FitTanSipWcsTask Fit a TAN-SIP WCS given a list of reference object/source matches.

## Processing summary¶

Measure the distortions in an image plane and express them a SIP polynomials.

Given a list of matching sources between a catalog and an image, and a linear Wcs that describes the mapping from pixel space in the image and ra/dec space in the catalog, calculate discrepancies between the two and compute SIP distortion polynomials to describe the discrepancy.

SIP polynomials are defined in Shupe at al. (2005) ASPC 347 491.

Note that the SIP standard insists (although it is only mentioned obliquely between Eqns 3 and 4) that the lowest three terms in the distortion polynomials be zero (A00, A10, A01, B00, etc.). To achieve this, we need to adjust the values of CD and CRPIX from the input wcs. This may not be the behavior you expect.

## Python API summary¶

from lsst.meas.astrom.fitTanSipWcs import FitTanSipWcsTask

classFitTanSipWcsTask(config=None, name=None, parentTask=None, log=None)

Fit a TAN-SIP WCS given a list of reference object/source matches...

attributeconfig

Access configuration fields and retargetable subtasks.

See the FitTanSipWcsTask API reference for complete details.

## Configuration fields¶

### maxScatterArcsec¶

Default
10
Field type
float RangeField
Range
[0,inf)
maximum median scatter of a WCS fit beyond which the fit fails (arcsec); be generous, as this is only intended to catch catastrophic failures

### numIter¶

Default
3
Field type
int RangeField
Range
[1,inf)
number of iterations of fitter (which fits X and Y separately, and so benefits from a few iterations

### numRejIter¶

Default
1
Field type
int RangeField
Range
[0,inf)
number of rejection iterations

### order¶

Default
2
Field type
int RangeField
Range
[0,inf)
order of SIP polynomial

### rejSigma¶

Default
3.0
Field type
float RangeField
Range
[0.0,inf)
Number of standard deviations for clipping level

## Examples¶

See lsst.pipe.tasks.photoCal.PhotoCalTask .. note:: Pipe task will require conversion before this link is usable.

## Debugging¶

FitTanSipWcsTask does not support any debug variables.