Class lsst::meas::astrom::SipReverseTransform

class SipReverseTransform : public lsst::meas::astrom::SipTransformBase

A transform that maps intermediate world coordinates to pixel coordinates according to the SIP convention.

The SIP reverse transform is defined as

\[\begin{split} \left[\begin{array}{ c } u \\ v \end{array}\right] = \left[\begin{array}{ c } u_0 + U + {\displaystyle\sum_{p,q}^{0 \le p + q \le N}} \mathrm{AP}_{p,q} U^p V^q \\ v_0 + V + {\displaystyle\sum_{p,q}^{0 \le p + q \le N}} \mathrm{BP}_{p,q} U^p V^q \\ \end{array}\right] \end{split}\]

with

\[\begin{split} \left[\begin{array}{ c } U \\ V \end{array}\right] = \mathbf{Z}^{-1} \left[\begin{array}{ c } x \\ y \end{array}\right] \end{split}\]

and

• \((u,v)\) are pixel coordinates.

• \((x,y)\) are “intermediate world coordinates” the result of applying the gnomonic (TAN) projection at sky origin CRVAL to sky coordinates).

• \(\mathbf{Z}\) is the \(2 \times 2\) linear transform ( \(\mathrm{CD}\)) matrix.

• \((u_0,v_0)\) is the pixel origin \(\mathrm{CRPIX}\) (but zero-indexed; the FITS standard is 1-indexed).

• \(\mathrm{AP}\), \(\mathrm{BP}\) are the polynomial coefficients of the reverse transform.

SipForwardTransform instances should be confined to a single thread.