Observing the Sun at 21 cm wavelength with a Two-element Radio Interferometer
Abstract
As part of the undergraduate astronomy course (Asro-191) at Harvard University, students have built pyramidal horn antennas with aperture size 75 cm x 60 cm, a rectangular waveguide, and a Software Defined Radio (SDR) receiver for 21 cm wavelength observations of the Galactic neutral hydrogen emission [Patel et al. 2014AAS...22441501P]. With two such antennas, a radio interferometer can be built rather simply, by just adding the signals using a power combiner, and feeding the output to the SDR receiver. The narrow bandwidth of this receiver (2.4 MHz) allows for easy detection of interferometric fringes, with minimal bandwidth smearing. Fringes are easily detected observing the Sun, with the two horn antennas along an east west baseline, with a fringe period of about 2.4 minutes for a 20 meter baseline. We compare our measurements of visibility amplitudes as a function of projected baseline length, with three theoretical models: 1) a disk of diameter 30', 2) a ring (due to limb brightening)of 43' diameter, and 3) a combination of disk plus ring models, also of 43' diameter. Our measurements appear to fit best the combination model, confirming the expectation of significant limb brightening and a slightly larger size of the Sun compared to the optical photospheric diameter. The radio interferometer has great educational value at both high-school and undergraduate levels. In addition to astronomy it is a useful tool to introduce concepts such as aperture efficiency, receiver noise temperature, and measurement errors.
- Publication:
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American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23511704P