5. The observations
SV observations were prepared with the same tools which are to become standard for the regular operations of the VLT, and which are already used for observing with the NTT and the 3.6m telescopes on La Silla. In particular, all the observation blocks (OBs) were prepared with the Phase 2 Proposal Preparation Tool (P2PP) in Garching ahead of the observing period. At Paranal we modified the observation blocks only in exceptional cases, e.g. to streamline the observations of standard stars. It is envisaged that visiting astronomers using the VLT will proceed in a similar way, by producing the OBs at home ahead of time and modifying them at Paranal.
The telescope performed very well for the complete SV period with a global loss equivalent to about two nights due to technical problems. The telescope operations worked very smoothly and did not provide any problems.
The observations themselves were arranged in a fashion mimicking the Service Mode of operation, i.e. several programs were mixed together in a single observing night with attention to the specific filter combinations, the appropriate calibration data, the observing conditions, and the sky background. The SV period included new moon, but lasted until two days into the second quarter with moon illuminations up to 70% during the last night. For most programs the observations were broken into integrations of typically 10 to 15 minutes exposure time to guarantee sufficient background illumination. The individual exposures were also offset by about . Standard stars were observed every photometric night.
The data were transferred to Garching shortly after the observations. Preliminary reductions were performed by the SV team in Garching and the results (and problems) reported to the observers. In particular, the progress and quality of the observations from the HDF-S observations were monitored in Garching. In this way, it was possible to optimize the observations according to the observing conditions and the scientific priorities of the programs.
The weather has been very unusual for Paranal and highly variable during the whole SV phase, perhaps an indication that the turmoil generated by the 1997 El Niño had not settled yet. While the fraction of photometric nights has been within expectation (70%), the seeing has been much worse than usual. Actually, with a monthly seeing average of , August 1998 turned out to be the worst month since the beginning of the seeing measurements on Paranal in 1988. Together with the limitations of the Test Camera this means that the SV data are not optimal. The future dedicated VLT instruments will provide a much better match for the capabilities of the VLT telescopes. In spite of these limitations, exciting science results have still been obtained, as demonstrated by the papers included in this Letters issue.
The final data reductions had to await a detailed analysis of the flat-fielding problem. The best solution was to combine the science data to produce a flat frame which matched the color of the sky. This was mostly due to the strong color sensitivity of the large blemish of the CCD. Two sets of flatfield frames were produced for each broad-band filter. The reductions of the science images were attempted with both flatfields and the results examined visually. The better result was then kept. In case of several exposures the images were combined using integer pixel shifts. Given the small pixel size and the large over-sampling, this did not limit the resolution in the result image. The combined images, together with the raw data, were made available as a data product to the community. All raw data were further archived in the VLT data archive in Garching.
The photometric calibration was achieved through the observations of Landolt standard stars. The large oversampling of the test camera allowed us to integrate on these bright star for 10 seconds in all broad band filters thus avoiding any problems with uneven illumination due to the shutter speed. We took care to observe a significant color range for the standards to measure the color terms adequately. A photometric solution was established for every night in which sufficient standard star observations were available. Typically we observed four fields several times throughout the night averaging about 10 standard star observations in total. The coefficients were determined by measuring the total flux in a diameter aperture around the star. The average zero-points, color terms and extinction coefficients are summarized in Table 2. These values are only indicative as the photometric conditions for each of the science observations has to be established independently. The scatter around the mean for all nights in which a photometric solution was determined is given in units of 0.01 magnitudes in parentheses. These solutions should be used with great caution as they are averages over several nights. While this is a fair assumption for the color term, which is instrument dependent, this is not true for the photometric zero-point and the extinction coefficients. For individual applications it has to be made sure that the data obtained were indeed taken under photometric conditions. Some of the SV data do not fulfill this condition. The average values for the zero-point and the extinction coefficients are also not an accurate determination for an individual night. These values are available from ESO. However, we measured a very small scatter in the zero-point as well as in the extinction term, which is an indication of the stable atmospheric conditions at Paranal. The absolute range for the zero-points is always less 0.20 magnitudes and 0.14 magnitudes for the extinction coefficient. These values give an indication of the accuracy which can be achieved by applying the average photometry parameters. For the narrow-band filters we also observed two spectroscopic standard stars (Feige 110 and EG 21; Hamuy et al. 1992, 1994).
Table 2. Broad band average photometric solutions
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999