The ORCA-TWIN qCMOS Project I. Commissioning at Calar Alto Observatory

Aims. We describe a pilot study to explore a new generation of fast and low noise CMOS image sensors for time domain astronomy, using two remote telescopes with a baseline of 1800 km. Methods. Direct imaging with novel qCMOS image sensor technology that combines fast readout with low readout noise. Synchronized observations from two remote telescope sites will be used to explore new approaches for measuring solar system bodies, precision stellar photometry, and speckle imaging. Results. A fast-track installation of an ORCA-Quest 2 camera at the Calar Alto Observatory (CAHA) 1.23m telescope has demonstrated the potential of the qCMOS technology for time domain astronomy. Conclusions. qCMOS technology generally outperforms classical CCDs for high-cadence imaging on 1-m telescopes, although EMCCDs remain competitive, and in some cases slightly superior, for very short exposures and faint sources.

💡 Research Summary

The paper presents the first commissioning results of the ORCA‑TWIN project, a pilot study that evaluates a new generation of fast, low‑noise CMOS image sensors (referred to as qCMOS) for high‑cadence time‑domain astronomy. The authors installed a Hamamatsu ORCA‑Quest 2 camera on the 1.23 m telescope at Calar Alto Observatory (CAHA) and performed a series of technical tests and on‑sky observations. The ORCA‑Quest 2 employs a back‑illuminated 4096 × 2304 pixel sensor with a 4.6 µm pitch, a floating‑diffusion node in each pixel, and on‑chip correlated double sampling (CDS). These design features enable an exceptionally low read‑out noise of 0.3 e⁻ and a frame‑rate of up to 25 fps in low‑noise mode, allowing single‑photon detection and a high conversion gain.

The authors compare the qCMOS performance against a conventional scientific CCD (Andor iKon‑L) and an EMCCD. Simulations using the optical parameters of the CAHA 1.23 m telescope (focal length 9.8 m, plate scale 20.9 arcsec mm⁻¹) show that, for exposure times longer than ~0.1 s, the qCMOS delivers a significantly higher signal‑to‑noise ratio (SNR) than the CCD, thanks to its negligible read‑out noise and fast read‑out. For ultra‑short exposures (≤ 10 ms) and extremely faint targets (≈ 20 mag), the EMCCD still retains a modest advantage because of its electron‑multiplication gain, despite its higher multiplication noise and limited dynamic range. The qCMOS, however, offers a much broader dynamic range, enabling simultaneous imaging of bright stars and faint transients without saturation.

The scientific concept of ORCA‑TWIN relies on synchronized observations from two remote sites separated by ~1800 km (Calar Alto in mainland Spain and the STELLA telescope on Tenerife). By time‑stamping each frame with GPS precision (millisecond accuracy) and using high‑cadence imaging, the system can effectively cancel atmospheric turbulence and perform three‑dimensional triangulation of near‑Earth objects (NEOs), low‑Earth‑orbit satellites, and space debris. The authors illustrate the potential with a historic example: simultaneous observations of asteroid 4179 Toutatis from Paranal and La Silla yielded a 40 arcsec parallax over a 513 km baseline. Scaling to the 1800 km ORCA‑TWIN baseline, sub‑arcsecond astrometry becomes feasible with exposures as short as 0.1 s, allowing distance determinations out to several astronomical units.

On‑sky tests at Calar Alto demonstrated that the ORCA‑Quest 2 can achieve photometric precision better than 0.05 mag for fast‑rotating NEOs, M‑dwarf flares, and hot subdwarf pulsators, outperforming a comparable CCD by a factor of two in the same exposure regime. The high frame‑rate also enables speckle‑interferometry‑like measurements: two independent lines of sight sample different atmospheric turbulence cells, and information‑field‑theory reconstruction can retrieve sub‑milliarcsecond stellar diameters or resolve diffraction patterns during asteroid occultations.

The paper concludes that qCMOS technology outperforms classical CCDs for high‑cadence imaging on 1‑meter class telescopes, while remaining competitive with EMCCDs; each detector type excels in different regimes (EMCCD for the shortest exposures and lowest fluxes, qCMOS for broader dynamic range and longer high‑speed exposures). The successful commissioning at Calar Alto paves the way for installing the second camera on the STELLA telescope in early 2026, completing the ORCA‑TWIN network. Once operational, the network will enable rapid triangulation of NEOs, real‑time monitoring of transient phenomena, high‑contrast Bayesian imaging with photon‑counting cameras, and coordinated electromagnetic follow‑up of gravitational‑wave events, thereby opening a new observational window for time‑domain astrophysics.