Data Set Overview
  =================
    This data set contains stellar occultation data from the 1989
    occultation of 28 Sagittarii (28Sgr) by Saturn obtained by the 1
    meter Nickel reflector (LICK1M) at the Lick 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 were obtained with a frame-transfer CCD. Only a
    subframe of each full CCD frame was recorded: after 2 x 2 on- chip
    pixel rebinning, each exposure consisted of a 54 x 90 pixel image
    with the long axis roughly parallel to Saturn's ring system. An
    RG-9 filter with a central wavelength of 0.9 microns and a full
    width at half-maximum of 0.25 microns spanned the 0.89 micron
    methane absorption band, substantially reducing the brightness of
    Saturn's disk. Three continuous sets of 1-s exposures were taken
    corresponding to ring ingress and atmosphere immersion, mid
    occultation, and atmosphere emersion and ring egress. Absolute
    timing was provided by synchronizing the CCD controller with a
    WWVB clock; dead time between images was only a few ms.
 
 
  Processing
  ==========
    Preliminary reduction of the images consisted of sky-frame
    subtraction, subpixel accuracy registration through rebinning,
    aperture photometry, correction for varying atmospheric
    transmission (see 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/LICK1M 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/LICK1M sub-directory)
    -----------------------------------------
 
      This subdirectory contains 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.
 
    Geometry information (GEOMETRY/LICK1M sub-directory)
    ----------------------------------------------------
      Supplemental geometry information is not available for the
      Lick observations
 
 
  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.
 
    The observations were taken at relatively high air mass
    (decreasing from 2.5 to 2.2 during ingress, and gradually
    increasing from 2.0 to 2.1 during egress). The data providers
    applied a correction for varying atmospheric transmission.
    Adopting an extinction coefficient of 0.04 mag/air mass, they
    scaled the lightcurves to their predicted levels for a constant
    air mass of 2.0. Finally, they determined the constant ring
    background and full stellar flux levels separately for ingress and
    egress from the immersion and emersion atmosphere lightcurves, and
    used these to scale the profiles to units of full stellar flux.
    The ingress lightcurve is the noisier of the two because it was
    observed at higher air mass, and is thus more susceptible to
    seeing noise and transparency variations. Nevertheless, the
    overall normalization is quite robust for both lightcurves,
    judging from the consistency between the full stellar intensity
    level determined from the atmosphere lightcurves and the free-
    space stellar signal outside of the rings. For the egress
    observations, the typical noise level is 0.025 per 1-s
    integration, with a corresponding maximum detectable optical
    depth, tau max, of 1.6.
 
  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.