Background

The Keck Cosmic Web Imager (KCWI) is a wide field, seeing limited, integral field spectrograph (IFS) optimized for precision sky limited spectroscopy of low surface brightness phenomena. KCWI features high throughput and flexibility in field of view (FOV), spatial sampling, bandpass, and spectral resolution. KCWI provides full wavelength coverage (350-1050 nm) using optimized blue and red channels. The development of KCWI is being done through collaboration among CIT, UCSC, and WMKO. KCWI's blue channel provides wavelength coverage of 350-560 nm, and KCWI's new red channel provides wavelength coverage of 560-1080 nm. KCWI is based on the Cosmic Web Imager commissioned at the Palomar Observatory in 2010. CWI is a single channel instrument with a fixed spatial sampling scale and a single adjustable VPH grating and an articulated camera with 2k x 4k mid-band optimized CCD. KCWI builds on the heritage of CWI with a two channel design offering three selectable spatial sampling scales and a selection of VPH gratings.

KCWI operates on the Keck II telescope at the right Nasmyth focal station. The instrument is equipped with an annular MAGIQ guide camera using wavelengths 560-695 nm. The annular field of view has an inner radius of 50 arcsec and an outer radius of 130 arcsec. The instrument operates in fixed gravity with field rotation compensated by a K-mirror for the science FOV. Figure 1 shows a view of KCWI-B with the outer enclosure transparent. A mannequin is shown as an indication of the overall size.

A large, custom made optical bench supports all of the instrument’s components, and three kinematic mounting points are used to accurately position the instrument at the telescope focus. The optical and mechanical configuration of KCWI is illustrated in block diagram form in Figure 2 and the optical layout is shown in Figure 3. The KCWI key performance requirements and verified compliance are listed in Table 1. All Level 1 Science Requirements have been met. Commissioning tests are required to fully verify the sky subtraction performance.

Figure 2 shows an isometric view of the KCWI optical layout. The science light passes first through a window to a K-mirror image de-rotator to a selectable image slicer stack located at the telescope focus. The slicer stack sits on a linear stage that selects among 3 slicer formats and a direct imaging alignment camera. The measured imaging plate scale is 0.147”/pixel.  A calibration system with a deployable periscope mechanism directs calibration light onto the image slicer. Calibration images provide the geometric information that are the basis of conversion of raw slicer images to rectified data cubes.
 
KCWI’s integral field unit (IFU) uses three selectable slicer mirror stacks to provide 0.35", 0.69" or 1.38" spatial resolution (with commensurate 8.4", 16.5", or 33.1" FOV in one axis and a 20.4" FOV for all configurations on the other axis, all based on measured slicer properties) and 24 slices for each format.

Light from the pupil array proceeds to a spherical collimator and then to a wavefront-correcting cylindrical fold mirror (FM1), completing the portion of the optical path common to both the red and blue channels. In the original blue channel only version of KCWI, a mirror was used to direct all the light to the blue side. In the completed instrument with both blue and red sides, the dichroic beam splitter reflects the blue light to the blue channel and passes the red light to the red channel optics and detector.

Slit limited spectral resolution has been demonstrated by test, enabling resolution depending on grating and slicer choice from R~1000-20,000. The demonstrated imaging resolution (0.3" FWHM) proves that imaging parallel to the slices will always be seeing-limited. 

Each spectrograph channel consists of a selectable bandpass filter (blue only); selectable, variable tilt gratings; and an articulated CCD camera. The blue channel fold mirror (FM3) and the remainder of the blue channel optical path are located below the KCWI optical bench. KCWI also incorporates a selectable nod-and-shuffle (N&S) mask, which covers 2/3 of the CCD active area when deployed to enable two patches of sky to be observed alternately (one patch of sky is hidden under the mask while the other is observed) during the course of a single integration for improved background subtraction with only a single detector readout. The N&S mask mechanism has been verified to position repeatably and to be robust over many cycles.

A focal plane camera is provided along with the 3 slicers in order to rapidly image the science field falling on the IFU. The spectroscopic throughput (corrected for the K-mirror reflectivity based on manufacturer witness coating tests) is 46% at 450 nm for the wideband low resolution BL grating, exceeding the requirement of 40% and the goal of 45% (instrument only). Medium (BM) and high resolution (BH2) gratings meet the 40% requirement, while BH3 shows a shortfall, achieving 30%.

Light-leakage and scattered light inside the instrument is very low. Contrast and ghosting requirements are met. Instrumentally induced sky subtraction errors are below the sky subtraction requirement, and the on-sky performance will likely be limited by sky background variations. Detector read noise (<3 e- rms) and dark are low and meet requirements.

The KCWI software package provides a full graphical interface to the user. There are easy means for configuring the instrument and taking exposures. FITS files contain extensions with detailed information on everything in the instrument configuration. Scripts can be written to execute a series of actions, and considerable attention has been given to error-trapping. The KCWI Data Reduction Pipeline is based on the PCWI pipeline and has been tested using both PCWI science data and KCWI calibration data. The pipeline is working and has been delivered with the instrument.