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I am interested in doing some amateur radio astronomy. Mainly at 1420MHz, hydrogen line. I have a RTL SDR stick. For those who don't know what that is, it's USB DVB-T receiver that can receive anything between 24 – 1766 MHz.

Now, there is a finished program for what I am asking, but I don't like using something that I don't know how it works. This is the source for finished program https://github.com/mariocannistra/radio-astronomy-fftw

SDR stick outputs I/Q data. I/Q data is just amplitude data but the amplitude is sampled at the same time 90 degrees away from the other sample. Amplitude of the signal is sqrt(I^2+Q^2). (I don't know how to insert equation, sorry). No problem so far.

I can record a sample of the signal and there is a library for Raspberry PI for doing Fast Fourier Transform on that sample. This is where I got stuck. How to find power spectral density from the Fast Fourier Transform?

I got the impression that it's just amplitude squared - I^2+Q^2? Can it be that simple?

I apologize for any nonsense that I may have said here. And of course I wrote FTT instead of FFT in the thread title. If I wrote everything correctly that would be weird. Moderators, please correct that.