# Sip¶

class astropy.wcs.Sip(*a, b, ap, bp, crpix*)

Bases: object

The Sip class performs polynomial distortion correction using the SIP convention in both directions.

Parameters: a : double array[m+1][m+1] The A_i_j polynomial for pixel to focal plane transformation. Its size must be (m + 1, m + 1) where m = A_ORDER. b : double array[m+1][m+1] The B_i_j polynomial for pixel to focal plane transformation. Its size must be (m + 1, m + 1) where m = B_ORDER. ap : double array[m+1][m+1] The AP_i_j polynomial for pixel to focal plane transformation. Its size must be (m + 1, m + 1) where m = AP_ORDER. bp : double array[m+1][m+1] The BP_i_j polynomial for pixel to focal plane transformation. Its size must be (m + 1, m + 1) where m = BP_ORDER. crpix : double array[2] The reference pixel.

Notes

Shupe, D. L., M. Moshir, J. Li, D. Makovoz and R. Narron. 2005. “The SIP Convention for Representing Distortion in FITS Image Headers.” ADASS XIV.

Attributes Summary

 a double array[a_order+1][a_order+1] Focal plane transformation matrix. a_order int (read-only) Order of the polynomial (A_ORDER). ap double array[ap_order+1][ap_order+1] Focal plane to pixel transformation matrix. ap_order int (read-only) Order of the polynomial (AP_ORDER). b double array[b_order+1][b_order+1] Pixel to focal plane transformation matrix. b_order int (read-only) Order of the polynomial (B_ORDER). bp double array[bp_order+1][bp_order+1] Focal plane to pixel transformation matrix. bp_order int (read-only) Order of the polynomial (BP_ORDER). crpix double array[naxis] Coordinate reference pixels (CRPIXja) for each pixel axis.

Methods Summary

 foc2pix() sip_foc2pix(foccrd, origin) -> double array[ncoord][nelem] pix2foc() sip_pix2foc(pixcrd, origin) -> double array[ncoord][nelem]

Attributes Documentation

a

double array[a_order+1][a_order+1] Focal plane transformation matrix.

The SIP A_i_j matrix used for pixel to focal plane transformation.

Its values may be changed in place, but it may not be resized, without creating a new Sip object.

a_order

int (read-only) Order of the polynomial (A_ORDER).

ap

double array[ap_order+1][ap_order+1] Focal plane to pixel transformation matrix.

The SIP AP_i_j matrix used for focal plane to pixel transformation. Its values may be changed in place, but it may not be resized, without creating a new Sip object.

ap_order

int (read-only) Order of the polynomial (AP_ORDER).

b

double array[b_order+1][b_order+1] Pixel to focal plane transformation matrix.

The SIP B_i_j matrix used for pixel to focal plane transformation. Its values may be changed in place, but it may not be resized, without creating a new Sip object.

b_order

int (read-only) Order of the polynomial (B_ORDER).

bp

double array[bp_order+1][bp_order+1] Focal plane to pixel transformation matrix.

The SIP BP_i_j matrix used for focal plane to pixel transformation. Its values may be changed in place, but it may not be resized, without creating a new Sip object.

bp_order

int (read-only) Order of the polynomial (BP_ORDER).

crpix

double array[naxis] Coordinate reference pixels (CRPIXja) for each pixel axis.

Methods Documentation

foc2pix()

sip_foc2pix(foccrd, origin) -> double array[ncoord][nelem]

Convert focal plane coordinates to pixel coordinates using the SIP polynomial distortion convention.

Parameters: foccrd : double array[ncoord][nelem] Array of focal plane coordinates. origin : int Specifies the origin of pixel values. The Fortran and FITS standards use an origin of 1. Numpy and C use array indexing with origin at 0. pixcrd : double array[ncoord][nelem] Returns an array of pixel coordinates. MemoryError Memory allocation failed. ValueError Invalid coordinate transformation parameters.
pix2foc()

sip_pix2foc(pixcrd, origin) -> double array[ncoord][nelem]

Convert pixel coordinates to focal plane coordinates using the SIP polynomial distortion convention.

Parameters: pixcrd : double array[ncoord][nelem] Array of pixel coordinates. origin : int Specifies the origin of pixel values. The Fortran and FITS standards use an origin of 1. Numpy and C use array indexing with origin at 0. foccrd : double array[ncoord][nelem] Returns an array of focal plane coordinates. MemoryError Memory allocation failed. ValueError Invalid coordinate transformation parameters.