Astronomy Online

Astronomical Observations: General Instructions


Welcome to the select club of astronomical observers! We hope you will enjoy reading this section of Astronomy On-Line and that you will find it useful for you and your group!

By entering the General Instructions area, you have indicated that you are interested in knowing more about how to perform observations with an astronomical telescope. Perhaps you already have a well specified idea about what you want to observe or maybe you are just curious about how this is done?

In both cases you will find the information on this page useful. It contains basic facts about the scientific process, and helps you to prepare an Observing Proposal, much as it is done by professional researchers who are active in the fields of astronomy and astrophysics.

It is not difficult to understand the main lines of this procedure, as they are described below. We expect that these guidelines will be sufficient to allow you to prepare a proposal for observations and thus to carry out your own astronomical programme. We believe that it will give you and all members of your group the feeling of `real' observational astronomy and at the same time much pleasure! Nevertheless, you will not be surprised to learn that some simplifications have been introduced into the present procedure, as compared to genuinely professional studies, since those have features which can only be fully understood by professionals or advanced amateur astronomers.

Your astronomy background

In order to make use of this information in this area, we expect that you have some very basic knowledge about astronomy. For instance, we assume that you know what the term magnitude (brightness) means and that you are able to use this to calculate the brightness difference between two objects when their magnitudes are indicated. You are also familiar with the equatorial co-ordinate system in the sky (Right Ascension and Declination; just like longitude and latitude on the Earth) and you have at least a basic knowledge about the wavelengths of electromagnetic radiation (the colours of the spectrum).

If you do not know some of these terms, we suggest that you consult your teacher or an introductory astronomy textbook.

Amateur astronomers who have performed observations themselves will most certainly be familiar with this and should have no problems in following the present guidelines.

A few words about the scientific process

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 a 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.

This process is typical for the basic sciences which study our surroundings. Astronomy is one of these, but it is somewhat special because most of the objects studied by astronomers are so far away that we cannot possibly expect to visit them in any foreseeable future. Thus, observations made at a distance (referred to as remote observations) play an extremely important role in our science. Most of the knowledge we now have about the enormous space which surrounds us and which you may read about in astronomy books, has been obtained through patient and painstaking observational programmes by many generations of astronomers.

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.

Astronomy and the development of technology

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. 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 pentabytes of data (1 pentabyte = 1000 terabytes = 1,000,000 gigabytes)!

About two decades ago, in the second half of the 1970's, new, high-performance digital detectors, like the Charge-Coupled Devices (CCD's) became available. And soon astronomers will be able to use new giant telescopes like the ESO Very Large Telescope for their investigations. By means of the new technology, they will be able to look further out in space and further back in time than ever before!

Astronomical observations

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 is 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 important parameters that define an optical astronomical observation, like those that will be performed during the programmes proposed by the Astronomy On-Line participating groups, are the position in the sky, the time of the observation, and the wavelength of the light that is recorded.


The position indicates in which direction the observation is made. If we think of a normal CCD frame, the position will indicate the celestial co-ordinates, that is Right Ascension (R.A.) and Declination (Decl.), of the centre of the recorded sky field. This is normally also the position of the object of interest, but there may of course be several such objects which can be observed at the same time, if their positions are near each other.

In order to make sure that the observer can point the telescope accurately for your planned observations, you must give the position with an accuracy to at least one-tenth of a minute in R.A. (this corresponds to 90 arcseconds at the equator) and one arcminute in Decl. (60 arcseconds), and preferably better. Remember that the field size of most of the available CCD cameras is only some arcminutes! Objects which subtend a larger angle in the sky cannot be observed in one single exposure; it may then be necessary to obtain a series of images which cover the complete field of the objects.

