The system temperature versus turret position has shown a bump (increase) for some receivers at the turret focal position. The receivers that had this increase also had platform focus curves where the platform focus position was lower than expected (or the horn was too high).
On 19aug02 the lbn horn was moved by 3.84" inches towards the tertiary to place the phase center in the correct position relative to the optics. After the focus change a 20 deg amplitude sine wave was done using the turret about the turret focus position. Calonoff's were measured while the turret moved. The plot shows the difference in Tsys versus turret position before and after the focus change. You can see a bump starting to appear in the "after" curve. This lends support to the idea that the Tsys bump is caused by the illumination of the tertiary by the horn and the sidelobes structure related to this.
The before swing was done in feb02 with the dome at 5. degrees za while the after swing was done at 16 deg za. The Tsys offset is caused by the za difference. The cal values between the two dates had been changed so the before data was scaled to the newer cal values.processing: x101/020823/dotursw.pro