General description

The calibration plan aims to ensure that all data stored in the archive can be fully calibrated, regardless of the specific needs of the visitor's program. Indeed, it is easy to imagine a situation that the data may be used later -- after their proprietary period -- from the archive for the purpose of another object. The execution of the calibration plan means that any limitations in that respect will not come from the lack of appropriate calibrations.

Next, the calibration plan allows to monitor the long-term evolution of the instrument. In practice, this is done by observing photometric "standard star of the month" to check the instrument throughput via the zero points. The user should be aware that some time -- usually during twillight before the image analysis can be done -- will be used to carry out the calibration plan. Typically, the observation of a single star in four broad band filters takes 12-15 min.

Therefore, daily calibrations are taken as a mater of routine, both by observers wishing to calibrate their own data and by the observatory support staff. The standard procedure is to run every morning after the end of the nigh time observations a special tool calobBuilt that inspects all files taken during the night and determines what calibrations are necessary. Furthermore, this tool prepares a calibration OB that can be loaded directly into BOB and executed.

However, observers are encouraged not to rely on our calibration plan to provide them with these data, but to take their own calibration data in the afternoons before their observations start. This has a number of advantages: they would do their own quality control of the calibrations, and they will have at their disposal appropriate calibrations if they wish to reduce the data during the night.

The day-time calibration plan includes:

• Imaging: special dome flats for all filters used (including narrow band filters).
• Spectroscopy: arcs and flats for each spectroscopic mode and slit combination. Images are taken with the lamps ON and OFF for bias/dark/stray light subtraction.
  
• Polarimetry: flats.

The night-time calibration plan includes:

• Imaging: at least two photometric standards from the list of Persson et al. (1998), observed at different airmasses: one near the zentih, and another at sec(z)>1.5; for purpose of long-term monitoring on of these is "the telluric standard of the month", defined in a table bellow.
• Spectroscopy: one telluric standard per target observed at airmass within 0.15 from that of the target; it is either a solar analog or a hot star, selected from the HIPPARCOS list of stars with known spectral type.

The long-term calibration plan includes:

• Spectroscopy: sensitivities for all spectroscopic modes.