A&A 371, 343-349 (2001)
DOI: 10.1051/0004-6361:20010321

The PHESAT95 catalogue of observations of the mutual events of the Saturnian satellites[*]

W. Thuillot1 - J.-E. Arlot1 - C. Ruatti1 - J. Berthier1 - C. Blanco2 - F. Colas1 - W. Czech3 - M. Damani4 - V. D'Ambrosio2 - P. Descamps1 - G. Dourneau5 - N. Emelianov6 - S. Foglia4 - G. Helmer7 - T. R. Irsmambetova8 - N. James9 - P. Laques10 - J. Lecacheux11 - J.-F. Le Campion5 - C. Ledoux12 - J.-C. Le Floch13 - G. Oprescu14 - M. Rapaport5 - R. Riccioli2 - B. Starosta15 - V. G. Tejfel16 - E. M. Trunkovsky6 - B. Viateau5 - C. H. Veiga17 - D. T. Vu1

1 - Institut de Mécanique Céleste et de Calcul des Éphémérides, Observatoire de Paris, UMR 8028 du CNRS, 77 avenue Denfert-Rochereau, 75014 Paris, France
2 - Instituto di Astronomia dell'Universit… di Catania, Catania, Italy
3 - Stuttgart, Germany
4 - Osservatorio S. Zani, Italy
5 - Observatoire de Bordeaux, France
6 - Sternberg Astronomical Institute, Moscow, Russia
7 - Observatoire de la Côte d'Azur, France
8 - Crimean laboratory of the Sternberg Astronomical Institute, Moscow, Russia
9 - Chelmsford, UK
10 - Pic du Midi observatory/Midi Pyrénées Observatory, France
11 - DESPA, Observatoire de Paris, France
12 - European Southern Observatory, La Silla, Chile
13 - Clichy, France
14 - Institutul Astronomic, Bucuresti, Romania
15 - Fan Mt. Obs. of the Univ. of Virginia, Charlottesville, USA
16 - Fesenkov Astrophysical Institute, Almaty, Kazakhstan
17 - Observatorio Nacional, Rio de Janeiro, Brasil

Received 14 September 2000 / Accepted 26 January 2001

In 1994-1996 the Sun and the Earth passed through the equatorial plane of Saturn and therefore through the orbital planes of its main satellites. During this period, phenomena involving seven of these satellites were observed. Light curves of eclipses by Saturn and of mutual eclipses and occultations were recorded by the observers of the international campaign PHESAT95 organized by the Institut de mécanique céleste, Paris, France. Herein, we report 66 observations of 43 mutual events from 16 sites. For each observation, information is given about the telescope, the receptor, the site and the observational conditions. This paper gathers together all these data and gives a first estimate of the precision providing accurate astrometric data useful for the development of dynamical models.

Key words: Saturn - satellites of Saturn - Astrometry

1 Introduction

Observations of mutual events of the natural satellites have been obtained since 1973 and are accurate way to obtain astrometric measurements. Many such events involving the Galilean satellites of Jupiter have been observed. In 1994-1996, similar events occurred in the Saturnian system and we organized and coordinated an international campaign in order to record these rare events. This campaign, named PHESAT95, allowed us to collect 66 lightcurves of 43 mutual events through international network of 16 sites.

We report in this paper the observations collected. A second paper will provide an analysis of the data and corrections to the theoretical models deduced from this set of observations. The aim of the present paper is to provide the photometric data and the observational parameters useful for future works on the improvement of dynamical models as well as for models of the surfaces of the satellites. These data can be accessed from the NSDC database dedicated to the natural satellites (http://www.bdl.fr/nsdc.html).

2 The mutual events and the eclipses

The Earth and the Sun cross the equatorial plane of Saturn every fifteen years (compared with six years for the Jovian one). The Saturnian declinations of the Earth and the Sun are then zero and since the orbital planes of seven of the main Saturnian satellites are very close to the equatorial plane of Saturn, the satellites occult and eclipse each other or with Saturn: transits, occultations, umbra and eclipses can be observed.

The 1995 period was favorable since it happened during the opposition of Saturn and the Sun. The phenomena involved S1-Mimas, S-2 Enceladus, S-3 Tethys, S-4 Dione, S-5 Rhea, S-6 Titan and S-7 Hyperion and they spanned a period of several months. The high inclination of S-8 Iapetus prevented us from observing any phenomena involving this satellite during the same period. Several eclipses by Saturn were in fact predicted by Soma (1992) but no observation was published to our knowledge.

