Nooelec’s main RTL-SDR receiver families are:SMArt Series– current flagship R820T2-based RTL-SDR, 100 kHz–1.75 GHz, 0.5 ppm TCXO, aluminium case.– similar to the SMArt but includes a built-in Bias-T for powering LNAs.– E4000 tuner version with extended tuning range to about 2.3 GHz.– XTR plus Bias-T.Mini SeriesMini 2+Original MiniThese use MCX connectors and are lower-cost designs.Nano SeriesNano 2+Nano 3Very compact dongles intended for portable or embedded applications.Legacy XTR SeriesXTRXTR+Older E4000-tuner models covering approximately 55 MHz–2.3 GHz.For your radio astronomy work, the models I’d pay attention to are:Model Tuner Bias-T NotesR820T2 No Best general-purpose Nooelec RTL-SDRR820T2 Yes Useful if powering an LNA at the feedE4000 Yes Extended frequency range to 2.3 GHzE4000 No Good above 1.7 GHzFor 1420 MHz hydrogen-line observations, I’d choose the SMArTee (if you want to power an LNA through the coax) or the SMArt v5 (if your LNA has separate power). The E4000-based XTR models tune higher but generally aren’t preferred for hydrogen-line sensitivity.An alternative view to the last sentence regarding E4000-based SDRs is that they are reasonable for sensitivity, but have gap around 1100 MHz. This does not matter for hydrogen line at 1420 MHz.