Can amateurs still contribute usefully to the development of astronomical
science ? Until the beginning of the XXth century, this question did not apply
and many amateurs had built or possessed instruments which were as powerful as
the ones used by professionals : noteworthy examples are William Herschel who
was a musician before he became a famous astronomer or Pons, the janitor of
the Marseilles Observatory, who, seeing his employers examine the sky night
after night, decided he would do the same and discovered 34 new comets ! In the
middle of the XIXth century, the world's largest telescope belonged to
Lord Rosse, an English nobleman, and it was superseded in 1918 only when the Mount
Wilson 100-inch telescope was commissioned.
Nowadays, modern techniques have so much improved that it seems difficult for
amateurs to contribute to the progress of astronomy. There remain however a few
fields in which their contribution can still be very helpful, notably in the
study of variable stars. In an 1880 report of the Advisory Committee on
Provincial Astronomical Observatories addressed to the Ministry of Education,
Mr. Loewy, its reporter, wrote :
" Through the analysis of the characteristics and the length of the period of such wonderful variations of brightness, very probable conclusions can be reached as to the physical constitution of these celestial bodies, their rotation period, their distance from the Earth, the stage of their stellar evolution and many other questions of the same order. "
Since this report was published, the interest of astronomers for variable stars
has kept on growing. The number of catalogued variable stars is now well over
40,000. This number is so high and the characteristics of these stars so varied
that it is impossible for professional astronomers to monitor and study all of
them. As early as 1916, Michel Luizet (1866-1918), France's most famous variable
star observer, wrote :
" Since every year new variable stars are added to the already quite long list of stars recognized as variable, the number of observers, and therefore, the number of observations is obviously insufficient. And yet, no other astronomical work is easier to tackle than the study of these stars! Many variable stars can indeed be observed with the unaided eye ; [...] with a small refractor or a good pair of binoculars, the field of study widens. So simple an equipment completed by a celestial atlas is obviously available to most amateur astronomers. With this equipment and some willingness, method and regularity, an observer can certainly do an excellent and useful work in this branch of astronomy.
Using the observations of the amateurs
It is by using simple methods of visual photometry that American, German,
British, Russian, Japanese and also French observers have made our knowledge
on variable stars progress. Even the best known variable as Mira Ceti or SS
Cygni (every eruption of which has been followed since 1896) must be permanently
monitored because their light curve often shows an anomalous behaviour or
unexpected accidents that are always related to a modification of their spectrum.
Thus, a team of radio-astronomers at Nan‡ay has used the observations of some
mira-type stars made by AFOEV observers to prove the existence of a correlation
with a time-lag between optical and radio variations. Many novae have been
discovered by amateurs, sometimes before they had reached their maximum
brightness ; immediately informed, professional astronomers have thus been able
to observe them by spectrography and the spectra thus obtained have allowed the
nature of these objects to be better known. Moreover, orbital telescope time
allotment is limited and the contribution of amateurs has become essential
to "locate" the star on its light curve at the very moment when the
professional astronomer has the telescope at his disposal.
Associations of amateurs
The beginner in the observation of variable stars, often equipped with a small
instrument and whose leisure time may not allow him to collect a dense dataset
of observations may ask himself whether a few random observations made during
in the course of a week-end off or through a clearing in the cloud cover are
of any value at all. Andr, Danjon wrote :
" In astronomy, every observation has its own value. A well-made observation will one day or another find its use and an observation that failed leaves an irreparable gap "
The scientific interest of the observation of variable stars is therefore not
negligible and it would be a pity if the observations of an amateur were kept
locked in a drawer. However, publishing his own observations and putting them
at the specialists' disposal is obviously very difficult for an amateur. This
problem finds its solution in the regrouping of amateurs in an association that
publish the observations of its members and make them available to professionals.
An isolated observation, apparently of no use, becomes one of the links of a
chain forged by the whole group. Today, the observations collected by the AFOEV
are computer-processed and stored in digital form at the "Centre de
Données Astronomique" (Center for Astronomical Data) of the Strasbourg
Observatory where they can be freely used by the professional astronomers of
the whole world (and they don't refrain from using this possibility).
Selecting the stars to be observed
Team work entails some discipline. Although the programme of the association is
by no means irremediably frozen and new variable stars are added to the initial
list, it is necessary to set some limits. The AFOEV programme includes mostly
long-period variables (mirae and semi-regulars). The amplitude of variation of
most of these stars is large enough for individual errors to have a lesser
relative importance. Thus, it is in the interest of the beginner who makes his
own programme to select stars that are well followed by other observers ; he will
thus have the possibility of comparing his observations with those of his
fellow observers and thus evaluate the acccuracy of his estimates. Cataclysmic
variables are also well observed at the AFOEV : however, these stars are rather
faint and reserved to trained observers.
During his probation period, the beginner will observe the same stars as often
as possible so as to get more and more familiar with their field. Then he will
progressively make his observations less frequent - long-period variables should
be observed once every 8 or 10 days, which will allow him to increase rapidly
the volume of his programme.
