A.

Introduction

The guide line for the development of the test bench was to allow the full characterization of a spectrometer including a free from grating, part of a spectro-imager. The design of this spectrometer is shown in Figure 1 and is based on an original modified-Offner design for imaging spectrometer with a convex gratings ruled on a Free-Form surface [1]. This required an imaging system that shapes the input beam as the one provided by the imaging system.

Fig. 1.

Spectrometer optical design including a FFG (Free Form Grating)

Fig. 2.

Spectrometer breadboard

B.

Tests to be performed

Imaging spectrometer main performances to be characterized are the following:

Each test will be briefly described in section V. To do these characterizations, the calibration setup needs:

The full calibration facility is described in next section.

C.

Input light shape constraints

The input light should be similar to the output of the coming imaging system. In this case, it will be a TMA telescope. In particular, the design of the telescope is telecentric (the variation of the chief ray incidence along the FOV can be neglected), nearly achromatic, with a F/# = 5. Moreover, it is requested to illuminate in one shot the full spectrometer entrance slit (62 mm long).

D.

Proposed solution

The calibration test setup has to image an object placed at a finite distance. A trade-off study indicates that no commercial devices are able to fulfill all requirements. It was thus necessary to go to an homemade telecentric achromatic design.

To be close as possible to the telescope properties at spectrometer entrance slit, the proposed concept is based on an Offner design (see Fig. 3).

Fig. 3.

Offner relay proposed for light shaping.

This design uses simple spherical mirrors. The system is achromatic, telecentric with a magnification of 1, and is designed with a F/# < 5 to fit with the imaging F/#.

A.

Test setup elements

The final design and configuration is depicted in Fig. 4 and Fig. 5.

It is composed of:

Fig. 4.

Scheme of the test facility with an imaging-spectrometer placed at the exit.

The calibration facility generates a quasi-monochromatic (with tunable Δλ and λ) and uniform (thanks to the IS) illumination on the entrance slit of the spectrometer under test. For each performance test, a dedicated object target is placed at the IS output (see section III.B): it will be imaged (ratio 1:1) exactly on the spectrometer entrance slit. Each element position is adaptable (with accurate translation/rotation stages) for fine tuning alignment.

Fig. 5.

Pictures of the calibration setup.

B.

Target objects

For each performance test, a dedicated target object has been performed at CSL. Some pictures are depicted in Fig. 6. The targets are inserted in specific housing adapted for an accurate positioning in the mounting aligned in the calibration setup under the IS.

Fig. 6.

Different objects that will be placed at the entrance of CSL Offner relay. (Left) 7.5μm wide slit for alignment and MTF. (Center) Knife-edge for the straylight measurement. (Right) Array of 50μm wide slits for keystone measurement.

C.

Test setup fully automatized

Because of the large number of parameters to record, the calibration facility is completely automatized. The Labview program controls:

The program manages the positions and wavelength scanning for each test configuration. Images recorded by the camera at each step are automatically classified to be analyzed.

D.

Setup alignment

Alignment consists mainly to co-align both mirrors (M1 and M2), and to localize exactly entrance and exit slits. For this purpose, slits are materialized by metallic spheres.

A first alignment was done with an API Radian laser tracker, completed after by an interferometric alignment. This interferometric alignment uses those metallic spheres to measure the WFE across the Offner Relay. The aberrations are minimized by tuning mirrors and entrance slit positions. At the optimum configuration, all positions are fixed. Metallic spheres at the exit of the Offner Relay are then placed at the focal point of the interferometer.

The WFE measurement for the final frozen configuration of the CSL Offner relay is shown in Fig. 7. Astigmatism is the main contributor to the WFE, but the RMS value is considered as sufficiently low.

Once Offner relay has been aligned, the second step is to align the spectrometer: its entrance slit shall coincide with the Offner relay exit slit (materialized by metallic spheres). A first alignment has been carried out by laser tracker: positions of the exit slit metallic spheres are recorded and entrance slit of the spectrometer is placed with the X,Y,Z motorized translation stages, at that position. Then, it is the image on the spectrometer camera that is used to refine the alignment: a slit theoretically thinner than a pixel is placed at Offner relay entrance, the focus is adjusted in order to image this slit on about 1 pixel on the camera.

Fig. 7.

WFE obtained after alignment adjustment.

A.

MTF

The MTF curve corresponding to the WFE of the Fig. 7 is better than 88% at 15 lp/mm.

