STUDY SUMMARY - IPMU

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<strong>SUMMARY</strong> REPORT WIDE FIELD FIBER-FED OPTICAL MULTI-OBJECT SPECTROMETER (WFMOS) Figure 3.13-1: The CCDs will be located within an evacuated enclosure supported by a narrow duct carrying a cold finger from a Joule Thomson (low vibration) closed-cycle cooler outside the spectrograph beam. CCD columns will be oriented parallel to the dispersion direction to allow faster region-ofinterest readout for DEHZ and GALR surveys, which use only half the detector area. This orientation also supports rapid binning of spectra to just a few lines to support rapid measurement of total flux during raster scans for calibrating positioner coordinates. A custom CCD will be procured, since this offers significant CCD performance and optical advantages. The optimal CCD size for good optical performance and minimum spectrograph cost is about half way between the standard large-format CCD size (61 mm) and a mosaic of two such CCDs. To minimize the speed of the camera and thus improve optical performance, the CCD size has been increased to 46.5 mm by 93 mm, the maximum allowing two CCDs to fit into the available 138 mm diameter on a 150 mm silicon wafer. The 16 μm pixel size has been optimized for the best compromise between resolution (for high signal-to-noise GA surveys) and S/N for the DE surveys where exposures are shorter and read noise has some impact. This leads to CCD dimensions, 2,880 columns and 5,760 rows, with 3 to 3.5 pixels across the FWHM of the image of the fiber after aberrations and lateral charge diffusion are taken into account. Quantum efficiency is projected to be very high over the entire 420-nm–970-nm wavelength range (Figure 3.13-2) and compares favorably to competing CCDs, particularly at the extremes. This remarkably high QE will be achieved by combining several recent, but proven, innovations: 1. Thick (450 μm), fully depleted p-channel CCDs, designed by LBL and fabricated by DALSA, will substantially enhance the QE at the longest wavelengths. Full depletion for such thick CCDs has been made possible by LBNL’s now mature design, which supports particularly high backside bias voltages (tested to 220 V). Notwithstanding the greater thickness, lateral diffusion and defocus with absorption depth can be shown to be negligible compared to the size of the fiber image. 51
<strong>SUMMARY</strong> REPORT WIDE FIELD FIBER-FED OPTICAL MULTI-OBJECT SPECTROMETER (WFMOS) 2. The delta doping process developed by JPL specifically for these p-channel CCDs will be applied to eliminate the absorption in the backside contact, providing reflection-limited blue response not otherwise seen shortward of 500 nm. 3. JPL will deposit a custom dual-layer sputtered TiO 2 /SiO 2 coating to reduce reflectivity to less than 4%, from 420 nm to 970 nm. 4. The low dark current seen in the delta doped LBL detectors allows them to be operated at –100°C or warmer, providing enhanced red QE due to the shorter absorption length at higher temperatures. Figure 3.13-2: Theoretical 1-R and lower bound on QE compared to next best competitor. The Hamamatsu can be improved in the red by increasing thickness but only to 300μm, and by applying the Ti0 2 /SiO 2 coating designed by JPL. LBL will perform cold wafer probing prior to backside processing and packaging. Packaging by JPL will assure less than 10 μm height variation across the mosaic of two detectors, while providing for small gaps between each CCD and its neighbor (or vacuum housing). Caltech will characterize and integrate the CCDs into the cameras. Full-frame readout time will be reduced to 16 s to match the fast reconfiguration time of the Cobra fiber positioner, while also reducing overheads between multiple exposures on the same field to
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STUDY SUMMARY - IPMU

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