1.1

Flight System Description

The EnMAP Hyper Spectral Imager (HSI) is a state-of-the-art satellite-based hyperspectral Instrument for Earth observation in Push Broom configuration. The main geometrical and spectral performance Parameters are summarized in the following Figure and Table:

Table 1

Geometrical and spectral key Specifications.

It has to be noted that delivering accurate radiometric data is also a key feature of the EnMAP mission and thus there are stringent requirements on Radiometric performance of the Instrument. Radiometric calibration is however not within the scope of the OGSE setup presented in this paper and will not be discussed in the following.

For more detailed information the reader is directed to Guanter et al:, The EnMAP Spaceborne Imaging Spectroscopy Mission for Earth Observation.¹

The EnMAP instrument optical system consists of a common three mirror anastigmat (TMA) telescope, a field splitter and two separate prism-based spectrometers. Figure 1 shows a schematic of the optical layout:

Figure 1

The EnMAP satellite

Figure 2

EnMAP HSI Optical beam path

In addition an On-board calibration system will be used for recurring in-flight radiometric and spectral calibration. It consists of a diffuser reflecting sunlight into the Telescope entrance aperture as well as a calibration light source that can be fed into the beam path in front of the Spectrometer entrance slits.

1.2

On-Ground characterisation

In order to verify the System Performance on ground as well as to acquire relevant calibration data an extensive on-ground characterization campaign is performed employing various optical Ground Support Equipment (OGSE-) setups. Apart from functional testing aspects the activities can be split into three main blocks aiming to measure:

Table 2:

main parameters subject to on-ground verification and calibration

Radiometric characterisation is split into activities on detector level and System level. The latter will be performed in a dedicated measurement setup employing a 1.5m diameter integrating sphere² already used for Ground Calibration in other programs. Description of this setup is not within the scope of this contribution.

In the course of the HSI integration several alignment steps precede the final end-to-end characterisation mentioned above. Some of them require Full aperture illumination of the Telescope and hence are to be done in a very similar setup as the End-to-End characterisation activities mentioned above. It was thus a natural decision to look for an OGSE solution that could already be employed during these alignment steps

2.1

Telescope assembly(TA) to spectrometer alignment:

Requirement.

Method:

2.2

Spectral response Function (SRF) characterisation

Requirement:

Method:

2.3

Line of sight (LoS) and Imaging contrast (MTF) characterisation

Requirement (LoS)

Requirement (MTF)

Method:

MTF and LoS characterisation are derived from the same measurement:

Figure 3

Determining the LoS of a pixel using a knife edge. (a) the image of a knife edge is scanned across the pixel of interest (b) the signal of the pixel as function of the knife edge position is the ESF of the imaging system including detector (c) the derivative of the obtained signal wrt the knife edge position yields the LSF again convoluted with the pixel size. The pixel LoS is defined as the centroid position of this function

3.1

Goal

It was the goal to develop a modular OGSE Architecture that maximizes synergies of recurring tasks and requirements between spectral and geometrical characterisation as well as certain alignment activities.

Reducing the overall amount of OGSE Items for alignment and test has clear commercial advantages. In addition the Integration and alignment team benefits from working with a multi-purpose setup. It allows to thoroughly understand the systematic behaviour and technical peculiarities of the involved devices prior to the final calibration campaign which typically suffers from significant schedule pressure.

3.2

OGSE Elements Overview

To give a full picture of the modular approach advertised in the previous section Figure 4 shows an overview of all involved OGSE devices and optical interfaces. The individual devices are detailed in Section 4. Some Items are grayed out as they have only been used in earlier steps of the integration and alignment process which is outside the scope of this paper.

Figure 4

Overview of the general OGSE architecture. For a more detailed description of the individual Items c.f. Section 4 The Items DWS and SpecSid are shown for completeness only, they are not within the scope of this paper.

The Items FAI (scene generator) & WIRAL (monochromator) can be remotely controlled using a central control authority which also controls the HSI focal plane detectors. This way time synchronized batch script operation involving flight detectors and OGSE is possible. Synchronization lags are typically <10ms between the individual devices, which is sufficient for the envisaged use cases.

3.3

Optical Fiber Interface

In a modular setup as described above it is crucial to use an easy to control yet flexible optical interface. In our case Silica-core step gradient Multimode fibers where chosen to transport light from the Light source (OGSE) to the OGSE element interfacing with the flight hard ware. All fibers are equipped with standard FC/PC connectors. This sets some constraints on the design of the individual components but pays off in terms of flexibility and compatibility of the Setup.

