Freeze the product, not just an archive landing page
The source is UVIS_HSP_2008_261_ALPARA_E_TAU01KM.TAB: an egress occultation on 17 September 2008, with one-kilometer radial sampling and a 110–190 nm band. The product label and volume README identify dataset CO-SR-UVIS-HSP-2/4-OCC-V3.0, published in 2020, and product creation on 27 November 2020. The general UVIS web page still describes version 2; the actual downloaded label takes precedence here.
The frozen table contains 1001 exact source records from 125000 to 126000 km, 121 bytes each, with all twelve original columns. Its SHA-256 is 6baadf1d91701ad8e868745f7aad9e49d853cec169ce45fea51b9a4f30fdd94c. The original label, README and provenance are downloadable alongside it. Only the declared HTTP byte range was retrieved; no full-product hash or raw-telemetry reduction is claimed.
All retained quality flags are zero. Earlier trial candidates carried binary-star or unmodeled-background flags and were rejected for this example. In a different product, do not erase a nonzero flag merely because a curve looks attractive. Keep its original value and define an analysis rule for it.
Reading: Cassini UVIS alpha Arae2008-261 egress, original PDS3 product label · NASA PDS Cassini UVIS archive README, version3.0 · Colwell et al., Cassini UVIS stellar occultation observations of Saturn’s rings
Read the units before taking a logarithm
The archived mean signal is the photon count per raw integration averaged within a radial bin. Background has not been subtracted from that column. The star model is the unocculted star signal, and the background model is separate. Subtract background once from the mean signal, divide by the star model, then take the logarithm and apply the sine of the absolute ring elevation 54.4 degrees.
The published normal optical-depth column has already passed through the archive reduction. It is an independent comparison column here; subtracting the background from it again would have no meaning. The reconstructed depths differ from the printed archive values by at most about 0.00171, The reconstruction uses rounded printed models and elevation; the unrounded original geometry is unavailable in this cutout.
Radius is distance from Saturn’s center in the ring plane. Longitude and the ring-plane look angle are degrees. The two time columns are TDB seconds since J2000: photons leave the ring before reaching Cassini. Keep that distinction if you later combine the profile with a moon’s pattern phase. The light wavelength is fixed ultraviolet; our searched wavelength is spatial, along the radius.
Reading: Cassini UVIS alpha Arae2008-261 egress, original PDS3 product label · Colwell et al., Cassini UVIS stellar occultation observations of Saturn’s rings
Carry the counting uncertainty without inventing calibration precision
The number-of-samples column tells how many raw integrations were averaged in each radial bin. Under independent Poisson counting, the variance of that mean is the mean count divided by the number of samples. Propagate it through the background subtraction and logarithm as shown below.
This conditional estimate holds the archived background and unocculted-star models fixed. Their uncertainty, finite stellar size, diffraction, residual instrument effects and radial correlations are not included. The maximum-detectable-depth column is a specified counting-sensitivity threshold in the label, not a per-row error bar. Do not substitute it for a standard deviation.
Bins with no samples, bad flags or nonpositive net signal need explicit censoring or a likelihood treatment. The script checks the retained dimensions and flags, and its derivation function preserves invalidity. It never replaces a censored optical depth with a convenient high value.
Reading: Cassini UVIS alpha Arae2008-261 egress, original PDS3 product label · Colwell et al., Cassini UVIS stellar occultation observations of Saturn’s rings
Run the complete frozen analysis
Save the Python script, exact table, label, README and provenance together. The command verifies the table hash, reconstructs the optical column, saves all measured arrays and flags, then performs the declared local searches and sensitivity checks. Its output is cassini-replay-rerun.json; the reference cassini-replay.json remains available for comparison.
The small public scientific replay is separate from the protected interactive engine. Its optional companion reanalyzes saved synthetic particle positions and counts independently. Neither command downloads new observations, imports the Lab engine or turns simulated ice into measured Cassini particles.
python3 -W error replay-cassini.py
python3 -W error replay-synthetic.py
| Window start / km | 1-km-bin scale / km | 2-km-bin scale / km | Reduced chi-square |
|---|---|---|---|
Reading: Cassini UVIS alpha Arae2008-261 egress, original PDS3 product label · NASA PDS Cassini UVIS archive README, version3.0
Measure a local spatial scale, then inspect the residual
The displayed 1000 kmsegment includes the strong structure near the Janus 4:3 resonance region. Three predeclared 80 kmwindows begin at 125310,125410 and 125510 km. Each search fits a sine and cosine together with a quadratic radial baseline, using the counting-only depth errors. Spatial periods range from 3 to 60 km on 1200 uniform frequency points; each exact grid is saved.
The dominant components are 16.53,5.943 and 5.172 km. A separate three-bin smoothed crest finder gives uneven successive spacings, including 26,20,16 and 9 km, depending on which local crests pass the declared threshold and separation rule. These observations support progressively finer radial structure, without asserting that every crest belongs to one linear wave.
The important diagnostic is the reduced chi-squared: roughly 104,107 and 54. Counting noise cannot explain those residuals. Nonlinear crests and overlapping structure remain. Adding 256 independent counting-noise perturbations gives narrow sensitivity ranges in the first two windows, but these are not physical-wave confidence intervals. The last window already switches between competing components.
Reading: Colwell et al., Cassini UVIS stellar occultation observations of Saturn’s rings · Salo & Mondino-Llermanos, Photometric modelling of self-gravity wakes and overstable oscillations · Goldreich & Tremaine, The excitation and evolution of density waves
Change the resolution using actual counts
Combine neighboring radial bins using the numbers of raw integrations as weights. Average the source and background models with the same weights, sum the integration counts, then reconstruct the optical depth from the combined intensity. Averaging the already logged depths would produce a different statistic.
The paired-bin searches enforce a minimum searched wavelength no shorter than twice their median radial spacing. The first two dominant components remain about 16.49 and 5.916 km, but the last becomes about 4.006 km. A result this sensitive to resolution and local model choice should not be reported with only its narrow counting-noise range.
The replay therefore stops at a measured optical profile and declared local spatial scales. It does not infer surface density, damping viscosity or ring age. A research analysis would isolate the appropriate wave, model Saturn’s gravity and the changing co-orbital moon forcing, and validate its treatment of nonlinear structure before making those inferences.
Reading: Cassini UVIS alpha Arae2008-261 egress, original PDS3 product label · Colwell et al., Cassini UVIS stellar occultation observations of Saturn’s rings · Goldreich & Tremaine, The excitation and evolution of density waves
Keep the source history with your result
Cite Colwell, Jerousek, Becker and Esposito’s 2020 PDS version 3.0 dataset and the 2010 UVIS analysis paper. The original README preserves its citation request. These are publicly accessible NASA PDS scientific data; this tutorial does not claim a separate Creative Commons grant from the data authors.
For another occultation, freeze its exact product and label first. Carry the count normalization, geometry and flags through every step, then compare spatial scales under another window or bin width. A useful result includes both the structure you recovered and the fit that told you to stop interpreting it.
Reading: Cassini UVIS alpha Arae2008-261 egress, original PDS3 product label · NASA PDS Cassini UVIS archive README, version3.0 · Colwell et al., Cassini UVIS stellar occultation observations of Saturn’s rings