INTRODUCTION Michel C. Festou(1) Observatoire de Besancon, Unite associee 389 du CNRS 41 bis avenue de l'Observatoire F-25044 Besancon - France Abstract: Background information is presented on both low and high dispersion spectra of comets that have been taken between 15 October 1978 and 14 September 1989 with the IUE. Characteristics of individual exposures are given in the form of an extented version of the IUE Merged Log of Observations including information on the geometrical factors relevant to each object and observation as well as comments about the pointing accuracy. This primary data set is re-organized in four additional tables in which the information is arranged by comet name, heliocentric and IUE-centric(2) distance, and heliocentric velocity. Complementary tables give the list of high resolution spectra, that of FES images and FES count rates. References of papers in which published observations and discussions of IUE observations have appeared are given in the appendix A. The list of comets observed with the IUE is given in the appendix B. The guide is illustrated by raw images, line by line spectra and both low and high dispersion spectra of comets taken with the IUE whose list is given in the appendix C. (1) Permanent address as of 1 July 1990 : Observatoire Midi-Pyrenees, URA 285 du CNRS, 14 avenue E. Belin, 31400 Toulouse, France. (2) This expression will be used throughout the guide since the IUE satellite was at a non negligible height above the surface of the Earth (approximately equal to 36,00O km). 1- IUE observations of comets 1.1 The motivation Although comet observations constitute only a small subset of the IUE archive (730 spectra and 235 Fine Error Sensor (FES) images have been collected between 15 October 1978 and 14 September 1989), the user of that data base might need (and wish) some help to wander through this wealth of information. Since comets are observed in geometrical conditions that differ widely from one observation to another, organization of the available data as a function of the main observational parameters is desirable. The official log of the IUE observations provides some of the necessary information but, unfortunately, it also contains errors and, in many respects, is incomplete. In particular, comets are moving objects and knowledge of the tracking record while the exposures were taken is of paramount importance to interpret the fluxes measured, especially so since comets are spatially and temporally variable sources. This guide provides an updated log of IUE observations of comets complemented by additional tables in which the data are presented to facilitate the search of images according to selected geometrical parameters, i.e. the heliocentric distance (a parameter that fixes the energy used to evaporate cometary material and that controls the extend of the coma), the heliocentric velocity (a parameter that determines the wavelength shift of solar spectrum structures with respect to cometary absorption wavelengths), and the IUE-centric distance (a parameter that principally determines the spatial resolution of the observations). Although the ULDA only concerns the low dispersion IUE spectra, lambda(1180A - 3200A), we have included information of similar nature about the 55 high resolution spectra that have been taken during the same period. Finally, a comprehensive list of FES images and the complete record of the FES count rates together with the position of the comet in the FES plane, when available, are included. The, IUE spectrographs allow to observe the main compounds of cometary comae: atoms, radicals and ions that are the end products of the destruction chain of the parent molecules that evaporate from comet nuclei. Most comets can only be observed once and consequently the quality and the reliability of the instrument employed to record their behavior is of paramount importance to make a fruitful use of the observations that have been recorded. Considering the difficulties associated with ground-based observations of comets, and although the quality of those latter has greatly improved over the past recent years, it is fair to write that the IUE observations have created the first true data base of comet spectrophotometric observations that allows a reliable extensive intercomparison of the emissions of comets. Much has already been done in the field of ultraviolet spectroscopy of comets and many discoveries were made thanks to the IUE observations (see the reviews by Feldman et al., 1976, Feldman, 1982, Festou, 1982, 1986, Festou and Feldman, 1987, A'Hearn, 1989). A large fraction of the available data set has not yet been analyzed to date and we hope that