Observation by amateurs has been traditionally viewed with some suspicion by the professional astronomical community, especially in the case of transient events(1). For example, data from variable star monitoring where information from many observers can be integrated over long intervals, have a higher weight than visual observations of an asteroid occultation. Serious amateurs who choose to contribute to the science face a continuing challenge to maintain the highest standards.

In an effort to upgrade the quality of information transmitted to professional institutions, we have experimented since 1984 with a baseline design of mobile video equipment which can be coupled to objective lenses ranging from 28mm to 625mm aperture (2).

Grazing occultations are always visible at the projections onto the surface of the Earth of the lines tangent to the lunar limb that are created by the motion of the Moon relative to the star being occulted.

Most information about the lunar profile continues to be provided by visual observers using simple equipment. Everyone is to be encouraged to join expeditions to observe grazes, because the accuracy of the observed profile is proportional to the number of stations. The use of grazes improves knowledge of the lunar profile (needed for the analysis of solar eclipse timings), data about close binaries, and galactic rotation from stellar reference-frame determination. Contact timings made near the edges of a solar eclipse track, for example, enable the diameter of the Sun to be derived relative to the lunar diameter, the latter being determined from occultations.

We present a new technique in double star photography leading to high accuracy measurements in amateur-size instruments. The trailing method has been improved by the use of a chopper that cuts the trail into a number of segments. The chopper period is accurately known, so it is the length of each segment on the plate. In this way the scale factor is measured in each exposure. Fine-grain films, such as hyper-sensitized Kodak 2415, are used and all measurements are performed at the microscope. The same technique may also be applied to CCD cameras. A statistical analysis of errors shows that Δθ ≤ 30′ and Δρ/ρ ≈ 0.01 for most binaries with ρ ≥ 3″. This method has been applied to professional-amateur joint programs dealing with multiple systems with variable components and a survey of poorly-observed wide binaries.

Astrophotography is only a small fraction of the broad spectrum of amateur astronomers’ work. With the development of high-sensitivity emulsions, amateur telescopes of large focal ratios and good quality, cheap multi-purpose cameras (both 35- and 70-mm), and intensive public relations work done by observatories as well as by individual, advanced amateurs, the methods and techniques of astrophotography have become accessible to all amateurs. With suitable equipment, experience, and familiarity with astronomical problems (e.g. by close contact with professional astronomers), they are even able to make valuable contributions to science.

The Dutch variable star section of the ‘Nederlandse Vereniging voor Weer-en Sterrenkunde’, founded in 1960, has about 10 to 15 regular observers. Depending on the weather conditions they perform 5 to 10 thousand observations a year. Our observation-recorder Henk Feijth collects the observations on paper and, since 1985, also on computer-cassette. From these contributions Feijth compiles a list on paper for the A.A.V.S.O. The cassette is written in a special format using the BASICODE program, see 2.

The reduction of the observations can be described as following:

1. 1. Every observer and computer-owner enters each observation as a DATA statement in a BASIC program. So each DATA statement is a record representing one observation, each DATA statement contains 20 characters:

2. 1- 6 Starname

3. 7-10 int(JD-2440000)

4. 11-13 fractional part (3 decimals)

5. 14: or <

6. 15-17 estimate without decimal point

7. 18-20 observer code

8. So a month of observations is simply a BASIC program consisting of just DATA statements.

9. 2. The Dutch broadcasting organization (NOS) has designed a kind of ‘Universal’ BASIC called BASICODE (version 2). With the aid of BASICODE computer X can read the programs of computer Y. The observer translates his data statements (the observations) into the BASICODE format. This is saved on cassette and sent to our recorder.

10. 3. Our recorder enters the monthly observations onto the BBC micro with the aid of the BASICODE translation program. Now the recorder runs a program that changes the star name into the Harvard number. This is done for the observations of every observer. After that the whole bunch of observations is sorted in ascending order of Harvard number and JD.

11. 4. The file of observations obtained in 3 is used as a source for our own reports, a graphic representation of the observations, and for simulating A.A.V.S.O. record sheets. This ‘end product’ we send to the A.A.V.S.O.

An iterative method of refining the orbital parameters of visual double stars was described. The sum of the least-square differences is minimized by the Levenberg-Marquardt method. The application to two examples was described, including one highly inclined orbit, ADS 8862 = Hussey 664 (i = 94.3 degrees).

A suite of programs written in BASIC for use on Apple (II, IIe, etc.) computers was described. A data file, extracted from the general catalogue by Marsden is incorporated. The programs are applicable to any geographical location and allow the determination of positions (in both celestial and horizontal coordinates), times of rising, transit and setting, and the calculation of ephemerides from orbital elements.

