# Which techniques are used to convert radio signals received by antenna to images?

I've written this question about experimental techniques as a companion question in order to move my answer from there to here.

I can think of at least four very different ways that "astronomers convert radio signals received by their antenna to images."

## 1. Scanning

This is the oldest and easiest to understand way to image. The answer to the question How did single dish (or single receiver) radio telescopes originally generate images? sums it up concisely:

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.

Here is an early 1D projection of scanning the plane of our Milky Way galaxy from The galactic system as a spiral nebula Oort, J. H.; Kerr, F. J.; Westerhout, G. MNRAS 118, (1958) p. 379. This image is reversed tone for better clarity.

For more information see answer(s) to Why the blank wedges in this very early 21 cm map of the Milky Way? (Oort et al. 1958)

## 2. Interferometry (most common technique today)

If you receive an incoming wavefront from two or more different locations, which in this case means two or more antennas, you can use math to reconstruct an image. This is called interferometric imaging and these days it's done by feeding the signals from each antenna to a large, fast computer known as a correlator. However in the early days of interferometric imaging the interference was actually done electronically, with amplified signals from each antenna combined in a mixer and other electronics.

In fact, one of the earliest techniques of radio interferometry used a single antenna which collected both a direct signal from the source and a signal taking a longer path by reflecting off of water! This is called Sea interferometry and in this case the water acts as a Lloyd's Mirror. This is one dimensional imaging only.

Read more about this in Radio Astronomy at Dover Heights as well as in the paper Galactic Radiation at Radio Requencies V. The Sea Interferometer J. G. Bolton and O. B. Slee, Australian Journal of Physics, vol. 6, p.420, December 1953.

above: Very Large Array (Source)

You may have seen images of (material around) a black hole recently. In this case the signals from each antenna were recorded on to hard drives, which were then shipped to a central location before being put into a computer correlator, but mathematically it is the same process. See answer(s) to How does the Event Horizon Telescope implement the interferometry?

## 3. Direct imaging using focal plane arrays

The question What is the highest granularity focal-plane array on a dish radio telescope? Or is this the ONLY ONE? shows the first image below, of the Parkes 21cm Multibeam Receiver and seems to be dated 1997. In this answer links to First image with Apertif: a new life for the Westerbork radio telescope which is shown in the second image.