The integral field spectrograph for the Gemini planet imager

Abstract

The Gemini Planet Imager (GPI) is a complex optical system designed to directly detect the self-emission of young planets within two arcseconds of their host stars. After suppressing the starlight with an advanced AO system and apodized coronagraph, the dominant residual contamination in the focal plane are speckles from the atmosphere and optical surfaces. Since speckles are diffractive in nature their positions in the field are strongly wavelength dependent, while an actual companion planet will remain at fixed separation. By comparing multiple images at different wavelengths taken simultaneously, we can freeze the speckle pattern and extract the planet light adding an order of magnitude of contrast. To achieve a bandpass of 20%, sufficient to perform speckle suppression, and to observe the entire two arcsecond field of view at diffraction limited sampling, we designed and built an integral field spectrograph with extremely low wavefront error and almost no chromatic aberration. The spectrograph is fully cryogenic and operates in the wavelength range 1 to 2.4 microns with five selectable filters. A prism is used to produce a spectral resolution of 45 in the primary detection band and maintain high throughput. Based on the OSIRIS spectrograph at Keck, we selected to use a lenslet-based spectrograph to achieve an rms wavefront error of approximately 25 nm. Over 36,000 spectra are taken simultaneously and reassembled into image cubes that have roughly 192x192 spatial elements and contain between 11 and 20 spectral channels. The primary dispersion prism can be replaced with a Wollaston prism for dual polarization measurements. The spectrograph also has a pupil-viewing mode for alignment and calibration.

Integral Field Spectroscopy of the Low-mass Companion HD 984 B with the Gemini Planet Imager

“…The star HD 984 was observed with the GPI integral field spectrograph (IFS, Chilcote et al 2012;Larkin et al 2014) on 2015 August 30 UT during the GPIES campaign (program GPIES-2015B-01, Gemini observation ID: GS-2015B-Q-500-982) at Gemini South. The GPI IFS has a field of view (FOV) of 2.8×2.8 arcsec 2 with a plate scale of 14.166±0.007 miliarcseconds/pixel and a position angle offset of −0°.10±0°.13 (Macintosh et al 2014a;De Rosa et al 2015).…”

Polarimetry With the Gemini Planet Imager: Methods, Performance at First Light, and the Circumstellar Ring Around Hr 4796a

“…Figure 17 shows the prism and translation stage. See also Larkin et al (2014) for further details on the GPI IFS and its mechanisms.…”

“…Halfway through the H sequence, the IFS cryocoolers were throttled down to reduce their induced vibrations. Modifications to the IFS and AO control laws in early 2014 have mitigated the level of vibration for subsequent observations, and further mitigations are ongoing (Larkin et al 2014;Poyneer et al 2014;Hartung et al 2014). The waveplate was rotated between P.A.s of 0…”

“…The imaging polarimetry mode is implemented primarily by a polarizing Wollaston prism beamsplitter within the Integral Field Spectrograph (IFS; Larkin et al 2014) and a rotating half-wave plate within the calibration unit, but depends on the overall instrument system to achieve its performance goals.…”

“…GPI is a dedicated system for directly imaging and spectroscopically characterizing extrasolar planets. It combines very high order AO (Poyneer & Dillon 2008;Poyneer et al 2014), a diffraction-suppressing coronagraph (Soummer et al 2011), an integral field spectrograph with low spectral resolution but high spatial resolution (Chilcote et al 2012;Larkin et al 2014), and an interferometric calibration wavefront sensor unit, plus an optomechanical support structure and extensive electronics and software infrastructure. First light of GPI occurred on 2013 November 12 UT; its initial performance achieved sufficient contrast for detection of β Pic b in single raw 60 s exposures without any postprocessing.…”