Equipment for digitizing 24 × 36 mm negatives was described. The sensing unit is a 1024-element CCD. Apart from the optical train, the transparency is held in a rigid carrier that is moved by a stepper motor to scan one row at a time. The whole operation is controlled by computer, and results in a 170.5 kb file for each image.

Who is an amateur-astronomer? We would certainly find several definitions if we tried to get answers from the audience. I am not trying to force anyone to accept my definition, in fact I do not know if I really have one. Someone who likes to read books about astronomy? Professional scientist, expert in other areas of science, but interested in astronomy? Science fiction writer who writes about space travel? Retired professional astronomer who is no longer paid for his work in astronomy? Constructor of telescopes or astronomical instruments? Well, each of these could be discussed. But we will rely, on this occasion, on common sense and take any one who is interested in astronomy and contributes to its progress either by observation or by construction of astronomical equipment and has not an official education in astronomy. He/she simply likes the idea that he/she is contributing by his/her work to our knowledge of the universe and feels proud of it.

The light-curves of many variables, prepared from a large number of individual estimates by many different observers, have a lot of scatter, ranges greater than one magnitude being common, although it is generally accepted that the eye can detect differences of roughly 0.1 magnitude, at least for stars of moderate brightness. The question arises whether these gross discrepancies can be reduced. We think that this is possible if observers make better use of their observational capabilities.

For certain types of variable stars, e.g. long-period ones, the lack of accuracy may often be balanced by the vast number of estimates available. But for other stars, especially those exhibiting small-amplitude fluctuations, this process of combining observations leads to broad light curves that completely hide any possible fluctuations. Variable-star observers are familiar with these problems, but it is desirable to try to improve the accuracy that can be obtained.

Stellar occultations are usually observed visually, but this can also be done photographically. Our method has some advantages:

1. – the obtained moments of contacts refer to the smoothed lunar limb,

2. – the error in the determination of contacts is calculable,

3. – the error may be less than in visual work,

4. – the reduction of photographs is fast, due to the application of an “on-line” computer [1].

In order to achieve all this, it is necessary to make a series of photographs of the Moon and the object being occulted before the first and after the last contact. This is a crucial step in our method, because the photographs must be of high quality. This demands two things:

1. – the images of the occulted object and the Moon must be reliably recorded and

2. – the illuminated limb of the Moon must give optimal darkening on the film.

A region of approximately 270 000 km2 near the south pole of the Moon has not been mapped by spacecraft and Dr John Westfall of ALPO proposed the “Luna Incognita” programme in 1972 to try to cover this area. A brief summary of the problems of observing this limb region was given, together with the author’s experience using the T60 and 1-m telescopes at Pic du Midi.

Occultations of stars by minor planets and comets can be used to determine the diameter of the occulting body. With photoelectric equipment, it might also be possible to measure the diameter of the occulted star or to resolve close binary systems at the same time.

On 1983 May 29, (2) Pallas occulted the 4.7-mag. star 1 Vulpeculae. A star this bright is occulted by one of the four largest asteroids only once every 60 years. Timings were sent by 130 observers in Florida, Louisiana, Texas, Arizona, Sonora, and Baja California, making it the best-observed asteroidal occultation to date. Asteroidal grazing phenomena were confirmed for the first time. The star is a spectroscopic binary. Several observers timed contacts of the 6th-mag. companion, determining the separation to ± 0.0004 arcsec. An elliptical fit to the timings projected onto the plane of the sky has been made. Unfortunately, clouds prevented timings from near the southern limit, so about 100° of Pallas’ circumference was not observed.

The 1986 Minor Planet Ephemerides call observers’ attention to the need for astrometric observations of 120 objects on the “critical list”. Their magnitudes are generally between 15 and 18 at opposition. The author’s programme of observation with simple means was described. The equipment used was a Newtonian 41-cm, f/4.8 reflector, a standard camera back, and Kodak TP2415 film, hypersensitized in forming gas. Out of 25 observations attempted, 20 photographs were obtained, 8 of which were of sufficient quality for the positions to be measured and to be published in Minor Planet Circulars. On two photographs, the minor planet (1982 KM) was detected, but there were no suitable reference stars within the field. (The lack of reference stars brighter than mag. 10 in certain regions should be noted.) The return of measured positions versus photographs obtained (40%) appears very acceptable.

Video recordings of two occultations were shown, the first concerning the star AGK3+29 0398 and (93) Minerva on 1982 November 22, 03:31 UT, using the 1-m telescope at Pic du Midi. The occultation was also observed in Spain and in the U.S.A. The second event involved AGK3+17 1309 and (146) Lucina on 1982 April 18, 20:23 UT, using the 1-m telescope at Meudon. A short secondary occultation (0.6 sec) was observed, not caused by the minor planet. A possible interpretation is a small satellite (5.7 km minimum diameter) near Lucina.

