# Freeform Wide Field-of-View Spaceborne Imaging Telescope: From Design to Demonstrator
## 4 Discussion and Future Perspectives  
### Short-Term Perspectives and Comparison with the State-of-the-Art  
Pushbroom non-scanning wide FOV instruments, such as OMI, TROPOMI, and the future UVN and UVNS instruments, have inspired the design of our telescope. As with the TROPOMI telescope, we use two freeform mirrors featuring an extremely wide FOV [11]. However, unlike TROPOMI, which shows a FOV of approximately 108\({}^{\circ}\), our telescope features a FOV of 120\({}^{\circ}\), which is the widest field angle that has ever been achieved for a space-based telescope. Our extremely wide FOV nearly allows Earth observation from limb to limb from an altitude of about 700 km. On top of that, our spatial resolution equals 2.6 km \(\times\) 2.6 km, which is about 2.7 times better than TROPOMI. Consequently, our design not only surpasses previous instruments in terms of FOV but also in terms of spatial resolution, all while fitting within 1 CubeSat Unit.  
The operation of our novel freeform telescope is evaluated within a first qualitative proof-of-concept demonstrator setup. The realization of this proof-of-concept demonstrator involved tackling advanced freeform manufacturing and precise mounting of the mirror surfaces. In general, a similar optical performance is observed for the simulations and measurements, validating the imaging performance and potential of our future space-based telescope. Possible further improvements might be achievable by a re-evaluation of the mounting structure of the mirrors. More specifically, we believe the performance might benefit from the following five improvements: (1) adaptation of the lead-in and lead-out positions of the diamond tool, avoiding overshoot of the machining causing defects close to the optical surface; (2) use of a black coating on the structure holding the mirrors to reduce stray light; (3) use of a physical aperture located at the entrance pupil, in combination with a baffle located at the entrance of the system, to ease the alignment and reduce stray light; (4) adding an adaptable tilt to the mirrors, especially for the primary mirror, as the angle between the light source and the primary mirror appears to be critical; (5) adaptation of the detector positioning tools inducing additional degrees of freedom allowing compensation for tolerances, by using the distance between the third mirror and the detector as a compensator.  
Future research will also include a detailed analysis of the radiometric budget. A back-to-the-envelope calculation, in combination with a comparison with the TROPOMI telescope, gives confidence that a sufficient signal-to-noise ratio (SNR) can be achieved. Our novel telescope features a small entrance pupil diameter (2.52 mm for our instrument vs. 3 mm for TROPOMI), targets an excellent spatial resolution (2.6 km vs. 7 km for TROPOMI), and thus a short maximum integration time (371 ms for a spatial resolution of 2.6 km at nadir vs. 1 s for 7 km for TROPOMI), which tend to decrease the SNR. However, this is balanced by the wider FOV (120\({}^{\circ}\) vs. 108\({}^{\circ}\)) and smaller f-number (7.23 vs. 9-10), which tend to increase the SNR.  
Figure 14: Scattered stray light beam evaluation when considering backward ray-tracing and illumination: (**a**) simulation result, (**b**) spot measurement, where the color corresponds to the beam intensity distribution. Both simulations and measurements show a similar performance.