In this connection, it is important to remember that due to the slow precession of the Earth's rotation axis (it makes one turn per 26,000 years), a position must always be accompanied by the epoch (this word means time in astronomy) to which it belongs. Professional astronomers nowadays most often use the year 2000 for this, meaning that the co-ordinate system refers to the direction of the Earth's rotational axis on January 1, 2000. Thus, when you submit the positions of the object(s) you want to observe, please try to specify them in this co-ordinate system. This is indicated by adding J2000 to the co-ordinates. For instance, the good-precision co-ordinates of the bright star Alpha Canis Minoris (Sirius) would then be given as: R.A. = 6 hours 45 min 9 sec; Decl. = -16 deg 42.7 min (J2000.0). However, if you are not able to do so, you may also submit the co-ordinates in the current system (J1996.5). That is normally the position which will be given in the tables in your almanac or sky calendar. In any case, just be sure to state the system year!


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 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 which 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: 1996 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 colours. Some filters are wide, meaning that light of a wide wavelength range (several different colours) 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 which only transmits light near 656 nm, that 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.

The telescopes and instruments

In order to prepare your Observing Proposal, it is important that you know about the technical capabilities of the professional telescopes and instruments which are available for you. Here, the word telescope refers to the optical system (mirrors and lenses) that focuses the light from the sky and the instrument is the device (camera or spectrograph) in the telescope's focal plane that records the image.

The telescopes are located at various observatories and they are all different. So are their instruments, but most differences are not so important for the types of observations considered here.

All instruments are equipped with CCD cameras, but the size of the sky field recorded in one exposure and the scale will be different from telescope to telescope. Normally, a larger telescope will be able to record fainter objects during the same exposure time, but this is not always the case. The magnitude of the faintest objects recorded (referred to as the limiting magnitude) will also depend on the focal length of the telescope, the sensitivity of the detector, the filter used and several other factors.

Professional astronomers refer to images (exposures) obtained with CCD cameras as frames. In astronomical terminology, a frame is therefore just the image of a sky field.

Thus, it is recommended that you read carefully the information about the individual telescopes and instruments available on the corresponding Web-pages. You may enter this area from here. Only thereafter will you be able to select the filters you want and to estimate the exposure times needed for your project and indicate them on your Observing Proposal.

Note that some of these Web-pages contain a lot of other information, some of which may be a bit complicated. However, you should have no problems in understanding the main features and capabilities of the telescopes.

From these pages, there are numerous links by which you may reach other interesting information about the individual observatories and the scientists and engineers who work there. Some of them also display nice images obtained with the telescope, further helping you to decide about your own observations.

Defining your Observations

Now you are ready to prepare your own Observing Proposal!

You start out by deciding which object(s) you would like to observe. What are their positions in the sky and how bright are they, expressed in magnitudes? Which filters do you want to use, in order to record the light of particular wavelengths from these objects?

How many exposures shall be made with each filter? How long do you estimate that the exposure time must be for the object to be well recorded? Would you prefer to obtain several exposures of one object, or rather one exposure each of several objects?

Once you have decided this, it is easy to write down a list of exposures, giving for each entry the position (Right Ascension, Declination; preferably in the J2000 system), the filter to be used and the exposure time.

Remember that not all objects in the sky can be observed at any given time! A telescope can only be pointed towards an object, if it is well situated in the sky and not too close to the horizon! You must therefore also give some consideration to this question and for instance compare the position of your objects with a skymap to be sure that they are observable in the month of November. In this connection, the Declination of your object(s) also plays a role; while northern objects (Declination is positive) may best be studied with the telescopes in Europe, objects in the southern sky may be easier to reach from the ESO observatory at La Silla in Chile.

Do not forget, that if your chosen object is moving (for instance a comet), it is necessary to write down the exact time that corresponds to the indicated position in the sky. A list of such positions at successive times is called an ephemeris (plural: ephemerides) and tells the observer where to point the telescope at a given moment, in order to have the moving object within the field. Note that the Software Shop will contain useful information about the calculation of ephemerides.

Do not forget that many other groups will also wish to use the telescopes. For this reason, do not ask for more observing time than you really need. Moreover, it is easier to fit smaller programmes into the schedule, so they normally have a larger chance of being carried out than very large ones!

This is the basic information needed by the observer and which must in any case be included in your Observing Proposal.

