Iterative Astrometric Sigma Clip

Iteratively clip residuals using recalculated sample standard deviation, retaining original indices and a bounded iteration count.

Description

Sigma clipping repeatedly recalculates a residual sample's center and spread after rejecting outliers.

Sigma clipping repeatedly recalculates a residual sample's center and spread after rejecting outliers. Retaining original indices makes the selected subset traceable to the input observations.^[1]

Iteratively clip residuals using recalculated sample standard deviation, retaining original indices and a bounded iteration count.

When to use Iterative Astrometric Sigma Clip

Use this tool to iteratively clip residuals using recalculated sample standard deviation, retaining original indices and a bounded iteration count.

Prepare the source, detector or coordinate measurements. Confirm frame, epoch, angular units, axis directions and the stated RA proper-motion convention before entering values.

How to use Iterative Astrometric Sigma Clip and understand its results

Prepare the source, detector or coordinate measurements. Confirm frame, epoch, angular units, axis directions and the stated RA proper-motion convention before entering values.
Enter residuals, sigmaThreshold, maximumRounds using the field table and calculation rules, then run the tool.
Read retainedIndexes, rejectedIndexes, clippingRounds. The explanation below defines the units, signs, ordering and any flags or placeholder values.
If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.

Input field	Meaning	Value type; see rules below	Requirement
residuals	Residuals	list of number	Required
sigmaThreshold	Sigma threshold	number	Required
maximumRounds	Maximum rounds	integer	Required

Enter 2–10,000 finite residuals, positive sigma threshold and integer maximum rounds 1–100. Each round uses current mean and n−1 SD. Stop for unchanged membership, zero SD, fewer than two retained points or the round bound; removing every point is an error. Return original zero-based retained/rejected indices and rounds that actually changed membership.

retain if |residual−currentMean|≤sigmaThreshold × currentSampleSD

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
retainedIndexes	Retained indexes	list of number
rejectedIndexes	Rejected indexes	list of number
clippingRounds	Clipping rounds	number

json

{
  "input": {
    "residuals": [
      0,
      0,
      0,
      0,
      10
    ],
    "sigmaThreshold": 1,
    "maximumRounds": 10
  },
  "output": {
    "retainedIndexes": [
      0,
      1,
      2,
      3
    ],
    "rejectedIndexes": [
      4
    ],
    "clippingRounds": 1
  }
}

Limitations and assumptions

No fitted astrometric model or per-observation uncertainty is used. Clipping is not proof of bad data, and thresholds/iteration limits affect the result.

Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Lists are limited to 100,000 entries; the rules above may require smaller limits. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use Astrometric Reduced Chi Square: Evaluate weighted astrometric residual chi-square and a supplied reduced-chi-square criterion; independent correctly estimated errors are assumed. For a complementary task, use Occultation Step Candidates: Find discrete sampled flux-drop candidates at a supplied threshold; two steps are not by themselves evidence for an occulted binary.

References

Iterative Astrometric Sigma Clip — tool.io

Similar or alternative tools

Astrometric Reduced Chi Square
Evaluate weighted astrometric residual chi-square and a supplied reduced-chi-square criterion; independent correctly estimated errors are assumed.
Occultation Step Candidates
Find discrete sampled flux-drop candidates at a supplied threshold; two steps are not by themselves evidence for an occulted binary.