The AFOEV programme includes practically no Algol-type variables and no cepheids
as the observation of this type of stars requires an availability which is not
compatible with the various obligations, mostly professional, of an amateur. As
a matter of fact, for this type of variable, an isolated observation is of
practically no value and only complete series of observations covering a whole
night or at least a great part of the night will give an observer the
opportunity to monitor the eclipse of an Algol-type binary or the maximum
brightness of an RR-Lyrae variable.
First of all, the unaided eye allows studying several bright variables but,
notwithstanding the fact that the method offers limited possibilities, it
entails several risks of errors, mostly because the comparison stars are not
necessarily in the vicinity of the variable. A pair of binoculars is the best
instrument to get trained in the study of variable stars as its wide field
facilitates the identification of the asterisms around the variable, thus
allowing an accurate localization of the star. But in this case also, one has
to be extremely careful if the comparison stars are too distant from the
variable. Whenever the choice is possible, a reflector is preferable to a
refractor. A refractor's drawback is its long focus with, as a consequence, a
limited field compared with a reflector whose extra advantage is, diameter for
diameter, a cheaper price and the possiblity of making it by oneself.
If for the larger instruments, an equatorial mounting fitted with accurate
setting circles or, nowadays, computer-driven makes pointing at a faint
variable easier, an altazimuth mounting will be easier to manage. The
instrument must however be fitted with a good finder with which one can find
the variable by hopping from star to star.
Star charts can be supplied at cost price to AFOEV members. The original
tracings were drawn by Antoine Brun, the founder of AFOEV. As they became
tarnished and brittle with time, they were redrawn and several new charts were
added. These photocopied 13x18cm (5"x7") charts are used to locate the variable
and include the sequence of comparison stars with their identification letter
and their magnitude given to one decimal place. The comparison stars are
indentified by letters (a, b, c, d,...) and their visual magnitude is given as
an integer, for instance 114 for visual magnitude 11,4 as a decimal point may
be mistaken for a star. Indices, for instance c1 , mean that the star was
added to the sequence between c and d.
These charts give all the necessary information on the star designation, its
position for equinoxes 1900, 1950 and 2000, the value of its annual precession
in right ascension and declination, its type of variability, its mean
brightness at maximum and minimum, its period and its spectrum.
For a given variable, the number of available charts depends on its brightness
at minimum light. Usually, there are four overlapping charts A, B, C,and D.
When they join the association, the observers are provided with an updated
catalogue of the available charts.
Searching for the variable and identifying it
For a beginner, searching for the variable and identifying it is the most
delicate and most tedious part of the job, though as an amateur astronomer he
will certainly be familiar with locating celestial objects. It is highly
advisable to begin with "conveniently-situated" variables i.e. stars situated
in the vicinity of easily identifiable fairly bright stars. This search is the
only serious difficulty that a beginner may experience, and with some practice
and training, both easily obtained, even "difficult" stars will be quickly
found and when one year later the celestial round brings a given constellation
back, he will be surprised and delighted to find almost at first glance a
star which he had observed a year before.
If the instrument is on an equatorial mounting, the variable and its field will
be easily found either with the help of setting circles or of a computerized
object locator. In the case of an altazimuth mounting, it will be necessary
first to identify the region of the variable with the help of chart A. Then,
using the finder and starting from a known bright star, the observer will
go star-hopping to the brightest star in chart B. Then with a low-magnification
ocular giving the widest field and making use of star alignments or remarkable
patterns of his own choice, the observer will finally come to the variable's
field. This is done step by step, using if necessary the chart showing the
faintest stars and more and more powerful oculars. Once the field has been
clearly identified, the observer checks whether the variable is visible and
if so, proceeds to an estimate of its brightness.
During this search, observers should avoid going to the extreme limit of
tiredness. If the search proves to be too difficult, it is preferable to
temporarily give up, have a rest and try again at the next observing session.
Estimating the brightness of the variable
Except when using a photometer, a visual observer does not "measure" the
brightness of the star. The observation consists in estimating its brightness
by comparing it with nearby non-variable stars whose brightness has been
measured with a photometer or more recently by photoelectric photometry or
Once the variable has been located and the comparison stars identified, the
observer looks for the star which appears to be just a little brighter than
the variable and the star which seems to him just a little fainter. Let "c" be
the star slightly brighter than the variable (designated as "v") and "d" the
star which appears slightly fainter. Estimating the brightness of the variable
consists in determining by how many steps (hereinafter, "degrees") "c" differs
in brightness from the variable on the one hand, and "v" from "d" on the other
hand, the brighter star always coming first. The idea of "degree" was invented
by Herschel and refined around the midddle of the XIXth century by Argelander,
the author of the famous Bonn catalogue (BD). He defined them thus :
Having compared the variable "v" with the brighter star "c", the same method
is applied to the comparison between "v" and "d", the fainter star, so as
to obtain finally an expression such as, for instance :
The estimated magnitude of the variable is then derived by applying the
where x and y are the number of "degrees" between "c" and "v" and "v" and "d"
respectively. The operation finally amounts to a mere rule of three. Let us
assume values 79 and 85 (i.e. magnitudes 7.9 and 8.5) for stars "c" and "d"
respectively. 3+2 = 5 "degrees" are equal to a difference of 8.5 - 7.9 = 0.6
magnitudes. In our example, the value of the "degree" is 0.6/5 = 0.12
magnitudes whence the concluded magnitude for "v":
The result of this arithmetical operation must be rounded to the nearest tenth
of magnitude, that is 8.3 in our example. Writing 8.26 entails a risk for the
beginner to believe that visual estimates enable him to reach an accuracy of
one hundedth of magnitude. Such an accuracy is obtained by photoelectric or
CCD photometry at the cost of complicated and difficult procedures.