Arlot & Thuillot (1993) made predictions of all the 1995 events using Dourneau's theory (Dourneau 1987, 1993) of the motion of the Saturnian satellites. 163 dates of eclipses by Saturn and 182 dates of mutual events were computed. Before 1995, very few events were observed during the previous occurrences; in their catalogue of astrometric observations of the Saturnian satellites, Strugnell & Taylor (1990) referred to 14 mutual events observations made in 1980. These observations, with older observations of 24 eclipses by Saturn and 2 mutual events made before 1980 and found in the literature, were used by Arlot & Thuillot (1993) to validate their predictions.

Since there is no thick atmosphere around the Saturnian satellites (the one on Titan is not troublesome for our observations), the photometric observations of such phenomena are very accurate for astrometric purposes. The results obtained after similar observations of the Galilean satellites (Arlot et al. 1997), show that a high astrometric accuracy may be obtained: an accuracy better than 30 mas is expected.

This allows us to provide data necessary for the improvement of the theoretical models of the orbital motions. Since 1995, a new dynamical model, TASS, has been available (Vienne & Duriez 1995). This model is based on a general theory of the motions of the Saturnian satellites (Duriez & Vienne 1991; Vienne & Duriez 1991, 1992) where, for the first time, all the gravitational interactions among them are considered and simultaneously analyzed. New accurate data could therefore be applied to determine the theoretical constants and consequently could also be applied to the interpretation of the space observations of the Saturnian system by the CASSINI space probe.

3 The PHESAT 95 campaign

The observation of these phenomena required an international campaign in order to collect a significant amount of data. These events occur over a short time period, so, numerous observers located in several sites are necessary in order to avoid meteorological problems and to observe from different longitudes to record different events. We thus invited observers previously involved in the PHEMU campaigns of observations of the mutual events of the Galilean satellites to join the PHESAT95 campaign.


  Almaty, Fesenkov Astrophys. Inst. (Kazakhstan)
Table 1: Main sites of observation

Main observatories
Reflector Longitude Latitude elevation
  cm h m s   $^\circ$ $^\prime$ $^{\prime\prime}$   meters
  60 5 7 49 E 43 11 10 N 1450
Assy, Fesenkov Astrophys. Inst. (Kazakhstan) 100 5 11 31 E 43 13 20 N 2750
Bordeaux (France) 60 0 2 6.6 W 44 50 7 N 73
Bucarest, Astron. Inst. of Acad. (Romania) 50 1 44 23.1 E 44 24 50 N 86
Calern, OCA/CERGA (France) 150 0 27 41.2 E 43 45 17 N 1282
Catania, M.G. Fracastoro Station (Italy) 91 0 59 55.0 E 37 41 30 N 1725
Charlottesville, Fan Mont. Obs. (USA) 80 5 22 11.3 W 38 2 0 N 100
Chelmsford (UK) 30 0 1 58.9 E 51 44 40 N 40
Crimean Lab., SAI Moscow Obs. station at Nauchny (Crimea) 60 2 16 4 E 44 43 37 N 600
ESO, La Silla (Chile) 50 4 42 55.1 W 29 15 26 S 2200
Itajuba, OPD/Lab. Nacional de Astrofisica (Brazil) 60 3 2 19.8 W 22 32 4 S 1864
Lumezzane, S. Zani Obs. (Italy) 40 0 40 57.8 E 45 39 59 N 830
Meudon, Paris-Meudon Obs. (France) 100 0 8 55.5 E 48 48 18 N 162
OHP, Haute-Provence Obs. (France) 80 0 22 52.0 E 43 55 46 N 665
Pic-du-Midi, Midi-Pyrénées Obs. (France) 105 0 0 34.2 E 42 56 12 N 2861
Stuttgart (Germany) 30 0 36 32.0 E 48 42 0 N 100

This campaign, coordinated by the Institut de mécanique céleste involved the locations given in Table 1. This table gives the diameter of the telescope used at these sites, their names, longitudes and latitudes.

For the observations of the mutual events usually relative photometry is possible but is sufficient. Since the elevation of Saturn above the horizon may be very small, the air mass is often too large and absolute photometry is then not possible. Two kinds of devices were used to perform this photometry: photoelectric photometric single channel recorders and two-dimensional CCD cameras. These receptors, and references to their characteristics, are listed in Table 2.


