A.

Concept

For a linear 1D field of view (FOV), the principle is to use a 2D micromirror array (MMA) component to select the wavelengths by acting on intensity (Fig. 1). Indeed, this component is placed in the focal plane of a first diffracting stage (using a grating for instance) and is used as a wavelength selector by reflecting or switching-off the light by deflection. On the MMA surface, the spatial dimension is along one side of the device and for each spatial point, its spectrum is displayed along the perpendicular direction: each spatial and spectral feature of the 1D FOV is fully adjustable dynamically and/or programmable for each exposure or even during an exposure by using the temporal behaviour of the component. It becomes then possible to realize a programmable and adjustable filter in lambda and delta-lambda. A second stage recomposes the beam after wavelengths selection by exactly the same dispersive component (same grating), leading to an output 1D image. This kind of implementation allows to tune the filtering function in spectral bandwidth for each spatial pixel quasi instantaneously.

Fig. 1.

Principle of the programmable wide-field spectrograph using a MOEMS device (DMD from TI) for wavelength tailoring on each field pixel.

B.

Slit generator

Digital Micromirror Devices (DMD) from Texas Instruments could act as field and wavelength selection reconfigurable mask. The largest DMD chip developed by TI features 2048 x 1080 mirrors on a 13.68μm pitch, where each mirror can be independently switched between an ON (+12°) position and an OFF (-12°) position. This component has been extensively studied in the framework of an ESA technical assessment of using this DMD component (2048 x 1080 mirrors) for space applications (for example in EUCLID mission). Specialized driving electronics and a cold temperature test set-up have been developed. Our tests reveal that the DMD remains fully operational at -40°C and in vacuum. A 1038 hours life test in space survey conditions (-40°C and vacuum) has been successfully completed. Total Ionizing Dose (TID) radiation tests, thermal cycling (over 500 cycles between room temperature and cold temperature, on a non-operating device) and vibration and shock tests have also been done; no degradation is observed from the optical measurements. These results do not reveal any concerns regarding the ability of the DMD to meet environmental space requirements [6].

In Europe an effort is currently under way to develop single-crystalline silicon micromirror arrays for future generation infrared multi-object spectroscopy (collaboration LAM / EPFL-CSEM). First arrays with 2048 micro-mirrors have been successfully designed, realized and tested at 160K [4,5]. On a longer time scale, these arrays could be used in programmable wide-field spectrograph for Earth Observation.

C.

Flight model

A flight model of a programmable wide-field spectrograph has been designed by TAS with LAM collaboration. Parameters of this instrument are: 30km field-of-view (FOV) on ground, with 15m ground resolution, spectral range 0.4-2.4μm, spectral resolution 2nm / 5nm /10 nm; intermediate focal plane (DMD) 2000x1000 pixels (pitch 15μm); focal plane Si pixel detector (pixel size 15 μm); spectrograph entrance slit size 30mm x 15 μm.

Prism and grating-based designs have been studied and the grating concept has been selected. This concept has been adapted for the demonstrator design (see next paragraph).

A.

Demonstrator optical design

Our goal is to make a robust and efficient instrument for a space mission. Selecting a good starting point was really important. In order to simplify as much as possible the optical layout of the system, we fixed some constraints:

Even if complex, we succeeded to design such a system, developing ideas proposed many years ago for the JWST near-infrared multi-object spectrograph [9], and developed more recently for BATMAN project [10].

Our optical design is based on a double Offner relay system placed in serial, with a 1:1 magnification between input field elements and the DMD pixels, and the DMD pixels and the detector pixels (Fig. 2). DMD orientation is at 45° (rotation around z-axis) with respect to the bench, due to the fact that the micromirrors are tilting along their diagonal. A simple spectrograph layout has been set up, based on two identical spherical mirrors acting as collimator and camera, and a low density convex grating to disperse light. We have ordered two identical spherical mirrors with a diameter of 115mm and a radius of curvature of 285mm. The most critical component of the system is the convex grating, due to complex manufacturing and tight alignment tolerances. A Jobin-Yvon (Horiba group) grating has been chosen from their available list of gratings, leading to a component with 150 gr/mm line density, 140mm radius of curvature and 40mm in diameter.

Fig. 2.

Optical design of the programmable wide field spectrograph demonstrator. Light from the telescope is coming from below (1D entrance slit), passing through two Offner-type spectrographs in serial separated by the DMD, up to the detector at the top of the drawing.

This makes the system simple and efficient, not suffering from chromatic aberrations. Delivered image quality onto the DMD and onto the detector is high enough to not degrade resolving power and spatial resolution, too. Typical monochromatic RMS spot diameters are 20μm at the intermediate focal plane (DMD surface) and 10μm at the focal plane, over the whole FOV for wavelengths between 450nm and 750nm.

B.

Demonstrator opto-mechanical design

The general mechanical design of the mock-up consists of a main optical bench supported by three legs (Fig. 3). This bench supports two spectrographs in serial. An OLED at the entrance simulates the field of view. Earth images are displayed using their RGB values. We have developed a software in Matlab in order to drive the input beam with adjustable image size on the OLED, programmable slits (shape, width), colors selection as well as the possibility to have a scanning slit-FOV, mimicking a push-broom type Earth Observation.

Fig. 3.

