DESCRIPTION |
Instrument Overview
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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.
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REFERENCES |
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.
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