Table 2: Receptors used for the observations

Code as
given in Description references
the tables    
  Single channel receptors  

photomultiplier EMI9789QA (Catania, Italia) Blanco (1999)
PM2 photom. EMI9789QA (K2CsSb) (ESO, Chile) -
PM3 photom. Hamamatsu R943-02 (GaAs) (ESO, Chile) -
PM4 photom. EMI9502B (Bucarest, Romania ) -
PM5 photom. WBVR single ch. (Nauchny, Crimea) Emelianov et al. (1999)
PM6 photom. 1P21 (Lumezzane, Italy) -
PM7 photom. RCA 931B (Charlottesville, USA) -
PMB photom. TELOC II B channel (Calern, France) Froeschlé et al. (1988)
PMV photom. TELOC II V channel (Calern, France) Froeschlé et al. (1988)
PMR photom. TELOC II R channel (Calern, France) Froeschlé et al. (1988)
  Two-dimensional receptors 

CCD THX7863 target (Bordeaux, France) Le Campion et al. (1992)
CCD2 CCD THX7863 target (OHP, France) -
CCD3 CCD Astriane, THX7863 (Pic-du-Midi, France) -
CCD4 CCD CAM2 (Itajuba, Brazil) -
CCD5 CCD ST-6V (Assy and Almaty, Kazakhstan) Emelianov et al. (1999)
CCD6 CCD Lynxx (Stuttgart, Germany) -
CCD7 CCD Sony ICX027BL (Chelmsford, UK) -
CCDV1 video mode uncooled intensified CCD MXRII Arlot et al. (1989)
  HCS Vision Techn. (Meudon, France)  

4 Lightcurves reduction procedure

Lightcurves were deduced from photometric measurements either from relative photometry performed with photoelectric photometers or CCD cameras.

For observations made with CCD cameras in video mode, the digitized signal was computed by digitizing boards. The lightcurves were obtained by aperture photometry. For CCD observations, images were measured by Gaussian photometry of the ASTROL software (Colas 1996). Two dimensional measurements allow us to calibrate the signal of the involved satellite to the signal of a nearby satellite, to substract the sky background in real time and so to obtain data under very difficult conditions (see for example Arlot et al. 1997).

The determination of the date of the minimum of light and of the value of the magnitude drop was obtained from a fit of the lightcurve with a sample polynomial. The errors on these determinations are given in Table 3.

5 The catalogue

5.1 The data

Table 3 gives for each date, the data related to the prediction of the event, conditions of observation and the data measured for analysis of the lightcurve.

We give the predictions of the timing of these events in Terrestrial Time as computed with Vienne and Duriez's ephemerides TASS 1.7 (Vienne & Duriez 1995). These predictions include the estimated magnitude drops; an asterisk means infinite magnitude drop for the total eclipses. Note that the values computed from the Dourneau's ephemerides (Dourneau 1987, 1993) are available in Arlot & Thuillot (1993).

The phase effect, computed by means of the Aksnes et al. (1986) method, is given in seconds of time and is included in the O-C value.

Under the predicted times and magnitude drop we show the observed ones in the UTC time scale. Observational data are sequentially given as explained in the caption. Note that two specific filters were used at the Pic du Midi observatory. The first one, labelled OG590, is a Schott long pass filter with transmittance of 0.99 above 640 nm. The second one, labelled DH710b, is a MTO interferential passband filter with spectral band from 530 nm to 900 nm.

The lightcurves shown in Fig. 1 give the magnitude drop versus UTC time for each observation described in Table 3.

These data and lightcurves are available through the Natural Satellite Data Center (NSDC) WEB server (http://www.bdl.fr/nsdc.html) and on the NSDC ftp anonymous server (ftp://ftp.bdl.fr/pub/NSDC/saturn/raw_data/phenomena/mutual/) of the Institut de mécanique céleste.

