeliteLabradoriteMagnetite

wavelength (micrometers)