optics required to define illumination of the sample, which is anticipated to be collinear with the electron pulse axis.

Condenser-objective lens implementation in fs-TEM
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Manipulating the electron pulse spot size and convergence angle at the sample facilitates real or reciprocal imaging modes, and controls magnification. Optimal illumination minimises imaging time, crucial when the stability of the apparatus or sample places temporal limitations upon image acquisition. When utilising single or few electron pulses to remove or alleviate space charge induced pulse broadening, the matching of beam dimensions to sample dimensions is essential to ensure practical imaging times.

![image](fig5.jpg){width="\textwidth"}

Placing the sample at the focus of a condenser lens restricts the fs-TEM system, as it is impossible to alternate between spatially focused or collimated illumination without major modification to the lens pole pieces. Furthermore, a highly divergent beam is favourable on exit of the sample plane to achieve high magnification, which is difficult to achieve in a traditional condenser lens.

In TEM, a condenser-objective (CO) lens combines the transport of the incoming electron pulse to the sample while also maximizing the post-sample divergence. A typical CO sees the sample placed equidistant between two strong magnetic windings, with B-fields approaching saturation in 