We describe the Cosmic Web Imager (CWI), a UV-VIS integral eld spectrograph designed for the Hale 200"
telescope at the Palomar Observatory. CWI has been built specically for the observation of diuse radiation.
The instrument eld of view is 60"40" with spectral resolving power of R5000 and seeing limited spatial
resolution. It utilizes volume phase holographic gratings and is intended to cover the spectral range 3800A to
9500A with an instantaneous bandwidth of 450A. CWI saw rst light in July 2009, and conducted its rst
successful scientic observations in May 2010.
We are developing the Cosmic Web Imager (CWI) to detect and map emission from the intergalactic medium (IGM).
CWI will observe the strong, redshift UV resonance lines of Lyα 1216, CIV 1550, and OVI 1033 over 3600-9000 Å to
trace IGM at 1 < z < 7. CWI is an integral-field spectrograph designed for the Hale Telescope at Palomar Observatory.
CWI combines in a novel way three mature and extensively used instrumental techniques. The Integral Field Unit (IFU)
provides a wide 2D field of view of 60 × 40 arcsec2 for observing extended emission over a large region. The
spectrograph using Volume-Phase Holographic gratings have high peak diffraction efficiency and are tunable for
covering a large bandpass with a single grating. A low read noise CCD combined with source/background shiftand-nod
allowing control of systematics and Poisson-imited sky subtraction to observe the low surface brightness universe. With
a resolution of R=10,000 CWI is sensitive to limiting surface brightness ranging from 25 - 27.5 mag/arcsec2 (10 min - 8
hours integration). Recent high resolution simulations predict Lyα Fluorescence from IGM at 100 - 1000 LU1. CWI with
sensitivity of ~200 LU improves the current observational effort by an order of magnitude and enables us to explore
wide range of overdensity (δ ~ 30 - 104) testing the standard model of structure formation in the universe. CWI also
serves as the counter part to the balloon borne integral-field spectrograph Faint Intergalactic medium Redshifted
Emission Balloon (FIREBALL) currently being built and planned to be launched in Summer 2007. FIREBALL will
observe Lyα Fluorescence from IGM at z = 0.7. CWI combined with FIREBALL will enable us to observe the evolution
of IGM and the low surface brightness universe.