the information given in this guide will be an incentive for many researchers to further study this unique set of data. 1.2 The sources of information Thee information contained in this guide has been collected from various sources. First the official IUE Merged Log of Observations, a document established from the information entered in real time by the Resident Astronomers who performed the observations with the Guest Observers. Second, we used the forms filled by the IUE Resident Astronomers in real time after each observation has been completed (i.e. the so-called handwritten log) to get additional information, especially on the starting times of the exposure segments or on the target tracking history. Third, in about 70% of the cases, we had access to personal notes written by the observers in real time. Those documents are extremely important since they often mention what really happened during the observations (computer crashes, pointing and tracking problems, non interruption of the exposures during maneuvers, restart times of exposures...) and these are the only documents that tell with accuracy where the slits were during the exposures (the case of comet IRAS-Araki-Alcock 1983 VII is the best illustration of this). We did our best to eliminate sources of error such as printing or typing errors of various kinds (on original documents, by Resident Astronomers, by Observers,...) by crosschecking the various sources of information. Consequently, this guide may contradict the information which is contained in any of the above mentioned documents: in case of contradiction, we have listed the information the most likely to be correct. In a few cases in which our information shows an unsolvable contradiction, we have put a question mark or indicated the nature of the problem encountered in detail. Hopefully, updates of this catalogue will remove most of those inexactitudes (to receive them, see the detachable form that is at the end of this guide). 2- The targets 39 comets have been observed with the IUE between October 1978 and September 1989, among which is the most famous of them, comet P/Halley (1986 111). Two comets were observed at two and three consecutive apparitions, respectively (P/Borrelly, in 1981 and 1987; P/Encke, in 1980, 1984 and 1987). 19 periodic, 8 long period (P ranging from 291 to = 50,000 years), and 9 parabolic comets were observed (three have become hyperbolic after the time of their perihelion passage, Bowell 1982 I, Cernis 1983 XII and Wilson 1987 VII). Two comets (IRAS-Araki-Alcock, 1983 VII, and Sugano-Saigusa-Fujikawa, 1983 V) passed very close to the Earth in 1983. A list of all comets observed with IUE is given at the end of this guide in appendix B. The observing conditions varied enormously from one target to another. 16 comets were observed on one day only , all others were observed during periods ranging from one week to about a year (P/Halley 1986 III and Wilson). Comets were observed over wide ranges of geometrical parameters: - the heliocentric distance varied from 0.71 AU to 5.87 AU (see Fig. 3 and 5, below); - the heliocentric velocity varied from -38.35 km/s to +34.20 km/s (see Fig. 4 and 6, below); - the IUE-centric distance varied from 0.0317 AU to 5.78 AU (see Fig.3, below); - the phase angle varied from 5.5 degrees to 116 degrees. Although most spectra were taken with the slit positioned at the center of coma, the spatial distribution of the coma emissions was studied in a few objects at both low and high dispersion. Table 1 gives all low and high dispersion images (including SWP sky exposures) obtained for each comet. One will observe that comet Halley stands by itself with 32% of the observations in the data base; the apparitions of comets Bradfield 1979 X, Encke 1980 XI, Austin 1982 VI, d'Arrest 1982 VII, IRAS-Araki-Alcock 1983 VII, Giacobini-Zinner 1985 XIII, Wilson 1987 VII, Borrelly 1987 XXXIII and Tempel 2 1987g were well covered too, at least on one leg of their orbits. 3- Content of the guide 3.1 General information Table 1: statistics on IUE observations of comets -------------------------------------------------- --------------------------------------------------------------------- Comet LWR LWP SWP FES INES Total Low High Low High Low High --------------------------------------------------------------------- 1974 II - - 1 - - - - - 1 1978 XV 6 3 - - 9 - - - 18 1979 X 27 5 40 36 - 108 1980 X 3 - - - 5 - 1 - 9 1980 XI 14 1 - - 22 - 5 - 42 1980 XII 1 - - - 3 - 1 - 5 1980 XIII 4 - - - 5 - 1 - 10 1981 II 1 - - - 2 - 1 - 4 1981 IV 1 - - - 3 - 1 - 5 1982 I 5 - - - 5 - - - 11 1982 IV 3 - 1 - 1 - - - 4 1982 VI 12 2 - - 13 - 5 - 32 1982 VII 6 - - - 14 - 1 - 21 1982 VIII 1 - - - 