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# Conclusion

The focus of this course has been on how astronomers use images to make measurements and map the contents of the Universe. These were some of the key learning points:

- An image consists of a two-dimensional array of information, recording brightness as a function of position. In the visible part of the spectrum (and some other wavelength regimes), images are typically made using CCD detectors.
- The main measurements obtained directly from images are observed quantities that reflect the light signal measured at the Earth, rather than the intrinsic properties of the source. These include flux or apparent magnitude, angular size, and surface brightness.
- Brightness measurements are obtained via photometric apertures, and surface brightness profiles can be used to optimise the size of the aperture used. Profiles can trace the point-spread function (PSF): the blurring due to telescope optics and atmospheric seeing.
- Angular sizes are measured in units of pixels from an image, and must then be converted to units of arcsec, arcmin or degrees using the pixel scale appropriate for the detector. Angular size can then be converted to physical size (in units of metres, or pc, kpc, Mpc) if the distance to the astronomical source is known. However, the effects of orientation must also be considered.
- For non-spherical objects, the shapes measured on an image may not reflect their true shape, due to the effects of orientation and 2D projection of a 3D object. If the inclination relative to the line of sight is known, then true proportions, and hence shapes, can be obtained. In some cases, particularly for more complex and irregular objects, there is insufficient information to determine the underlying three-dimensional shape.

- For a circular disc-like object, the observed major axis corresponds to the true diameter of the disc, and the inclination angle (tilt, ) relative to the plane of the sky is given by

  Equation label:(Equation 1)

  where is the measured semimajor axis and the measured semiminor axis.

- If a linear object is projected at an angle of to the plane of the sky, then its true length, , is related to its measured length, , by

  Equation label:(Equation 2)
- Astronomical objects can appear in images as point-like or extended, depending on whether they are resolved by the telescope. Extended objects have angular sizes larger than the image’s point-spread function.
- There are five main types of extended astronomical object in visible light images: open clusters, globular clusters, galaxies, planetary nebulae, and diffuse nebulae.
- Astronomical images taken with different telescope filters, spanning different wavelength ranges, can be combined to produce colour images. Quantitative measurements, such as colour index, are usually made by carrying out photometry separately on images from individual filters and then combining the measurements.
- Narrow-band images can be made using filters that allow through only light associated with a particular emission line. One of the most commonly used examples is , which highlights the locations of regions of hot gas.
- Images are also made across the electromagnetic spectrum. Multiwavelength astronomy involves using images spanning a wide range in wavelength, from radio waves to gamma rays, which highlight different regions and features.
- Images from small areas of sky can be combined together to make sky maps. The Milky Way spans a large area of sky and has been mapped in detail at many wavelengths. Surveys are also made of the extragalactic sky, producing wide-area images and maps of the locations of millions of galaxies beyond our own. Time-domain surveys compare images taken at different times to identify variable phenomena.

This OpenLearn course is an adapted extract from the Open University course S284 Astronomy.

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OpenLearn - Astronomy: images of the Universe
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