GPI 2.0: design of the pyramid wave front sensor upgrade for GPI
ABSTRACT
After more than six years of successful operation at Gemini South, the Gemini Planet Imager (GPI) will be moved to Gemini-North. During this move, the instrument will undergo a series of upgrades. One of these upgrades will be the installation of a new pyramid wavefront sensor (PWFS) with a low noise EMCCD detector that will replace the current Shack-Hartmann WFS. This upgrade is expected to significantly increase the sky coverage of GPI, providing increased level of AO correction and access to fainter targets. The new PWFS will be assembled on a standalone bench that will be aligned and tested independent of the GPI to ensure the required performance is achieved. Once the performance is verified, the completed subassembly will be installed in place of the current WFS hardware during the final integration into the GPI. In this paper, we will present the final design of the new GPI PWFS. Included will be a description of the optical performance simulations completed and their results, and a detailed overview of the opto-mechanical design of the new PWFS bench.
1. INTRODUCTION
The Gemini Planet Imager (GPI) is a facility-class instrument that was originally designed for and installed on the Gemini-South 8 meter telescope located in Chile. Since the commissioning of GPI in 2013 and the beginning of science operations in 2014, GPI has been used for a wide range of scientific programs ranging from large-scale exoplanet surveys to short-term, high-impact observations leading to the discover of new exoplanets.[1] The GPI instrument consists of three major components: an adaptive optics (AO) system, a calibration (CAL) system and an integral field spectrograph (IFS). These systems work together to correct the bulk atmospheric distortions, attenuate the light from the star while transmitting the light from any nearby orbiting exoplanets, correct any residual aberrations not corrected by the AO system and complete the spectral analysis of the remaining exoplanet light.
With the decision to move GPI to Gemini-North, the scientific community decided to leverage this opportunity by embarking on a significant upgrade to several GPI systems that will further enhance the capabilities of the new GPI2. The portion of the GPI2 upgrade that is covered in this study is the AO system upgrade. The original AO system, based on a Shack-Hartman wavefront sensor (WFS) design, will be replace with a new AO system utilizing a pyramid WFS (PWFS) which will enable access to lower magnitude stars while providing better correction than GPI is currently capable of delivering. Significant effort has already been expended to validate the PWFS concept through analysis and testing completed during the development of the PWFS design for the Thirty Meter Telescope’s (TMT) Narrow Field InfraRed Adaptive Optics System (NFIRAOS). Leveraging this work, using many of the same components designed for NFIRAOS, will provide a robust upgrade to GPI. The contents of this paper will describe the optical design and supporting simulations, and the final opto-mechanical design of the GPI2 PWFS AO system
2. OPTICAL DESIGN AND SIMULATION
2.1 PWFS Optical Design
The optical design concept, illustrated in Figure 1 below, receives an f/64 beam reflected from the GPI science beamsplitter (dichroic with a cut-off wavelength of 925nm) that passes through two flat steering mirrors: one with two angular adjustments (SM1) and one with two angular adjustments plus one linear adjustment to control focus (SM2). These two mirrors adjust the position and focus of the spot on the tip of the pyramid (pointing) as well as the position of the pupil images on the detector (centering). Additionally, SM1 is used to modulate and dither the spot around the tip of the pyramid. The pyramid optic is a glass double-pyramid that splits light into four channels, each of them going through a lens (camera lens) to create an image of the telescope entrance pupil onto a fast-readout low-noise detector (EMCCD) for image acquisition. Close to the tip of the pyramid, a field stop limits the FOV to reduce background contamination and limit source confusion. Absent from this design is an atmospheric dispersion compensator unit (ADC) since the expectation is to use the GPI wide-band ADC in the common path. The feasibility of using the GPI ADC was a significant risk in this design and was addressed first in the development of the GPI2 PWFS upgrade. This work is summarized in Section 2.3 below.
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