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The Messenger

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The Messenger THE MESSENGER No. 7 - December 1976 A Giant Leap for European Astronomy FIRST PHOTOGRAPHS WITH THE ESO 3.6 METRE TELESCOPE One of the first prime-foeus photo­ graphie plates taken with the giant ESO tele­ seope on La Sil/a shows a field with a newly diseovered irregu­ lar dwarf galaxy in the southern eonstellation Seulptor (see page 16). Ob­ server: Dr. S. Laust­ sen, ESO; expo­ sure 120 min on November 11, 1976; emulsion IIla-J baked in nitrogen; filter GG 385; blue eorreetor lens; smallest images 70 mierons = 1.3 areseeond. "First Light" for 3.6 m Telescope (see page 16) was on the programme, and a one-hour ex­ posure revealed its beauty and showed a good number of its brighter stars resolved. This first plate has been fol­ A first look at the sky through the ESO 3.6 m telescope with lowed by others of up to two hours exposure (see the its mirror aluminized was taken during the night of Novem­ cover photo). During the first five nights we have taken ber 7-8. The "first light" to a telescope is a unique moment, about 30 plates and the image quality looks very good. We and it was celebrated with a midnight tea. But apart from still have to determine the limiting magnitude, but it that, we had many troubles that night. Obviously you can­ should be at least 24m, possibly even fainter. not expect everything to work at once in an instrument as A lot of work has been done to getthis far. In the last issue complex as the 3.6 m telescope; much time must be of the Messenger we reported that the mechanical assem­ allowed for debugging the entire system. The people who bly of the telescope was completed. Since then, the tele­ work with it have to learn how to handle the various proce­ scope controls have been put to work by a team from the dures, but after a very successful aluminization, we had Electronics Group from Geneva who spent more than a nearly forgotten that something could go wrong. And so, month on La Silla.The optics have been installed and a sys­ during the night of first light, the images (what a disaster!) tematic alignment and test programme has been carried looked like sm all hearts. Very romantic, although not out by a team of three from the Optics Group in Geneva d ur­ exactly what astronomers are looking for in the sky. It took ing the last two months. As mentioned above, the mirror us two days to find and cure that fault. The 11-ton main has been aluminized. Not a small job either, but it was so mirror had chosen to rest on two ofthe back supports only. weil prepared that already the first aluminizing gave an ex­ From the third one it kept a respectful distance of 1 mm. cellent result. On these big jobs as weil as on hundreds of The third night (November 9-10) was much more suc­ smaller ones, the work has been progressing steadily, al­ cessful. The newly-discovered dwarf galaxy in Sculptor though not without minor setbacks. On August 26, a light- The ESO 3.6 metre telescape on La Silla. In the front end the prime-focus cage. 2 A look down in the prime-focus cage. To the right, the astrono­ Preparing observations. The astronomer is about to enter the mer's chair, from which he guides the telescope during the expo­ prime-focus cage, which he rides during the exposure. To faci­ Sures. In the centre the adaptor with an eyepiece for direct viewing litate entry, the telescope is brought to horizontal position. The and focussing. Above this the TV-camera (see text). exchange of top-ends, wh ich is a unique feature of the ESO tele­ scope, is also done in this position. ning struck the dome and caused a lot of damage to the pression that our work on the telescope is finished. So far, electrical installation and the newly-installed electron ics. A only the prime focus is in operation. A great many im­ few weeks later apart of the building was flooded with oil, provements and minor jobs still have to be made. But in and cleaning-up took several days. Never mind, we feel between, the observations contin ue. And it is our belief and that the photos of the last nights more than compensate hope that the percentage of time devoted to astronomy will the difficulties behind! from now on steadily increase. Now, however, we should be careful not to give the im- S. Laustsen, November 12, 1976 Optical Alignment of 3.6 m Telescope and First Tests The Optics Group from Geneva has been intensively occu­ at least an order of magnitude more precise than visual or pied for the last ten weeks with the alignment and testing of photographic assessments. the prime-focus optics for the 3.6 m telescope. As soon as the Hartmann plates have been measured on The basic alignment of the optics of the telescope per­ a more accurate measuring machine in Europe, a complete pendicular to the declination axis was completed about report of the test results will be published. These results three weeks ago. Since that time, an intensive period of will refer to the naked mirror and to the complete prime­ Hartmann testing has fully occupied uso focus system with the Gascoigne plate correctors. The trip­ let correctors will be available in a few months and will be The measuring facilities at present available on La Silla the subject of a further report. are not sufficiently accurate to give a final figure for the concentration of geometrical energy in a given diameter. The Cassegrain-focus alignment and tests should take However, there ist clear evidence that the specification of place about next March. 75 % within a diameter of 0.4 arcsecond should be ful­ R. Wilson, October 29, 1976 filled-we think probably by a clear margin. The computer analysis of the plates shows that the basic, lower-order­ aberration terms are smalI; while the workshop tests had already establ ished that the su rfaces are very smooth. Tur­ bulence effects in the dome and telescope seem, at pre­ The Prime-Focus Cage sent, to be the factors limiting quality and the precision of centring. However, even with the existing plate-measuring facilities, it has been possible to centre the system to within The first plates have now been taken with the 3.6 m tele­ 0.2 arcsecond of tangential coma, in spite of dome turbu­ scope. This was done in the prime-focus cage that allows lence and indifferent external seeing. the astronomer to ride in the front end of the telescope during the observations. In the following we shall explain External seeing has been mainly poor d uring the whole how the cage was equipped for the first test of the tele­ test phase, but the first photographs with the telescope scope. have shown very circular images of faint stars on IIla-J 1 plates with diameters of 1 to 1 / 2 arcsecond. With the In the cage there is room for one astronomer, an adaptor actual seeing conditions prevailing, the Hartmann tests are and some auxiliary equipment needed by the astronomer 3 d uring the observations. The adaptor may be used either as neath. The pedestal includes a high-precision motor­ a camera or as a su pport for other detectors. The adaptor is actuated focussing drive which may move the whole adap­ placed on a pedestal protruding into the cage from the unit tor (or any other instrument) along the telescope axis. The underneath the cage through a hole in the floor of the cage. speed is variable by means of a thyristor controller. The po­ That unit is supported by the structure of the telescope sition of the focussing drive is measured by an encoder and through the main spiders. The cage itself is supported by is shown on a digital display on the wall of the cage. add itional upper spiders. Due to this double-support struc­ The adaptor is seen on the photo. For direct prime-focus ture, only a minimum of vibrations are transmitted to the photography, the adaptor is mounted together with a sup­ adaptor from the cage motors or the astronomer. port carrying a one-element Gascoigne corrector lens The cage thus supports only the observing astronomer (not shown). The obtainable field is approximately 17 arc­ and the auxiliary equipment. The observer sits in achair, minutes when using this corrector lens. which is adjustable in height by means of a motor-drive. The adaptor is equipped with 6 filter-holders and 6 Furthermore, the whole cage may be rotated in order to plate-holders for photographic work. An eyepiece may be provide a maximum of comfort for the astronomer in all po­ mounted on one of the plate-holders. The TV-guide probe sitions of the telescope. All necessary control-panels are has a field of 1 arcminute, which may be placed anywhere situated on the wall of the adaptor and within the reach of along the periphery of a circle with a diameter of 27' and the astronomer. A TV-monitor displaying a guide-probe concentric with the plate field. One of four guide-probe fil­ image from the adaptor and a handset to control the tele­ ters may be selected. The TV-camera is equipped with an scope facilitate manual guiding from the cage.
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