Opto-mechanical design of the programmable wide field spectrograph, 3D general design view

The optical components are mounted in dedicated mounts with stops and screws and could be aligned along all degrees of freedom thanks to stages with micrometer accuracy. In order to minimize cost, the design intends to use only one type of material which is aluminium alloy (except for mirrors that are made in silica). The main bench is light weighted.

Due to the wide FOV covered by the DMD, we will cover it only partially and we add motorized stages on the input beam optical system as well as on the detector, in order to address the full FOV sequentially.

C.

Demonstrator integration

The demonstrator has been integrated and aligned on a damped table (Fig. 4). The optical beam is generated on the left-hand side by an OLED; the DMD is located at the top center and two spectrographs are located, one before the DMD for dispersing the light of each field pixel on the DMD, and a second one, identical to the first one, for combining the tailored wavelengths on each field pixel towards a unique output 1D line on the detector.

Fig. 4.

Integrated demonstrator picture Input/OLED is on the left, the DMD at the top, and the output/detector on the right.

Images are recorded by the CCD camera located at the right-hand side of the picture. The grey ribbons at the top of the picture consist of 300 wires for controlling the DMD. Optical path scheme is etched on the bench.

Alignment strategy uses a combination of mechanical and optical measurements. Each optical sub-system is referenced by its vertex and center of curvature from the Zemax file; alignment tools are placed at the subsystems location and mechanically located with a mechanical 3D measurement machine; actual positions away from the theoretical positions are calculated and optically aligned thanks to a micro-alignment telescope, using the screws mounted on each element, allowing the movement of the element within the 6 degrees of freedom.

Each spectrograph has been aligned independently before final integration on DMD both sides. First spectra have been obtained and measured; typical spot diameters are within 1.5 detector pixels (8.3μm detector pixels), and spectra generated by one micro-mirror slits are displayed with this optical quality over the whole visible wavelength range. This alignment procedure has been defined during the integration of ROBIN, the mock-up of BATMAN instrument, based on an optical design very close to the programmable wide field spectrograph design. [10]

In order to locate precisely images and spectra on the DMD surface, we have built an additional optical bench imaging the DMD surface, this bench being behind the spherical mirror M3-B as defined in Fig. 2. M3-B mount is precisely located using pins and shims and could be removed and replaced with a very high precision; when removed, DMD surface could be imaged thanks to the additional optical bench. This bench is made with two doublets with a 0.5 magnification allowing a full imagery of the DMD surface on an additional CCD camera. Fig. 5 shows images of the input (OLED) plane and intermediate focal plane (DMD surface). The OLED displays a white slit at 45° with respect to the horizontal (and vertical) axis. After passing through the first spectrograph, the slit is dispersed on the DMD surface; spatial and spectral directions are labelled on Fig. 5; 0-th order is partly imaged on the DMD surface as a white narrow stripe (image of the entrance slit), and -1 order is faintly displayed at the bottom-center of this picture. Using these images and exact wavelength locations, thanks to interference bandpass filters, we have determined the exact spectral dispersion to be 200nm wavelength range dispersed along 353 micromirrors, i.e. 0.57 nm / micromirror.

Fig. 5.

Images of the input (OLED) plane and intermediate focal plane (DMD surface). Dispersed spectra (1^st order) are displayed on the DMD surface, while 0th-order and -1 order are also recorded on the image. Spatial and spectral directions are labelled.

D.

Demonstrator in operation

A synthetic linear FOV mimicking Earth views is generated on the OLED and typical images have been recorded on a CCD camera at the output focal plane of the instrument. The FOV exhibits different features including bright objects and “colored” elements (Fig. 6).

Fig. 6.

Typical Earth-type images are displayed on the OLED as a “slit” image

When the DMD is set with all mirrors in ON position, this means that no lambda selection is made over the whole FOV, the output image is identical to the input image (Fig. 7). At the input, the OLED is set as a scanning slit with a one-pixel width; at the DMD level, the 0^th order is masked by switching OFF the mirrors where 0^th-order light is projected (see Fig. 5), and the output is the recombined slit, image of the entrance slit. The image is an intensity image on the CCD camera; spatial resolution is slightly degraded with respect to the one-pixel wide entrance slit, as it is expected by the optical design of our demonstrator. Note that removing optically the 0^th-order decreases the straylight in the instrument.

Fig. 7.

End-to-end measurement: image recorded at the output of the programmable spectrograph with all DMD pixels ON except those removing 0^th order light. DMD surface behavior is also shown (black pixels are OFF)

By tailoring the DMD, we could modify each pixel of the input image: for example, it is possible to remove the bright object by turning OFF all corresponding mirrors for this object on the DMD (spatial and spectral), see Fig. 8. We can also, for each spatial pixel modify the spectral signature in the DMD plane by turning ON and OFF the micromirrors located along the dispersed direction for this pixel; for example, in Fig. 9, both bright object as well as all “blue” color along the whole 1D image are removed.

Fig. 8.

End-to-end measurement: image recorded at the output of the programmable spectrograph; the bright object and the diffraction 0^th order light are removed. DMD surface behavior is shown (black pixels are OFF)

Fig. 9.

End-to-end measurement: image recorded at the output of the programmable spectrograph; the bright object, “blue” color along the whole 1D image and the diffraction 0^th order light are removed. DMD surface behavior is shown (black pixels are OFF)