1.1

Instrument Architecture

A key instrument on-board the TRUTHS mission is the Hyperspectral Imaging Spectrometer (HIS): a dispersive imaging spectrometer that, in conjunction with an on-board calibration system (OBCS), is capable of providing an accurate, continuously calibrated datasets of spectrally resolved earth, solar and lunar radiance [1]. It measures light in the near-UV, visible, NIR, and SWIR wavebands, utilising a novel SI traceable on-board calibration system. As a system, TRUTHS has a challenging aim of providing a goal absolute radiometric accuracy better than 0.3% over the whole spectral range, with k=2 coverage factor. The payload can be used within its own right, or to cross-calibrate data from other space instruments.

The HIS, a push-broom imaging spectrometer, covers the spectral range from 320 nm to 2400 nm, with spectral resolution from 0.3 nm to 7 nm. In orbit, it will cover ~100 km in swath, with a spatial sampling distance (SSD) of 50 m. The instrument broadly consists of the telescope and spectrometer optics, a cooled detector and cryostat, front end electronics (FEE) and the mechanical structure and associated thermal control.

1.2

General Architecture

The HIS instrument is composed of 3 optical subunits that can be built in isolation: the telescope, which focuses the image at the slit; the spectrometer, which spectrally separates the incoming light; and the polarization scrambler, used to remove any polarisation bias. The optical design is illustrated in Figure 1: light enters at the top right, and the detector plane is in the top middle of the image.

Figure 1:

Full HIS optical design schematic

The HIS focal length is approximately 229 mm with a 70 mm entrance pupil.

The very wide spectral band has necessitated development of a wide band high reflectivity coating. Dielectric coatings work on interference and hence are difficult to design over a wide spectral range; they can also display large peak to peak variation. Metallic coatings tend to exhibit low UV reflectivity. TRUTHS have therefore opted for a hybrid design where highly thermally and mechanically stable Zerodur glass substrates are metal coated for high reflectivity in the infrared (IR) and an IR transparent multilayer dielectric coating is added to boost UV/visible reflectivity. A protective overcoat is also employed to enhance durability for cleaning and handling.

1.3

Spectrometer Optical Design

All the mirrors are freeform, and although relatively simple, free form manufacturing is always a challenge. The prisms are made from fused silica of very high optical quality. This is necessary to achieve the needed transmission over this large waveband and to avoid aberration due to striae. Although this configuration has been used in the past (e.g. for Chris), the high resolution required for TRUTHS dictates much larger prisms. In addition, the diffraction limited design drives the prisms to have power (curved faces) to compensate for other aberrations in the system. Effectively, these prisms could be considered as lenses with a very large wedge. Manufactures can create lenses with very little wedge or flat faced prisms with large wedge; combining the two is a challenge.

This optical configuration, however, provides a very good optical quality spectrometer. Figure 2 shows the modelled modulus of the optical transfer function (OTF) – i.e. the modulation transfer function (MTF) – of the spectrometer at 1500 nm. The result is limited by diffraction.

Figure 2:

Optical MTF of a spectrometer only for some point on the slit at 1500nm. The sagittal curves are ACT and the tangential one ALT.

The Offner configuration tends naturally to almost suppress any smile or keystone and to be telecentric in both slit and detector planes. The distortion of the slit image on the detector at any wavelength (the smile) and the distortion of the spectrum for any point of the slit (the keystone) can be limited to a few parts of a pixel. These considerations generate constraints for the telescope design: it must be telecentric in the image plane and provide no smile-limiting distortions from Earth to detector.

1.4

Telescope Optical Design

The starting point for the telescope was a classic off-axis 3-mirror configuration, avoiding any obscuration in the pupil. This 3-mirror configuration, though able to provide the needed telecentricity in the image plane, is difficult to design compactly with no distortion. The design has therefore evolved to use a 4-mirror configuration, which offers more degrees of freedom to fully avoid distortion.

Figure 3:

Optical layout of current telescope

The telescope is composed of freeform mirrors and offers the possibility of accommodating an intermediate field stop after the first mirror to reduce stray light and limit the light entering in the spectrometer. The telescope is optimised so that the optical quality is limited by diffraction over that whole field of view.

