The RTL-SDR can be used for many interesting radio astronomy applications such as observing the Hydrogen line. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky and integrating the RF power over time, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot.
On his website Steve Olney has been writing about his experiments and results with using an RTL-SDR to observe the hydrogen line. On his website he writes that he uses a 3M dish, with an LNA at the antenna to reduce the system NF, a hydrogen line tuned bandpass filter to remove out of band noise, 2 line amps to overcome coax loss, and finally a second LNA just before the RTL-SDR dongle to optimize the signal strength for the ADC. The dongle he uses has been modified to use a TCXO, and is aircooled via a PC fan. He also uses a modified version of the rtlsdr.exe IQ file recorder and his own custom GUI for controlling the RTL-SDR and antenna tracking mechanism.
His results show that he was able to detect the Hydrogen in the Large and Small Magellanic clouds. He also shows a method for converting the 8-bit IQ data down to 1-bit to save disk space, and shows that while some noise is added, the overall result is preserved.
See the related posts for other hydrogen line experiments with the RTL-SDR.