In this paper, Section 2.1, the authors talk about using the ICRF2 to find a tuning frequency in search of the 21-cm absorption (of HI).

Since we already know the frequency of HI (1420.40575177 MHz), why is this so important? What am I missing about the functioning of radio telescopes?

This question doesn't really answer my query.

  • $\begingroup$ I wonder if looking for "known redshifts" might have something to do with it? $\endgroup$ – uhoh Mar 12 '19 at 2:11
  • $\begingroup$ Ummmm...I guess? Can you elaborate? $\endgroup$ – Jim421616 Mar 12 '19 at 7:24
  • $\begingroup$ It's just a thought/guess. I think someone more knowledgable will come by and post an answer. $\endgroup$ – uhoh Mar 12 '19 at 7:28

The very first sentence of that paper makes it clear that it is concerned with observations of the 21cm line in the redshifted universe, i.e. on objects so far away from us that redshift becomes significant. And just as any other electromagnetic emission, radio waves are affected by this. So, in section 2.1, the authors describe selecting a sample of radio sources with known redshifts so that they would know how much they had to tune their receivers downwards in frequency.

This is actually quite common and relevant in today's 21 cm radio observations. Redshifted signals are observed even down to 140 MHz and below as stated on LOFAR's Redshifted 21cm hydrogen Line page.

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