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.