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Register a new userThe Integral Field Unit for the Echellette Spectrograph and Imager at Keck II
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A. I. Sheinis
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We report on the design, development and commissioning of an Integral Field Unit (IFU) that has been built for the Echellette Spectrograph and Imager (ESI) at the W.M. Keck Observatory. This image slicer--based IFU, which was commissioned in the spring of 2004 covers a contiguous field of 5.65 x 4.0 arcseconds in 5 slices that are 1.13 arcseconds wide. The IFU passes a spectral range of 0.39-1.1 um with a throughput of between 45 % and 60 % depending on wavelength and field position. The IFU head resides in an ESI slit mask holder, so that ESI may be converted to the IFU mode remotely by selecting the appropriate slit mask position. This IFU is the first of a family of designs for the spectrograph, providing a range of field-coverages and dispersions. In addition, we present the first-light science imaging and spectroscopic observations of RXJ1131-123, a low-redshift, lensed quasar. These observations show the 4 spectra of the lens and lensed-images captured in a single pointing.
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We report on the design and performance of the Keck Cosmic Web Imager (KCWI), a general purpose optical integral field spectrograph that has been installed at the Nasmyth port of the 10 m Keck II telescope on Mauna Kea, HI. The novel design provides blue-optimized seeing-limited imaging from 350-560 nm with configurable spectral resolution from 1000 - 20000 in a field of view up to 20x33. Selectable volume phase holographic (VPH) gratings and high performance dielectric, multilayer silver and enhanced aluminum coatings provide end-to-end peak efficiency in excess of 45% while accommodating the future addition of a red channel that will extend wavelength coverage to 1 micron. KCWI takes full advantage of the excellent seeing and dark sky above Mauna Kea with an available nod-and-shuffle observing mode. The instrument is optimized for observations of faint, diffuse objects such as the intergalactic medium or cosmic web. In this paper, a detailed description of the instrument design is provided with measured performance results from the laboratory test program and ten nights of on-sky commissioning during the spring of 2017. The KCWI team is lead by Caltech and JPL (project management, design and implementation) in partnership with the University of California at Santa Cruz (camera optical and mechanical design) and the W. M. Keck Observatory (observatory interfaces).
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.
OSIRIS is a near-infrared (1.0--2.4 $mu$m) integral field spectrograph operating behind the adaptive optics system at Keck Observatory, and is one of the first lenslet-based integral field spectrographs. Since its commissioning in 2005, it has been a productive instrument, producing nearly half the laser guide star adaptive optics (LGS AO) papers on Keck. The complexity of its raw data format necessitated a custom data reduction pipeline (DRP) delivered with the instrument in order to iteratively assign flux in overlapping spectra to the proper spatial and spectral locations in a data cube. Other than bug fixes and updates required for hardware upgrades, the bulk of the DRP has not been updated since initial instrument commissioning. We report on the first major comprehensive characterization of the DRP using on-sky and calibration data. We also detail improvements to the DRP including characterization of the flux assignment algorithm; exploration of spatial rippling in the reduced data cubes; and improvements to several calibration files, including the rectification matrix, the bad pixel mask, and the wavelength solution. We present lessons learned from over a decade of OSIRIS data reduction that are relevant to the next generation of integral field spectrograph hardware and data reduction software design.
We are building an image slicer integral field unit (IFU) to go on the IMACS wide-field imaging spectrograph on the Magellan Baade Telescope at Las Campanas Observatory, the Reformatting Optically-Sensitive IMACS Enhancement IFU, or ROSIE IFU. The 50.4 x 53.5 field of view will be pre-sliced into four 12.6 x 53.5 subfields, and then each subfield will be divided into 21 0.6 x 53.5 slices. The four main image slicers will produce four pseudo-slits spaced six arcminutes apart across the IMACS f/2 camera field of view, providing a wavelength coverage of 1800 Angstroms at a spectral resolution of 2000. Optics are in-hand, the first image slicer is being aluminized, mounts are being designed and fabricated, and software is being written. This IFU will enable the efficient mapping of extended objects such as nebulae, galaxies, or outflows, making it a powerful addition to IMACS.
The Gemini Planet Imager (GPI) is a new facility instrument for the Gemini Observatory designed to provide direct detection and characterization of planets and debris disks around stars in the solar neighborhood. In addition to its extreme adaptive optics and corona graphic systems which give access to high angular resolution and high-contrast imaging capabilities, GPI contains an integral field spectrograph providing low resolution spectroscopy across five bands between 0.95 and 2.5 $mu$m. This paper describes the sequence of processing steps required for the spectro-photometric calibration of GPI science data, and the necessary calibration files. Based on calibration observations of the white dwarf HD 8049B we estimate that the systematic error in spectra extracted from GPI observations is less than 5%. The flux ratio of the occulted star and fiducial satellite spots within coronagraphic GPI observations, required to estimate the magnitude difference between a target and any resolved companions, was measured in the $H$-band to be $Delta m = 9.23pm0.06$ in laboratory measurements and $Delta m = 9.39pm 0.11$ using on-sky observations. Laboratory measurements for the $Y$, $J$, $K1$ and $K2$ filters are also presented. The total throughput of GPI, Gemini South and the atmosphere of the Earth was also measured in each photometric passband, with a typical throughput in $H$-band of 18% in the non-coronagraphic mode, with some variation observed over the six-month period for which observations were available. We also report ongoing development and improvement of the data cube extraction algorithm.
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