Seeing the Light Through the Dark J
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No. 103 – March 2001 Seeing the Light Through the Dark J. ALVES1, C. LADA2 and E. LADA3 1European Southern Observatory, Garching, Germany 2Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA; 3University of Florida, Gainsville FL, USA 1. Introduction: The Search for years of study, little is understood about posed of molecular hydrogen, which is the Initial Conditions for Star the internal structure of these clouds virtually inaccessible to direct observa- Formation and consequently the initial conditions tion. Because of its symmetric struc- that give rise to star and planet forma- ture, the hydrogen molecule possesses Stars and planets form within dark tion. This is largely due to the fact that molecular clouds. However, despite 30 molecular clouds are primarily com- (Continued on page 15) Figure 1: Visible and near-infrared images of Barnard 68. The images are a B, V, and I-band composite (left) and a B, I, Ks-band composite (right). At visual wavelengths the cloud is completely opaque owing to extinction of background starlight caused by small (~ 0.1µm) interstel - lar dust particles that permeate the cloud. The red stars detected at 2 µm through the visually opaque regions of the cloud (right) are the stars that will provide direct measurements of dust extinction through the cloud. 1 T E L E S C O P E S A N D I N S T R U M E N TAT I O N La Silla Telescope Status, a Great Achievement on Image Quality Performances A. GILLIOTTE, ESO Nowadays almost all La Silla tele- cell was designed for such a process, by the 3.6-m was found insufficient be- scopes deliver very good image quality, then on the 3.6-m and is soon to be car- cause the improvements in the per- routinely achieving sub-arcsec images. ried out on the 2.2-m as well. formance and dynamic range of the de- In some cases, the theoretically pre- The optimal frequency of the CO2 tectors showed more accurately the im- dicted performances of the telescope is cleaning, resulting in clean mirrors age quality achieved by the telescope. matched, or the limitations are at least without too much time spent on the op- With the availability of wavefront sen- understood. eration, was found to be one week. The sors – the first one available was a The image quality, however, is not NTT was the test bench for the process Shack-Hartmann type called Antares – only a function of the telescope optical with the mirror surface maintaining the optical quality of the telescope was set-up, it is also highly dependent on good reflectivity for a three-year period measured more often, and more pre- the cleanliness of the optical surfaces. with the regular CO2 cleaning and four cise aberration information was ob- “in-situ” washings (Fig.1). tained. On several occasions, large Main Mirror Maintenance Aphotograph of the washing operation aberrations were identified. The spher- Five years ago, systematic cleaning at the NTTis also presented in Figure 2. ical aberration, often mistaken for see- of the primary mirrors using CO2 was All the above-mentioned telescopes ing quality, appeared clearly and only implemented on the major telescopes: were realuminised during the year 2000 the decentring coma was adjusted after the NTT, 3.6-m, 2.2-m and Danish 1.54- and the reflectivity is still above 88% at measurements performed only on m. Main mirror cleaning is now carried 670 nm with low roughness. The 2.2-m zenith. It was found that almost all third- out by the telescope team as part of the main mirror suffered some water con- order aberrations were affecting the tel- routine maintenance operation. T h e tamination during the bad winter period escope. The most critical issue con- only restriction with this process is that and a first washing is scheduled for the cerned the instability of the aberrations the humidity cannot be too high. near future. Recently, the 3.6-m main at different telescope orientations as Indeed, high humidity causes the con- mirror also suffered water contamina- well as with changes in temperature. A densed water surrounding each cold tion from cooling liquid, and the wash- dedicated study with eventual periods CO2 particle to stick the dust to the mir- ing is already scheduled end of March. of closed telescope was really compul- ror surface, resulting in an accumula- The status of the mirror is verified on sory. In 1994, a complete study of the tion of dust on the mirror. a monthly basis with a measure of the 3.6-m with a contracted optical engi- A mirror water washing operation, R% and roughness. A database of the neer on La Silla was initiated to deter- performed by the opticians, was also mirror status for the main telescopes mine the status of the telescope and to implemented about three years ago. will soon be available on the La Silla find a way of improving it. The best period to wash the mirrors is web site. During this process, we gathered con- the end of the summer, when the high siderable experience in solving problems humidity is decreasing and hence the Telescope Image Quality with the telescope, but also a deeper frequency of the CO2 cleaning was dis- About nine years ago, the awareness knowledge of the limitations of the tele- rupted. The mirror washing was first im- of the observer towards the image qual- scope was reached. The 2.2-m, Danish plemented on the NTT, whose mirror ity increased and the quality delivered 1.54-m and the NTT have all benefited from this exercise. Of course, the active optics concept, greatly demon- strated with the NTT, allowed a better understanding of the local thermal con- tribution. The contributions of dome and mirror seeing were identified and a solution to decrease both has been found. Thanks to all the La Silla staff for their ongoing interest and success in im- proving the image quality at the tele- scopes on site. The NTT Status This telescope delivers the best im- age quality on La Silla with the active optics system fulfilling all expectations. Initially, the limiting factor for this tele- scope was found to be the image qual- ity of the La Silla site. Now, after a long period of operation, more image quality limitations have appeared. To monitor the image quality, the NTT has a dis- tinct advantage over the other tele- Figure 1: The variation of the reflectivity (R%) and roughness of the NTT main mirror during scopes as repeated image analysis is the period from August 1996 to August 2000. made during the observations. All re- 2 en to reach the same operational mode as used on the VLT. A complete maintenance of the asta- tic levers is underway to recalibrate and readjust each lever. For perfect imaging quality at the zenith, each lever must be adjusted to the medium range of the counterweight move, and the spring compensation tuned to compensate the spherical aberration constant term. This time-consuming operation will be performed during 2001. The possible upgrade of the active optics will also include a standardisa- tion of the present software to comply with the latest version at the VLT. The 3.6-m Status Excellent work at the 3.6-m has re- sulted in improved image quality due to Figure 2: The NTT main mirror being rinsed with distilled water. The last washing step before the minimisation of the spherical aber- drying the mirror surface. ration, triangular and astigmatism con- tributions, even when the telescope is sults are logged in a database, and sta- – Install a Nasmyth shutter to close inclined. In addition, the dome and mir- tistics can be performed with respect to the optical path not in operation. ror seeing contributions are now almost external meteorological conditions or The new baffle and fans have been negligible. The main limitations affect- other parameters. The image quality installed and the new results already ing image quality are the coma instabil- generally becomes poorer when the show a substantial improvement in the ity when moving the telescope far from wind speed is below 5 m/s or if the tem- image quality (cf. Fig. 3b). the zenith. Astigmatism is still slightly perature is dropping rapidly as demon- The air-conditioning of the instrument high, with a contribution coming from strated by Figure 3a. rooms was slightly improved but an op- the axial fixed points. A new design of Several recommendations were timal solution could not be implemented the axial fixed point contact on the mir- made to the NTT team to improve the because of high cost. ror back side is compulsory to reduce a image quality: The timing of a complete image lateral stress applied on the mirror. – Install high flux fans to produce ar- analysis and mirror correction cycle is A new support for the M2 unit is cur- tificial wind to reduce mirror seeing. unfortunately long in comparison with rently under study. It will mainly be a – Install a new M2 baffle with open the VLT active optics system. On some copy of the present NTT M2 adjusting shade to avoid warm bubble forma- occasions, more frequently than be- support. The improvement to image tion. fore, the active optics is not converging quality will be significant, with the coma – Improve the air conditioning in the properly. Hence the telescope does not variation kept under control at a low instrument room to avoid a positive gra- always operate in closed-loop correc- value. We are still awaiting the green light dient.