Continuum Observations with the 12m telescope

Figure 1: The continuum image of the Galactic Center region made with the 12m telescope. The image was made at a frequency near 8.2 GHz with a bandwidth of ~150 MHz.

The 12m telescope room temperature receivers are now integrated with the Mock backend and regular observations are in progress (the telescope schedule is available here). Ongoing observations include continuum imaging of the Sun near 8.7 GHz (observing bandwidth 1 GHz), monitoring of bright pulsars near 2.2 GHz and spectral line observations near 8.5 GHz. We have now implemented and tested a new continuum observing mode, which will be useful for imaging weak (source brightness < a few percent of the system temperature) and extended (angular size larger than several tens of arcminute) sources in the Galaxy. Making a radio continuum image of a weak and extended source with a single dish is challenging due to system gain variations - often referred to as the 1/f noise. For the 12m receiver system, the 1/f noise dominates for integration times about a fraction of a second when observing with bandwidth greater than ~150 MHz. The new observing mode applies a winking calibration noise at 25 Hz rate while obtaining data with the Mock spectrometer. The noise toggling is synchronized with the data acquisition system such that ‘noise on’ and ‘noise off’ spectra are provided by the spectrometer. These spectra are then used to remove the gain variation with time. Observing modes to scan the telescope along right ascension and along declination (thus allowing ‘basket weaving’) are also available for observations. An example image of the continuum emission toward the center of our Galaxy made using the new observing mode is shown in Fig. 1. These observations were made near 8.2 GHz.