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When making an observation using a radio telescope, is it possible to filter out noise coming out from a known source by using another telescope that will focus on that noise source?
By filtering, I mean processing the observation on a computer and calculating the filtered image using the second telescope source. Assume that both of the telescopes are connected and can easily sync on time and location and exchange other information.

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    $\begingroup$ I appreciate your selection of my answer. It's common for a question asker to give the community plenty of time to answer before accepting a best answer. This allows the rest of the community time to comment on existing answers and even formulate alternate more thorough answers. If a better answer than mine is later provided, please feel free to switch your acceptance! $\endgroup$ – Connor Garcia Nov 30 '20 at 19:55
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    $\begingroup$ @ConnorGarcia Yes of course. It still puzzles me why the Cosmic Microwave Background cannot be filtered. I try to ask about this, once I learn more about the subject. $\endgroup$ – Ilya Gazman Nov 30 '20 at 20:02
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    $\begingroup$ These are excellent and deep questions. The photons in the CMB got pretty well randomized, so the set of photons hitting telescope number one won't hit at the same times as the set of photons that hit telescope number two. Also, the CMB is mostly isotropic, so it's impossible to focus a telescope on it. A signal from a satellite, on the other hand, isn't random at all, and can be pointed to. $\endgroup$ – Connor Garcia Nov 30 '20 at 20:22
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    $\begingroup$ I usually give questions a week before accepting answers, some contributors don't necessarily check in every day. It's totally your choice of course, but you might consider unaccepting per the request above? $\endgroup$ – uhoh Nov 30 '20 at 23:22
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Coherent interfering signals are usually referred to as RFI (or Radio Frequency Interference). One could certainly digitally filter out a coherent interfering signal from a telescope by observing from an additional telescope focused on the source of the interfering signal, given the proper geometry. Here is a research paper on Radio Frequency Interference Mitigation that outlines some of these very techniques in section 3.4: Post correlation studies.

Incoherent interference is typically referred to as noise, and won't be the same at the two telescopes. This means an astronomer can't use one telescope to filter out the noise at the other. Some examples of noise sources are:

  1. Noise that comes from thermal from the telescope's electronics
  2. Ground backscatter into the antenna receiver
  3. Inaccuracies introduced by analog to digital conversion and other signal processing.
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  • $\begingroup$ I've added an answer. $\endgroup$ – uhoh Nov 30 '20 at 23:33
  • $\begingroup$ @uhoh I just edited my answer to eliminate discussion of CMB to give you full stage to discuss it in your answer. $\endgroup$ – Connor Garcia Dec 1 '20 at 1:15
  • $\begingroup$ yikes! Now I've got stage fright! $\endgroup$ – uhoh Dec 1 '20 at 1:20
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I'd like to address something I saw in comments as a supplemental answer.

I don't think we can view the CMB as incoherent noise. The CMB is just as much of an extended yet coherent source as any extended object for which radio images exist. Any source that's distant compared to the size of the array is going to have coherence, since all antennas receive roughy the same waves, just delayed differently by their path lengths1.

Just for example:

The only problem is that CMB is not localized so it will be present no matter where one points a radio telescope. In other words, it's not "noise" because it's really there!

If you could build a second antenna so far away that parallax moved your intended object substantially (if for example you were making a radio image of Jupiter using an array of radio telescopes in a big halo orbit around a Sun-Earth Lagrange point) then in that case you might be able to

  • Characterize it as noise, since the contribution would be different in each receiver
  • Remove it somewhat from your image of the foreground object since it would be more local interference or incoherent thermal noise in each receiver2.

But of course Jupiter is pretty loud, so it might not be necessary.


1Although I struggle with the concept, we can think of each radio photon as entering all telescopes and interfering with itself, even if that happens six months later after the hard drives have all been shipped to a central location for processing!

2Actually it would be almost exactly like thermal noise, about 2.72548±0.00057 K in fact. (Slightly related: Why doesn't thermal radio emission from a DSN “hot dish” completely swamp the benefits of a cold LNA?)

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    $\begingroup$ The 21CMA's frequency range looks too low to observe the CMB. The paper says they are trying to observe celestial sources from the same time the CMB was formed. Still, you may be correct about coherence in the CMB. When I worked for the National Radio Astronomy Observatory we never treated it as a coherent source, but I suppose that doesn't mean it can't be. I laughed sooo much when I was directed back to my own answer! +1 $\endgroup$ – Connor Garcia Dec 1 '20 at 0:47
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    $\begingroup$ @ConnorGarcia ya my ploy didn't work as effectively as planned because I didn't read that carefully. There are several Earth-based interferometric arrays that do coherently image the CMB, (e.g. 1, 2, 3) but at 70 – 200 MHz that isn't one of them. $\endgroup$ – uhoh Dec 1 '20 at 0:53
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    $\begingroup$ The ironic thing is I was actually aware of the CBI. I am still pretty new on Astronomy Stack Exchange. I'm inclined to leave my flawed answer up and let it be downvoted or bypassed. Is it considered bad form to leave up a flawed answer rather than correct it? $\endgroup$ – Connor Garcia Dec 1 '20 at 1:03
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    $\begingroup$ @ConnorGarcia hmm, well my answer is now flawed as well as you've pointed out. I'm going to "fix it" by adjusting to address only comments under the question and label it as a "partial answer" or "supplemental answer". Why don't you go ahead and refine yours as well. Stack Exchange (in my opinion) is "Collaborative effort to generate good answers to on-topic questions" with future readers as our customers. I try not to focus on myself or reputation or being right, but instead on delivering the best information to those who come after us. $\endgroup$ – uhoh Dec 1 '20 at 1:09
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    $\begingroup$ A bit of mentorship goes a long way. It's much appreciated. $\endgroup$ – Connor Garcia Dec 1 '20 at 1:36

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