6415 & 6435 Front panel connector is not exactly at zero

The front panel location of the 6415 and 6435 is found each time you switch the instrument into a different length mode.  

The software uses the RL from the front panel connection transition to air to identify where zero is.  It is important that there not be anything connected to the instrument when the instrument is finding the zero location.  The control laptop will display a dialog box reminding you to disconnect all devices from the instrument when switching length modes.

Please make sure that you disconnect all devices from the instrument to ensure that it accurately finds the zero point. If you leave a jumper connected, then the instrument won't see the fall-off from Rayleigh Scatter to noise floor that is expected at the front panel connector. 

Additionally, the presence of a bright reflection, for example, from a PC connector transitioning to air, can raise the noise floor so that the RL and IL from the front panel connector is completely obscured.  In this instance, the instrument will set the zero-point based on the brightest peak from the noise floor, not the reflection from the front panel endface.  In this case, the zero point can be off by up to +/- 2 inches (5.1 cm).