Data Set Overview
    =================
      This dataset contains images and spectra of the Saturn system
      taken at the William Herschel Telescope (WHT) using the
      Auxiliary Port Imager (API) and the Intermediate Dispersion
      Spectrograph and Imaging System (ISIS) in early August, 1995.
      This period precedes the actual August plane crossing by some
      ten days. The viewing geometry was such that Earth and the Sun
      were on opposite sides of the ring plane, and the ring opening
      angle as viewed from Earth was approximately 0.2 degrees.
 
      Every image showing Saturn, its rings or the region of the
      inner satellites has been included in this data set, regardless
      of the original intended purpose.
 
      The WHT observing program whose results are included in this
      data set is:
 
       Proposal: GR/K94546
       Title:    Unknown
       PI:       Carl Murray (Queen Mary, University of London)
 
    Objectives
    ==========
      There were two primary objectives for this set of observations:
 
      1. Observe the inner Saturnian system.
 
        a. Attempt to detect suspected but unconfirmed small
           satellites [GORDONETAL1995]
 
        b. Reacquire small inner satellites for astrometry, especially
           Prometheus and Atlas following the questions arising from
           the April 1995 HST observations  [BOSH&RIVKIN1996].
 
        c. Attempt to detect the mysterious new features detected in
           the April 1995 HST observations [BOSH&RIVKIN1996].
 
        d. Monitor the atmosphere of Saturn for disturbances. This was
           included since the planet would be in the field of view for
           all of the inner system observations and with a suitable
           filter would not be saturated except in the longest
           exposure images. A narrow band methane filter centered at
           8920 Angstroms with Full Width at Half Maximum of 50
           Angstroms was used for all of the inner system
           observations.
 
      2.  Obtain spectral observations of the E ring to determine if
          the ring does have a strong bias in the blue end of the
          spectrum as indicated in observations made during the 1980
          ring plane crossing [LARSONETAL1982].
 
    Observation summary and conditions
    ==================================
      The primary instrument for these observations was the
      Intermediate Dispersion Spectrograph and Imaging System (ISIS).
      This is a narrow slit instrument; consequently, absolute spectra
      would not be obtained.  The program called for also obtaining
      ISIS spectra of Enceladus, known to be spectrally neutral, then
      by subtraction obtain a relative spectrum of the E ring. At the
      outset, the seeing was too marginal to allow useful observations
      with ISIS due to partial cloud cover and Sahara dust in the
      upper atmosphere. Consequently, on each night a set of API
      images were obtained of the E ring using different filters as
      insurance. Late on the second night the seeing improved to
      approximately 0.1 arcsec and a good spectra for both the E ring
      and Enceladus were obtained with ISIS. As a consequence, the
      insurance observations were never fully processed. They are
      included as raw data only.
 
    Parameters
    ==========
      The PDS label for each file contains a broad variety of
      additional parameters enabling the user to determine image
      geometry and to convert pixel values to physically meaningful
      quantities.
 
    Processing
    ==========
      The calibrated images have been processed using IRAF. This
      includes: analog bias correction, dark current correction,
      flat field correction, and sky correction. Spectra wavelengths
      were registered with a copper-argon reference source and
      compared to known absorption lines in the spectra.
 
    Data
    ====
      The data provided here are images in FITS format and also in
      ASCII tables.  For each data file, a detached PDS label is
      provided containing additional parameters.
 
      Most users will prefer to focus on the fully calibrated images
      and reduced spectra found in the DATA/IMAGES/CALIMAGE/ and
      DATA/SPECTRA/EASYISIS/ subdirectories respectively.
 
      The other potentially useful subdirectory is entitled
      DATA/IMAGES/BROWSE/, which contains easy-to-display GIF files
      showing the calibrated images at reduced resolutions.  These
      files are especially useful when the user desires only a quick
      look at what the images contain.
 
 
       Subdirectory        file name      File contents
 
   DATA/SPECTRA/RAWISIS/  FxxxxxxR.IMG   raw 2D spectrographs
   DATA/SPECTRA/CALISIS/  FxxxxxxC.IMG   calibrated 1D spectra
   DATA/SPECTRA/EASYISIS/ aaaaaa.TAB     ASCII table reduced spectra
   DATA/SPECTRA/RAWAPI/   FxxxxxxR.IMG   raw filter sequence images
   DATA/IMAGES/BROWSE/    FxxxxxxR.GIF   reduced resolution images
   DATA/IMAGES/CALIMAGE/  FxxxxxxR.IMG   calibrated images
   DATA/IMAGES/RAWIMAGE/  FxxxxxxR.ZIP   raw images
 
 
    Ancillary Data
    ==============
      Additional calibration files are provided to assist in the
      analysis and interpretation of the data.
 
      The appropriate subdirectories under the CALIBRATION
      subdirectory contain bias, flat field, sky, and spectral
      standard files as well as filter profiles for most of the
      filters used in this data set.
 
    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.
 
 
    Media/Format
    ============
      This data set is archived on compact disc (CDROM) media.
      Organization and formats are according to PDS and ISO 9660
      level 3 standards.

Confidence Level Overview
    =========================
      Marginal observing conditions for much of this data limit the
      amount of good information; see 'Data Coverage and Quality'
      below.
 
      ISIS spectra were calibrated using a Copper-Argon reference
      standard and known absorption features in the spectra.
 
      API CCD scale and orientation were obtained by fitting relative
      positions of the available satellites from the set {Enceladus,
      Tethys, Dione and Rhea} in each with predictions based on
      [HARPER&TAYLOR1993], and [HARPER&TAYLOR1994].
 
    Review
    ======
 
      This data set passed peer review on January, 30, 2004.  The
      members of the peer review panel were James ('Gerbs') Bauer,
      Imke de Pater, Douglas Hamilton, and Steve Larson Ron Joyner and
      Steven Adams represented the PDS Central Node at JPL.
 
    Data Coverage and Quality
    =========================
 
      At the outset, the seeing was marginal, and entirely unsuitable
      for the use of ISIS, throughout the first night and well into
      the second night.  Consequently, several filter series of the
      E Ring were obtained with the API as insurance. As a result of
      improved seeing late on the second night, ISIS spectra were
      obtained and as a consequence, the API filter sequences were not
      processed. However, the raw images are included.  For
      approximately the last two hours of the second night, the seeing
      improved to approximately 0.1 arcsec. The best images of the
      inner system begin with image F161225X.IMG.  During this period
      good spectra were obtained with ISIS for the E ring and
      Enceladus (to be used to obtain relative spectral information
      for the E ring).  E ring spectra were obtained near the east
      ansa in a region which was free of satellites.
 
      The field of view of the API imaging system is sufficient to
      allow a single main ring image to extend beyond both ansae
      including the satellite orbits to beyond the orbit of Dione. All
      four of the classically known inner satellites were imaged,
      including a sequence showing the eclipse of Mimas by the rings.
      In addition, several of the small inner satellites, including
      Prometheus can be identified in several images. A few features
      can be detected moving with the mean motion of the F Ring,
      including at least one detection that has been identified as a
      prediscovery detection of 1995/S6. With the exception of the
      longest exposure images, main ring images contain some details
      of Saturn's upper atmosphere.