Instrument Information
Instrument Overview
    ALICE (a name, not an acronym) is a simple, light-weight
    spectrometer for the spectral range 700 to 2050 Angstroms and
    having a spectral resolution of 8 to 12 Angstroms.  It employs an
    off-axis, prime-focus telescope, with aperture 40x40 mm^2 and
    focal length 120 mm, feeding the entrance slit of a Rowland circle
    spectrograph at normal incidence.  The spectrograph is centered on
    a concave, toroidal, holographic, reflection grating, with 1600
    grooves/mm and a radius of curvature of 150 mm in the dispersion
    plane, used in first order.  The detector is a micro-channel-plate
    array with 3 stacks providing a Z configuration with a
    cylindrically curved surface having radius 75 mm (the radius of
    the Rowland circle) and a double delay line readout. The detector
    has a 1024x32 pixel format (wavelength x spatial) with two
    side-by-side, solar-blind cathodes separated by a small wavelength
    gap. A potassium bromide cathode on one half of the detector
    covers the spectral range 700 to 1200 Angstroms, while a cesium
    iodide cathode on the other half covers the spectral range 1230 to
    2050 Angstrom. A 30-Angstrom gap between the two cathodes is used
    to attenuate the extremely bright H I Lyman-alpha emission at 1216
    Angstroms. The instrument has two doors - one for the instrument
    as a whole that can be used repeatedly (automatic closure when
    high ion densities are reported by the ROSINA instrument) and one
    for the detector that was opened during in-flight commissioning
    and can not be operated again. Both doors have MgFl windows for
    testing and to provide a backup in case the doors fail to open.
    ALICE has three modes for collecting data.  In Image Histogram
    Mode, the 1024x32 pixel array is mapped to a 1024x32 2-byte array
    in memory. Each detected photon increments the count in the
    appropriate location of memory, ultimately creating an image that
    is linear in photon-count. Either the entire array or a subset of
    windows in the array can be saved for downlink.  In Pixel List
    Mode, the pixel coordinates of each detected photon are recorded
    with time-words inserted into the list at regular intervals to
    enable study of the temporal variability of the spectrum. In this
    mode, specific areas of the detector can be omitted from the
    recording, for example to eliminate a very bright line while
    measuring weak lines.  In Count Rate Mode, the total number of
    counts from the entire array is recorded, thus functioning as a
    broad-band ultraviolet photometer.
    Details are included in an article to be submitted to Space
    Science Reviews.
REFERENCE_DESCRIPTION Stern, S.A., D.C. Slater, J. Scherrer, J. Stone, M. Versteeg, M.F. A'Hearn, J.L. Bertaux, P.D. Feldman, M.C. Festou, J.W. Parker, and O.H.W. Siegmund, Alice: The Rosetta Ultraviolet Imaging Spectrograph, Space Sci. Rev., 128(1-4), 507-527, Feb 2007.