XMM-Newton Science Analysis System


epicbscalgen (ccftools-1.36) [xmmsas_20230412_1735-21.0.0]

Description

The XMM Current Calibration File contains the constituent CCF::Boresight [1] which holds for each instrument a tuple of three angles $(\phi, \theta, \psi)$ describing the misalignment of the respective instrument boresight with respect to the satellite coordinate frame (SACCOORD – see documentation of package cal. The angles are to be interpreted as 3-2-1 Euler rotation angles[2] through which the SACCOORD frame is transformed into the instrument boresight (IBS) frame as depicted in Fig. 1 for the case of EMOS2. Formally, the IBS is defined as follows:

• The $+X$ axis points along the optical axis of the telescope of the respective instrument from the focal plane into the sky.
• The $+Z$ axis is orthogonal to the $+X$ axis and lies in the plane spanned by the $+X$ axis and the $+Z$ axis of the CAMCOORD2 (see documentation of package cal frame.
• The $+Y$ axis completes the right-handed, orthonormal Cartesian coordinate frame.

Figure 1: Coordinate frames used in epicbscalgen

The computation of the angles is a purely geometrical problem and, thus, should have a straightforward analytical solution. However, due to the large number of involved independent variables (Ra/Dec of pointing, position angle, position of cameras in satellite frame, uncertainties in focal lengths, position and extent of sources being looked at, etc.) and the fact that math with Euler angles is non-trivial epicbscalgen employs an empirical scheme which is outlined below. The scheme has the following advantages:

• The process is fully automatic in the sense that is does require only minimal input from the person running the task.
• It is bound to always give the right answer (chances of making mistakes are small).
• The correctness of the algorithm can be tested with SciSim.