A.

Families of concept

During the study several families of concepts have been identified, using the following rules:

Chosen family of concepts is based on a different separation between the UV/VIS/NIR channels and the SWIR. The UV/VIS/NIR channels are separated with the help of a spectrometer whereas the SWIR channel is directly imaged on a second adapted focal plane. This separation was found to be the only way to keep the SWIR option in the race. The number of modules in this family of concepts is one or two.

Fig. 1

Chosen family of concepts

B.

Rationale for concepts selection

At the end of the first phase of the Study, the Concepts Review led to the selection of two instrumental concepts to be investigated in more details during the second phase. The rationale of the method proposed for assessing the best candidates was as follows:

C.

Possible optical concepts

The instrument can be of two kinds:

The constraint on instrument volume strongly favors the double-pass concept.

Fig. 2

Classical instrument and constrains on pupil (left) and Double-pass instrument (right)

D.

Design characteristics and constraints

Design drivers are as follows:

Also, in double-pass design, the dual-pass optic is used as collimator and imager, so that the collimator focal length is equal to the imager focal length. Furthermore, the telescope focal length is equal to the instrument focal length.

At last, the design of the instrument is constrained by the position of the different pupils (cf Fig. 2Erreur ! Source du renvoi introuvable.). Indeed, at slit level, the telescope Exit Pupil shall be equal to the collimator Entrance Pupil, and the collimator Exit Pupil and imager Entrance Pupil shall be real and shall be on the disperser. These are strong constraints on the optical design.

E.

Telescope design

Many telescopes designs were studied (reflectives solutions : TMA, Schwarzschild, Walrus,… but also refractive designs). For each concept, a set of performance and characteristics was assessed :

Fig. 3

Studied telescopes at the same scale

Among them, few telescopes were able to meet simultaneously all the instrument needs (compact / few modules / accessible entrance pupil, and compatibility with spectrometer entrance pupil). None of them was based on a single module (70° FoV).

In the following table the telescope designs are divided in 4 groups depending on their assets. In each group the best candidate is highlighted.

Fig. 4

Groups of candidate telescopes

F.

Spectrometer design

As explained above, priority was given to double-pass spectrometers in order to reduce the overall instrument volume. We have thus studied the telescopes listed above in dual-pass configuration. In order to reduce volume, the grating was put in pupil plane. Optics with no real exit pupil have thus not been studied because they led to large volumes (ex: tma4, tma5,…). Some telescopes were too compact and could not be used as double-pass spectrometers (ex: ma4_2, tma10, walrus, 5ma) because of lack of space to accommodate them.

Beside these double-pass spectrometers, we also studied Dyson/Offner spectrometers, and two examples of classical spectrometers (collimator+imager).

For each of the spectrometers the following characteristics and performances were assessed :

Fig. 5

Studied spectrometers at the same scale

Among them, a few was able to meet simultaneously all the instrument needs (compact / few modules / accessible Entrance Pupil, and the constraint with telescope exit pupil) and none was based on a single module.

In the following table the spectrometer designs are divided in 5 groups depending on their assets. In each group the best candidate is highlighted.

Fig. 6

Groups of candidate spectrometers

G.

Outcomes of the concepts review: solutions at instrument level

The combination of telescopes and spectrometers led to 20 possible instrument concepts. From the telescopes and spectrometers listed above, the following best candidates were discussed at the Concepts Review:

Fig. 7

Selected telescopes and spectrometers

The following two concepts were selected for further design in the second part of the Study, emphasizing compactness versus the preference of having a single-module instrument:

MA4_4 (four mirrors astigmatic) – TMA16DB : 2 modules, flat grating, compact. Later on, the telescope has been replaced by a TMA.

FlatSchwarz6 (Schwarzchild) – Dyson5: 2 modules, concave grating, very compact.

A.

Concept 1

In the first concept, represented in Fig. 8, the scrambling window is placed in the entrance pupil of the instrument, and a folding mirror enables to accommodate the telescope and the double-pass TMA in a small volume. The telescope images the Earth on the slit, and the double-pass TMA images the slit to infinity. The grating disperses light, and the double-pass TMA images this dispersed light on the detector. The instrument has a focal of 81.4mm, f# of 3.3 and FOV of 70° (35° for each module).

Fig. 8

Instrument overview – Concept 1

B.

Concept 2

In the second concept, represented in Fig. 9, the scrambling window is placed in front of the telescope, but not in a pupil. The telescope images the Earth on the slit, and the Dyson spectrometer disperses and images the slit on the detector.

Fig. 9

Instrument overview – Concept 2

The instrument has a focal length of Focal=81.4mm, a f# of 3.3 and a FOV of 70° (35° for each module).

C.

Estimated performance of both concepts

For the two concepts, requirements apportionment toward sub-systems was made. Mirrors reflectivity and grating efficiency are the main drivers for the optical components. The grating efficiency is particularly stringent for Concept 1, whereas it is relaxed for the Concept 2 and is marginally compliant with classical gratings (35% in the UV instead of the 50% required. Gratings based on new technologies could achieve the required level of performance.

Grating solutions able to meet the requirement were studied extensively by Thales Research and Technology in the frame of this Study. Candidate gratings are based on sub-wavelength binary structures. The nano-structured patterns are made of SiO2 or Si3N4 and the effective index depends on the fill factor (i.e. the width-to-period ratio). These type of gratings have already been developed but not for a design in reflection and with a curved surface. Some solutions in silica have been tested in transmission on small samples and evaluated numerically for a transitive design. They present high efficiency (up to 60-80%), a flexibility to adapt the shape of the spectral efficiency profile and a good polarization behavior. However specific development are clearly needed, in particular regarding the expected size and shape of the component to be realized.

The performance of both instrument concepts have been estimated, and main outcomes are listed in Fig. 10.

Fig. 10

Performance of both concepts

Concept 1 main non-compliance concerns spectral co-registration. Such concept would therefore need to implement smile and keystone correction by processing. Further optimization of these performance at optical level was not further investigated.

Concept 2 meets all the requirements.

In order to be fully aware of gains obtained with these new designs, we have tuned Concept 2 so that it be compliant to the OLCI requirements only (i.e. twice relaxed versus the LFOV requirements). Thanks to that we could estimate the gain on volume for two equivalent designs.

The volume of one optical module compliant to the LFOV requirements is 5.3L. The volume of one optical module compliant to the OLCI requirements only is 1.7L.

Fig. 11 provides with the same scale the volume of the Concept 2 OLCI-tuned (2 modules), compared to the OLCI concept (5 modules).

Fig. 11

Concept 2 compliant to OLCI requirements (2 modules) versus OLCI optical design (5 modules) at the same scale