Table 3: Data from the observations of the Saturnian mutual events in 1995. One line gives the computed data i.e. the date and type of the phenomenon (for example 2E3 means mutual eclipse of S3 by S2, 5O4 means mutual occultation of S4 by S5), its timing, the predicted magnitude drop and also in Cols. 5 to 7, Ph.: phase effect included in the O-C; Dist.: apparent distance from the involved satellites to the center of Saturn in planetary radii. The folowing lines give the observed data i.e. the name of the location, the date of the maximum of the magnitude drop and its value and also in colums 8 to 17, O-C: Observed minus calculated dates of the maximum in seconds of time, C being issued from the TASS prediction; Ap.: aperture of the telescope in centimeters ; Rec.: code of the used receptor in column (cf. Table 2); El. Sat.: elevation of Saturn upon the horizon ; El. Sun: elevation of the Sun upon the horizon ; Cd.: observational conditions : [1] means very good conditions; [2] means acceptable and [3] very difficult conditions; Filt.: filter eventually used during the observations in column; no filter used is denoted by "-''; T. int.: integration time of the measurements in seconds; a variable integration time is denoted "v''; Dia.: size of the diaphragm when used; Sat. in dia.: satellites in the diaphragm or involved in the CCD measurements, i.e. the satellites, the global magnitude drop of which was observed

Table 3: continued
\includegraphics[scale=0.92]{ds1933tab3B.ps} \end{table}

Table 3: continued
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5.2 Discussion

This catalogue intends to provide observational information and reduced data from the PHESAT95 campaign. A subsequent paper will provide an analysis by means of orbital longitude corrections for the two main theories used, Dourneau and TASS of Vienne and Duriez.

The conditions of observation of these phenomena were difficult because of the faintness of the magnitude drop of many of these events in combination with the closeness to Saturn and its bright ring. Furthermore, the local conditions of observation of phenomena affect the quality of the data and several poor quality lightcurve resulted from small elevations above the horizon.

The quality of each lightcurve may be judged either by the value of the errors on the determined parameters (time of the minimum of light and lightflux drop) or from the appearance of the lightcurve itself.

We note that, as for the previous catalogue of such events, we computed the errors as following. The error on the lightflux drop is deduced from the standard deviation from the fit to the model light curve. The error on the date of the minimum is deduced from the error on the magnitude drop combined with the speed of the decrease of the lightflux during the event. Thus, this error depends on the number of points, on the integrating time and on the depth of the light curve. Therefore, the error bars may be compared only between events made with the same time constants and, preferably, with the same equipment, in order to determine an observational error and a measurement of the quality of the observation.

Partial analyses of some of these data have already been published. For example, Thuillot (1996), Thuillot & Descamps (1999) showed that an astrometric accuracy better than 48 mas can be deduced from this kind of data. Emelianov et al. (1997, 1999) published results obtained at the Crimean laboratory and at the two astronomical observatories in Kazakhstan. Workshops gathered many of the participants of this campaign, first in Bucharest (Arlot & Stavinschi 1996), and later in Catania in Italy; several partial results were also given there (Arlot & Blanco 1999).

6 Conclusion

We give in this paper the results of the PHESAT95 campaign. Unfortunately data from some observers could not be used, generally because of the faintness of the observed signal and its poor signal/noise ratio and sometimes because of the uncertainties in the time scale. The present catalogue gives the results obtained by all the participants of this campaign who obtained significant results. In order to record the maximum number of events, it was necessary to organize international campaigns such as the PHESAT95 campaign. For the first time, we have obtained numerous photometric observations of the Saturnian mutual events. These phenomena occur every 15 years and in the same way as for the Galilean satellites, they lead to very accurate astrometric measurements which are very difficult to obtain with other techniques from the ground. Furthermore, they may allow us to determine surface parameters by comparison of lightcurves with synthetic models.


These observations have been made thanks to the CNRS (Centre National de la Recherche Scientifique), the INSU (Institut National des Sciences de l'Univers) and the CNES (Centre National d'Études Spatiales) through the PNP (Programme National de Planétologie) who supports the PHESAT95 campaign and the Institut de mécanique céleste/Bureau des Longitudes.

We thank several colleagues, amateur astronomers and students, J. Bourgeois, H. Bulder, L. Cardon, Dr. V. Kornilov, Dr. A. Krysczynska, Dr. P. Bouchet, Dr. Ph. Robutel, A. Rouhan, M. Savoia, R. P. Wilds, for their efforts and their help during the campaign.

We are indebted to Dr. A. Vienne and Dr. L. Duriez for providing us the solution TASS1.7 of their general theory of the motion of the Saturnian satellites.

We also wish to thank the staff of the observatories where these observations were made for their help during this campaign, especially the staff of the Observatoire de Haute-Provence (OHP).



Online Material

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Copyright ESO 2001