Spectrometer Observing Modes

As previously noted, the GBT Spectrometer can be used to support a host of astronomical research efforts. Figure 1 illustrates some of the many scientific areas the GBT will explore. In considering specific types of observing, namely spectral line, pulsar, and continuum observations, a small example of how the Spectrometer will handle these areas is included below:

Figure 1

Spectral Line

For spectral line observing, the Spectrometer has the capability to support up to 40 input signals of varying bandwidth. Its 32 low-speed samplers support bandwidths of 12.5 MHz and 50 MHz. The high-speed samplers, of which it has eight, support bandwidths of 200 MHz and 800 MHz. The Spectrometer also works in various modes. These modes include simple and double Nyquist sampling and 3 to 9 level sampling. The term Nyquist refers to the Nyquist Theorem. This theory states that the highest frequency that can be accurately represented is one-half its sampling rate. So, if you want to accurately measure a specified frequency, you need to sample it twice as fast. An important benefit of this design is the large number of different configurations possible to support a vast range of observing strategies. All of these aspects are important to quality astronomical observing.

Pulsar

For pulsar observing, the Spectrometer can be configured to produce an array of lags (very short integration's of data representing total power within a channel) versus time. An example would be a configuration to produce an array of 256 x 4096 numbers representing total power as a function of frequency and time. In this example, a time interval corresponding to a pulsar period could be broken into 4096 intervals, each producing a 256-channel "spectrum". The time resolution of the Spectrometer is expressed as 1.3 msec over 256 since each of the 256 correlator chips can be independently controlled (blanked). Other configurations of time and frequency are possible to include manipulating the data output into sub-arrays to measure a feature's polarization. As illustrated, the Spectrometer is capable of supporting a vast number of observing plans.

Continuum Observing

The Spectrometer is capable of supporting work in the realm of continuum observing. Such work however, is expected to be very limited as the GBT already has a backend instrument, the Digital Continuum Receiver, designed to satisfy needs in this area. For continuum observing, an analog signal is converted to a digital format. This digitized information is then integrated and processed with the resulting output being broadband continuum data representing either total or switched power as determined by the observer.

System Configurations

Examples of system configurations, to further illustrate Spectrometer capabilities and how it can be used, are shown in the following tables.

800-MHz BW (3-level, Nyquist Sampling)

200-MHz BW (3-level, Nyquist or twice Nyquist Sampling)

50-MHz BW (High-speed Samplers, twice Nyquist Sampling)

50-MHz BW (Using low-speed Samplers, Nyquist Sampling)