The goal of this project was to replace the existing CCD in the red side of LRIS with a 2 x 1 mosaic of 2k x 4k fully-depleted LBNL CCDs to increase the sensitivity of the red channel of the instrument and eliminate fringing. Other improvements include a modest increase in spatial and spectral coverage, improved image uniformity due to a flatter detector, and increased reliability for the detector readout electronics.
There are four areas in which we can realize significant gains in LRIS-R scientific productivity.
We can improve the LRIS-R throughput at all wavelengths and by up to 50% at the reddest wavelengths.
Field size/slit length
LRIS-R is currently detector-limited in the spectral and spatial dimensions. The proposed new focal plane is larger by 25% in each dimension. We are updating a ZEMAX model of the camera/filter wheel/shutter to see exactly how much of this increase translates into increase slit length and spectral coverage.
Fringing, flexure and sky subtraction
LRIS-R suffers from flexure at the level of around 5 (24µm) pixels peak-to-peak for 180 degree rotation. The current LRIS-R Tektronix CCD also has fringes in the red whose amplitude varies with spectral resolution and atmospheric conditions, but is typically about 10% peak-to-peak amplitude longward of 700nm. The combination of flexure and fringing makes it very difficult to correct for the fringing and to do proper sky subtraction in the affected regions. The high-ρ CCDs have thusfar unmeasureable fringe amplitudes.
The current Tektronix CCD requires 73 seconds for a full chip, unbinned readout. The proposed new CCD system will decrease this to ≤40 seconds.
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