a key issue about vacuum chambers is to use self stable components. a round component is self stable (the pressure load is symmetric), a flat component, e.g. the sides of the top or the top itself, is not self stable. here, using two 360 cf flanges on top instead of the previous top part, we have almost as much access to the experiment as with the top part. and a more self stable geometry of the chamber. another option would be to stick to the previous top part, but make it elliptic seen from the top, and round with respect to the main axis of the chamber. this would be a complicated expensive option. viton o-rings for sealing the top part holds down to 10-8 mbar, good enough for our experiments (i.e. berlin clusters) and imaging of nanoparticles and fib structures.
