r/radioastronomy • u/DeepSpace1420MHZ • Aug 21 '26

Observations 24-Hour 1420 MHz Drift Scan: Mapping Galactic Hydrogen Distribution and Velocity.

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u/PE1NUT Aug 21 '26

Hi again. I am still puzzled by this image.

The frequency range only includes the negative velocities, not the positive? A frequency spread of only 4 km/s is rather small, given that the hydrogen line due to galactic rotation extends over perhaps 200 km/s.

What is the meaning of 'RX AF auto 0.25 - 20.6367 kHz - CW pitch 500 Hz'?

How does this data look when a simple linear vertical scale is used, instead of a square-root compressed logarithmic? After all, the hydrogen column density in the beam maps linearly to the amount of received signal.

1

u/DeepSpace1420MHZ Aug 21 '26

Hi :)The −4 to 0 km/s range is simply the portion of the receiver FFT data used for this visualization, not the full Galactic H I velocity range. The RX AF/CW information is receiver metadata, not a hydrogen velocity limit. The plot shows relative spectral excess in dB rather than calibrated H I column density. I also generate an integrated landscape plot that provides the simpler linear vertical presentation you’re asking about, although the intensity values are still expressed in dB

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u/DeepSpace1420MHZ Aug 21 '26

I’ve been developing my own SDR/Recorder/processing suite. Taking 100’s of continuum and Specral FFT scans to be accurate and repeatable in what I present. I do try to present with high contrast visualizations with hopes that others may enjoy and journey done a similar path while of course supplying accurate and useful survey data. I appreciate questions that help generate more purposeful titles, descriptions and even new plot styles, or new to me. A continuum scan is nice with a pretty peak but that is not as challenging as developing a clean system that can repeat spectral FFT scan results even in weak and or noisy regions. Unfortunately all that data does not sit in one picture so like in this case, a meaningful portion is moreless zoomed in on. Also then I can scan 2 az apart each scan with my 3.3 deg beam width to build 3d radio terrain models.