Présentation à l’équipe OCO : 27/01/2011
Transparent 1:
So, my name is Veronique Pascal and I am in charge the microcarb instrument. I submit a
dispersive instrument concept conceived in my team in CNES by senior engineer Christian
Buil. For this first and preliminary study, several persons participate as the optic expert
Jacques Loesel, the optoelectronic expert Laurie Tauziede and the mechanical and thermal
expert.
Transparent 2:
For Microcarb, the main mission aims are :
- To mesure CO2 gas with good accuracy more precisely better than 1.5 ppm
- 3 bands are used : one near 763 nm, the second at 1576 nm and the third at 2061nm
Low cost drive this project; So we plan to implement the instrument on micro satellite. The
mass, the consummation and the volume are constrained.
These elements drove the choice of dispersive optical components for high radiometric and
spectral performances. And the Echelle grating allows to compact instrument. More precisely,
we use the capacity of spectral multiplexing and so we used the same echelle grating for the
three bands. This idea is patented by CNES with the title: “high performances compact
echelle grating spectrometer with double pass telescope”
Transparent 3:
In this slide, some details are given. So we see the classical principle of spectrometer with a
entrance slit, a collimator optic, the dispersive element and the objective lens that focus
spectrum on detector plane. The power resolution power or is the rapport of wavelength on
Instrument Line Spread width  is given by this equation.  is the angular slit width, D is
the input telescope diameter,  is the incidence angle on the grating, d1 is the internal
spectrometer pupil (d1 defined the spectrometer size), k represent the diffraction order and m
the groove density. The dispersive element is an echelle grating. For achieving maximum
grating efficiency, we use the littrow configuration. The incidence angle is nearly blaze angle
in this case and the diffractive angle is 2° out off littrow angle typically.
For the power resolution, the minimum target value is 25000. So we choose a large incidence
near 70° and a high large diffraction order but we take a low groove density.
Transparent 4:
In fact, the value of the power resolution value depends of product k m. Spectrum is
segmented in narrow pieces (one piece is equal to one diffractive order). This parameters of
grating was chosen so that efficiency is maximum in each spectral band. Useful orders are
diffracted in the same direction so the instrument is compact (bunt ?) but in this case, we
need a physical separation. Because useful orders are diffracted in the same direction we need
a physical separation for each band.
Two methods are possible for physical separation :
- we can use a cross disperser element and in this case, the separation is spatial. We
illustrate this method here
- we can use filters with bandpass about where k is the diffraction order. We
preferred this spectral separation for our application because we can use specific
detectors of each band and we can use all long the entrance slit for each band
Transparent 5:
A probative breadboard made by CNES team confirm us the ability of this kind of grating.
The breadboard is only functional and not representative of final design. We use a standard
Richardson grating and the optical elements, detector available in laboratory.
Transparent 6:
The optical configuration of the breadboard is simple as you can see on this figure. So, the sun
light cross the atmosphere and is come to the spectrometer throught a and it is receipted on
diffusion screen. A simple objective lens focus the After, the lens image the sun light on
entrance slit. The collimator and camera lens function are devoted to a single objective. Here
the echelle grating. Then, the lens spectrometer led the light to echelle grating. A folding
mirror redirect diverts the light on detector. Finally, after the detector we have a processing
unit. The narrow band filters are put here on the slit because it is easier. We can see here a
zoom on echelle grating (back more precisely) and here the camera that is used.
Transparent 7:
You can see an example of observed 2D spectrum. Here is the traverse axis or spatial axis
parallel in slit height. And here is the spectral axis dispersion axis on the other dimension of
detector. Each spectral line is a monochromatic image of the slit.
Transparent 8:
We remove dark signal on 2D image and we equalize the non uniformity gain. We correct
also the geometrical distortion as smile effect or keystone effect.
Finally, we add several pixels on column and we obtain the spectral profile. In horizontal axis
line we have the pixel number (uncalibrated spectrum here) of the detector and in column the
relative intensity.
Transparent 9:
We can observe the evolution of spectral line profile with the sun air mass. Different
monitoring are represented on the figure. The spectral resolution is 0.038 nanometer. The
power resolution value is 41000. Look also a solar line – the profile is independant of the air
mass value.
Transparent 10:
After the presentation of the concept and the preliminary results of the breadboard of this
concept, we study a preliminary implementation of microcarb spectrometer. We associate the
efficient echelle grating with the compactness tri-mirror anastigmat optics.
The input telescope is a cassegrain telescope (TBC). The light is focus on slit then the light
cross the TMA and arrive on echelle grating . The same TMA is used for the by the diffracted
beam light and dichroic optics separate the different spectral bands. It is easier to insert
bandpass filters with detection system. The system is very compact for 3 bands. If we want to
add an another band we can select the spectral band by exchanging the filter (for example at
1.6µm we can select CO2 or CH4). An another advantage of this system is an chromatic
system.Note that the system is achromatic.
Transparent 11:
We have a more advanced version use a quadric quadri mirror anastigmat (QMA). The light
path is different before and after the grating. The image quality is better and the
implementation of focal planes is easier as you see here.
Transparent 12:
For the mission, a sub pixel cloud detection is necessary. So we add to 3 band spectrometer an
imaging system. The main characteristics are the following ones:
- same Field of view as spectrometer
- the target of sampling pitch is 100m and 200m maximum
- the spectral band is centered on 0.625µm and the bandwith is 0.15µm
- The signal noise ratio is around 100 at radiance 86W/m/µm/SR
In respect of compact instrument, we plan to use the same front optic as spectrometer if it is
possible.
Transparent 13:
This slide describes the preliminary microcarb platform implementation. We choose a
configuration with scan mirror. The ability of the pointing mirror is +/- 45° along the swath
and 15° for the pitch.
We take into account the calibration. I describe the first idea just after this slide.
And we take also into account the polarization effect. So we add a scrambler in the first study
then we consider a decreasing of available signal when we determine the preliminary work
point.
Transparent 14:
Just before an example of preliminary working point. I precise the principle of spectra
extraction. In fact, we divide the swath. For example, in this case, 3 FOV are obtained from
the detector available information. The pixels are merged and for these Nbin pixels a spectral
profile are given like described before. So, we obtained 3 independent spectra for each FOV.
Transparent 15:
For the same final performance of CO2 gas concentration, different working point can been
chose. We indicate in this slide an example of preliminary working point of Microcarb.
The size of FOV can be : 10,9Km2
For a swath of 36,3km we can have 11 FOV. The slit width at nadir is around 400m.
The pupil diameter is 30mm and the focal length is 90mm.
We merge 23 pixels. And with all these parameters, we obtain an a signal to noise ratio equal
to 410 for the NIR band B1, 500 for SWIR B2 and 200 for SWIR B3. The power resolution
value is always 25000.
Last point, Statistically 5 FOV can be used among the 11 FOV depending on the cloud
coverage of the scene.
Transparent 16:
The expected spectral band are showed for each band with a power resolution equal to 25000.
Transparent 17:
As agreed, some elements for the calibration : we put the calibration device here during the
preliminary study. We consider 3 lines of sight for calibration : one for flat field, one for
pointing the sun (through a diffuser) and the third for cold space or black body.
In conclusion, I describe you our concept based upon echelle grating. A preliminary
implementation and a preliminary budget are made in first phasis of microcarb project. But
it’s the beginning and we have many questions. I propose you to discuss on these questions.
Transparent 18 :
So our first question is……