Eucentric focus¶

Eucentric Focus is the microscope image focusing that makes an object at eucentric height at Gaussian focus.

In operation, you go through these steps to reach that focus value.

1. Use alpha wobbler to move the object on the specimen holder to eucentric height where the movement of the object at the center of the screen is minimal during the wobbling.
2. Looking at the object in the microscope and adjust defocus until the object is in-focus.

The eucentric height is defined by the goniometer alpha tilt axis, and therefore grid independent. Therefore, focus value (in term of current applied to the lens ) is also an invariant. Therefore, it is a very good reference point. Microscope manufacture aligned and calibrated magnification at eucentric height and encentric focus. We'd like to do the same in Leginon.

Please note that there is a "eucentric focus" button on FEI scopes. The button is not calibrated to the operational definition above unless you've done so or request engineers to recalibrate it regularly. Do not assume that pressing that button will bring the focus to the Eucentric Focus with 100 um if not calibrated.