To get even more certain and check the first estimate, it is advisable, if the sequence allows it , to compare "v" with more than two stars, for instance : assuming c = 79, d = 85 and e = 88,
When it is impossible to observe the slightest difference between the two stars
and after going several times from the variable to the comparison star and
conversely, it seems that each star appears alternatingly brighter, they are
said to be of equal brightness and the observer will write :
Another method of visual estimate is Pogson's fractional method, derived from
Argelander's method. The magnitude difference between the comparison stars
whose brightness frame the variable's brightness is divided into ten "degrees".
In our example, the estimate would be something like this :
Lastly, Pickering's method consists in dividing arbitrarily the difference in
magnitude between the two comparison stars into as many "degrees" as there are
tenths of magnitude, a "degree" being therefore equal to 0.1 magnitudes.
Taking the example above, the magnitude difference between "c" and "d" is 0.6
i.e. 6 "degrees". If for instance, the brightness of "v" appears to be halfway
between the two comparison stars but slightly closer to the faintest star,
the observer will write
Recording the observation
The observation is written down in a notebook or even a mere sheet of paper.
All the observations will be written down in chronological order. The observer
should mention the date on top of the page or of the series of observations and
for each observation : the variable's designation, the time of observation
(hour and minute ; the latter is important in the case of a cataclysmic
variable) and the detail of the comparison. A dim light, either an
under-powered bulb or a bulb screened by a red transparent paper will be used
to examine the charts and make note of the results which will be arranged like
|R Leo |21hr. 00 |1 v 2 m | |RX Boo |21hr. 30 |d 5 v 2 f | |TW Vir |21hr. 45 |< k | |R CrB |00hr. 37 |c 1 v 2 d | |etc. | ||
The estimates written down in the copy-book or on the sheet of paper will then be copied in an observation logbook containing the following indications :
For instance :
|21hrs.00 |0942+11 |R Leo |l 1 v 2 m |7.0 |II |2 |J80 |fairly good| |23hrs.00 |1419+26 |RX Boo |d 5 v 2 f |7.9 |II |2 |T200x80 |fairly good| |23hrs.25 |1140-03 |TW Vir |<k |<13.9|I |I |T200x160 |good |
Reporting the observations
Special report forms can be obtained from Emile Schweitzer, 16 rue de Plobsheim
F-67100 Strasbourg, the recorder of the AFOEV database. The observations,
however, may be sent on any medium provided that some conditions be respected.
They are meant to facilitate the editor's task and include the mention of the
observer's name and first name, the month and year, the total number of
observations in the month, the instruments used and the main site of
observation. The stars should be sorted by increasing right ascensions and the
observations relative to each star in chronogical order.
The reports are to be sent monthly to the bulletin's editor, as far as possible
during the first days of the following month. The observations are more and
more frequently sent by e-mail.
Accuracy of the observations - Possible causes of errors
The errors inherent in the methods of observations are impossible to avoid as
these methods are subjective. Errors can be accidental (misidentification of
the variable, for example) or systematic (arising either from the instrument
used or from the observer). The main causes are :
Photographic observation of variable stars
The photographic observation of variable stars is less and less practiced
although it has the advantage of offering an indisputable and impersonal evidence.
To be comparable with visual observations, photographic observations must
however respect particular spectral characteristics as regards the type of
film. Commercial photographic films should therefore be used with a filter.
Photoelectric and CCD observation of variable stars
The photoelectric observation of variable stars requires a fairly elaborate
equipment whose assembly requires a solid knowledge of electronics if a
reasonable cost is looked for. Moreover, it can be implemented with fairly
large aperture telescopes only. On the other hand, it can reach an acccuracy of
a few hundredths of magnitude and the observation of a variable in different
spectral bands is possible. It is mainly directed at the study of low
amplitude variables. CCD imagers are more and more easily available to
amateurs and allow observing faint variables.
The French Association of Variable Star Observers (AFOEV)
AFOEV is a registered association whose statutes were adopted in 1927 and
modified twice in 1973 and in 1986. Its headquarters are at the Strasbourg