2 - - - 3 1983 V 3 - - - - - 1 - 4 1983 VII 15 1 - - 10 - 11 - 37 1983 XI 1 - - - - - - - 1 1983 XII 1 - - - - - 2 - 3 1983 XIII 1 - - - 1 - - - 2 1984 IV - - 4 - 6 - - - 10 1984 VI - - 7 - 6 - 2 - 15 1984 VII - - 2 - - - 1 - 3 1984 XXI - - 1 - 2 - 1 - 4 1984 XXII - - 7 - - - 1 - 8 1984 XXIII - - 1 - - - 1 - 2 1985 XIII - - 24 3 19 - 13 - 59 1985 XVII - - 9 1 6 - 1 - 17 1986 III - - 140 20 72 4 78 - 314 1987d1 - - 3 3 2 - 4 - 12 1987g - - 17 - 7 - 15 - 39 1987 III - - 5 - 5 - - - 10 1987 VII - - 26 7 17 2 25 - 77 1987 XIII - - 1 1 - - 1 - 3 1987 XXIX - - 8 - 4 - 6 - 18 1987 XXXII - - 1 - 1 - 2 - 4 1987 XXXIII - - 10 1 4 - 8 - 23 1989a - - 1 - - - 1 - 2 1989g - - 2 - - - 1 - 3 1989o - - 20 1 12 - 8 - 41 1989 III - - 15 - - - - 15 30 1989 VI - - 3 - 1 - - 3 7 1989 XV - - 2 - 1 - - 2 5 1990 V - - 30 - 21 - - 39 90 1990 XIV - - 4 - - - - 4 8 1990 XX - - 43 - 13 - - 35 91 1990 XXI - - 4 - 1 - - 6 11 1991 I - - 2 - - - - 2 4 1991 XXI - - 2 - - - - 2 4 1992 III - - 2 - 2 - - 2 6 1992 XXVIII - - 6 - 4 - - 7 17 1993 III - - 7 - 1 - - 7 15 1994 V - - 4 - - - - 6 10 1994 XIV - - 9 - 4 - - 9 22 1994 XXX - - 12 - - - - 12 24 1994 XXIV - - 11 - 2 - - 11 24 1994 XXVII - - 2 - - - - 2 4 1995 o1 - - 9 - - - - 9 18 1996 B2 - - 10 - 6 - - 10 26 --------------------------------------------------------------------- Pointing and tracking Acquiring the comet: in almost all cases, the center of brightness is found using the FES. This center of brightness is assumed to be the nucleus. An appropriate drift is given to the spacecraft (S/C) to compensate for the motion of the comet. Then one of the three following techniques can be used to control the comet position: 1- the "nucleus" is put at the "reference point" (see its position in the FES camera focal plane in Fig. 1) and from there is put into the desired aperture by a standard offset maneuver; an inverse maneuver should bring the comet back to the reference point after the observation. In practice, because of the occurrence of various S/C drifts during the exposure, the comet is not found to be exactly at the reference point and the quality of the tracking is then measured by the amount by which the comet should be moved to go back to the reference point. This maneuver is generally repeated every 20 to 30 minutes. On average, the mean drift that is recorded is of the order of 1-2 arc seconds if the exposure is shorter than 10 minutes and it increases to 3-4 arc sec when the exposure time goes up to 30-40 minutes. 2- if one does not wish to put the comet nucleus in the slit and if the desirable offset is larger than the slit dimensions, one can either proceed as above or use the center of brightness as a guide (if the comet is not too faint, it is possible to locate the center of light of a stellar like source in the FES focal plane and to maintain it at that position). 3- in the case the comet position is very accurately known: one can use as guide a star whose distance to the nucleus is known ("offset" mode). This technique is well suited for faint comets that are not easily detected by the FES and for which an accurate ephemeris is available (it can not be applied to newly discovered objects since, in the first weeks after a new comet has been found, the uncertainty on the position of the objects is of the order of 1 to 5 arc min). Tracking is obtained by giving the S/C the appropriate drift rate and by monitoring the path of the offset star at regular intervals (approximately 10 min) in the FES plane. 3.2 General lists All comet observations are first listed in the form of a general table (Table 2) that is the essence of this guide. Then part of that information is displayed again as a function of a selected key parameter, for the mere purpose of helping the reader to easily find specific observations related to that selected parameter, i.e. the comet name (Table 3, the only table that contains an information not given in Table 2, references to publications in which the observations have been published), the heliocentric distance (Table 4), the IUE-centric distance (Table 5) or the heliocentric velocity (Table 6). Each individual IUE observation is identified by a number that appears in the first and last columns of Table 2. This identification number is repeated throughout the guide to allow the reader to retrieve easily the totality of the information that is available on a specific observation. Additional lists of high resolution spectra (Table 7), FES images (Table 8) and FES count rates (Table 9) are given. We shall give below a complete description of the contents of each table. 