Data Set Overview
  =================
    This data set contains stellar occultation data from the 1989
    occultation of 28 Sagittarii (28Sgr) by Saturn obtained by the 2.7
    meter telescope (MCD27M) at the MacDonald observatory . Data
    include occultation profiles for both ingress and egress which
    each cover a radial slice from the outer edge of Saturn's
    atmosphere to beyond the orbit of the F-ring.
 
    Ring profiles are archived as resampled normal optical depth
    profiles. Geometry solutions and noise models are also provided.
 
 
  Parameters
  ==========
    The raw data consists of nearly 40,000 individual frames
    taken with a four channel visual aperture photometer using
    a 58 x 62 pixel InSb detector array using a 2.1 micron
    circular variable filter with a spectral resolution of 0.02.
    The data were recorded at a time resolution of 0.2 sec per frame.
    Both ingress and egress ring occultations were observed.
 
 
  Processing
  ==========
    Preliminary reduction of the images consisted of sky-frame
    subtraction, 0.1 pixel accuracy registration through rebinning,
    aperture photometry with corrections for nonlinear detector
    response and corrections for varying atmospheric transmission
    (NICHOLSONETAL1995, NICHOLSONETAL2000).
 
    The fully processed and resampled ring profiles have been
    generated as follows:
 
    (1) Observations of the 28 Sgr occultation made from several
    observatories were combined with Voyager PPS and RSS data sets
    to determine precisely the ring radial scale and the Saturn
    pole orientation (FRENCHETAL1993, HUBBARDETAL1993,
    NICHOLSONETAL2000).
 
    (2) Using the improved ring radial scale, the time-series of
    raw samples was converted to a uniformly-spaced series of radial
    samples. The spatial sampling interval used was 10 km,
    corresponding to Nyquist-sampled data based on the apparent star
    diameter of approximately 18 km.
 
    (3) Raw data DN levels were converted to normalized flux based
    on the viewing geometry, and subsequently to normal optical depth.
 
 
  Data
  ====
 
    Derived profiles (DATA/MCD27M sub-directory)
    -------------------------------------
      The resampled data are presented in seven column ASCII tables.
      The columns are (1) radius(km), (2) normalized flux, (3) sigma
      flux, (4) normal optical depth(tau), (5) tau lower, (6) tau
      upper, (7) quality flag.
 
      The quality flag has five possible values. They are: 0=ok, 1=tau
      greater then tau_max, 2=non-photometric, 3=corrupted data,
      4=missing data. For detailed discussion of the model used to
      determine noise and reasonable values for optical depth see
      MODEL.TXT in the DOCUMENTS sub-directory.
 
      In general, for corrupted or missing data, dummy values of
      9.9990 are used in columns 3-6.
 
 
  Ancillary Data
  ==============
 
    Browse data (BROWSE/MCD27M sub-directory)
    ----------------------------------------
      This subdirectory contains two types of plot for each
      observation run.
 
      Plots of radial profiles of stellar flux normal optical depth of
      the rings as derived from the observations of the 28 Sgr
      occultation by the rings of Saturn. Each plot is based on the
      corresponding profile data file in the corresponding
      subdirectory of the DATA directory.
 
      Plots of ancillary geometry information. Each plot is based on
      the corresponding profile data file in the corresponding
      subdirectory of the GEOMETRY directory.
 
    Geometry information (GEOMETRY/MCD27M sub-directory)
    ----------------------------------------------------
      The geometry information is provided in separate files for
      ingress and egress. Data are arranged in four columns:
 
        a) Equatorial Radius (Km),
        b) Observatory received time (UTC),
        c) Ring Plane inertial longitude of the intercept point (deg),
        d) delta time (s). The latter is a correction factor used when
           determining the line-of-sight distance between the ring
           intercept point and the observer. The data are presented
           with uniformly varying radius (1000 km increments) between
           73,000 km and 142,000 km.
 
      The data are arranged in the order of increasing radius
      regardless of whether the observation is ingress or egress.
      Geometry data were obtained using a polynomial fit. The
      polynomial coefficients can be found in DOCUMENTS/POLY_CO.TXT
 
 
  Coordinate System
  =================
    All geometric quantities appearing in the labels are in J2000
    coordinates.  In this coordinate frame, the z-axis points
    northward along the Earth's J2000 rotation axis and the x-axis
    points toward the First Point of Aries.  In some cases, B1950
    coordinate values are also included.
 
  Media/Format
  ============
    This data set is archived on compact disc (CDROM) media.
    Organization and formats are according to PDS and ISO 9660
    level 2 standards.
 
    Most binary data files are in least-significant-byte first, which
    is the native format for PCs and Digital workstations.  Users of
    Suns and other workstations may need to swap bytes in some data
    files before use. Note that the software tools provided on this
    volume swap the bytes automatically if this is necessary.

Confidence Level Overview
  =========================
    Overall the confidence level is high. Flawed data are clearly
    identified by non-zero values for the quality flag.
 
    Photometric uncertainties in the normalized lightcurve arise from
    several sources. For a 3'' aperture radius, the array read noise
    is ~0.0026. Photon noise in the stellar signal is negligible by
    comparison. In practice, the dominant noise source near full
    stellar intensity (low optical depth) is seeing noise, which was
    measured to be 0.02 per 0.2-s integration. At low stellar
    intensity (high optical depth), the dominant source of error is
    uncertainty in the level of background ring flux present in the
    aperture, which was found to be 0.005. Other sources of error
    include vignetting in the optics and variable atmospheric
    extinction, which would introduce slow variations in the total
    signal. The data providers have attempted to correct these to
    better than 1% of the full stellar intensity. Photometry
    experiments show that systematic uncertainties in the B Ring are
    about 0.006, which sets a limit on the maximum detectable optical
    depth, tau max of 2.2.
 
  Peer Review
  ===========
    Peer review of this data set was completed in February 2005. The
    peer reviewers were Shawn Brooks (JPL), Richard G. French
    (Wellesley College), Colleen A. McGhee (Wellesley College), Cathy
    Olkin (South West Research Institute), and Leslie Young (South
    West Research Institute). The data set was also reviewed at the
    PDS Engineering Node by Steven L. Adams.
 
 
  Data Coverage and Quality
  =========================
    For the approximately last half of the egress observations, cloud
    cover significantly affected the observations. Ring features can
    still be detected, but the observing team judged that absolute
    flux values are not valid. These data are flagged as non-
    photometric in the resampled data files.