Coronography at La Silla: High Resolution Imaging of Faint Features Near Bright Objects F. PARESCE and c. BURROWS, Space Telescope Science Institute, Baltimore, USA, and Astrophysics Division, Space Science Dept. of ESA

Introduction of this article and available from the sponds to 5.10-3 square arcseconds in authors. the sky with a total rectangular field of Some of the more interesting as­ view of 22.5 by 35.9 arcseconds. tronomical sources happen to lie very The optical and detector system just close to bright objects. If the source is Experimental Setup described was calibrated absolutely us­ very faint, it becomes extremely difficult ing bright, stable stars as calibration to discern it against the glare of the The optical configuration of our ob­ sources. Using standard Johnson S, V, bright object. This situation arises, for servational setup at La Silla is shown and Rand Cousins I filters available at example, when attempting to image the schematically in Figure 1. Light from the La Silla (ESO filter numbers 445, 446, ionized tori around satellites such as 10, object under investigation is gathered 447 and 465), we obtained overall peak protoplanetary or circumstellar systems and focused by the f/8 MPI 2.2-metre counting efficiencies between 4,000 and of nearby stars, accretion disks within Cassegrain telescope onto a blackened 8,000 Aof 0.2 to 0.4 CCO counts per close binaries, faint emission photoetched mask located in the focal photon depending on bandpass. The nebulosities in the vicinity of old novae plane. This occulting mask is shaped in transmission of the achromatic doublet and possible fuzz around bright the form of a long thin wedge which can decreases rapidly below 4000 Aand is quasars. In all these cases, it is impera­ be moved longitudinally by a microme­ essentially opaque below 3500 A. Thus tive to reduce dramatically the scattered ter in order to vary its projected width in for U band measurements the lens light in the wings of the bright object the sky from 2 to 10 arcseconds de­ should be replaced by a singlet of fused seeing disk and/or to prevent the bright pending on seeing conditions, source quartz or silica and the detector object from saturating the detector and brightness, etc. It can also be moved changed to the UV sensitive GEC CCO thereby disabling it in the adjacent transversely to occult any desired por­ available at the 2.2 metre. areas. The latter effect can be important tion of the field of view. Following the even for relatively faint objects such as mask, an achromatic doublet reimages 15th magnitude quasars if long integra­ the telescope focal plane with a magnifi­ tion times are required. cation of 5 onto the detector so that the Observing Methodology Accomplishing this objective would effective focal ratio of the system be­ First, the coronograph is focused by open up a very fertile field of inquiry; one comes f/40. adjusting the position of the lens mount that is ideally suited to the performance An apodizing mask especially de­ until the image of the occulting wedge characteristics of modern high resolu­ signed to reduce diffracted stellar light under flat field illumination comes into tion, large aperture telescopes and de­ from the 2.2-metre telescope pupil is sharp focus. The coronograph comes tectors. Success requires a well-de­ located in the exit pupil and is rigidly equipped with internat LEOs to provide signed coronograph coupled to a high attached to the lens mount to ensure such illumination so that the focusing quality photon collecting device located that it remains there as the coronograph can be carried out even when the tele­ on a site with excellent seeing condi­ is focused. The mask obscures 30 per scope is not in operation. Second, a tions. This note briefly describes the cent of the exit pupil close to the images suitable star is made to fall somewhere techniques we have devised in our first of the entrance pupil edges. After this on the unocculted area of the detector attempts with a simple coronograph mask, the light passes through optical through a broad band filter and the re­ mated to the 2.2-metre MPI Cassegrain filters in a rotating commandable two­ sultant image is used to focus the tele­ telescope at La Silla and some of the wheel assembly mounted just in front of scope secondary. The coronograph results obtained in the first year of oper­ the detector. In our two runs of Sep­ need not be refocused if a filter is ation. A more complete description of tember and November 1986 at La Silla, changed since the filters are in an f/40 the experimental and data analysis we employed the 512 x 320 30 micron beam. Third, a suitably opaque neutral techniques and the scientific results can square pixels RCA CCO. In this particu­ density filter (N03 or 4) is moved into be found in the references at the end lar configuration, each pixel corre- the beam and the bright primary source acquired. Using the telescope TV autoguiding system described by Ouchateau and Tele~cope Coronagraph ~---- 1\ Ziebell in the Messenger No. 45, 1986, \ ( the source is placed behind the occult­ ing wedge in aseries of short acquisi­ tion exposures. With aseries of increas­ ingly smaller offsets, the source is pre­ ---- cisely centred behind the occulting wedge. This is accomplished by making the spillover light distribution above and below the wedge obtained with progres­ -- - sively less ND attenuation as symmetri­ cal as possible. Finally, any ND filter remaining is removed and the long ex­ 1: Optical------configuration. Figure posure in the specified bandpass be-

