1 Images and measurements

Astronomical images record distributions of brightness, and how that depends on an object’s position on the sky. For optical astronomy this is now usually done using CCD (charge-coupled device) detectors. These are similar to cameras in modern mobile phones, although typically larger and/or more sophisticated.

Making images in parts of the electromagnetic spectrum beyond visible light can require less intuitive methods, such as interfering electric signals together from multiple antennae to produce radio wavelength images. But however an image is made, the basic end result – as shown in Figure 1 – is a two-dimensional (x, y) array of pixels: a grid that records the brightness at related locations on the sky.

Figure 1 How information is stored in a simple (greyscale) astronomical image. The data are stored as an x, y grid, where each pixel has an associated brightness.

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The figure shows a graph with two axes, labelled x position and y position. Each axis is marked at regular intervals, so that a regular grid of squares is defined that covers the entire graph.

The contents of each grid square are the pixels in the image. These pixels are shaded from dark to light, with the darker pixels forming the shape of a spiral galaxy. The colour scale to the right of the graphs shows that this image is a negative, so that light pixels show areas of least brightness, while dark pixels show the areas of greatest brightness.

Figure 1 How information is stored in a simple (greyscale) astronomical image. The data are stored as an x, y grid, where each pixel has ...

Additional information, such as the astronomical coordinate system and details of when the observation was made, is typically stored within the image file as metadata.

We need to employ careful methods to make measurements from images such as Figure 1 if we are to make reliable inferences about stars and galaxies. In this section, you will examine how these measurements are made, and what they can tell us.