THE ING NEWSLETTER No. 5, October 2001
    GENERAL
    SCIENCE
    TELESCOPES AND INSTRUMENTATION
    OTHER NEWS FROM ING
    TELESCOPE TIME

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    The ING Red Sensitive CCD Project

    Simon Tulloch (ING)

    This  project aims to improve the red sensitivity of our instruments through the use of low-fringing high-Quantum Efficiency (QE) CCDs. These are being produced by MIT Lincoln Labs for a consortium of observatories. Gerry Luppino manages the consortium at the University of Hawaii. His Company, GL Scientific, is providing CCD packages and cables. The ING commitment to the consortium is $141,000. This will give us at least three science grade devices.

    The wafers will be divided and put through two different processes. The first process, called BIV, will give excellent red response but very poor blue performance. The second process, called UV or MBE, will improve the blue response whilst leaving the red response intact. The MBE process is still being developed (as of spring 2001). We have requested that our first chip be from the BIV process. This chip will be used at ISIS RED as its blue response is not important. Our subsequent chips will hopefully come from the MBE process, depending on its success. Two of these devices will be incorporated into a mosaic camera for use with WYFFOS long, UES and possibly for prime focus imaging on the WHT. If the wafer run has good yields we can expect a fourth device for use with OASIS. Additional sources are being investigated for a fourth chip should the yields from this contract be lower than expected.
     

    CCD Characteristics

    Size: 2048 × 4096 pixels 
    Type: CC1D20
    Pixel size: 15 × 15 microns

    Two outputs with a high sensitivity of 15µV/e–

    Fringing at 1000nm £ 10%

    Manufactured from 40-micron thick high-resistivity silicon

    The Physics of Deep Depletion CCDs

    Standard thinned CCDs are typically 15 microns thick. As the wavelength approaches 1 micron, the absorption depth of silicon increases rapidly and the CCD becomes transparent. The red sensitivity suffers accordingly. There is an additional problem, called 'fringing' which in some applications is an even more serious drawback than poor QE. As the transparency  of the chip increases at longer wavelengths the CCD acts as a Fabry-Perot cavity with light reflecting back and forward between the front and rear surfaces. Interference is produced that heavily modulates the spatial uniformity and reduces the SNR of the observations. The solution is to make the CCDs thicker than the absorption depth of the silicon, incident photons will then be absorbed on their first pass and reflection from the rear surface will be greatly reduced. Our CCDs will be 40 microns thick. Standard silicon cannot be used for this process since it cannot sustain the high electric field throughout the full depth of the device that is so important for good QE. Instead a special grade of high-purity high-resistivity silicon must be used.
     

    Fringing Figure 1. Image showing the effects of fringing in a thinned astronomical CCD. [ GIF | TIFF ]

    Latest Quantum  Efficiency Data

    QE data is available for the BIV CCD with a broad band coating. The red response is impressive; up to three times better than a thinned EEV. QE data on the MBE (blue boosted) CCD is only available for a device with an anti-reflection coating optimised for the blue. The red response of this device is not fantastic but should approach that of the BIV, once a broad band coating is applied. Marconi have also started to produce deep depletion CCDs, and the QE of their device is shown in Figure 2.
     

    Comparison plot
    Figure 2. Quantum Efficiency comparison between current and new red CCDs. [ GIF | TIFF ]

    The first of the ING cameras is complete and awaiting the delivery of the first CCD. For additional information, please see the project web site : http://www.ing.iac.es/~smt /redsense/redsense.htm.
     

    First Red CCD Camera Figure 2. A photo of one of the ING cameras (with a test chip mounted) complete and awaiting the delivery of the first red CCD. [ JPEG | TIFF ]


    Email contact: Simon Tulloch (smt@ing.iac.es)


    Previous: First Light on the New Small Fibre Module of Autofib2/WYFFOS Up: Table of Contents Next: First Results from the Rayleigh Laser Guide Star Project

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    © Isaac Newton Group of Telescopes, La Palma, 2001.