McStas: DiskChopper bug when using isfirst=1

[willend]

In reactor settings (i.e. with isfirst=1) it seems that the calculation for emission time has an error:

The left and right sides of the beam are transported at different times, effectively giving rise to a splitting into two pulses... See attached graphics with a ToF and an x vs ToF monitor

A first analysis shows that the effect seems two-fold:

1. The chopper will effectively have two windows, which will affect the intensity ~a factor of 2 when using only the first "halfpulse"

2. What is worse, that the chopper then behaves as if it has only half the witdth and is put off the center of the beam...

Do not be too alarmed though - it seems that the component behaves nicely in the "normal"/"default" isfirst=0 setting.

[willend]

It turns out the behaviour was related to placing the DiskChopper above the beam by an y-translation, which makes the atan2 calculation of beam location wrt. centre of rotation go to the woods...

TODO: Fix docs in the component to recommend using rotation for placing the chopper above the guide!

[willend]

Updated docs in place. Closing