Main Advantages:

IR-Damping:

One major drawback about using Silica-core fibers is the high damping rate at wavelengths >2μm. for this reason, which essentially limits the Silica core fiber length to ~ 1m for all practical purposes. To overcome this limitation a ZrF Fiber was used to transport the light across the distance between Light source and Illumination OGSE (~25m) however to maintain interface compatibility small pieces of Silica-core fiber were used at both ends interfacing with the OGSE devices

Etendue matching

Unobscured, unobstructed optical imaging systems as e.g. the FAI or the HSI optical system have the property to conserve the illumination etendue ε defined as the product of illuminated area and illuminated solid angle.

In our case the required illumination etendue at the Telescope entrance ε_Telescope is set by the dimensions of the telescope pupil and illuminated area on the HSI focal plane detector. Given that full aperture illumination by the FAI needs to overfill the Telescope entrance pupil and corresponding field for the individual measurement cases, the optical fiber interface at the FAI input has to provide an etendue ε_Fiber > ε_Telescope.

As mentioned above the Fiber NA (and thus the illuminated solid angle) is kept constant for technical reasons, the only possibility to adjust the etendue is to select a Fiber with a sufficiently large core (i.e. illuminmated area). Fortunately cores up to 1mm diameter are available which is sufficient for all practical purposes.

Homogenisation:

For Knife edge measurements (as well as for spectral characterization) a very homogenious light field, both in real (focal plane-) space as well as in angular space is desirable. In addition the light field must be incoherent.

In reality using a monochromator like the WIRAL as light source the intensity pattern as well as the spatial coherence properties of the light exiting the monochomator depend on the selected wave lengths and monochromator slit widths.

A well-established technique to homogenize light fields and get rid of spatial coherences is to couple the light into an Ulbricht sphere. In our case however this would have led to inacceptable losses in the overall optical path. A convenient overview of technologies for fiber-based light field homogenization is given in³. Here an optical throughput well above 50% was achieved. At the same time homogenous and incoherent illumination scenarios at the fiber exit could be created, independent of the input light field. Figure 5 shows measurement examples of pupil and focal plane intensity patterns measured at the FAI input fiber port.

Figure 5

Intensity profiles at the FAI input Fiber port. Left: cut through the focal plane intensity pattern, Right: cut through the angular intensity pattern.

4.1

High dynamic Range optical Autocollimator (HDAC)

The HDAC is a custom developed electronic Autocollimator and as such is used as master reference for angular measurements in the frame of Line of sight characterisation. The device was developed by Möller-Wedel-Optical GmbH upon OHB specifications. It is based on a standard platform used in the commercial line of Electronic autocollimators offered by Möller-Wedel-Optical. An important aspect of the Project was to calibrate the Autocollimator traceable to national standards with sufficient accuracy. This task was also performed by Möller-Wedel-Optical GmbH.

Key performance parameters are:

Figure 6

High dynamic range Electronic Autocollimator (built by Möller Wedel Optical GmbH)

4.2

Full aperture Flat mirror (FAM)

The FAM consists of a precise two sided reference Flat in a tip-tilt gimbal mount. It has two main purposes:

The FAM was developed upon OHB Specifications by Airbus-DS in collaboration with Aperture Optical Sciences Inc.

Key performance parameters are:

Figure 7

Full aperture mirror FAM

4.3

Wide Range adjustable Light Source (WIRAL)

The Wiral consists mainly of a commercially available Grating Monochromator (iHR320) by HORIBA Scientific, fed by several light sources and a fibre coupling interface.

The WIRAL serves as spectral reference for the described measurement cases it is calibrated traceable to national standards using a PTB-calibrated Echelle spectrometer (GWU_LambdaScan_Spectrometer, provided by DLR-IMF²). A dedicated recurring calibration plan employing Gas-Cell line sources ensures validity of the calibration throughout the EnMAP onground calibration campaign.

Key performance aspects include

The monochromator is equipped with three Gratings that allow to cover the entire EnMAP spectral range with sufficient throughput. In addition the gratings can be exchanged by a mirror, allowing to use the monochomator in a white light configuration. Bandwidth and power of the emitted light can be adjusted by choosing the entrance and exit slit width of the monochromator in conjunction with the used grating. Several filter wheels are installed to block higher diffraction orders of the gratings and select additional attenuation factors.

