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tokens: contain groups of background QSOs. The narrowband filter has an FWHM of 40 Å, corresponding to the radial separation of 42 cMpc. The survey volume of the 2622 groups is cMpc cMpc cMpc cMpc 3 . Thus, BOSS1441 can be regarded as the most extreme system selected from a ≈1.0 × 10 8 cMpc 3 survey volume using our selection technique. 3. Deep Imaging and Spectroscopy in the BOSS1441 Field After selecting the BOSS1441 field, we conducted deep narrowband and broadband imaging and multiple-slit spectroscopy to search for LAEs in this field. 3.1. KPNO-4 m/MOSAIC Narrowband + Broadband Imaging We used the MOSAIC1.1 camera (Sawyer et al. 2010 ) at the f /3.1 prime focus of the 4 m Mayall Telescope at the Kitt Peak National Observatory to conduct deep narrowband and broadband imaging. We obtained deep narrowband images with the NB403 narrowband filter. As shown in Section 2.2 , NB403 is used for probing the Ly α -emitting galaxies at . The BOSS1441 overdensity was observed on 2013 March, and 2014 April and June. The total exposure time is 17.9 hr, which consists of individual 15 or 20 minute exposures with a standard dither pattern to fill in the gaps between the eight chips. The seeing ranges from 11 to 17, with a median combined seeing of 132. We observed with the broadband filter (Bw) for 3 hr to match our selection criteria. These observing conditions enable us to achieve a narrowband magnitude at 5 σ using a circle aperture of 25 and a Bw -band magnitude m B = 25.9 at 5 σ using the same aperture. The narrowband sensitivity limit corresponds to a Ly α surface brightness of erg s −1 cm −2 arcsec −2 . 3.2. LBT Multiple-wavelength Broadband Imaging Our multiband imaging was conducted with the Large Binocular Cameras (LBC; Giallongo et al. 2008 ) at the Large Binocular Telescope (LBT) on Mount Graham in Arizona. LBC is characterized by a unique optical system. Each of the two mirrors has a collecting area of 8.4 × 8.4 m 2 and a field of view of 23′ × 23′. For a complete selection of star-forming galaxies at , we used the LBT to conduct deep multiple broadband observations in the U , V , and i filters (Steidel et al. 2005 ). Multiple broadband imaging can also eliminate the low-redshift [O ii ] interlopers that are the main contaminants of LAEs at z = 2.3. We use the standard z = 2 star-forming galaxy selection (BX selection) technique (Adelberger et al. 2005 ) to eliminate the low- z interlopers. The one-night LBT/LBC deep multicolor imaging was carried out in 2014 March. The LBC binocular mode was used, and the LBC blue channel was used solely for deep U -band observations. On the red side, we observed the same field in the V and i filters. We conducted 6 hr deep imaging in U , 2.5 hr in V , and 3.5 hr in i . The individual exposure was six minutes with a standard five-point dither pattern to fill in the CCD gaps. The conditions were partially cloudy, with a median seeing of 06. Our observations reached at 5 σ , at 5 σ , and at 5 σ . These depths are