3.2.1 General list of the observations (Table 2) Table 2 contains information on all 380 long wavelength (38 comets) and 295 short wavelength (29 comets) low resolution spectra, 235 FES images (30 comets), 49 long wavelength (13 comets) and 6 short wavelength high dispersion spectra (2 comets). The following information is provided for each observation: N: identification number of individual observations. It is given in all tables and it will allow the reader to establish a correspondence between the content of the specialized tables and the full information that appears in Table 2 on each individual observation (except for Table 3 that contains references to studies in which IUE data were published). Date: day of beginning of observation (dd mm yyyy). Exposure start (E.s.): time of beginning of the exposure (hh:mm:ss). Exposure time (E.t): exposure time in seconds. The exposure time that has to be considered to interprete flux measurements may differ from this value, e.g. some time may have been spent at the reference point 2 (RP2) to perform tracking checks or a drift may have occur during the exposure. See the notes (indicated by numbered asterisks) at the bottom of each page of Table 2. Middle of exposure (M.e.): time (decimal fraction of the day; a value > 1 corresponds to an exposure begun shortly before the end of the day) for which all geometrical elements relevant to the observations have been computed. Because pointing checks and various system maneuvers often occur during exposures, M.e. is in general larger than E.s. + [(E.t.) / 2]. In the case of the FES images, M.e. is assumed to be equal to E.s. because the exposure time of FES exposures is very short. Image n(degrees): camera identifier and image number in the IUE data base ("LWR", "LWP", "SWP" and "FES" stand for Long Wavelength Redundant camera, Long Wavelength Primary camera, Short Wavelength Primary camera and Fine Error Sensor camera, respectively). D: spectrograph resolution identifier. "L" stands for low dispersion (nominally approximately 1-3 10^2) whereas "H" stands for high dispersion (nominally approximately 1-3 10^4). In the case of the FES images, the letters indicate the type of exposure mode used. Ap: status of the apertures. If the mention "L,S" appears, the object was in the large slit and the two slits were open. If the mention "S" appears alone, the large slit was closed and the object was in the small slit. The mention "L,S" also indicates that the data for each slit were processed, although the target was in the large aperture. In the case letter "L" appears alone, the target was in the large aperture and only the large aperture data were processed. Comet: comet identification number as given by the International Astronomical Union. Full names appear at the end of the guide in appendix B. Rh: distance to the Sun in astronomical units (AU) of the comet at time M.e. The orbital elements used in our calculations were taken in Marsden (1986, Catalogue of Cometary Orbits, 6th edition) or in recent IAU Circulars. Unless observations were carried out far from the epoch for which the orbital elements are exactly osculating to the comet orbit (generally the date of perihelion passage), the accuracy of the numbers given here is better than 0.00005, AU. All geometrical parameters are computed for the time M.e. dRh/dt: heliocentric velocity (km/s) of the comet at time M.e. (accuracy = +/- 0.005 km/s). delta: distance (A.U.) of the comet to the IUE S/C at time M.e. (because of the high altitude of the IUE S/C, this parameter may differ significantly from the distance to the Earth, especially in the case of comet IRAS-Araki-Alcock 1983 VII). In our calculations, we have assumed that the IUE was constantly 5 Earth radii above a point situated at longitude 270 degrees E and latitude 0 degrees. Since this is only an approximation for the actual orbit of the satellite, this approximation may introduce errors of order +/- 0.0001 AU. d(delta)/dt: velocity (km/s) of the comet at time M.e. relative to the theoretical point representing the IUE observatory defined above. Because of the large and rapidly varying velocity of the IUE S/C, only partially accounted for in our calculations, our velocity may differ significantly from the true IUE-centric velocity of the comet, say by approximately +/- 0.5 km/s. SEC: Sun-IUE S/C-Comet angle (ddd.dd) at time M.e. (+/- 0.01 degree, except in the case of comet IRAS-Araki-Alcock for which the error might be larger). SCE: Sun-Comet-IU S/C angle (ddd.dd) at time M.e. (+/- 0.01 degree, except in the case of comet IRAS-Araki-Alcock for which the error might be larger). RA: geocentric right ascension (hh mm.m) of the comet (epoch of the date, not 1950.0) at time M.e. Dec: geocentric declination (dd mm) of the comet (epoch of the