France

France has always been a centre of excellence for double star studies. In the last century observers such as Robert Jonckheere and Paul Muller were very active observers and discoverers. The latter also developed the doubleimage micrometer. The leading amateur was Paul Baize who was not only a prodigious observer but also computed orbits, many of which remain in the catalogue today. Antoine Labeyrie developed speckle interferome-try which has had a profound effect on the observation of very close visual binaries and which has allowed large telescopes to be used to their full resolution capability.

For the present generation, the leading professional figure is undoubtedly Paul Couteau (Figure 20.2) with more than 2700 discoveries to his credit and 25,500 measures. Dr Couteau has spent a great deal of his career at the Observatory of Nice where today double star research still continues.

Figure 20.2. Dr Paul Couteau (right) with Bob Argyle at Santiago de Compostela in August 1996 (Angela Argyle).

Figure 20.2. Dr Paul Couteau (right) with Bob Argyle at Santiago de Compostela in August 1996 (Angela Argyle).

Monture Quatoriale Observatoire

Figure 20.3. The

50-cm refractor at Nice

Figure 20.3. The

50-cm refractor at Nice

Under the auspices of the Commission des Etoiles Doubles of the Société Astronomique de France, a team composed of Guy Morlet, Maurice Salaman and René Gili has for some years now been taking advantage of the capabilities of the CCD imaging technique using the 50- and 76-cm refractors at Nice Observatory (Figures 20.3 and 20.4).

Whilst the 17.89 m focal length of the 76-cm refractor did not require any change, the 7.50 m focal length of the 50-cm refractor has been brought to 15.50 m using a 2x Barlow lens (Clavé). The CCD camera presently in use is a French LE2IM, a Hi-SIS 23 with a Kodak matrix KAF 401E (758 x 512 square pixels of 9 |im).

The imaging software is either QMIPS 32 or QMIPS. Short exposures of 1 s down to 0.02 s are taken. For

Figure 20.4. The plate at the back of the refractor can be shifted in the focal plane. It supports both the CCD camera and the eyepiece used for visual control of the field (Courtesy R. Gili).

Figure 20.4. The plate at the back of the refractor can be shifted in the focal plane. It supports both the CCD camera and the eyepiece used for visual control of the field (Courtesy R. Gili).

every pair, 200 images or so are currently saved on the hard disk of a portable computer.

Observations are later reduced after the 10 or 15 best images have been selected and composited (i.e. shifted and added) using MIPS. The measurement of composite images is achieved using specific software for determining the position angle, angular separation and magnitude differences.

From 1997 to 2000, seven observing sessions have been conducted at Nice Observatory and the team measured some 300 different pairs down to 0'.'4 with the 50-cm refractor and to 0'.'3 with the 76-cm refractor, demonstrating that the CCD imaging technique fits the needs of double star measurement well, giving very reliable results and allowing the best use of observing time.

Jean-Claude Thorel (Figure 20.5) is one of the leading visual observers in France today. His interest in

Figure 20.5. JeanClaude Thorel in his office at Nice. (Courtesy J.-C. Thorel)

astronomy started during a childhood illness when he was kept in isolation and his father brought him a book on astronomy to pass the time. It was some 15 years later that the interest in astronomy returned and he bought a 60-mm refractor to use at his home in Villepreux, close to Versailles. This was followed by a 20-cm Schmidt-Cassegrain and his early interests included lunar and planetary drawing and deep-sky observation. His first serious work was comet observation, resulting in a published guide on to how to observe and draw them.

He then became involved in work to resolve some inconsistencies in double star catalogues during the construction of the Hipparcos Input Catalogue. This involved two trips to use the 1-metre telescope at Pic du Midi in 1986 and 1987. This expanded into a general programme to measure neglected and problem pairs in the double star catalogues using the 50-cm and 76-cm refractors at Nice. He has recently been working on a programme of checking the double stars discovered by the Tycho mission on the Hipparcos satellite, some 4800 of which are visible from Nice. This had meant travelling from Villepreux to Nice three or four times a year, a return trip of 2,000 km but his job now means that he is able to live in Nice and take advantage of the proximity of the telescopes there.

He has made 6000 micrometric mean measures with the refractors at Nice, including four new pairs (JCT1-4) and has also published a biography of Robert Jonckheere amongst other works.

Figure 20.6. The

205-mm reflector used by Jean-François Courtot in Chaumont, north-east France

Figure 20.6. The

205-mm reflector used by Jean-François Courtot in Chaumont, north-east France

Images Nice France Telescope

Meanwhile in North-East France, Jean-François Courtot has been engaged in double star research since 1993 but he has been interested in astronomy from youth. He uses a homemade 205-mm Newtonian from Chaumont (Figures 20.6 and 20.7).

For wide pairs, a chronometric method, the transit method, is often used. The angular separation is derived from the time needed by components to successively cross the same thread because of diurnal motion. Each measurement consists of six alternate readings (±180°) of the position angle and 20 determinations of the transit time. The mean internal error for the position angle is usually ± 0°2 and ± 0'.'3 for the angular separation.

Figure 20.7. The

RETEL micrometer attached to the 205-mm reflector of Jean-François Courtot

For closer pairs, a filar micrometer has been installed to measure separations occasionally down to 0'.'66, the practical diffraction limit under good seeing with the 205-mm telescope. Each measurement consists also of six alternate readings of the position angle while three double-distance measures of separation are taken. For pairs close enough to be observed at the same glance under magnification x500 without darting rapidly from one star to the other, the filar micrometer allows the mean internal error to be kept typically within ±0?1 and ±0'.'03. This latter limit is the equivalent reading accuracy allowed by the screw constant and the overall focal length.

To compensate for various seeing conditions and more or less controllable errors, the measurement of a given double is usually repeated on 3 or 4 different evenings. For the closest pairs, bright components (V < 7.5) and stable seeing are needed. Wide pairs accommodate to fair conditions and can sometimes be measured down V = 10.

So far, some 3000 measurements of 800 different doubles have been completed, published and included in the WDS database, and a few of these pairs having never been observed before. Aside from observations of orbital and neglected systems, proper motions of optical pairs are checked using historic double star

Figure 20.7. The

RETEL micrometer attached to the 205-mm reflector of Jean-François Courtot

measurements as a start point and new determinations are proposed at times.

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