P.I.: J. S. Miller, M. Bolte, R. Guhathakurta, D. Zaritsky (UCO/Lick Observatory)

The ESI instrument is a versatile, multi-mode spectrograph and imager, with high throughput one of its most important goals. ESI began commissioning in August 1999. There are two distinct spectroscopic modes:

• a medium-resolution echellette mode with prism cross dispersion; and
• a high-throughput mode using prism dispersion only.

The spectrograph modes cover the full wavelength range of the Keck II silvered mirrors (3900–11,000 Å) in a single exposure with a velocity resolution as low as 22 km/sec FWHM (using the 0.3 arcsec wide Echellette slit). The low-resolution, prism-only mode provides the same spectral range, multi-object capability, and high throughput. ESI also provides an imaging mode with a field of view of 3.0 arcminutes. An Epps refracting camera and a single 2K×4K detector are used for all three modes.

The ESI collimator will be an off-axis paraboloid, similar to the collimator in LRIS. A single 2K×4K Lincoln Labs CCD with 15 micron pixels is used with a Leach-2 CCD controller. A single, all-refracting camera, is used for all three operating modes. There is a 1K×1K PXL acquisition/guide TV camera that views part of the slit plus a stationary, offset mirror. Three 5-position filter-wheels will be provided in the science beam: one in the focal plane containing slits, and two beneath that providing either a mask (for the echellette slits, which are all on a single slit plate) or filters. Another wheel carries filters for the guide/acquisition camera. At any one time, a single echellette grating is available, fixed in position and in rotation. Changing gratings is a manual operation and normally will not be done during the night. A flat mirror bypasses the grating for switching to the prism-only (low-dispersion) mode. A second flat mirror bypasses both the prisms and the grating to provide the imaging mode.

A page on sensitivities contains useful information for planning exposure times, etc.

General specifications
Location	Keck II Cassegrain (f/15)
Focal plane scale	0.725 mm/arcsec
Scale at detector	0.1542±0.0010 arcsec/pixel
Wavelength range	0.39–1.1 microns
Detector	MIT-LL 2048×4096 CCD, 15 micron pixels
CCD sensitivity	QE of 10% (at 0.32 m), 61% (0.4 m), 82% (0.5 m), 80% (0.6 m), 77% (0.7 m), 69% (0.8 m), 45% (0.9 m), 11% (1.0 m)
Readout electronics	Leach 2 DSP programmable
Readout modes	Single or dual amplifier; slow, normal, and fast (the default) modes
Readout times	39 sec (dual-amp, default [fast] speed) 70 sec (simgle amp, default speed)
Readout noise	2.5 electrons/pixel (normal readout speed)
Grating	175 lines/mm, 32.3° blaze angle
Cross dispersion	Two prisms, first used in double-pass mode.
Slit/filter wheels	Three, only first (slit wheel) in focal plane. Five "slots" in each wheel. Each slot can accomodate up to three filters. Normally one slot is left clear in each wheel.
Wavelength calibration lamps	Xe, HgNe, CuAr, quartz
Guider	Slit viewing with offset field 1024×1024 Photometrics PXL CCD Total FOV 3×3 arcmin. 0.233 arcsec/pixel 50×50 mm filters, initially only ND filter provided.
Echellette mode
Wavelength range	0.39–1.1 microns
Spectral resolution	As low as 22.4 km/sec (FWHM) using 0.3 arcsec slit
Resolving power (R=λ/Δλ)	As low as 22.4 km/sec (FWHM) using 0.3 arcsec slit
Slit length	20 arcsec
Slit widths	0.3, 0.5, 0.75, 1.0, 1.25, 6 arcsec (all on same slit plate, selectable using mask in second wheel)
Order blocking filters	None
Low-dispersion mode
Wavelength range	0.39–1.1 microns
Spectral resolution	R=6000 (0.39 microns) to R=1000 (1.1 microns)
Slit length	8 arcmin
Slit widths	0.5, 0.75, 1, 1.25, 6 arcsec (each on a separate slit plates). Currently only the 1 and 6 arcsec slits are available.
Order blocking filters	None needed; dispersion provided by prisms, which have no higher-order spectra.
Slitmasks	Not available initially.
Imaging mode
Field of view	2×8 arcmin (full field) 2.1×3.5 arcmin (for 93×154 mm filters)
Scale	0.1542±0.0010 arcsec/pixel
Filters	B, V, R, I (93×154 mm; 128×212 arcsec FOV) full-field R filter (2×8 arcmin FOV) holders for two 100×100 mm filters (138×138 arcsec) holders for three 50×50 mm filters (69×69 arcsec)
User Interface
Control electronics	intelligent Galil controllers, parallel stage control
Computer	Sun Ultra 2, OpenWindows
Image display	FIGDISP with tcl/tk GUI
Control GUI	Dashboard (tcl/tk-based)
Data reduction	IRAF, IDL, Vista, custom echelle reduction software