Prime Mission Lessons Learned
• Large variations in apparent optical
depth with different occultation
geometries.
• Greater than anticipated signal in low B
occultations.
• Multiple low-B and moderate-B
occultations enable measurement of 3D
properties of small-scale structures (e.g.
self-gravity wakes).
June 11, 2007
UVIS Team Meeting
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Prime Mission Lessons (cont.)
• Grazing (“chord”) occultations reveal finescale structure over narrow radial range.
• Direct measurement of self-gravity wake
length scale (possibly even largest ring
particles) with highest resolution occultations.
• Joint occultations with VIMS valuable for
some stars.
• Spectroscopy riders work well with CIRS
radial scans, others hampered by data rate
and secondary axis orientation.
June 11, 2007
UVIS Team Meeting
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Prime Mission Lessons (cont.)
• Spectroscopy riders need to be scans around
Z axis at high inclinations; prefer Z axis
oriented radially in the rings at low
inclinations.
• Higher data rate (1 ms vs 2 ms integration
period) valuable for occultations.
• Higher data rate (at least 5 kbps vs 1 kbps)
needed for FUV spectroscopy riders.
• Rain Happens.
June 11, 2007
UVIS Team Meeting
3
Prime Mission Observations
• Ring impact observations (discontinued with
null result).
• Stellar occultations.
– 55 completed (2 lost entirely; 5 heavily corrupted)
– 24 more planned before end of prime mission
– ~15 of these are faint star replacements of impact
observations
• Grab bag of spectroscopy riders.
• Solar occultations (joint with VIMS).
June 11, 2007
UVIS Team Meeting
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Occultations as CAT Scans
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UVIS Team Meeting
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Outer B Ring Optical Depths
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UVIS Team Meeting
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Alp Leo Turnaround
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UVIS Team Meeting
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Signature of Viscous Overstability
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UVIS Team Meeting
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Possible Propeller Detection
Power Spectrum
Estimated abundance of
propellers would give signal
in roughly 1 out of 40 A ring
occultation cuts.
Red: UVIS Counts
June 11, 2007
UVIS Team Meeting
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XM Science Objectives
• Probe the highest optical depth regions of the
rings.
• Measure the 3D properties of small-scale
structures in the rings, including self-gravity
wakes, viscous overstability waves, bending
waves, and ring edges.
• Study fine-scale structures and particle sizes
directly.
• Map variations in ring composition.
• Study azimuthal variations within the rings.
• Study temporal variations in density waves
and elsewhere.
June 11, 2007
UVIS Team Meeting
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XM Observations
• High incidence angle stellar occultations:
– Identified as a priority 1 tour driver for the XM.
– Unique opportunity in the Cassini mission to probe
the most opaque regions of the rings.
– Two stars meet this observation requirement:
• Alpha Crucis (B0.5 IV, B=-68.2, I=950,000 Hz,
RA = 186.65, Dec = -63.1)
• Beta Centauri (B1 III, B=-66.7, I=940,000 Hz,
RA=210.96, Dec = -60.4)
– Candidate occultations have been identified and
tabulated.
June 11, 2007
UVIS Team Meeting
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Geometry for Beta Cen Occ.
Beta Cen
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UVIS Team Meeting
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XM Observations (cont.)
• Low incidence angle stellar occultations:
– Identified as a priority 1 tour driver for the XM.
– Fewer occultation opportunities than higher
inclincation occultations.
– Useful in conjunction with other occultations for
analyzing 3D structure of the rings.
– Useful for studying low-optical-depth regions of
the rings.
– Best candidate occultations have been identified
and tabulated. Alpha Leo is particularly good
(B=9.5, I0=40,000 Hz)
June 11, 2007
UVIS Team Meeting
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Geometry for Alpha Leo Occ.
Alp Leo
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UVIS Team Meeting
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XM Observations (cont.)
• Stellar occultations of opportunity at various
incidence angles:
– Extend coverage for azimuthal and temporal
variations (e.g. clumps).
– Use chord occs to get high resolution
measurements of various regions of interest in the
rings (similar to Alpha Leo rev009 in prime
mission).
– Identified occs are mainly with Alpha Arae
• Joint occultations with VIMS (e.g. Alpha Lyrae
(same star), and Alpha Scorpii (binary star)).
June 11, 2007
UVIS Team Meeting
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XM Occultation Requests
• 12 full radial cuts (inner edge to outer edge,
not ansa-ansa) with Beta Cen or Alpha Cru.
• 2 Alpha Lyrae occultations with VIMS (A ring
only, 65 minute duration).
• 18 Low-B occultations (typical duration is less
than 90 minutes).
• 13 additional occultations with a range of B
and a range of turnaround radii.
• Spreadsheet of occultation times and
geometries assembled by Brad Wallis from
input from Josh Colwell.
June 11, 2007
UVIS Team Meeting
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XM UVIS Ring Riders
• Ride with VIMS on solar occultations.
• Ride with VIMS and CIRS on ring imaging
(and ISS on a case-by-case basis depending
on data volume and observation design).
– Low-phase observations provide the highest
signal.
– UVIS has just (last week) identified an instrument
configuration that eliminates a large light leak in
the EUV channel. New low-phase EUV
observations in the XM will effectively be the first
measurements of ring reflectance at short EUV
wavelengths.
• Requesting 5 kbps data rate on VIMS and
CIRS observations.
June 11, 2007
UVIS Team Meeting
17
RWG Statement on UVIS XM Observations
• UVIS should get 8 A-cru or B-cen occultations at priority 1
and 5 more at priority 2.
• UVIS should get 18 low-B occultations at priority 1, with
typical duration less than 1.5 hours.
• It was agreed that UVIS should get 13 additional
occultations by Alpha Arae or other stars which emphasize
turnaround radii at new radial locations.
• In addition, for spectral mapping, it was agreed that UVIS
should get priority 1 consideration for 5kb/s data rates on
CIRS ring scans, which are optimally paced, and 10kb/s
on VIMS stellar occultation observations (where possible).
• Occultations of opportunity and joint occultations with
VIMS should be sought.
June 11, 2007
UVIS Team Meeting
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UVIS Team Meeting
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UVIS Team Meeting
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