It has been more than one year since the appearance of the third issue of the Calar Alto Newsletter. A number of fascinating activities have been carried out since then, among them the modification of the 3.5m dome and the introduction of longer blocks of service observations for the regular astronomical programmes. Below, we are summarising some of the recent activities, and include a status report of our image quality programme at the 3.5m telescope. This fourth edition of the Calar Alto Newsletter also includes articles by Rossa & Dettmar Bejar, Rebolo & Zapatero-Osorio and Leinert et al. on recent scientific activities on Calar Alto.
In order to improve the image quality at the 3.5m telescope, a significant modification of the dome has been carried out by Dillinger Stahlbau. A total of 16 ventilation doors provide a net opening of 20 square meters. The telescope is operational again since 20-10-01 and first results show that the vents provide a very efficient ventilation of the dome, particular at times when the dome has been closed for a few days because of bad weather. The astronomy group is presently gaining empirical knowledge to determine an opening and closing procedure for the vents which will best benefit astronomimcal night time observations. The efficient ventilation of the dome will help to quickly remove local seeing degradations which are by temperature gradients within the dome and to the ambient, outside air. Apart from local turbulence, static telescope aberrations may degrade the image quality at the 3.5m telescope. We have carried out curvature sensing and Shack - Hartmann tests (using Puntino, which was brought to Calar Alto and for which we thank Dr. Rajiv Bhatia) at the telescope which may indicate that the mirror cell support is not working in the most perfect way. We are presently planning to install load cells to measure the actual force distribution. Those measurements should tell us without any doubt if the mirror support works the way it is supposed to. We have also introduced an optical engineering programme for the 3.5m and 2.2m mirrors. We regularly measure reflectivity and scattering using a commercial device from DMO. Based on those measurements, we determine washing and CO2 cleaning intervalls. We aim to maintain the reflectivity of the mirror above 87% at any given time.
Our differential image motion monitor delivers data on-line at the telescope in 20sec intervals routinely now. A few months of data in total show that the median seeing is 0.9 arcsec, with a dispersion of 0.28 arcsec. There is still an unclear issue about the absolute calibration of the DIMM. A recent comparison with IACDIMM, the instrument operated on La Palma, remains inconclusive. During a 3-week campaign on La Palma, our DIMM measured seeing values which were about twice as large as the values provided by IACDIMM, with median values from night to night ranging from 0.9 - 4.9 arcsec. We note, however, that IACDIMM was operated with a camera on loan from the ING after the original camera on that system broke down. This may have caused a problem with the software gauging of the IAC DIMM which is not understood yet.
In order to fully characterise Calar Alto, determine the seeing, and the optical parameters of turbulence, Dr. Aziz Ziad from the Universite de Nice Sophia Antipolis will bring his generalised seeing monitor (GSM) to Calar Alto. Measurements are planned in parallel with the May ALFA run. We trust that after his run, together with ALFA and our continued DIMM measurements, a very clear understanding of the site characteristics of Calar Alto will be available.
In recent years, service observations combined with queue scheduling (service/q) have been exploited at many observatories in order to be able to carry out a scientific programme at the atmospheric conditions required, thus avoiding the disadvantage of a rigid schedule which often forces the scientific programme to adjust to the actual conditions (seeing, transparency, to some extent moon). The main driver for service/q observations is contained in the argument that it is more sensible to have fewer runs with scientifically meaningful data rather than many runs with marginal data sets. Another way of putting this is to say that it is not desirable to have programmes near the cutoff line receive the very best nights by pure luck while the programme with the highest ratings gets clouded out. Calar Alto should probably aim to deliver quasi-guaranteed data for the highest rated programmes, ie assume the commitment to complete a given programme whenever it is possible, even if this takes more than 1 semester. In the spring semester of 2002, the five most highly rated programmes on Calar Alto were completed in service mode. Requests for service observations are rising, with presently more than 50% of all observations at the 3.5m telescope and some 30% at the 2.2m telescope being carried out in service mode. With the presently built joint control room for the 3.5m and 2.2m telescopes, about 2/3 of all programmes may be carried out in service in the near future.
The contribution by Hopp & Fernandez demonstrate the large variability in the near-UV to near-IR excinction coefficients which are possible on Calar Alto. Their measurements cover the 1986 to 2002 period and provide an extremely useful reference to the extinction variations one might expect. The authors point out the problems in using a standard extinction curve for Calar Alto. In order to get a good determination of the extinction at any given night, Ulli Thiele of the Calar Alto Astronomy group has been working on the installation on an extinction monitor, which will deliver extinction measured in V to the telescopes. The extinction monitor is in its testing phase but seems to work quite reliably. We expect to deliver extinction values to the fits header of regular Calar Alto integrations within the next few weeks.
The scientific contributions demonstrate once again the most interesting science which is being carried out using medium-sized (2-4m) telescopes. The manuscript by Rossa & Dettmar summarises the results of observations gathered with CAFOS at the 2.2m telescope on the diffuse ionised gas in the halo if nearby edge-on spiral galaxies. From their Halpha survey it appears that DIG is not a unique phenomena in a few galaxies only, but that it is present in a large fraction (40.5% in their sample) of spiral galaxies. Bejar et al. provide a census of the brown dwarf population in the Sigma Orionis cluster. As a major new finding, they suggest that isolated planets are as numerous as stars in the cluster. The question arises whether these objects form dominantly as stars, or as planets. Leinert & friends summarise their lunar occultation runs on the quasar 3C273 carried out using the fast readout mode of OmegaCass (32x32 pixel subarray) at a frequency of 8 milliseconds.