43 set mainly by the time necessary to read ing. Gosmic ray events and local GGO out, process, and display the relevant defects are also located by comparison GGO acquisition images. A detector with neighbouring pixels using standard with real time display capabilities or a statistical tests and iterating to avoid faster data handling system for the GGO including bad pixels in the averaging would obviously shorten considerably process. Finally, the edges of the this set up period and allow a faster turn occulting mask are defined interactively around. This is a critical factor for a and stored in a file associated with the lengthy survey programme. image. No further user interaction is re­ quired in the subsequent processing which can proceed automatically. Data Analysis and Results The results of this process are shown All the raw images are processed in a in Figures 2 and 3 that represent images standard way to prepare them for scien­ of Beta and Alpha Pictoris, respectively, tific analysis. The GGO bias level and the taken with the system we have just de­ variations in response across the detec­ scribed with the R band filter on the tor area are removed by subtracting a night of November 27, 1986. Beta Pic­ constant bias level and dividing by an toris is a star suspected of having a appropriate flat field. Overclocked, satu­ giant protoplanetary system seen edge­ Figure 2: Image of Alpha Pictoris. The image rated pixels, and bad columns are easily on in emission surrounding the central has been flat fielded, and minor blemishes located and masked out so that they will star while Alpha Pictoris is used as the have been removed. The horizontal bar is the not contribute to the rest of the process- reference star. Both images are mainly occulting mask and has been masked black. The masked circular area in the centre with triangular appendages is due to saturated and bleeding pixels. The deviations from cir­ cular symmetry are due to instrumental scat­ tering and diffraction effects. The image is truncated at 5,000 counts, and has been displayed with the image greyscale changed to give equal numbers of pixels at any given intensity. (Histogram equalization.) gun. If a faint feature is expected to be lost in the light from the wings of the bright object seeing profile, the tele­ scope is moved to a control star nearby and the procedure just described re­ peated. After some practice, the over­ head time used to acquire and precisely register the source can be kept within approximately 20 minutes. This time is

Figure 3: An image of Beta Pictoris with trun­ Figure 4: R band image of Beta Pictoris circumstellar disk .obtained as a difference image of cation at 3,000 counts. It is otherwise treated Figures 2 and 1. North is up and East is to the left. The image displayed is about 23 arcsec in identically to that of Figure 1. All instrumental width. The disk extr:mds diagonally from NE to Sw. The dWk portions of the image are due to effects are similar but there is an additional saturated pixels or the focal plane mask. The bright line rUlining vertically through the frame bulge in the image at bottom left (NE) through centre, and light close to the mask particularly to the right of centre are due to residual top right, due to the circumstellar disko diffractive scattering. 44 composed of the stellar seeing disk with small contributions due to scattering within the instrument and, of course, for Beta Pictoris, the circumstellar disko This feature, however, is almost lost in the image shown in Figure 2 without benefit of a direct comparison with the reference image shown in Figure 3. Although a feature running in a rough­ Iy NE to SW direction through the Beta Pic image, without a corresponding one to be discerned in the control image, stands out rather clearly in the image shown in Figure 2, considerably more analysis effort has to be expended in order to generate a photometrically true or correct image of the immediate surroundings of Beta Pictoris. Details of the complex data processing tech­ niques used to subtract the reference image of Alpha Pic shown in Figure 3 from that of Beta Pic in Figure 2 are given in Reference 1. The basic problem to be solved is to make the result of the analysis as free as possible from assumptions, biases and subjective judgements as possible. In other words, it was taken as axiomatic that no user interaction would be permissible and no assumptions about the characteristics or even the existence of circumstellar features would be allowed. This is a crucial but often neglected attribute of any method applicable to the interpreta­ tion of astronomical data of this type. The best algorithm chosen essentially Figure 5: A narrow band Nil image of R Aquarii, a variable binary witl1 known emission of consisted of a truncated least squares ionized knots along a collimated axis. Tl1e knots are resolved clearly in tl1e raw data but are fit between the two images allowing the overexposed in tl1is image to reveal tl1e presence of faint loops of material to tl1e Nortl1 of tl1e relative intensity, overall background star. Tl1e directions are tl1e same as in tl1e previous figures. and spatial registration to vary. The best values for these parameters obtained this way were then used by the pro­ fact that the variations in brightness of hotter blue companion accreting a por­ gramme to subtract the reference from the disk in the broad B, V, Rand le tion of that mass via an accretion disk the Beta Pictoris image. bands quite closely mimic the stellar (see Solf and Ulrich, 1985, and refer­ The result of this procedure for the spectrum, almost certainly implies