• Barbara Jesline

Do you know: how do astronomical observations do?

Everyone knows that astronomers study the sky. But what sorts of measurements do they make, and how do these translate into data that can be analyzed to understand the universe? This article introduces astronomical observations to guys in related fields (e.g., engineering, statistics, computer science) who are assumed to be familiar with quantitative measurements and computing but not necessarily with astronomy itself.

All scientific work starts with an idea which is frequently formulated as a question to which the scientist would like to find an answer. Two examples of such questions are How hot are those stars? or How far away is that galaxy? although most will be more complex. This is followed by an investigation, that takes the form of observations or experiments, or it may be more theoretical. Then follows careful scrutiny of the obtained data and the process ends with some kind of interpretation of this material, hopefully producing the answer to the question posed and also resulting in new, exciting knowledge. Very often, this new knowledge soon leads to new ideas being formulated and the entire cycle starts again.

Through many centuries, astronomers have tried to improve the observational technology and thereby to improve their observations. This effort dates all the way back to the stone age, when the Neolithic people built large stone monuments, some of which can still be admired. Many of these were `astronomical observatories' with which observations were performed that served to establish the calendar of the local communities. Stonehenge on the Salisbury Plains in Southern England is one of the most impressive examples of the building skills of our distant forefathers, but there are also stone settings in many other European countries.

From the simple instruments which were used in antiquity, astronomers and craftsmen through the ages contributed to the never-ending technological development of their instruments. The first sighting instruments were only able to measure very approximate positions of the objects in the sky and the eye was the only light-detector available. From the early 17th century, the telescope has accompanied the observers, opening entirely new vistas towards near and distant regions of space. 250 years later, in the middle of the 19th century, the photographic emulsion was introduced in astronomy and has since helped to provide a lasting record of the images of the sky. Nevertheless, since the opening of the space age, barely 40 years ago, we have also begun to explore some of the planets, moons and comets in the Solar System by means of visiting spacecraft. This technique is referred to as in situ (on-site) observation and has given us extremely important information about these objects. Do you know that over 2 million photographic plates are now available in the archives of the world's observatories? If they would all be scanned and all the information on them would be stored in a computer, there would be several petabytes of data (1 petabyte = 1000 terabytes = 1,000,000 gigabytes)!

There are several different types of astronomical observations. Some consist of taking pictures of an area of the sky in which the object of interest is seen. For this, a telescope and a camera with a light-sensitive detector are needed. This is the most common type of observation performed by amateur astronomers. Other types of observations include spectroscopy and polarimetry. And while ground-based observations record visible light, infrared light or radio emissions, special instruments in space observatories may also collect ultraviolet light, X-ray and gamma-ray emission. Astronomical observations have become extremely efficient, and it is possible to observe many objects in a short time. A large professional, optical telescope with a state-of-the-art CCD detector may obtain several hundred images every night, showing fine details of relatively nearby objects and also stars and galaxies which are exceedingly far away.

The fundamental measurement that an astronomer makes is the amount of radiation from the sky, as a function of direction, time, wavelength or frequency, and polarization. The long history of astronomical measurements began with observations made by the human eye and brain, later aided by architectural constructs like Stonehenge or devices like the sextant. The invention and adoption of the telescope transformed astronomy.

The most important purpose of astronomical telescopes is to act as a ‘light bucket.” In a rainstorm, a bucket with a larger top opening will collect more rain. Similarly, the rate at which a telescope can collect photons from a given direction in the sky depends on the diameter of its main mirror or lens (its aperture size D), and is proportional to D2. As the number of photons detected from an astronomical source increases, the uncertainty of the corresponding measurement decreases. Because astronomical objects are far away, only a small amount of their radiation reaches us; every last photon can be important. Using larger telescopes allows us to collect more photons, so we can detect fainter objects more quickly, or more finely subdivide (e.g. in time or wavelength) the light received from brighter objects.

The second major purpose of an astronomical telescope is to precisely determine the location in the sky from which radiation is emanating. Focusing the radiation can be done in one of two ways: refraction or reflection. Refraction is familiar from other optical instruments such as eyeglasses and microscopes and involves bending of light through a lens material (usually glass). Very large lenses are heavy and can only be supported around their thin edges. Most modern large astronomical telescopes focus light using reflection by curved (usually in the shape of a conic section) mirrors that can be supported from the non-reflecting side. These mirrors can be made of materials similar to familiar everyday mirrors—silver- or aluminum-coated glass—or rather different, such as the beryllium mirrors on the James Webb Space Telescope (JWST)3 or the wire surfaces of a radio telescope.

