THE ASTRONOMICAL OBSERVATION

The instruments for astronomical observations

The astronomy is the study of everything that is in the space that surrounds us . So we must observe, first to know what we see in this space. This has been done since the dawn of time. Astronomy has changed only a little during millennia. With the progress and development of astrophysics, the situation has radically changed. Astronomy is now trying to explain what we observe . Its working method is a deductive reasoning based on the measurement, like all other branches of physics. The result of these observations will always be ultimately, numbers. These numbers are then used to attempt to discover , through laws - formulas - already known, new relationships between physical quantities and thus leading to new conclusions to be confronted to new observations that confirm or , on the contrary , leading to abandon these conclusions.

This scenario, summarized here in a few lines , is actually a very complex and is still a matter of patience and perseverance . Its completion requires years or even decades. It is now always the result of the involvement of many astronomers working together.

The instrumental configuration.

Astronomy is multiple. There is not one astronomy but astronomies . This is also true from the instrumental point of view. In all cases , however , there is something that does not vary in the instrumental equipment , it is the combination of three materials , a collector, an analysis device and a detector.

1. The collector

The collector is, as its name suggests, an instrument designed to collect light . These are astronomical telescopes. More the lens or mirror is larger and more the amount of light received is large, which allows to see faint objects . The first of all collectors that ever existed on this planet is the human eye. This is because the pupil is very small that we tried (and succeeded through Galilee) to replace it with something else larger. We will see the different types of existing telescopes and the difference between refractors and reflectors.

2. The analysis device

It is that will provide to the astronomer the quantity to be measured . We can define four main types of devices :

imaging : the goal will be to collect an image of a piece of sky emphasizing: the spatial stability of the sensor ( be sure that the distances measured on the detector correspond to something real on the sky) and and the resolution , that is to say, the ability to distinguish fine details in the images collected. As stated in the chapter on the celestial sphere , we only observed angles or angular distances and the angular resolution of an analysis device is measured in angle : the higher the resolution , the more detailed is the image (we can measure small angular diameter). We will see that this resolution is limited by the telescope used ;
photometry : its purpose is to collect light in a range of given wavelength (selected by a filter) and measure its amount over time . Fixed stellar sources are taken as references ;
spectrography : it separates the received signal into its various components for each wavelength and allows to trace back to the nature of light radiation emitting body . We'll see how the principle of spectroscopy in astronomy is used ;
polarimetry : it uses the property of the light to be polarized after passing through a particular medium.

There are many devices useful to researchers , imagers ( CCD targets ) , photometers and spectrographs . Besides these standard devices , some very specific devices are developed , usually by teams who will use to perform a very specific observation. Examples include all photometers built for exceptional events such as occultations of stars by solar system objects. These fast photometers serve only for this type of observation . Standard or not, these devices are always complex and often expensive instruments because they must be very powerful , so working to the ultimate performance of its components.

3. The detector.

The detector is placed behind the analysis device . Photographic plates were the first modern detectors. It has rendered immense services and we now have with it a memory of astronomy that could one day be greater than that of information technology . Today, the electronic CCD receptor consists of a mosaic of photosensitive elements and is almost universally used.

This " trio ", collecting , analysing and detecting (and recording) , is found in all branches of astronomy. Let us briefly examine this point. We can roughly distinguish between:

- fundamental astronomy (astrometry): it is only interested in the position of the celestial bodies in the sky and in their motions so imaging is used. The main instrument of this branch was the refractor in the past and the reflector today associated with the eye (in the past) or photographic plate (during the XXth century) or CCD camera ( today ) . Note that measuring the radial velocity of stars is practiced using a spectrograph but is related to astrometry.

- astrophysics are interested in composition , evolution, formation, life and death of the physical bodies of the solar system and of stellar objects. The goal is also to understand the ohysics and internal structure of objects through theories. For that spectroscopy and polarimetry are extensively used.

The collector is always the telescope but the analysis that associates devices are much more numerous because the instrumental investigation methods are very diverse. These are cameras for imaging, spectrographs for spectroscopy , photometers and bolometers for measuring stellar fluxes , polarimeters for the study of polarized light , etc. Today, the detector is most often a CCD target.

We have discussed so far the analysis of only the electromagnetic radiation of the optical field (that is to say, the wavelength of which is between 10 nanometers and 10 micrometers ) which includes ultraviolet, visible (from 400 to 800 nm ) and infrared. Telescopes used for other wavelengths will be very different than telescopes used in optics . For examples, radio astronomy, whose purpose is to also astrophysical studies, but differs mainly by the wavelength domain it seeks to explore. Its instruments fall within the radio field (centimeter and millimeter ) and the collector is of a different type . Taking advantage of the long wavelength of the field compared to the optical domain , we make the economy of a polished mirror , adopting in its place, a simple grid with a cell of about size of the wavelength . Regarding the X-ray astronomy and gamma-ray astronomy , all the instrumental chain had to be rethought.

Credit : L. Vapillon/observatoire de Paris

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