2T flatness

Two different kinds of flatness problems can be distinguished.

One: A tilting slope. Can be present at one energy, other energys are OK. This can be corrected by changing the 2T (1T-1R) current. Investigate if seesaw is possible. (minimum tilt of 2% before dose drops)

Second: A step form at the centre of the field. The field is more or less flat at both sides of this step. This step is caused by a miss alignment of focus position and flattening filters. Because of the profile of the photon filters the error for Xhigh will be larger than for Xlow. Trying to correct this profile using 2T current will give bad results. If electron flatness is tilted as well look for miss alignment of focus position. We once found a washer on one side of the first bending magnet. This washer altered the fringing fields of this bending magnet moving the focus in the AB plane.
When only photons are effected, investigate the moving and locking mechanism of the secondary filters. The moving mechanism must have slack when the filters are locked.