In 1985–86 when spaceprobes flew past P/Giacobini-Zinner and P/Halley, many amateurs observed these comets and submitted their results to national or international (IHW) organisations. Their visual, photographic, photometric and spectroscopic observations made a contribution to scientific knowledge. It is pertinent to ask if space programmes under consideration will allow similar cooperation to be continued.

The least uncertain project is that of another cometary fly-past by GIOTTO. Following orbital corrections made between 1986 March 19 and April 2, (ΔV =115 ms−1), the European probe should pass 22 300 km from the Earth on 1990 July 2. The gravitational assist from our planet should then put it onto an orbit to pass P/Hartley 2, P/du Toit-Hartley or P/Grigg-Skjellerup (1992 July 10). This last comet appears to be the most desirable target; although the final decision will not be taken until autumn 1989, recommendations have been made (Morley, 1986) for an international astrometric campaign.

Most professional astronomers do not conduct specific comet-discovery programmes and a new comet is, in general, an intrusion into an observational programme, although one that does, of course, receive all the attention it deserves. Comet discoveries are by-products of professional work. This may also be the case with some amateurs (G.E. Alcock discovered his fifth comet while looking for novae). Most amateur discoveries are the result of active and systematic work. Over the period 1951–70, on average 9 comets per year were discovered by amateurs.

The time spent scanning the sky before discovering a comet is typically hundreds of hours. The times of W. Bradfield between his first comets were 260, 306, 145 and 106 hours. Astronomers at Skalnaté Pleso discovered eight comets in six years, averaging one comet for 206 hours of searching. R.D. Austin and R. Meier spent 151 and 105 hours, respectively, between discoveries. These figures are only indicators, because many other factors (including luck) play a part. Alcock’s two comets in six days is quite exceptional.

Various methods of determining the rotation of P/Halley’s nucleus give 2 periods: 53 hours, and 7.4 days (1). Cometary light-curves are commonly a function of heliocentric magnitude versus the logarithm of heliocentric distance: the result is a set of scattered points. The average light-curve is calculated by the least-squares method. The method proposed here is a reduction of scatter by computation to derive the comet’s specific light variations. The factors causing scatter are: observer experience (physiological and psychological effects), instrumental factors, comet morphology, methods of comparison, and observing conditions. Recently Morris and Edberg (2) have shown that the first three of these were the most important factors. In fact, the prime necessity is to consider visual magnitude estimates as a statistical sample with variability caused by scattering factors. This effect is different for each sample of estimates. Mathematical correction will be applied only after a statistical analysis of the effect of the factors causing error. The method consists of four steps:

1. 1) Computation of heliocentric magnitude (mH) and study of the number of observations for each observer. Observers with less than 10 are omitted (because the least-squares method is not reliable in such a case).

2. 2) Correction of instrumental factors and observer effect. Computation by least-squares method is carried out between (mH) versus instrument diameter (D) and exit pupil (P). The diameter and exit-pupil correction is made only if the correlation is significant and not systematic. The correction for observer effect is achieved (for each observer) by least-square computation between mH and logarithm of heliocentric distance. The O–C magnitude is plotted on a graph and a preliminary curve drawn.

3. 3) Correction of bias: when a magnitude estimate is made on one night, an estimate the next night will tend to be equal because of psychological effects. It is possible to detect this effect by comparing the number of equal and different magnitude estimates (±0.1 mag) made at time-intervals of 24 hours, and over 24 hours. This comparison is made with the χ2 test. If the test is significant the biased estimates are omitted.

4. 4) Other factors (systematic error, photoelectric sequence of comparison stars) are detected by multi-factorial analysis and deleted.

This work was carried out by an expedition organized by the S.A.F. in 1986 April to La Réunion. The aim was to observe and photograph P/Halley, within the framework of IHW, as part of the Island Network in the southern hemisphere. To be more precise, our work consisted of studying large-scale phenomena: the structure, dynamics and possible disconnection events in the plasma tail. We were lucky enough to observe one of the latter on the night of April 11/12, and describe it here.

For the Island Network, IHW had a number of Schmidt telescopes (Celestron 8). One was lent to the S.A.F. and this is what we used. This telescope has a focal ratio of 1.5, with a 200-mm (8-inch) objective and 300-mm focal length. We used only Kodak TP2415 film, hypersensitized in forming gas (24h at 60°C).

During a photographic campaign on Comet P/Halley in 1986 April, a considerable number of photographs were obtained, especially on April 8, of a disconnection event in the plasma tail. Initial details were presented at the Symposium in Heidelberg at the end of 1986.

The photographs were obtained with a camera five examples of which were specially constructed (by Carron), and known as the K600. The objective is a 100-mm OG, focal length 600 mm and the image is corrected with a field lens. The camera body is a Hasselblad, taking 120 roll film, which was Agfa 1000.