that ences therein). Although this system is Beta Pictoris case is shown in Figure 3. the emission we detect is due to scatter­ known to be one of the most complex This figure dramatically illustrates the ing of the stellar radiation from particles observed so far, probably, mainly be­ power of the technique briefly described of average size much larger than one cause it is so close (200-300 parsecs here. The image is photometrically micron diameter. This, in turn, indicates by most estimates), some of the most accurate in that counts at any point in that some sort of accretion process has exciting phenomena occur deep within this image are linearly related to the true been active in the protoplanetary nebula the inner nebulosity extending out to emitted intensities of the protoplanetary to form large grains from the approxi­ 10-15 arcseconds at most from the disk through the absolute calibration of mately 0.1 micron sized particles com­ central object. The morphology and the optical and detector system used. A monly found in interstellar space. It physical characteristics of this nebulosi­ standard ratio image, for example, should be noted that the IRAS infrared ty are not weil known, principally be­ would not satisfy these basic require­ excess emission reported for this star by cause of the difficulty of imaging accu­ ments and, thus, cannot represent a Gillett, 1986 (Reference 3) that initially rately and reliably within a few arc­ true image of the stellar surroundings triggered interest in this particular sys­ seconds of the bright Mira whose visual dependent as it still is on spurious and tem does not necessarily have to origi­ magnitude varies periodically from mv = contaminating effects of the specific de­ nate from the same material responsible 6 to 11 in 387 days. tecting system employed in the investi­ for the scattering observed in the op­ An obvious solution to this problem is gation. This and other images of the Beta tical. offered by the coronographic technique Pictoris disk taken at La Silla in the B, V, A slightly different application of the whereby the bright Mira is occulted by Rand le bands represent the first visible coronographic technique just described the 2 arcsecond wide wedge as shown images of this fascinating feature. is shown in Figure 4 wherein an area in Figure 4. This allows long integration The physical implications of these around the symbiotic Mira variable R time exposures through narrow band measurements on Beta Pictoris and Aqr is investigated in some detail. This filters tuned to bright emission lines other candidates are described in detail fascinating system is quite likely to con­ such as Ha and the forbidden [N 11] in Reference 2. For Beta Pictoris, the sist of a mass losing Mira variable and a 6583 A. Short duration broad band R 45 exposures appropriately subtracted establishing and elucidating the construction and calibration of the from the narrow band images allow an mechanism responsible for the ob­ coronograph, to B. G. Taylor, F. Mac­ even better view of the line emission served activity in this enigmatic system. chetto and H. S. Stockman at ESA and region around this object. The image More observations of a number of in­ the ST Scl for constant encouragement shown in Figure 4 also illustrates graphi­ teresting objects with this technique at and financial support, to P. Bely, R. cally the potential of this technique as La Silla are being planned for the near Prange, R. de Grijp and A. Vidal-Madjar both previously known and unknown future. The authors welcome sugges­ for assistance in many aspect of this bright and faint features throughout the tions from readers of this publication for programme. We also wish to thank S. di inner nebulosity can be readily dis­ improvements, additions to and ideas Serego Alighieri for the flat field he kind­ cerned and accurately measured down for new applications of the basic tech­ Iy supplied and F. Noriel, for expert to approximately one arcsecond of the nique described here. If you have a editorial assistance. Mira without much trouble. Especially favourite object that might benefit from obvious is the famous jet made up of an investigation with our coronograph, References several knots extending in a generally please contact us so that we may ex­ 1. Burrows, C., F. Paresce, and A. Evzerov. northern direction towards the bottom plore the feasibility of a joint effort. Instrumentation and Data Reduction of the figure but faint wisps, knots, and a Techniques for the Detection of Circum­ counter jet extending to the limits of our stellar Material, BAA.S., 18, 1027, 1986. image in the southwest are also clearly 2. Paresce, F. and C. Burrows, Wide Band Acknowledgements discernible against the sky background. Imaging of the Beta Pictoris Circumstellar Disk, BAA.S., 18, 1027, 1986. Direct comparisons of images taken in None of this work would have been 3. Gillett, F.C., "IRAS Observations of Cool the light of several emission lines of even remotely possible without the en­ Excess around Main Sequence Stars", in elements in varying ionization stages thusiastic support of Daniel Hofstadt Light on Dark Matter, ed. F. T. Israel, D. show remarkable differences revealing a and his operations group at La Silla, in Reidel, 61,1986. complex temperature and electron den­ particular Paul Le Saux, Gerardo Ihle, 4. Solf. J., and H. Ulrich. "The structure of sity structure within the nebulosity. Michel Maugis, and A. Urquieta. We are the R Aquarii Nebula", Astron. Astrophys., These data should prove quite useful in especially grateful to Anatoli Evzerov for 148,274,1985.

