# Freeform Wide Field-of-View Spaceborne Imaging Telescope: From Design to Demonstrator
## 3 Telescope Proof-of-Concept Demonstrator
### Laboratory Demonstrator Setup
#### 3.2.2 Performance Validation Using NIR Laser Source  
The imaging performance is evaluated using a near-infrared laser source at 1550 nm, at the center of the spectral range that is targeted with this telescope (1100-1700 nm), and within the sensitive operation region of the Xenics sensor (900-1700 nm). For this, an APEX AP2080C tunable laser source was used and set to an output wavelength of 1550 nm and optical output power of 339.63 \(\upmu\)W. An optical fiber is connected to the light output unit, to guide the light to the telescope, supplemented with a collimating lens to achieve a collimated input beam. The Xenics Bobcat 320 GigE 100 was used as a SWIR camera, featuring an InGaAs sensor with a 320 \(\times\) 256-pixel resolution and a pixel pitch of 20 \(\upmu\)m.  
A full FOV of 120\({}^{\circ}\) was successfully observed. The measured spot sizes, at different field angles, show a good consistency with the simulated spot sizes (Figure 9). Note that we observe geometric spots and not the RMS spot sizes. Nevertheless, the spot sizes are of the same order of magnitude as when considering the simulations executed in the sequential mode. The shape of the spots also shows a good resemblance. A deviating spot arises at \(\pm\)24\({}^{\circ}\), where we observe stray light reaching the detector. We suspect this stray light arises from a manufacturing error, as a small defect near the primary mirror is present at this field (Figure 10). An accurate inspection of the mirror surface indicated some minor defects just at the edge of the mirror area, induced by small overshoots at the lead-in and lead-out positions of the diamond tool. In theory, these points are outside of the mirror surface and thus not illuminated in the simulations. However, since the RMS spot size is considered during the simulations, the tails of the Gaussian input beam might reach these minor defect points in the demonstrator, inducing light reflection and scattering.  
Figure 10: Small defects, highlighted by red circles, can be observed in the primary mirror and the mounting structure. One is particularly important because it is located exactly at the \(-24^{\circ}\) field, as indicated by the bottom field plot, probably causing the stray light that is observed for this field. The different colors in the bottom figure correspond to the different fields.  
Figure 9: Simulations versus measurements at the detector. The top row shows the simulated spots for different field angles, while the bottom row presents the measured spots. The measurements validate the simulations, except for \(\pm\)24\({}^{\circ}\), where we suspect stray light caused by a manufacturing error.  
Next, a more quantitative comparison is performed by evaluating the simulated and measured spot diameter. By rounding the simulated spot sizes to the number of pixels needed to image them, we enable a comparison with the illuminated pixels and thus spot sizes using the laboratory demonstrator (Figure 11). The measured and simulated spots show a similar size along the full FOV. For both the measurements and the simulations, we consider both the full spot diameter and the peak intensity. The simulated full spot diameter shows a good match with the measured peak intensity. However, the measured full spot diameters are consistently giving slightly larger values. Several factors might affect these latter spot sizes, such as stray light, possible manufacturing defects, or too large tolerances coupled with the absence of a compensator, as the distance between the fold mirror and the detector is fixed and imposed by the mounting plate. Additional simulations indicated the high importance of this compensator as minor detector movements strongly impact the spot size on the detector surface.