Post date: Oct 20, 2009 4:46:42 AM
"Productive" amateur radio astronomy is generally pretty difficult to put into practice. A fairly short list of setups capable of actually detecting radio energy from celestial objects fit into a typical amateur's budget. Some favorites include:
1. Neutral hydrogen detection and mapping at 1420 MHz
2. Solar/lunar detection at 10 GHz with offset dishes/LNBs
3. Detection of a limited number of strong radio sources at VHF and UHF frequencies.
4. Jupiter decametric radiation at 20-30 MHz
5. Meteor ionization via foward-scatter radar at VHF frequencies
For items 1, 4, and 5, software defined radios have begun to add more capabilities than in the past, however, little or no new information is likely to be built upon, even with much larger antennas. Some of the best amateur systems have been barely able to detect the strongest pulsars, but for the most part, they are beyond reach for amateurs at the current time.
This all sounds pessimistic, however, advances in software defined radios and computational power are opening up new possibilities, particularly for digital interferometry. Amateur-level interferometry using classic methods have been the most effective in detecting radio sources so far. Some very nice examples of successful work with interferometers can be found at these sites:
In particular, I was impressed by how effective low frequency interferometry was, even when using a pair of simple dipole antennas. At low frequencies, synchrotron emission from radio sources is vastly stronger than it is at microwave frequencies, making their detection with low gain antennas much more feasible. When coupled with the resolving power of an interferometer, the potential for detecting hundreds of radio sources should be possible at the amateur level. Along with increasing availability of software defined radios and increased computational power in consumer-grade computers, much more powerful observing is within reach.
Synchrotron and thermal radio sources are inherently broadband in nature, so observations at large bandwidths are beneficial. Traditional communication-grade radios are not a good match in this regard; the ability to detect as large an amount of bandwidth as possible is desirable. Higher end SDRs like the BladeRF have a maximum bandwidth of ~30 MHz, and at low frequencies, this can constitute a large fractional bandwidth which is an important consideration for digital interferometry. Compared with a classical, narrow-bandwidth interferometer with say a 30 kHz bandwidth, 1000 times more energy can be captured with a software defined radio based interferometer. Because of 'bandwidth smearing' channelization of power into frequency bins is essential, something an SDR is good at doing.
Using an FX correlator (that I discuss elsewhere in detail), it is possible to break up the passband into say 1000, 30 kHz frequency channels that can be individually cross-correlated. This will allow for a number of things:
1. Excision of RFI: RFI is a big problem that would render a simple, wide band, total power receiver useless.
2. Bandwidth synthesis: Each frequency has its own distinct baseline, allowing for a much better filling of the
u-v plane for imaging synthesis.
3. At low frequencies, an SDR can achieve a higher fractional bandwidth.
4. Additional algorithms such as de-dispersion can be employed to search for transients, including pulsars.
5. Multiple baselines from 3 or more antennas can be analyzed.