PYRAMIR: Exploring the On-Sky Performance of the World’s First Near-Infrared Pyramid Wavefront Sensor

Abstract

This paper presents the on-sky performance of the unmodulated infrared pyramid wavefront sensor PYRAMIR mounted on the ALFA adaptive optics system at the 3.5 m telescope of the Calar Alto Observatory. The performance of the system is compared with the performance of the Shack-Hartmann wavefront sensor of the ALFA system. We carried out a series of measurements to characterize the performance of PYRAMIR under open-loop tip-tilt compensation, and high-order closed-loop conditions, using stars of different magnitudes. We measured the tip-tilt jitter by following the centroid position of a stellar image on a fast series of frames. Additionally from the pyramid wavefront sensor data we could estimate the tip-tilt jitter in closed-loop. Under closed-loop conditions we also measured the long-exposure Strehl ratio. We compared the results of the wavefront sensor measurements with those of the Shack-Hartmann sensor on the same telescope, especially regarding the distribution of the error budged over the Karhunen-Loève modes, and the power spectral density. Our first finding is that we can indeed start up this nonmodulated pyramid system, even under bad seeing conditions. Under good conditions the Strehl ratio reaches ≥60% in K 0 band. We found that the minimum signal-to-noise ratio (S/N) in each subaperture required to close the high-order loop is only 0.4. This is a surprisingly low number. To compare the performance to existing systems, we introduce the S/N per subaperture per loop cycle as a device-independent measure. Using this scheme, we find that the ratio between the low-order residuals and the high-order residuals in the case of PYRAMIR is lower than that of the Shack-Hartmann system, especially in the faint flux regime. This is an important finding because it means that the pyramid-based system removes the halo, i.e., light scattered by the atmosphere, around the target star better than a Shack-Hartmann sensor-based system. A comparison of the power spectral density of the PYRAMIR and the Shack-Hartmann sensor measurements, and a comparison of the noise propagation coefficients of PYRAMIR with theoretical predictions from the literature, confirm this superiority of the pyramid over the Shack-Hartmann sensor.

Expected gain in the pyramid wavefront sensor with limited Strehl ratio

“…Even if this analytical expression is not directly used in the following, here it is reported to show how the SH-WFS global reconstruction noise is proportional to the photon noise, enabling us to gauge the P-WFS noise to that of the SH-WFS, assuming the same reconstructor. The prediction given in 1999 was confirmed with high confidence for low radial orders (up to Q = 7) with a laboratory experiment called Pyramir (Peter et al 2010). We contacted the authors and were informed that their data were obtained in a laboratory and that the Strehl ratio (SR) on the pyramid pin was of the order of 90−95% (priv.…”

“…From a course inspection of Fig. 8 of Peter et al (2010), a level of confidence better than 5% is estimated. The lower propagation coefficients, for the P-WFS in closed loop, translate into a gain ∆ MAG > 2.5 mag, which can be achieved with respect to a SH-WFS, for the generation of ELTs in the visible and IR bands (Fig.…”

High-Speed Imaging and Wavefront Sensing With an Infrared Avalanche Photodiode Array

“…Recent innovations in infrared avalanche photodiode (APD) detectors, wherein the avalanche gain of photo-generated electrons occurs within the HgCdTe substrate, have reduced the effective read noise of sizable pixel arrays to below the critical 1 e − threshold (Feautrier et al 2014;Finger et al 2014). When paired with correspondingly low dark currents, there is the potential to drastically improve the many current and future applications of infrared arrays in astronomy, e.g., infrared photon counting (Beletic et al 2013;Rauscher et al 2015), improving the sky coverage of laserguided star adaptive optics (AO) systems using sharpened infrared tip-tilt stars (McCarthy et al 1998;Dekany et al 2008;Wang et al 2008;Wizinowich et al 2014), increasing the sensitivity of pyramid wavefront sensors (Peter et al 2010) and interferometers, e.g., S. Guieu et al (2015, in preparation), decreasing noise in post-coronagraphic and speckle nulling wavefront sensors in high-contrast systems (Martinache et al 2012;Cady et al 2013), and improving temporal bandwidth and sensitivity for IR photometric observations (Rafelski et al 2006;Mereghetti 2008). To prove this maturing technology in a challenging observing environment, we demonstrate the use of a Selex ES Advanced Photodiode for High-speed Infrared Array (SAPHIRA) with the Robo-AO visible-light laser AO system mounted to the robotic Palomar Observatory 1.5 m telescope (Cenko et al 2006).…”

Preprocessed cumulative reconstructor with domain decomposition: a fast wavefront reconstruction method for pyramid wavefront sensor

“…Since the pyramid sensor was introduced by R. Ragazzoni in 1990s [3,4], theoretical studies and numerical simulations have shown the enhanced and adjustable sensitivity as well as a better closed loop performance compared to the SH sensor [5][6][7]. The advantages of the pyramid over SH sensor were successfully demonstrated on sky [8,9].…”