CHART: Completely Hackable Amateur Radio Telescope
astrochart.github.io
astrochart.github.io
Although the more interesting of the two, radio astronomy, for the amateur, seemed like it wouldn't yield as interesting results as visible light astronomy.
A lot of smart amateurs discuss all things astro at the Cloudy Night forums; the subforum where topics such as radio astronomy are discussed is here: https://www.cloudynights.com/forum/88-scientific-amateur-ast... Could be worth a look if you're seeking ways to contribute to the area.
There are ways of getting around this: physically scanning the big antenna, multi-apertures for receive across the focal plane, or by abandoning actual imaging and measuring the correlations between arrays of lower gain (smaller) apertures.
But the best way (if the signal is loud enough) is just to have a smaller aperture. Real science monitoring for transient radio signals can and is being done with literally just using the feedhorn from a large dish by itself (with some precise clocks, expensive lna, receiver, and time stamping) to detect FRB (and other high power transients),
https://arxiv.org/abs/2001.05077 STARE2: Detecting Fast Radio Bursts in the Milky Way
Multiple stare2 type instruments distributed around the world are likely to do real radio astronomy in the near'ish future. This is amateur scale possible. In the "Completely Hackable Amateur Radio Telescope" they're using a pyramidal dish because it's easier to construct but for trying to contribute to "real" radio astronomy you'd want a choke ring feed horn or at the very least a conical horn. The idea is to minimize sidelobes which otherwise prevent you from knowing for sure the signal you received came from the direction the horn is pointing.
2. Another low hanging amateur accessible "real science" is actually doing something when you're receiving solar radio emissions. Calculate the velocity of a CME shock by the frequency drift as it travels upwards through decreasing magnetic field. Or even more audacious, use a hackrf (or ettus usrp b200 or something) to frequency hop at 8 GH/s over a few hundred MHz bandwidth and record the fine structure present at milli and microsecond timescales precent in many types of solar radio bursts. Things like spectrogram zebra patterns and fiber bursts and other unexplained highly coherent fine timescale patterns which still lack accepted physical explaination today. Particularly in that their very short timescales indicate the emission regions have to be only ~a handful of kilometers in scale but outshine the rest of the sun. Whatever "small" solar features are making these short intense radio emissions are open to even amateur interpretation at this point.
https://www.aanda.org/articles/aa/full_html/2023/01/aa42905-...
As for the fine timescale structure of solar radio bursts, depending on frquency, and for this you're probably wanting to cover all of l-band and up, you only need about a meter^2 of aperture. And to see the fine-structure you only need a fast enough receiver with wide bandwidth. That's a few hundred to a thousand bucks. And you definitely don't need to do any sort of imaging or array stuff. All you need is a single receiver and to look at the spectrogram with a time precision of milliseconds. It is very amateur accessible despite the fact that even the largest, most complex, radio telescopes are looking at it too.
I wish the software was in fact a little slicker. For my brief toying with it, the apps felt like either Swiss Army knife apps with too many knobs, and/or had kind of janky UI.
Radio astronomy seems like another cool use of SDR that could draw me back in to learn more about it.
I remember reading that interferometers are usually all connected by physical cables with physical loops to make sure the incoming data is combined at exactly the right time. But are we at a point now where that can somehow be done intelligently in software? Or are these little RTL-SDR's not accurate enough to even begin trying that?
The accuracy needs to be within about 1/10th of a period, give or take. I'm not sure the min/max frequency range for this system, but I saw 1.4 GHz in a screenshot, which would yield a tolerance of about 0.1 * (1 / 1.4 GHz) = 70 picoseconds.
That is achievable with the right hardware, but unfortunately the RTL-SDR doesn't include an option to use an external clock reference. As a result, each dongle's ADC sample timing and RF synthesizer phase will constantly be wandering relative to the others. It's not a one-time calibration; it's an ongoing random walk that changes on a millisecond-by-millisecond basis.
In theory you might be able to pull it off if you had a separate emitter in view of each antenna, calibrate each unit based on that signal, and then synchronize everything in software. But at some point it's easier to just use hardware that has an external clock input, and avoid the whole problem.
When your math starts requiring relativistic physics, it's a lot easier and cheaper to just run some fiber.
https://en.wikipedia.org/wiki/Hanbury_Brown_and_Twiss_effect
The two options are to keep those offsets under control (i.e., lock everything to a common clock) or to rapidly measure the offsets as they change and try to compensate in software (difficult).
(Another at first surprising thing is that radiation received from celestial sources is only coherent because of their very small apparent size -- the sources themselves are not coherent at all, because their physical size is very large)
I admire the guy for having as much fun with his failures as his successes.
I focus more on maximizing what i can get out of a simple hardware setup, which means skipping anything that involves complex digital analysis or extremely sophisticated and sensitive equipment; it means more time having fun and less time debugging problems where I actually don't know enough to debug the problem.
Is there a better frequency available to amateur hardware that would give tolerance within more reasonable limits?
Without a shared external clock reference, i.e. over longer distances, how expensive does the hardware get if you want to be able to accurately measure the time/phase wandering to correct in software?
Just curious if it’s a limit of the low cost RTL-SDR or if it’s a harder problem than that (or both?).
Over longer distances, this is an active area of research with many different approaches for various applications.
Two recent well-known examples are the "Event Horizon Telescope" (the network of radio telescopes that has been generating images of black holes) and optical frequency combs (a recent demo published in Nature achieved time-transfer accuracy of a few femtoseconds).
[0]: https://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/...
Looking around on Google, I don’t see anyone that has tried using this for interferometry yet but the KrakenSDR team explicitly mentions that it can be used for interferometry.
The wiki is still public afaik.
maybe not but thats how it came off
[1] https://www.pictortelescope.com/observe [2] https://www.pictortelescope.com/Observing_the_radio_sky_with...
It would certainly be cool if it were possible.
Check out the Galaxies tab here: https://public.nrao.edu/gallery/