Beam Settings CP

In the Beam Settings Control Panel various settings related to the illumination system are controlled. In addition a control is present to reset the defocus display of the system to zero.

The Tecnai microscope has eleven fixed settings for the spot size (the first condenser lens or C1). The numbering is such that spot 1 is the largest and spot 11 is the smallest spot. The actual spot size also depends on the microscope mode, with larger spots in the microprobe mode and smaller spots in the nanoprobe mode. The spot size is changed by moving the scale indicator along the track bar.

Microscopes equipped with a TWIN-type objective lens have two operating modes of the lens, the microprobe and nanoprobe modes. The difference between these modes lies in the way the minicondenser lens is used. In the microprobe mode, the minicondenser acts in the opposite direction of the main field of the objective lens, creating a separate lens. In nanoprobe, the minicondenser works in the same direction as the main field and no separate lens field exists.

Switching between the two modes is achieved by clicking on the microprobe or nanoprobe buttons. The active mode is highlighted by the yellow color of its button.

Characteristic	Microprobe	Nanoprobe
Purpose	TEM imaging	Analysis, STEM, CBED
Spot size (appr.)	~250 to10 nm*	~10 to 1 nm*
Field of view	>largest area seen at lowest Mi magnification	Appr. 1/40 of the size of the condenser aperture*
Convergence angles	Low (millirads)	High (tens of millirads)

In order to prevent damage to a specimen by unintentionally focusing the beam too much, it is possible to block focusing the beam (Intensity) further than a certain point with the Intensity Limit function. Set the Intensity (either underfocus or overfocus - overfocus being clockwise with the knob when the beam has gone through focus) to the maximum degree of focusing desired and press the Intensity Limit button. The button will become yellow (active). Henceforth it is not possible to focus the beam more than that setting. To switch Intensity Limit off, click on the button again.

Note: The Intensity Limit function only works after the Spotsize-Intensity calibration has been done in the Alignment procedures.

When the magnification is changed, the field of view changes and the screen intensity will be going down (high magnification) or up. To keep the same screen intensity, the operator can adjust the Intensity setting. The Tecnai microscope can also automatically change the Intensity setting as a function of magnification, spreading the beam when the magnification is lowered and focusing it when the magnification goes up. To select this function, define a good Intensity setting for the current magnification and then click on the Intensity Zoom button. The button will become yellow (to indicate Intensity Zoom is active). From then on the microscope will keep the screen Intensity constant.
At very high magnifications, it may not be possible to achieve the same screen intensity anymore because the beam is already completely focused. In that case the Intensity Zoom will stop and resume again when the magnification is lowered.
Intensity Zoom can be used at the same time as Intensity Limit. In that case, the Intensity Limit defines the maximum focusing of the beam.

Note 1: The Intensity Zoom function only works after the Spotsize-Intensity calibration has been done in the Alignment procedures.

Note 2: The Intensity Zoom function is decoupled between LM, Microprobe (HM) and Nanoprobe (that is, it can be on or off in any of the three independently). When switching between LM and HM, the Intensity Zoom functions reverts to its previously defined state (when going to Microprobe, if Intensity Zoom was off in Microprobe it will be off, and vice versa).

When the Reset beam button is pressed, the user beam shift (as normally set with the left-hand track ball) is reset to zero. The beam will then be in the position defined in the alignments.

When the Focus beam button is pressed, the Intensity setting is changed to the value at which the beam is focused. This value is defined in the Spotsize-Intensity calibration procedure and the function will only work when this procedure has been done.