date) at time M.e. The IUE observatory position induces parallax effects that must be accurately computed to achieve correct pointing and guiding. Rho: distance (arc seconds) of the nucleus to the center of the slit used (as indicated in column "Ap") as seen from the IUE S/C. Most SWP sky exposures were taken 2 degrees away from the nucleus in the "sunward": this means the slit was displaced along the - pitch direction (i.e. exactly "sunward"). For most offset exposures, mentions such as "sunward" or "tailward" that are found in documents related to the observations indicate often slit motions that may not exactly correspond to displacements along the ± pitch axis of the S/C (see Fig. 1). The position angle of the nucleus center of slit vector (Pa_NS) given in Table 2 allows to know which offset maneuver was actually performed. As for the FES images, the position of the nucleus in the FES2 plane, when known, is indicated in X,Y units (one X unit = 0.268", one Y unit = 0.2617") or by the mention "RP2" when the comet was at the reference point of the FES camera (concerning the FES camera distortion and the FES photometric accuracy, see the various ESA and NASA IUE Newsletters). Pa_NS: position angle (degrees, positive towards east) of the nucleus-center of slit vector. See Fig. 2. Pa-Sun: position angle (degrees, positive towards east) of the nucleus to sun vector. Differences by less than one degree (mod 180 degrees) between the two position angles represent round off errors in our calculations (the mean nucleus position uncertainty in the FES is about equal to 1 arc sec, not to mention slit drifting motions). Orient: position angle (degrees, positive towards east) of the long axis of the slit. No value is given when the approximately circular small slit is exposed. See Fig. 2. Tracking: a parameter of great importance to interpret the results. Here is given the motion (arc seconds) that was necessary to bring the comet back to its initial position (at RP2) after each exposure. Consequently, the comet drift had a sign opposite to that indicated here. When no information was available, we have estimated the drift (in an unknown direction) based on observed values for exposures having similar exposure times: if the exposure time E.t. is smaller than 3 minutes, the drift d is on average of the order of 1 arc sec or smaller; if 3 min < E.t. < 15 min, d is approximately 2"; if T.e. = 30 min, d is approximately 3"; if E.t. < 60 min, d is approximately 5". One will assume that drifts were linear during the exposure segments when exposures are taken in multiple segments, the value given here is the average of segment drifts. One should remember here that the spectrographs pixel size is approximately 4 x 4 arc sec. Sat: information on the saturation level of the exposures. When the number given is < 10, the following code is used to indicate which part of the spectrum is saturated: 1- HI Lyman alpha; 2- OH (0-0); 3- OH,(1-0); 4- OH(1-1); 5- OH (0-0) has a few pixels saturated; 6- OH (1-0) has a few pixels saturated; 7- CS (delta_v = 0); 8- CS (delta_v = 0) has a few pixels saturated; 9- continuum above 2900 A is saturated. When the number given is >= 10, it represents the rough value of the raw signal (in "IUE data numbers", i.e. always < 255) that was recorded. Usually, that information is found when processing the original images. Here, we used the information provided in the IUE merged log and in the observer's log books (when images were displayed in real time), completed by a critical examination band flux values we have measured in those spectra. 3.2.2 Observations listed by comet name (Table 3) Table 3 gives for each observation the comet name, the image number, the dispersion, the aperture through which the comet was observed, the heliocentric distance, the heliocentric velocity and the "lUE-centric" distance of the comet at the time of middle exposure, the distance from the slit center to the nucleus (arc sec), identification numbers for references to papers in which the observations have been published or discussed (complete references are given in the appendix A), and finally the guide identification number (N) assigned to each observation in Table 2. Many spectra identifications are not given in those papers. When missing, identifications were made by the author of this guide; uncertain identifications are mentioned by a question mark. 4. Low resolution spectra 4.1 List of spectra ordered by heliocentric distance (Table 4) Table 4 gives for each observation the heliocentric distance, the heliocentric velocity, the "IUE-centric" distance of the comet at the time of middle exposure, the distance from the slit center to the nucleus (arc sec), the image number, the comet name, and the identification number (N) of Table 2. In Fig. 3 below, one will observe that observations are numerous enough to almost completely cover the (IUE-centric distance, heliocentric distance) domaine in which water vaporizes easily. For heliocentric distances in the 0.7-1.8 AU range, the intercomparison of observations made at very similar heliocentric and IUE-centric distances is possible. 