CFI is the CONTOUR Forward Imager, designed for high sensitivity and
responsivity to ultraviolet wavelengths. CFI will perform its measurements
while CONTOUR is approaching the nucleus and at a range of >2000 kilometers,
so it looks out the front side of the spacecraft through an opening in the
protective dust shield. Its main functions are:
 
    *
 
      to locate the target comet against the star background days to weeks
before an encounter
    *
 
      to take color images of the nucleus, any gas or dust jets, and other
features in the inner coma
    *
 
      to image the inner coma in wavelengths sensitive to major species of
ionized gas
 
 
             CFI Vital Statistics
 
Telescope aperture       60 mm
Telescope focal length   300 mm
Field-of-view            2.5deg x 2.5deg
Pixel field-of-view      43 mrad (0.002deg)
Wavelength range         308-920 nm
Spectral channels        10
Detector type            Marconi 47-20
Pixels per image         1024 x 1024
Typical exposure times   90 ms to 10 sec
 
 
CFI is optimized for low-light and short-wavelength imaging of a comet during
CONTOUR's approach to it, including early detection for optical navigation,
imaging of the coma, and imaging of emissions from ionized gases.
 
CFI's reflective telescope focuses light through one of ten selectable
filters mounted in a filter wheel, of which one is for broadband ('clear'
filter) optical navigation images, three are for imaging in narrow wavelength
bands emitted by ionized gases in the coma, and six are for multispectral
imaging of the nucleus and coma. The digital image is captured by a
high-efficiency, back-illuminated charge-coupled device (CCD) manufactured by
Marconi. This is similar to the detector used in a standard video camera,
except that CFIs CCD is black and white only (with color obtained using
different filters) and it is manufactured to achieve higher sensitivity
especially at ultraviolet wavelengths (as short as 308 nm for coma emission
band imaging). The field of view is 2.5deg x 2.5deg, 1024 x 1024 pixels.
 
Instead of looking directly at a comet and into the oncoming stream of
impacting dust, CFI's telescope looks at one of four mirrors mounted onto the
side of a rotatable cube. Only one mirror is exposed at a time to the
oncoming dust, and a fresh mirror is put in place before each comet encounter
by rotating the cube. The most important CFI measurements at each comet are
for optical navigation and broad scale nucleus/coma imaging, and these are
made at distances of tens to hundreds of thousands of kilometers before the
mirror could be seriously damaged by oncoming dust particles.
 
 
 
         CFI Camera Filters
 
 
Wavelength  FWHM1       Purpose
 
'Clear'     350         Navigation / tracking
309.0       6.0         OH emissions
344.8       8.4         OH, CN continuum
387.0       6.2         CN emissions
445         10          Geology, 'blue'
514.1       11.8        C2 emissions
526         5           C2 continuum, 'green'
620         4           Geology, 'red'
840         9           Geology / near-IR 1
920         13          Geology / near-IR 2
 
FWHM = full width of the wavelength range passed by the filter at greater
than half intensity
 
CONTOUR's highest-velocity comet encounter (28 kilometers per second) will be
with Comet Encke in November 2003. During the days before the enounter, every
six hours a group of clear-filter images will be taken for spacecraft
navigation and downlinked to Earth. These show the comet's position against a
background of stars as dim as 11th magnitude, and will be analyzed to refine
targeting of the encounter. Science imaging of the coma begins 5 hours before
closest approach to the nucleus, with regular monitoring of activity in the
inner coma. The prime CFI science imaging begins only six minutes out and
continues through about 70 seconds before closest approach, with detailed
imaging of the inner coma and color imaging of the nucleus. The narrow
emission band filters are sensitive to hydroxyl (OH) from dissociated water
and to cyanide and carbon (CN and C2) from dissociated dust, and will show
the locations of jets of gas and dust driven off the nucleus by sublimating
ices. The last color images will show details on the nucleus surface as small
as 85 meters across. After this, the nucleus passes out of CFI's
field-of-view and is no longer observable.
 
From http://www.contour2002.org/instruments4.html
 
Copyright 2002 NASA Discovery Program
Used by Permission