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That actually sums up my question nicely: How did single dish (or single receiver) radio telescopes originally generate images? - or at least 2D intensity maps or contour plots.

Early radio telescopes were actually not much more than a Directional Antenna pointing up, connected to a sensitive receiver with various filters, maybe an IF stage or two, but no demodulation. You measured the "noise" signal strength as a function of time on a chart recorder - pen and ink.

I think there were radio images generated and published well before there were high granularity interferometers and computational correlators. The early ones were contour plots, on 2D plotters - again pen and ink.

How was this done? How did, say, a single dish antenna with a single feed generate radio maps?

edit: It doesn't necessarily have to be a dish antenna - it's the single receiver part I'm interested in.

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  • $\begingroup$ I've asked a somewhat related question. I got started on this after reading this good question which might benefit from an aditional answer. $\endgroup$
    – uhoh
    Commented Jun 15, 2016 at 2:32

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They scan the object, if you point the dish a a point in the sky as the Earth rotates the dish scans across astronomical objects, then move the dish to point at a slightly different position and let it scan across the object again, and again. After a while you can re-construct an image from the scan lines in a similar way to analogue TV.

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  • $\begingroup$ Sounds good! If the signal were weak you could build it up over many cycles, or move the telescope at a rate slightly faster/slower than the sky. But if there is drift in the gain of the telescope (this is the 'old days') or changes in the atmosphere (like water content) or even maybe the ionosphere, won't that cause parts of the image to be out of calibration with other parts? $\endgroup$
    – uhoh
    Commented Jun 15, 2016 at 8:03
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    $\begingroup$ Analog TV looked better than it actually was because it redrew many images per second and our eye averaged. The old analog scanning electron microscopes that might take minutes to scan one image to a piece of polaroid film for ultimate resolution were plagued by all sorts of drifts, sample charging, mechanical, emission current/emitter condition... oh it was such a pain before Zeiss made the first digital SEM and changed the world! (note: I have no affiliation with Zeiss - the old analogue SEMs were a royal pain!) $\endgroup$
    – uhoh
    Commented Jun 15, 2016 at 8:05
  • $\begingroup$ Actually I'll leave those to another day (and another question). Thanks! $\endgroup$
    – uhoh
    Commented Jun 15, 2016 at 8:27

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