4.2 List of spectra ordered by "IUE-centric" distance (Table 5) Table 5 gives for each observation, the "IUE-centric" distance of the comet (uncertain by no more than +/- 0.000 2 AU), the heliocentric distance and the heliocentric velocity of the comet at the time of middle exposure, the distance from the slit center to the nucleus (arcsec), the image number, the comet name, and finally the guide identification number (N) of Table 2. The IUE S/C-comet distance is the parameter that mainly determines the spatial resolution. In the case of the observations of comet IRAS-Araki-Alcock, the proximity of the comet to the Earth led to the spectacular discovery of the emission of S2. 4.3 List of spectra ordered by heliocentric velocity (Table 6) Table 6 gives for each observation the heliocentric velocity, the heliocentric distance and the "IUE-centric" distance of the comet at the time of middle exposure, the distance from the slit center to the nucleus (arc sec), the image number, the comet name, and finally the guide identification number (N) of Table 2. Figure 4 below shows the distribution of the observations as a function of the velocity of the comet relative to the sun (the parameter that governs the very important "Swings effect"). The two "tails" at very positive and very negative values correspond mostly to comet P/Halley 1986 III observations whereas the cloud of points at v_h < +/- 0.10 km/s indicates that most other comet observations were made at or near the time of perihelion passage. 5- High resolution spectra (Table 7) Table 7 gives for each observation the observation date, the image number, the IUE aperture, the comet name, the heliocentric distance, the heliocentric velocity and the "IUE-centric" distance of the comet at the time of middle exposure, the distance from the slit center to the nucleus (arc sec), the position angle of the nucleus-slit center vector, the orientation of the slit (see Figs. 1 and 2) and the guide identification number (N) of Table 2. Figs. 5 and 6 give the distribution of the high dispersion observations in the (heliocentric distance, "IUE-centric" distance) and (heliocentric velocity, heliocentric distance) planes, respectively. The distribution of the points in Fig. 5 indicates that collisional quenching and non equilibrium phenomena can be studied to some extend (except in the case of comet 1983 VII, the spatial resolution was never less than about 200 km - but measurements concern line of sight quantities - ). The distribution of points in Fig. 6 indicates that the Swings effect can be well investigated. The vast majority of spectra were recorded through the large long wavelength camera aperture and the main emission that was recorded is consequently that of OH (0-0). However, note that emissions from the other OH bands, those of CS, C2, C02+, NH and OH+ appear too in well exposed spectra, on top of the continuum spectrum seen down to approximately 2600A. In the SWP spectra, emissions of HI, SI, CI, OI, and CO are the only clearly identified spectral features. 6- FES data 6.1 List of FES images (Table 8) Table 8 gives for each observation the comet name, the image number, the heliocentric distance, the "IUE-centric" distance and, when available, some information on the position of the nucleus in the FES2 plane (in FES units, i.e. 0.268" and 0.2617" per unit along the X and Y axis, respectively), and finally the guide identification number (N) of Table 2. The information contained in FES images is not easy to interpret since i) the signal that is recorded is due (mostly) to two sources whose spatial distributions are very different, light scattered by dust particles (almost a point like source) and light scattered by C2 radicals (spatial distribution of a daughter species); ii) the FES camera pixel sensitivity has varied (in a special way near the reference point because of an excessive and necessary use); iii) the FES pixels do not have the same size, but distorsion in the FES plane is small and well known; iv) the sensitivity of the FES extends from 4500 A to 9000 A. A calibration of that camera of some quality exists (or might become available in the future) and use can probably be made of the information contained in those images (in addition