1.5

Polarisation Scrambler

The polarization scrambler considered in this phase for TRUTHS is a Dual Babinet Pseudo Depolarizer (DBPD). It is composed of 4 wedged prisms that are optically contacted to effectively create one flat plate. The DBPD generates a variable dephasing over the pupil and converts a given polarisation state to the sum of different states, so that the exit beam is considered unpolarised. The polarisation scrambler is more efficient at short wavelengths, the “depolarization power” varying with 1/λ.

The scrambler is realised in crystalline quartz. As illustrated in Figure 4, the machined flat input surface is aligned with the Z-axis (the optical axis). To create a single Babinet prism, two wedged prism are cut and contacted, and an effectively flat plate is created. Two Babinet prisms are bonded to create a dual Babinet configuration, with the second pair at 45 degrees with respect to the first, to further scramble the beam.

Figure 4:

Schematic principle of a DBPD

Figure 5:

Polarisation pupil map at the rear surface of the last prism in the current baseline system.

Placement of the scrambler within the telescope can be in a collimated or converging beam. That is to say, either at the telescope entrance or before the slit. In both cases it generates a degradation of the MTF due to the so-called “diamond effect”; in the case of the converging beam option it also adds chromatic aberrations. Obtaining large diameter crystals at a suitably high optical standard is seen as a risk, so the current baseline is the smaller scrambler nearer the slit.

In regards to the “diamond effect”: the birefringence of the crystal prisms generates a dephasing as well as a variation of deviation as a function of the direction of the incident polarization. This results in an image-walk-off per prism and thus, for the four prism scrambler, 4 images. This walk-off is controlled by the wedge angle of the prisms, and the location of the images by the rotation of the prism assemblies.

Figure 6 gives an example of the diamond effect in a simplified system where the scrambler is in the entrance pupil. With modifications to the design, these spots could be arranged to have the images in a straight line in X or Y.

Figure 6:

Illustration from a simplified paraxial 191 mm focal length system at 1500 nm wavelength where the prism are at 45 deg to each other. Airy’s disc is given for reference

The polarisation sensitivity (PS) of the full HIS instrument is plotted in Figure 7 as a function of wavelength for different field angles. The required PS is indicated in blue. In the current HIS design, the orientation and direction of variation are manageable with the design of the scrambler.

Figure 7:

Polarization sensitivity (%) of the full instrument as a function of wavelength for different field angles

1.6

Smile and Keystone

Keystone is the maximum variation of centroid position ACT with respect to the wavelength for each field direction. Smile is the maximum variation of centroid position in the spectral dimension with respect to the field orientation for each wavelength. In this design the smile is negligible whatever the wavelength and keystone varies as per Figure 8.

Figure 8:

Keystone representation for 7 field orientations

THERMAL CONTROL

Maintaining the HIS within strict thermal limits is complicated by the mission environment: the spacecraft orbit drifts through the year, and regular manoeuvres are required for calibration, solar and lunar measurements, simultaneous overpasses to other sensors, and the characterisation of Pseudo-Invariant-Calibration-Sites. The result is a wide range of possible illumination conditions for which the thermal design must meet performance requirements.

The HIS optical bench stability is ensured using a thermal enclosure covered with laminar heaters. The enclosure surrounding the optical bench is cooled radiatively using two radiators linked by embedded heat pipes to withstand direct solar flux. The mean temperature of the structure is controlled at 20°C with a maximal gradient of 2°C, minimising the structural and optical surface deformations due to thermoelastics. The FEE is regulated at around 20°C by use of a ‘warm radiator’ and a dedicated heating line to maintain stability.

Furthermore, variation in the thermal background incident on the detector must be kept to a minimum in order to meet the absolute radiometric accuracy requirement, which is critical to the ambitious mission objectives. One option is to cool the optical chain; however, if the thermal stability of the optical bench can be assured to within very strict limits, ambient operation of the optics becomes possible. Operating at ambient temperatures would greatly simplify the instrument design as well as the assembly, integration & verification (AIV) process. The immediate surroundings of the detector, within its cryostat, make a large contribution to the thermal environment. So, a cold shield is used, cooled to cryogenic temperatures using the same system as the detector.