STUDY SUMMARY - IPMU

SUMMARY 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.
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