Proto-NUX: A prototype telescope for ground-based near-ultraviolet observations

Abstract

The Near-UV-eXplorer (NUX) is a proposed ground-based, wide-field telescope array with a field of view of $\sim$70 square degrees, designed to operate over the 300-350 nm wavelength range and to achieve a target sensitivity of 20 mag in 150 seconds (5 sigma). Its main scientific objective is the detection and characterization of hot, rapidly evolving transients in the near-UV (NUV). Proto-NUX is a pathfinder instrument for NUX, based on an off-the-shelf 36 cm Celestron RASA wide-field astrograph that has been modified to enhance throughput and image quality in the targeted NUV band. The main objectives of Proto-NUX are: (1) to quantify the NUV sensitivity of the prototype and assess the feasibility of the full NUX facility; and (2) to characterize atmospheric extinction in the NUV, including its temporal variability and its dependence on zenith angle. Using three filter configurations, we aim to measure the wavelength dependence of the atmospheric extinction and to disentangle the contributions from Rayleigh scattering (dominating at wavelengths >325 nm) and molecular ozone-dominated absorption (dominating <315 nm). On-site testing is scheduled for 2026 at the Pic du Midi Observatory (France, 2877 m altitude) in order to evaluate on-sky performance under high-altitude observing conditions.

Figures (8)

• Figure 1: Decomposition of atmospheric attenuation in the NUV into its individual extinction components, calculated with the Cerro Paranal Advanced Sky Model for the La Silla site at 1.2 airmass. Shaded regions indicate the relative contribution of each component to the total attenuation. Vertical lines denote the bandpasses of the adopted NUX filters (see Section \ref{['section:filters']}).
• Figure 2: 3D drawing of the proto-NUX telescope. The optical tube and focusing mechanism are the original components of an off-the-shelf 36 cm Celestron RASA telescope. The front section of the telescope (left side of the image) has been completely redesigned and newly manufactured, including the Schmidt-corrector (blue), the corrector holder (black/yellow), and the lens assembly (teal). The CMOS camera (brown) is attached to the lens assembly via a tilt plate and a filter slider. A close up view is shown in Fig. \ref{['fig:lens_assembly']}.
• Figure 3: Close-up 3D view of the redesigned lens assembly. The Schmidt corrector (blue) supports the aluminum lens assembly housing (teal), which is secured by a retaining ring (black). The two lenses (red/yellow) are held in place by a threaded retaining ring (gold). A filter slider (green) and the CMOS detector are attached to the housing; the camera sensor and its protective UV-transparent window (purple) are shown.
• Figure 4: Spot diagrams of the proto-NUX optical design at different field positions across the field of view, roughly corresponding to the size of the Gpixel GSENSE2020 BSI sensor. The diagrams are shown for multiple wavelengths (top figure: 300nm and 325nm, bottom figure: 325nm and 350nm). The black circle represents the Airy disk, and the squares are about three times the pixel size (6.5 micron) of the CMOS detector.
• Figure 5: Transmission curves of the individual components contributing to the proto-NUX system throughput. The figure shows the transmission of the three NUX filter configurations, the atmospheric transmission (1.2 airmass, la Silla), the mirror coating reflectivity, the quantum efficiency of the CMOS camera, and the transmission of the Schmidt corrector. See the inset for the color coding of the different components.