High-Precision Photometry with a scientific CMOS Camera: II On-Sky Testing of the Marana camera at the NGTS facility

Abstract

Modern scientific CMOS cameras offer very fast readout speeds and low read noise. In this study, we evaluate the performance of the Andor Marana CMOS camera through on-sky testing carried out at the NGTS facility at the ESO Paranal Observatory in Chile. We mount the Marana camera to an NGTS telescope, and conduct photometric observations of bright stars. In particular, we target transit events around eight known bright exoplanet host stars. Simultaneous observations are carried out using an existing Andor iKon-L CCD camera on a neighbouring NGTS telescope. This allows for a direct comparison of the photometric precision between the CMOS and CCD cameras. We find that the Marana CMOS exhibits a similar level of photometric performance to the CCD camera, achieving 500\,ppm at a 30-minute timescale for a T $=10$\,mag star. Although the CCD has a slightly better quantum efficiency over the NGTS filter range (520-890\,nm), we find that the faster readout speed of the CMOS compared to the CCD means that the CMOS camera detects 20\,\% more photons per unit time for a solar-type star in our standard 10\,s exposure time operation mode. This results in the CMOS performing slightly better photometry in the photon-limited regime. We conclude that modern CMOS cameras, such as the Marana, are very well-suited for astronomical time-series photometry applications.

Figures (21)

• Figure 1: Top: QEs of the cameras as a function of wavelength. The filled grey area shows the NGTS custom filter. The blue line is the QE for the Marana CMOS camera and the red dashed line is the QE of the iKon-L CCD. Bottom: The ratio of the QE (CMOS/CCD).
• Figure 2: The Marana sCMOS camera, highlighted within the blue rectangle, is mounted on one of the NGTS telescopes while capturing flat-field images during evening twilight at Cerro Paranal, Chile.
• Figure 3: Target pixel frame (50 $\times$ 50 pixels) for CCD on the left and CMOS on the right. The selected photometric apertures for each camera are shown with green circles for target star KELT-10 (TIC-269217040, T $=10.27$ mag) observed on 2024 August 1.
• Figure 4: Photometric precision as a function of stellar magnitude for 10 seconds exposure predicted by the theoretical model and from observations during the new moon on 5th July 2024 (upper panels) and full moon on 22nd June 2024 (lower panels). The left panels represent data from the CCD, while the right panels correspond to the CMOS. The dark line indicates the total modelled noise, including contributions from scintillation (yellow), photon shot noise (green), dark current noise (purple), sky background noise (blue), and read noise (red). The data points are coloured according to the Gaia DR3 $\mathrm{G_{BP}}-\mathrm{G_{RP}}$ colour and represent the measured noise in the detrended light curves from relative photometry of 746 stars with $8<\mathrm{T}<14$ in a field centered on TIC-188620407.
• Figure 5: Ratio for CCD and CMOS camera noise contribution for different magnitude stars from \ref{['fig:noise_dark_bright']}. The data points are coloured according to the catalogued Gaia DR3 $\mathrm{G_{BP}}-\mathrm{G_{RP}}$ colour. The black dashed line represents the ratio 1 which is the case where the noise is the same for both cameras. Top: The noise ratio for a dark night (no-moon). Bottom: The noise ratio for a bright night (full-moon). The panels on the right show the histogram distributions.