I recently visited the Green Bank Telescope for an observer training workshop. Although my research is entirely theoretical, ultimately it has implications for use of radio telescopes like GBT. This post collects some very brief notes.
GBT is a 100m radio telescope. It is composed of a 100m parabolic dish and focus. Its characteristic feature is the offset focus. Typically radio telescopes have the focus in the centre of the dish. By having the focus offset this results in less scattering and better sensitivity and pointing down to 2’’.
Because the GBT is so sensitive, probing down to milli or even micro Jansky levels, radio frequency interference (RFI) is a key consideration. The entire GBT is set in a radio quiet zone, i.e. no phone signals, wifi etc. Closer to the GBT no microwaves, digital cameras or even petrol cars owing to the RFI given off by the spark plugs!
GBT pipeline is broken down into 2 primary sections:
- Pick ‘frontend’ - i.e. choose a receiver for specific observation frequency.
- Pick ‘backend’ - i.e. how the data is taken and processed, which depends on the observation type i.e. pulsars will have a different backend to spectral line studies.
Within the pulsar backed, there are 3 main observation modes,
Folding is used in the situation when we know something about the pulsar. Since most pulsar signals are very weak it can be impossible to detect a single pulse against the background noise. To detect the pulse it is necessary to ‘fold’ the signal every period.
If we don’t have any prior information on the pulsar we instead use a search mode. In this case we Fourier transform the pulsar signal into the frequency domain to inspect for periodic signals e.g. if in Fourier space we see a signal at 100 Hz we would then fold the data at 0.01 seconds. From this we can see how RFI can be especially problematic for pulsar studies; many anthropogenic RFI sources are also periodic. If we know the frequency we expect the RFI at (e.g. AC current at 60 Hz) we can ignore that frequency.
The calibration mode is simply used to try and calibrate for noise.
Pulsar signals are subject to dispersion by the ISM. We want to correct the signal for the dispersive effects - in order to do this the dispersion measure must be known. In a fold mode, a reduced chi-squared fit can be done to tell us the most likely dispersion measure. In search mode we must correct at multiple trial DMs. The DM can also be used to identify RFI - since RFI has a local origin is will not be dispersed by the ISM and have zero DM.
This DM correction is described as either ‘incoherent’ or ‘coherent’. With incoherent corrections, the signal is detected and split into channels across the bandwidth. Each channel is then corrected by an appropriate amount. Conversely, coherent corrections are applied directly to the voltage signal. Typically, for the same bandwidth, coherent corrections give better time resolutions (1/bandwidth vs Bandwidth/num_channels)