## 2 Materials and Methods
### The Instrument  
The GLADE reflecting telescope has a 0.3 m primary mirror, i.e., six times larger than the PROBA3 telescope. The instrument layout is shown in Figure 2: stellar photons proceed from the primary mirror (M1) to the secondary mirror (M2), then to the folding mirror (FM) and the tertiary mirror (M3), finally reaching the ring-shaped focal plane (FP).  
In fact, the central field is lost at the FM level due to the large central aperture required to allow optical beams from the desired annular field through to the FP. In addition, this is a benefit to our application because the Sun and inner corona, already largely rejected by the external occulter, still provide significant diffracted light, which in GLADE is dumped to space through the FM hole, taking some of the functions of a Lyot stop and/or internal occulter, as shown in Figure 3. Additional features, e.g., a baffling acting as a light trap, are not further addressed in the scope of the present paper.  
Figure 3: GLADE telescope optical layout: rejection of residual diffracted light from the Sun. Folding mirror FM and subsequent stellar optical paths not shown.  
Figure 2: GLADE telescope layout: optical path to the focal plane of rays from fields at \(\pm 0^{\circ}.7\) (blue and green lines, respectively).  
The optical design [23] is derived from a classical Korsch configuration [24; 25], further modified to achieve good imaging quality over a large annular field. The focal plane is populated only over a strip ranging from 35\({}^{\prime}\) to 50\({}^{\prime}\), i.e., approximately 0\({}^{\circ}\).58 to 0\({}^{\circ}\).83 (width \(\sim\) 15\({}^{\prime}\)), according to the layout shown in Figure 4 (left). The detection system is a set of 12 uniformly spaced 4\(k\times 4k\) CMOS chips with a pixel size of 4 \(\upmu\)m (chip size of 16.4 mm), compatible with current technology (e.g., Teledyne Photometrics (Tucson, AZ) Iris 15 or SBIG (Ottawa, Ontario, Canada) STC-428-P cameras, Sony ([[https://www.image-sensing-solutions.eu/](https://www.image-sensing-solutions.eu/)]([https://www.image-sensing-solutions.eu/](https://www.image-sensing-solutions.eu/)), accessed on 13 December 2023) IMX455). The well-spaced layout allows for the modular development of simple individual detector modules and also the easing of thermal dissipation issues.  
The light deflection values in the GLADE field range between 805 mas at 0\({}^{\circ}\).58 and 562 mas at 0\({}^{\circ}\).83, with an average value of 667 mas at the intermediate 0\({}^{\circ}\).7 radius. Therefore, the deflection amplitude is about two orders of magnitude larger than in Gaia.  
The detected signal is somewhat degraded by the finite pixel size, introducing some MTF degradation even on nearly ideal detectors. In Figure 4 (right), the different distributions of photons (solid line), as produced by diffraction in the optical system, and of detected photo-electrons (dashed line) are shown. The effects of a lower peak signal and a wider signal spread are taken into account in the performance assessment.  
The effective focal length (EFL) \(F=3.7\) m results in an optical scale \(s=55.75^{\prime\prime}\)/mm, so that the detector angular size is 0\({}^{\prime\prime}\).22/pixel and 15\({}^{\prime}\)/chip. The overall sky coverage is 0.77 square degrees. The 10\(\times\) smaller pixels of GLADE provide a 100\(\times\) reduction in the background with respect to PROBA3, corresponding to a faint sensitivity improvement of about 5 magnitudes.