The light source can be selected by actuating an automated fold mirror. Currently two distinct Light sources (LS1 and LS2) are installed along with a gas-cell based line source for Spectral calibration verification.

The core parts were procured as pre-aligned off-the-shelf products. Fine alignment and thorough characterisation as well as integration with the HSI functional environment was done by OHB.

Figure 8

Wiral OGSE. Left: schematic overview of the architecture as designed by HORIBA, Right: Foto of the monochromator Setup

4.4

Full Aperture Illumination OGSE (FAI)

The FAI is the Core Part of the OGSE Setup presented in this paper. It was developed, manufactured integrated, aligned and tested upon OHB Specifications by Bertin Technologies.

System architecture

The Full Aperture Illuminator (FAI) (Figure 9) is composed of :

Figure 9:

Architecture of FAI OGSE

Main functions

The FAI is a Swiss Army Knife that is used during several steps of HSI integration through several configurations:

Key performance

The FAI needs to provide a highly stable illumination situation and at the same time keep its very small wave front error. This led to an All-Invar-structure for the collimator with Zerodur® selected as mirror material.

The optical system has been manufactured and aligned to provide a wave front error on the order of λ/20 for the shortest measurement wavelength (see Figure 10). Including all other relevant contributors, the FAI Illumination provides a collimated beam with a guaranteed Wave front performance <40nm(rms) including Focus.

Figure 10

Wave front error map of the Collimator optical system for a central field point

Table 3 summarizes the most demanding performance requirements of the FAI and their link with each configuration.

Table 3.

Key performances of FAI and relations with the different use cases

The FAI is fully compliant to all these requirements.

5.1

Wiral working parameters

During the final commission phase, the radiometric and spectral performance of the WIRAL was optimized with an additional emphasis to run the individual measurements with optimal efficiency.

It was the goal to balance the output bandwidth versus the emitted power. While the bandwidth is required to be < 10% of the EnMAP Spectral channel width for the respective wavelength, the power needs to be sufficient to produce high-SNR signals on the illuminated Focal plane detectors. In total 6 wavelength ranges were defined, in which single values for the monochomator slit widths choice of grating and filters were applied. The wavelength is set by turning the monochomator grating. Figure 11 shows the results of this effort confirming that for almost all wavelengths sufficient margin is present both for power as well as for bandwidth with respect to the required values.

Figure 11

WIRAL wavelength ranges. Measured Power (light blue) and Spectral bandwidth defined as FWHM of the emitted spectrum (orange) are plotted against the minimum required power (solid blue) and the maximum required bandwidth (solid red)

The WIRAL absolute wavelength accuracy after calibration is determined by the reproducibility of the wavelength setting. Figure 12 shows that typical drifts of the bandwidth are on the order of 5% whereas the wavelength reproducibility is 50pm and thus <1% of the smallest EnMAP spectral channel width.

Figure 12

WIRAL bandwidth verification and wavelength drift characterisation based on Gas Cell reference lamp.

Figure 13

Wiral calibration results for one of the SWIR wavelength ranges. Measured Wavelength of light exiting the Wiral by a calibrated echelle spectrometer (GWU_LambdaScan_Spectrometer) before (left) and after (right) applying a calibration table. Please note the different x-scale of the two plots. (measurements performed by A. Baumgartnder (DLR-IMF))

5.2

Geometrical referencing Strategy

For line of sight characterization of the HSI the direction of the collimated input beam produced by the FAI must be referenced in the Telescope Line of Sight coordinate system CS_o_TA#1. The latter is operationally defined by a mirror reference cube (TA_RC#1) located next to the telescope entrance (Figure 14).

Figure 14

Operational definition of the Telescope Line of Sight coordinate system CS_o_TA#1 employing a mirror cube (TA_RC#1) next to the telescope entrance

Referencing is performed in a two-step process as illustrated in Figure 15

Figure 15

Schematic procedure to reference the FAI input beam with respect to the Telescope line of sight coordinate system CS_TA#1

The total accuracy of this procedure is analyzed in a budget to be <1arcsecond. Main contributors are:

Contributors a) and c) have been experimentally verified to be on the 1μrad level.

In order to assess the FAM adjustment accuracy b) measurements have been performed that show that thermal drifts within the FAM gimbal mechanics are limiting this accuracy. However if either sufficient time for thermalisation is given or the measurements of Step2 can be performed in a time synchronized manner, accuracies on sub μrad level are possible. Both options are currently under investigation.