I bet you're wondering what an old-fashioned hat and a chewy breakfast roll have to do with space science! Top hats and bagels are two types of electrostatic analyzers flown on sounding rockets to measure high-energy electrons in space. Scientists from the University of Iowa built top hat and bagel electrostatic analyzers for the CHARM sounding rocket.
The names of these two types of instruments come from the geometry of the detectors. The top hat (example shown below) consists of two metal hemispheres nested inside one another. The detector itself doesn't look much like a top hat. The name refers to the metal piece that sits on top of the two hemispheres like a "hat." When a voltage difference is applied between the two hemispheres, electrons entering the detector are deflected by the resulting electric field and follow a curved path. The amount of the deflection depends upon the detector voltage and the energies of the electrons, so only electrons with a selected range of velocities can enter the detector. By changing the voltage we can detect electrons with different velocities. Near the bottom of the detector are a pair of microchannel plates, which multiply the number of electrons to make the signal produced by the incoming electrons easier to measure. The CHARM rocket has two top hats that can measure the both the electron energies and the directions in which the electrons are traveling.
The basic idea behind a bagel detector is similar to a top hat, but the bagel has a different geometry. If you take apart a bagel detector (shown below), you will see that the inside of the detector looks like half of a bagel. The bagel half nests inside a hollowed out half-bagel (sort of like a bagel's crust). When we put a voltage on the inner bagel, electrons follow a curved path up through the hole in the center of the bagel. The hole is covered by a pair of microchannel plates, which multiplies the number of electrons to make them easier to measure, just like in the top hat detector. The CHARM rocket has eight bagels. Each of the eight bagels measures electrons at a specific energy traveling along the Earth's magnetic field.