B.

Spatial uniformity

The setup was designed to uniformly illuminate an exit slit around 60mm long.

The exit slit spatial uniformity of the Offner relay is mainly linked to the IS output. It has been measured with the set-up depicted in Fig. 8. The monochromator is working on the 0^th diffraction order, illuminating the slit with a polychromatic light. An optical fiber (20 μm width) with a NA=0.1 is used in order to analyze only infield light (F/#~5, comparable to the spectrometer F/#). The fiber is connected to a photodetector, and fixed on a translation stage to scan the full exit slit.

Fig. 8.

Exit spatial uniformity measurement set-up.

Results are presented in Fig. 9: the Offner relay reaches 99% uniformity over 59mm.

Fig. 9.

Uniformity measurement at CSL Offner relay exit slit

C.

Straylight

To measure Offner relay straylight at exit slit, for in field light, the set-up presented in Fig. 10 is used. It allows a scan of the Offner relay focal plane to investigate the transition of a knife-edge placed at the entrance slit.

Fig. 10.

Straylight measurement set-up, at the exit slit of the Offner relay. A knife-edge is placed at the entrance slit, its image at the exit slit is scanned by an optical fiber linked to a photodetector.

Measurement results are depicted in Fig. 11. Averaged straylight at 250 μm from the cut is around 0.13% ± 0.01%

Fig. 11.

Offner relay straylight measurement results.

D.

Irradiance

Irradiance at spectrometer entrance slit (F/#5 field) has been measured with a calibrated photodiode, depicted in Fig. 12. Even if the irradiance is small, for the tested spectrometer, enough flux is collected by the camera in its focal plane to achieve good S/N image.

Fig. 12.

Irradiance measurement at spectrometer entrance slit. The slit height seen by the calibrated detector has been measured at 7 mm. Slit width is fixed by the spectrometer entrance slit (30 μm width).

E.

Polarization

To test polarization sensitivity, the input light has to be nearly non-polarized. Offner relay output light polarization has thus been measured. The set-up is depicted in Fig. 13, using a linear motorized rotating polarizer to scan polarization angles.

Fig. 13.

Polarization characterization set-up. A rotating linear polarizer is placed in front of the Offner relay exit slit. Output light according to polarization angle is collected by an optical fiber linked to a photodetector.

Measurements are influenced by two main factors:

Fig. 14.

Sinusoidal variation to be subtracted from polarization characterization measurements coming from rotating stage behavior.

Fig. 15.

Polarization sensitivity of Offner relay output light.

Since the Offner relay polarization is known (Fig. 14 and Fig. 15) it can be removed, and by this way the tested spectrometer (including camera) polarization behaviour is known with an accuracy of 0.5%.

A.

Keystone

The Keystone is defined as the departure from straightness and parallelism of line spectra from white point sources at various positions in the image field. For this test:

Fig. 16.

Preliminary pictures capture on the spectrometer camera respectively for keystone, straylight, MTF and spectral registration configurations. Horizontal axis is the spatial dimension, vertical axis is the spectral dimension.

B.

Spectral response matrix

This characterization includes spectral range, spectral resolution, peak transmission, smile and tilt, and out-of-band rejection. The goal is to build a 3D spectral response matrix.

All this information can be extracted from a spectral scan (Δλ =2.5 nm, for λ =400 to 950 nm) performed by the monochromator. For this configuration, no slit in object plane is needed: a full line along geometrical dimension will be registered at different spectral positions according to λ.

C.

MTF

Two kind of MTF can be measured: in spatial direction, or in spectral direction. MTF is extracted from LSF. To do LSF measurement, there are two configurations:

D.

Polarization sensitivity

Polarization sensitivity measurement implies image registration according incident light polarization angle. For this purpose, a motorized rotating polarizer is placed in front of the spectrometer entrance slit. By removing the polarizer artifact and previously characterized Offner polarization signature (see section IV.E), the spectrometer polarization sensitivity can be extracted. This measurement can be done with polychromatic light or quasi-monochromatic light.

E.

Straylight

In-field straylight is measured by placing a knife-edge in the object plane of the calibration setup in order to mask half of the image plane surface (along spatial direction), see central picture of Fig. 6. Images have been recorded for different wavelengths (Δλ ~10nm) and for white light (example in the second picture of Fig. 16). Removing background, these images gives information on straylight with respect to the distance from the knife-edge.