to some information on the gross morphological structure of the target, e.g. the fan shaped coma of comet Encke, the dust tail of comet Halley). Note that the ion tail is never (clearly) seen in FES images and that most FES images show a nearly circular image corresponding to the C2 cloud and/or the center of the dust coma. 6.2 FES count rates (Table 9) Table 9 gives the number of FES counts recorded when the comet was at or near the reference point. The slow degradation of that region of the FES plane due to its extensive use over the years renders comparison between old and recent observations rather difficult. However, relative measurements on a short time scale are possible (e.g. the famous examples of comet IRAS-Araki-Alcock 1983 VII Feldman et al., 1984). Relative measurements over a few month period are perfectly valid (e.g. the example of comet Halley 1986 IR observations, Feldman et al, 1987). The designations "FU", "FO", and "SO" indicate the observing modes and stand for "fast track/underlap", "fast track/overlap" and "slow track/overlap", respectively. For measuring the FES count rates, the FES plane is scanned in four areas of 12.7 by 12.7 arc sec and the FES count rate is the average of the four values. In the overlap mode, the four areas overlap while in the underlap scan, they don't (see the IUE Newsletters for more details). The second mode is more appropriate to evaluate the brightness of a comet coma. 7. References A'Hearn M.F., 1989, in A decade of UV Astronomy with the IUE, ESA SP-281, 1, 47-58, "Comets". Feldman P.D., 1982, in Comets, L.L. Wilkening Ed., The University of Arizona Press, Tucson, 461-479, "Ultraviolet spectroscopy of comae". Feldman P.D., A'Hearn M.F., Millis R.L., 1984, Astrophys. J. 282, 799-802, "Temporal and spatial behavior of the ultraviolet emissions of comet IRAS-Araki-Alcock 1983d". Feldman P.D., Festou M.C., A'Hearn M.F., Arpigny C. et al., 1987, Astron. Astrophys. 187, 325-328, "lUE observations of comet P/Halley: evolution of the ultraviolet spectrum between September 1985 and July 1986". Feldman P.D., Opal C.B., Meier R.R., Nicolas K.R., 1976, in The Study of Comets, NASA SP-393, 773-796, "Far ultraviolet excitation processes in comets". Festou M.C., Feldman P.D., Weaver H.A., Keller H.U., 1982, ESA SP-176, 445-449, "The spectrum of comets as derived from IUE observations". Festou M.C., 1986, in New Insights in Astrophysics, 8 years of UV Astronomy with IUE, ESA SP-263, 3-9, "Comets". Festou M.C., Feldman P.D., 1987, in Exploring the Universe with the IUE satellite, Y. Kondo Ed., Kluwer Acad. Publ., 101 - 118, "Comets". 8 - Absolute calibration of the IUE spectrographs Detailed information will be found on the absolute calibration of the IUE cameras in the recent papers by Holm (1986, NASA IUE Newsletter 26, 11), Bohlin (1986, Astrophys. J. 308, 1001), Sonneborn and Garhart (1987, NASA IUE Newsletter 33, 78), Clavel et al. (1988, Astron. Astrophys. 191, 392), Bohlin and Grilmair (1988, Astrophys. J. Suppl. 66, 209; 6+8, 487), and in the various references those papers mention. Acknowledgments I would like to thank W. Wamsteker who encouraged me to compile the information contained in that guide and who critically reviewed its preliminary version. Many VILSPA staff members helped me during my numerous visits to VILSPA to better understand the technical problems attached to observing moving targets with IUE and this guide benefits from their advice. I would like to specially thank Roberto Gilmozzi, Jacques van Santvoort and Jean Clavel. Gathering the information contained in that guide is a many year effort and it would have been impossible to lead it to completion without the collaboration of the individuals who performed most of the observations at GSFC and let me examine their personal logs: I am deeply indebted to Paul Feldman, Elizabeth Roettger and Michael A'Hearn for their help. The most painful part of this work was not to collect the information, although that required some persistence in the effort, but consisted in actually building from my original data files the tables that this guide contains and to verify the accuracy of their contents. All geometrical calculations were done automatically on our micro-VAX using a preliminary version of the guide as an entry source. The code to manipulate the files on the VAX was written by Mrs Frangoise Gazelle. The ephemeris code employed was kindly provided by Don K. Yeomans. Most of these complex, lengthy and somewhat boring tasks were accomplished by my collaborator, Mrs Regine Bernhard. I would like to say how much I have appreciated her competence, patience during this project, and finally her efforts to track my errors.