for the fs-TEM are defined in Methods. Following a series of iterations, we present a fs-TEM design in Fig \[fig:fig2\], incorporating the source discussed above and in accordance with these five criteria.

![image](fig2.jpg)

Summing the radius of the electron pulse Airy disk and the spherical aberration in quadrature gives an indication of spatial resolution [@williamsandcarter]. For an electron wavelength of 4.5 pm and assuming a spherical aberration coefficient $C_{S}$ = 3 mm, an optimal convergence angle of 4.5 mrad is found, which corresponds to an optimal resolution of 0.7 nm. However, an electron pulse from a NSMT has a degree of inherent curvature which increases the apparent $C_{S} \geq$ 10 mm, increasing the optimal resolution to greater than 1 nm. There is a possibility that this could be improved with multipole aberration correction, however such devices add to the flight length of the instrument, hence gains in spatial resolution would compromise temporal resolution.

Temporal control is implemented both passively and actively: passive control via considerations of the space-charge influence of each component and minimization of the total source to sample distance, and active control via a laser-driven resonator. The resonator acts by modifying the position-velocity distribution such that a temporal 