Sip Distortion Evaluation

Validate and evaluate forward SIP distortion terms for reference-pixel offsets; linear and constant terms belong in WCS, not SIP.

Description

SIP describes nonlinear forward detector distortion using powers of reference-pixel offsets.

SIP describes nonlinear forward detector distortion using powers of reference-pixel offsets. Constant and linear terms belong to the base WCS and must not be duplicated as SIP terms here.^[1]

Validate and evaluate forward SIP distortion terms for reference-pixel offsets; linear and constant terms belong in WCS, not SIP.

When to use Sip Distortion Evaluation

Use this tool to validate and evaluate forward SIP distortion terms for reference-pixel offsets; linear and constant terms belong in WCS, not SIP.

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 Sip Distortion Evaluation 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 offsetX, offsetY, termPowers, xCoefficients, yCoefficients, polynomialOrder using the field table and calculation rules, then run the tool.
Read xDistortion, yDistortion, distortedOffsetX, distortedOffsetY. 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
offsetX	Offset x	number	Required
offsetY	Offset y	number	Required
termPowers	Term powers	list of list of number	Required
xCoefficients	X coefficients	list of number	Required
yCoefficients	Y coefficients	list of number	Required
polynomialOrder	Polynomial order	integer	Required

Enter offsets, order 2–9, distinct nonnegative integer exponent pairs and aligned x/y coefficient lists. There must be at least one term and at most 128, each total degree between 2 and the order. The tool sums both polynomials and returns each distortion plus the offsets after adding it.

Paired lists must be nonempty and have equal lengths.

dxDistortion=ΣA_pq x^p y^q; dyDistortion=ΣB_pq x^p y^q

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

Output field	Meaning	Value type
xDistortion	X distortion	number
yDistortion	Y distortion	number
distortedOffsetX	Distorted offset x	number
distortedOffsetY	Distorted offset y	number

json

{
  "input": {
    "offsetX": 2,
    "offsetY": 3,
    "termPowers": [
      [
        2,
        0
      ],
      [
        1,
        1
      ]
    ],
    "xCoefficients": [
      0.1,
      0.2
    ],
    "yCoefficients": [
      0,
      0.1
    ],
    "polynomialOrder": 2
  },
  "output": {
    "xDistortion": 1.6,
    "yDistortion": 0.6000000000000001,
    "distortedOffsetX": 3.6,
    "distortedOffsetY": 3.6
  }
}

Limitations and assumptions

It evaluates supplied forward terms without fitting coefficients, applying a CD projection or computing inverse SIP. Units and reference pixels must match the intended WCS, and unused polynomial terms can be omitted.

Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Matrices are limited to 128 rows and 128 entries per row; other lists are limited to 100,000 entries. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use Tan Wcs Pixel To Sky: Apply inverse gnomonic TAN projection after a CD transform; pixel and reference-pixel values must use the same indexing convention, and SIP is not applied here. For a complementary task, use Cd Matrix Image Axis Angles: Infer both image-axis angles in the WCS tangent plane from CD columns, exposing skew rather than assuming an orthogonal scalar rotation.

References

Sip Distortion Evaluation — tool.io

Similar or alternative tools

Tan Wcs Pixel To Sky
Apply inverse gnomonic TAN projection after a CD transform; pixel and reference-pixel values must use the same indexing convention, and SIP is not applied here.
Cd Matrix Image Axis Angles
Infer both image-axis angles in the WCS tangent plane from CD columns, exposing skew rather than assuming an orthogonal scalar rotation.