Electrostatic lens structure

An electrostatic lens comprising:

• a first conductive plate provided with a first aperture;
• a second conductive plate provided with a second aperture, the second aperture being substantially aligned with the first aperture;
• a voltage supply for supplying a first voltage to the first conductive plate and a second voltage to the second conductive plate, the second voltage being lower than the first voltage with respect to intended function of the lens;
• and
• an insulating structure for separating the first conductive plate from the second conductive plate;
• wherein the insulating structure comprises a first portion in contact with the first conductive plate and a second portion in contact with the second conductive plate, the first portion having an overhanging portion contacting said first conductive plate and the second portion having an indented portion at an edge of the insulating structure, and wherein a gap is formed between the overhanging portion and the second conductive plate.