Imaging is the most intuitive process of gaining access to celestial sources, because we are by nature familiar with it. Our eyes are optical instruments focusing light from our surroundings on the retina, where light-sensitive cells create a two-dimensional image which in turn is transported to the brain by nerves. Our eyes are therefore like cameras, the nerves being equivalent to a cables bringing the signals from the detectors to a processing computer - the brain.
Our eyes register photons continuously and they are propagated to the brain and processed there in a steady stream. We do not have any kind of storage or memory device that would enable us to accumulate photons over time. Which is a pity, because with that capability we could see at night (if we could keep our heads steady long enough so as not to smear out the accumulating image).
As things are, we are left with three ways out of our blindness at night (or, in other words, our insufficient sensitivity to very faint light):
- 1. Night vision binoculars, which collect more photons than our eyes because of a their larger objective lenses, or
- 2. look at other wavelengths, e.g. with infrared detectors, or
- 3. use light detectors that can accumulate photons over time
Option 1 is ok for applications on Earth and for
hobby astronomy, but not for going to the faintest
Option 2 is an alternative, but not a substitute for collecting optical photons, because in the two wavebands one does not see the same things. With respect to this, please see the pages on multi-wavelength astronomical observations. Which leaves us with
Option 3: Photographic film or other light-sensitive devices such as video cameras (making use of "charge coupled devices", CCDs), can indeed store photons (or rather, electric charges with an energy proportional to that of the incoming photon). These days CCD cameras are the most common astronomical imaging devices. Below an example of an image obtained with a CCD camera; this is a red light exposure of the southern galaxy NGC 1365, obtained with a 2048x2048 pixel CCD camera.
None of the objects visible in this image, not even the bright galaxy centre or the brightest stars, are visible to the unaided human eye. It takes the light-collecting power of a 4-m telescope (in this case the CTIO 4-m telescope) plus an extremely sensitive CCD camera to obtain such an image - by integrating photons for 12 minutes.
Imaging with a single radio telescope
What is described above refers to imaging with a two-dimensional detector. This could be the human eye (which images onto a two-dimensional array of cells on the retina), a conventional photo camera (where light is captured by a film of light-sensitive emulsion) or a digital (CCD) camera, where there are many pixel elements in a two-dimensional array, each of which can register light.
However, a radio telescope (single dish) normally has only one single receiver horn, i.e. a single detector element, like a single CCD pixel. With such a single horn, a telescope obtains only one measurement at a time, not a two-dimensional array (image). There are several technical reasons for this, most prominently the size of the required detector elements (which can be up to a metre in diameter for observations at 20 cm wavelength). To obtain an image of the sky, a single radio telescope must perform a raster scan of the area, measuring the brightness point by point, which is later transformed into an image in a computer once the observations are completed.
Still, there are a few so-called "multi-horn" receivers, such as e.g. the Parkes 21 cm Multi-Beam receiver, which has 13 horns. Another prominent example of a multi-horn receiver is displayed below, namely the "Submillimetre Common-User Bolometer Array, SCUBA".
Here the individual horns are a lot smaller and tiny little images with 37 datapoints at a time can be produced by it (there is another SCUBA, for a different frequency, that has 91 elements). This looks like a small imaging array for somebody used to dealing with modern CCD cameras, but for radio astronomy this is a big step forward. And a larger successor, SCUBA-2, which will have 4 arrays of 32x40 pixels each, is in the making.
Note that interferometry produces data that can be transformed into images, without the need for each telescope to have more than one single detector.