# Freeform Wide Field-of-View Spaceborne Imaging Telescope: From Design to Demonstrator
## 3 Telescope Proof-of-Concept Demonstrator
### Laboratory Demonstrator Setup
#### 3.2.3 Stray Light Analysis  
While we qualitatively demonstrated the proof-of-concept of our telescope, small deviations in the performance were observed, mainly originating from stray light that is induced by the tilt of the primary mirror and the manufacturing defects at the edges of the mirrors. To gain more insight into the stray light and its origin, an in-depth evaluation was performed using red laser light, replacing the visual observation of the stray light within the laboratory setup. For this, we used a collimated red laser (780 nm, beam diameter of 850 \(\upmu\)m), in combination with the Gentec BEAMAGE-4M beam profiling camera as the detector.  
The tilt of the primary mirror with respect to the laser source appeared to be a highly sensitive and important parameter influencing the stray light. Despite the presence of a black absorbing sheet to reduce the amount of stray light, a suboptimal tilt of the light source with respect to the primary mirror induces an elliptical vertical beam (Figure 12). In the current setup, and because of the monolithically mounting, the tolerance on this tilt could not be accurately determined. However, the presence of stray light was cross-checked with the  
Figure 11: Comparison between the number of pixels (pixel pitch = 20 \(\upmu\)m) needed to image the simulated spots and the number of pixels on which the spots were imaged using the laboratory demonstrator. For each field angle, the peak intensity of each spot (peak), as well as the full spot is considered. Error bars are given on the full spots. The error on each measured full spot is 1 pixel, while the error bars on the simulated spots are given by the estimated changes (dependent on each field) in the tolerance analysis.  
simulations using Zemax OpticsStudio non-sequential mode, indicating a vertical beam when the source is not properly tilted around X, thus confirming these laboratory observations.  
An additional evaluation was performed using backward illumination, by interchanging the source and detector. This evaluation enables us to focus on the stray light presence while excluding the potential positioning error on the detector, which was also used as a compensator during the tolerance analysis. To enable a cross-check between the simulations and measurements, the ray-tracing simulations are adapted according to the backward laboratory configuration, using the non-sequential mode (Figure 13). For this, the former detector is replaced by the collimated laser source, and a new detector is inserted at the position of the entrance aperture. During both the simulations and measurements, a vertical line is achieved on the detector (Figure 14). As the incident beam size slightly exceeds the size of the secondary mirror, this line signal is induced by scattering on the edges inducing stray light in the vertical direction. The thickness of the vertical beam approximately equals 1300 \(\upmu\)m for the simulation and 1815 \(\upmu\)m for the measurements. The measurements show a slightly larger beam due to the imperfections on the sides of the mirror as indicated in Figure 10.  
Figure 12: Stray light induced by a minor misalignment of the source and primary mirror, simulated within the non-sequential mode. When the source is not properly tilted around X, stray light occurs after reflection on the primary mirror, causing a vertical beam.  
Figure 13: Backward ray tracing from the fold mirror to the secondary mirror and primary mirror. The detector is located at the position of the former entrance pupil. An elliptical vertical spot is obtained at the detector.