Grab either (or both) of the faces of the Brewster crystal, and drag the crystal along the entire length between the two focusing mirrors.
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Initially, we will examine what happens to the mode and cavity stability as we drag optics around. We are interested in three parameters: The positioning of the crystal, the position of the focusing mirrors, and the positioning of the cavity end mirrors.
The cavity mode is independent of the precise crystal placement. This means we are free to place the crystal at exactly the waist.
The cavity has two stability modes. If the distance between the focusing mirrors is too short (< 200 mm) or too long (< 280 mm), the cavity is unstable. For some separations, the cavity is only stable in the sagittal plane, because of the off-axis curved mirrors.
The end mirror can be moved over an extremely large range without greatly changing the cavity mode. When the cavity becomes unstable for long separations to the end mirror, moving the focusing mirrors closer together by a few mm makes the cavity stable again.