Sensitivity levels can be increased at existing telescopes by increasing the recorded bandwidth at each site. The shift to digital electronics is making it possible to receive and record progressively larger bandwidths at modest cost.
The signal chain at a modern radio telescope mixes down the observed sky frequency to a much lower intermediate frequency (IF) that can be easily sampled. The available IF bandwidth at the telescopes that constitute the EHT varies but is typically a few GHz. Upgrades are being planned at multiple telescopes to ensure that they will be able to observe a bandwidth of 8 GHz to match the available bandwidth of ALMA at 230 GHz.
The IF at each site is sampled using digital backends. Several different types of digital backend have been used in EHT observations, including the first-generation DBE1 system, the Digital Base Band Converter (DBBC) system, and the ROACH Digital Backend (RDBE). These systems take in one or more IF inputs, digitally sample the analog signal, and channelize and format the data for recording.
The data are recorded on hard disk packs using the Mark6 system. Each Mark6 contains 4 modules of 8 hard drives that simultaneously record data at up to a total rate of 16 gigabits per second, which is the data rate of two 2-bit Nyquist-sampled 2048 MHz data stream. Several Mark6 systems are deployed at most of the EHT sites.
Recorded disk packs from each site are shipped back to a central location for correlation. EHT observations are correlated on either the Mark4 hardware correlator or the DiFX software correlator. Among other advantages, software correlation clusters are scalable, allowing astronomers to take advantage of Moore's Law advances in processing power.