What is the `Scientific Rationale'?

When professional astronomers request observing time at the large telescopes of the world, they must always explain why they wish to perform the observations described in their proposal. This justification is referred to as the Scientific Rationale. It simply means that there must be some sensible reason why these observations should be carried out. In practice, this takes the form of a written explanation of what the observations are expected to achieve, in terms of new knowledge and discoveries.

This is particularly important, since most professional telescopes are oversubscribed, indicating that more observing time is requested than is actually available. For this reason, professional observing proposals are almost always evaluated by an Observing Programmes Committee, which select the most promising, that is, most likely to result in new knowledge.

Thus, when professional astronomers compete for time at a large telescope, they go to great lengths to explain why their proposed observations are both timely and promising, in the hope that they may obtain a share of the sparse observing time!

In your case, you may have different reasons for requesting observations. Not all Observing Programmes within the Astronomy On-Line Programme are expected to lead to entirely new knowledge, although it is of course possible that unexpected discoveries will be made. But we would still like you to state why you want these observations to be performed. This will also help the observers at the telescopes to carry them out in the best possible way.

For instance, your Observing Programme may concern new images of an object you are particularly interested in; this may be an asteroid, a comet, a star cluster, an interstellar nebula, a galaxy or a cluster of galaxies, a quasar, etc. Maybe you would just like to own an image which was made specifically for you and your group with a large astronomical telescope? Perhaps you intend to exhibit this picture in your school or your club? Or you are an amateur astronomer who wants more observations of an object you are already observing yourself with your own telescope? Maybe you would like to see how a particular object appears when imaged in different colours in order to determine its temperature? Or you would like to combine such images in order to obtain a true colour image of the object?

There may be many reasons that you want an observation to be performed. In your Observing Proposal you must state these reasons, so that the astronomers who do the observations are aware of the aim of your observational programme.

Once you have written all this down, you must submit your proposal to the Astronomy On-Line Observing Programmes Committee. This is a group of a few astronomers and educators who will read your proposal. They will make sure that it is technically feasible and also that all the necessary information is available, so that the observer will be able to carry out your proposed observations with the telescope.

For this you have to use the Form for Submission of Observing Programme which is available on the corresponding Web page. Please be sure to read carefully through your proposal before you submit it. You obviously would not wish the telescope to be pointed in the wrong direction, just because the co-ordinates you have indicated are wrong!

List of Observing Programmes

Once your Observing Proposal has been accepted by the Committee, it will be announced on the Web in the corresponding List of Accepted Observing Programmes. This is where you will be able to follow closely what is going on. Here you will also be able to see which Proposals have been submitted by the other groups.

Please note, however, that the inclusion of your Observing Programme in this list is still not a water-proof guarantee that it will also be carried out. Even though a considerable number of professional telescopes will be at the disposal of Astronomy On-Line participants for a substantial, total number of nights and hours, it is possible that the weather will be less co-operative. There is, of course, also the possibility, that the total number of exposures requested would demand more time than what is available (oversubscription!). In that case a selection will have to be made. Participants should remain assured, however, that the organisers and the participating observatories and the observers will do their utmost to ensure that the largest possible number of accepted observing programmes will be carried out, at least partially.

Project status (November 5, 1996)

The remaining areas of the Astronomical Observations' Shop will open later, when all observatories have provided the information on the Telescopes and Instruments pages needed for the preparation of Observing Proposals by the groups.

We also expect various software for the handling of the CCD image files (frames) to become available. However, already now, Participating Groups may receive on request and free-of-charge, a CD-ROM with the complete ESO Munich Image Data Analysis System (ESO MIDAS). This system is has been developed at ESO and is used by professional astronomers for the computer treatment of their observational data. It also runs on a (powerful) PC (486 or higher). You may wish to consult the MIDAS Webpage for more information about this system and the computer requirements before ordering it by sending an email to E. Voelk at ESO.

Here will later also appear detailed information about the requirements for the report about your observing programme which you are expected to submit at the end of the project and which will be published in the Astronomy On-Line Newspaper.


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