Observe the galactic 21cm hydrogen line yourself! Its easier than it ever was! You need a horn, some filters, an amplifier, and a software defined radio (SDR). Here are some places to get started.
- Lightwork memo series. This is probably the best place to start to actually build. The epic build log by Trumbauer and Khashayar will delight and amaze.
- AAS poster by N. Patel. Another excellent summary of a completed project documented, from build to results.
- A Physics lab gives a detailed walkthrough.
- Tutorials geared towards high school teachers. By WVURAIL, same group as the Lightwork memos.
Horn Hacking @ ASU
The CHART* project puts students in the drivers seat to design, build and document their progress. They have a quite nice design for a cardboard horn that does not require a can for the waveguide. Their web site has many video tutorials about horn construction, using gnuradio to make sdr programs, solving linux problems and so on. The CHART designs and website was developed in the LoCo group by Adam Beardsley (now at Winona State) and several students.
*Completely Hackable Amateur Radio Telescope