A focusing telescope’s ability to precisely measure the direction of radiation – its spatial resolution – is limited by the size of the telescope and the diffraction of light. Diffraction occurs when waves pass through a narrow opening or across an edge: the waves spread out and interfere with one another. The diffraction of electromagnetic waves means that even a source of radiation of zero physical sizes, observed by a telescope of finite size, will generate an image with a finite size. Of course, astrophysical sources have non-zero sizes, but in most cases, they are far enough away to be considered points. We call such sources to point sources and the shape of their images the point spread function. The angular size of a point source as observed by a given telescope is a way to describe a telescope’s spatial resolution, usually quantified by the point spread function’s full-width at half-maximum (FWHM). In general, detecting details of an object on image scales smaller than a telescope’s spatial resolution isn’t possible, so point sources appear as pinpricks of light with no internal structure.

The spatial resolution of a telescope depends on the ratio of its aperture size to the wavelength of the radiation used (λ/D, where a smaller value is better). At a given wavelength, a larger telescope will have a better spatial resolution, but as wavelengths get larger, telescopes need to be larger to have the same spatial resolution. For example, radio telescopes commonly observed at a wavelength of 21 cm. To achieve the same spatial resolution at 21 cm as a visible-light telescope working at 600 nm, a radio telescope needs to be 350,000 times larger! Fortunately, as wavelengths get larger, telescope surfaces can be less precise. The shape of a telescope surface has to be smooth to a fraction of the wavelength of the radiation: visible-light telescopes must be made of very carefully polished material, but the surface of a radio telescope can be much rougher. This makes it feasible to build large radio dishes and is why the currently-largest telescopes are those that work at radio wavelengths.

Time plays a very important role in astronomy. In this connection, it has often been said that all astronomical observations are unique. This is because, even though two exposures may have been obtained with the same equipment and show the same sky field, they have not been made at the same time. Something may have changed in that field in the meantime. For instance, objects in the solar system move comparatively fast and an asteroid will leave a small trail during an exposure that lasts more than a few minutes. Moreover, many stars vary at a regular rate, others do so irregularly. Some stars brighten very considerably during an enormous outburst and other stars have been seen to fade slowly into obscurity. Thus, in order to fully document an astronomical observation, the time must be indicated, normally at the start and the end of the exposure. It can also be the beginning and the length of the exposure. Usually, the time is given in Universal Time (UT), in hours, minutes and seconds. An example of the time indication for an observation is, therefore: Start of exposure: 2019 October 12 06h 05m 30s; Duration of exposure: 180 seconds.

Most astronomical images are recorded through special optical filters that isolate particular regions of the electromagnetic spectrum. In the visible region of the spectrum (approximately 400 - 800 nm), they correspond to the well-known colors. Some filters are wide, meaning that light of a wide wavelength range (several different colors) may pass through, others are quite narrow, thus isolating the light emitted by particular atoms and molecules. A picture of a nebula obtained through a narrow filter may, therefore, record only the light from one particular type of atom and in this way show the distribution of such atoms within the nebula. If more filters are used, the distributions of several species may be studied and intercompared. Any astronomical observation must therefore also be described in terms of the wavelength of the light which is recorded. For instance, it is common to use the so-called wide standard B, V and R filters which isolate blue, green-yellow and red light, respectively. The corresponding wavelength regions are approx. 390 - 480 nm, 500 - 580 nm and 610 - 700 nm. Another example is a narrow, red filter that only transmits light near 656 nm, which is at a wavelength at which hydrogen atoms emit strongly. Images obtained through such an H-alpha filter will, therefore, show the distribution of hydrogen atoms in the field observed.

Astronomers observe a wide range of astronomical sources, including high-redshift galaxies, AGNs, the afterglow from the Big Bang and many different types of stars and protostars. A variety of data can be observed for each object. The position coordinates locate the object on the sky using the techniques of spherical astronomy, and the magnitude determines its brightness as seen from the Earth. The relative brightness in different parts of the spectrum yields information about the temperature and physics of the object. Photographs of the spectra allow the chemistry of the object to be examined. Parallax shifts of a star against the background can be used to determine the distance, out to a limit imposed by the resolution of the instrument. The radial velocity of the star and changes in its position over time (proper motion) can be used to measure its velocity relative to the Sun. Variations in the brightness of the star give evidence of instabilities in the star's atmosphere, or else the presence of an occulting companion. The orbits of binary stars can be used to measure the relative masses of each companion, or the total mass of the system. Spectroscopic binaries can be found by observing Doppler shifts in the spectrum of the star and its close companion. Stars of identical masses that formed at the same time and under similar conditions typically have nearly identical observed properties. Observing a mass of closely associated stars, such as in a globular cluster, allows data to be assembled about the distribution of stellar types. These tables can then be used to infer the age of the association.

For distant galaxies and AGNs, observations are made of the overall shape and properties of the galaxy, as well as the groupings where they are found. Observations of certain types of variable stars and supernovae of known luminosity, called standard candles, in other galaxies, allows the inference of the distance to the host galaxy. The expansion of space causes the spectra of these galaxies to be shifted, depending on the distance, and modified by the Doppler effect of the galaxy's radial velocity. Both the size of the galaxy and its redshift can be used to infer something about the distance of the galaxy. Observations of large numbers of galaxies are referred to as redshift surveys and are used to model the evolution of galaxy forms.