Search for Supernovae in Distant Clusters of Galaxies L. HA NSEN, Copenhagen University Observatory H. U. N0RGAARO-NIELSEN, Oanish Space Research Institute H. E. J0RGENSEN, Copenhagen University Observatory

Supernovae and Cosmology SNe I near maximum rival with galaxies It has recently been realized, how­ One of the main problems of cosmol­ in brightness (Mv = -19.7 for Ho = ever, that a subgroup named SNe Ib ogy today is to determine whether the 50 km 5-1 Mpc-'). At a redshift of z = 0.5 exists in spiral galaxies. This subgroup universe is open or closed, i. e. if it will the expected peak magnitude in V is is characterized by the absence of the continue to expand for ever or if it will about 22.7 depending on K-correction A 6150 absorption feature in the spec­ recollapse in a far future. The classical and the cosmological model assumed. tra. SNe 1b will hardly cause any major attempt to settle the question is to ob­ For Friedmann models with qo = 0.0 problem for cosmological applications serve some kind of standard candle out (open) and qo = 0.5 (transition to closed) as they are about 1~ 5 fainter than the to large redshifts, z, and measure the the difference is 0.28 magnitude. A majority of SNe I. If they are not dis­ positions of the objects in the Hubble modest number of SN I events could criminated by other means they may be diagram (log (z) versus apparent mag­ therefore provide the evidence for an discarded because of gross deviations nitude). The brightest galaxies in rich open or c10sed universe. Notice, that from the predicted apparent magnitude. clusters have for example been used for neither Ho nor the absolute peak mag­ this purpose, but significant evolution nitude need to be known. The Search Programme corrections are expected wh ich are hard SNe I as standard candles are not to determine with the required precision, supposed to be plagued by uncertain With the launch of the Hubble Space and no firm conclusion has been corrections, as are other candidates. Telescope (HS1) it will become possible reached as yet. The K-corrections can be accurately de­ to do photometry on distant SNe to A more promising candidate for a termined from nearby SNe I, and no magnitudes fainter than 25, and the cos­ standard candle is the type I supernova change of the supernova characteristics mological goal is then within reach. The (SN I). SN I events show spectra and with look-back time is expected. first and difficult problem is to find the light curves which are very alike, and the A well-developed theoretical model SNe I. G.A. Tammann (1) estimates that intrinsic scatter in peak brightness is for SNe I assumes the deflagration of a a Coma-like cluster at z = 0.5 will show a less than 0.3 magnitude. SNe I occur in white dwarf wh ich is pushed to the rate of 0.5 SN I per year within the field spirals as weil as in elliptical galaxies. Chandrasekhar limit by mass accreted of the Wide Field Camera of the HST. Events in elliptical galaxies are not ex­ from an evolving companion. In this pic­ However, observing time on the HST is pected to suffer from any significant in­ ture virtually the same event happens very expensive, and fortunately the job terstellar extinction in the parent galaxy, every time with no variation of mass and can be performed from the ground. The which would otherwise be difficult to chemical composition. This explains the Danish 1.5-m telescope at La Silla is correct for with the necessary accuracy. reproducibility of the phenomenon. ideal for the task. The observing time is 46