HYPERION

The Epoch of Reionization is one of the most exciting frontiers of modern day cosmology, and one of the most challenging to explore. Over the past ten years, astronomers have sought to detect the Epoch of Reionization with many different experiments, and primarily focused on using redshifted 21cm emissions to map out this period of the universe's history. In the case of PAPER and HERA, astronomers have built specialized low-frequency interferometers in order to observe the overall power spectrum of hydrogen in the early universe. The interferometric honeycomb design of HERA also enables the observer to potentially image growing ionization bubbles throughout the Epoch of Reionization. The LOFAR instrument also seeks to measure this signal through a more generalized instrument, with much longer baselines (on the order of kilometers compared to PAPER/HERA's baselines, which are on the order of decameters).

Astronomers have also sought to measure the sky-averaged 21cm signal, as a way of quantifying the overall neutral hydrogen content in the universe through the use of specialized single-dish experiments. The single dish is an important, if fraught, design choice as it enables direct sampling of the spatial monopole 21cm signal (i.e. the all-sky average signal). The trouble with single dish experiments is that it is extremely difficult to effectively calibrate every source of noise that the instrument itself contributes to the overall system noise, particularly in cases where that noise is frequency dependent. One of the main benefits of interferometric designs is that the vast majority of instrumental noise largely averages itself out, which abates the very strict calibration requirements demanded by Epoch of Reionization studies.

However, speaking from a purely Fourier background, an interferometer cannot be used to directly sample the monopole term, as interferometers work by imposing spatial frequencies across the sky which are then combined to form images. It is clear that a monopole signal, such as the 21cm global signal of reionization, would integrate to zero over any set of observed spatial frequencies, in the case of a perfect interferometer able to sample every mode and see the full 360 degree sky. However, HYPERION will circumnavigate this issue by using absorber baffles to impose an artificial horizon on the sky. This horizon interrupts the flat nature of the spatial monopole, which creates leakage from the DC-mode into modes with non-zero interferometric spacings. As discussed in Presley et al 2015, the optimal spacing between elements is approximately one wavelength. While the baffle structure will likely force a larger separation, one consideration to be made during design is ensuring that the spacing remains as tightly packed as possible to maximize our sensitivity to the monopole term.