The most commonly used observing configurations of each instrument are calibrated by means of the Observatory's Calibration Plan. Calibration Plans for each instrument and mode are described in detail in their corresponding User Manual, and their execution times are not charged to the Service Mode programmes.
Please check carefully in the User Manual whether or not the instrument configurations that you plan to use are included in the instrument's Calibration Plan. If this is not the case, you must provide additional OBs for calibration appropriate for each mode not included in the Calibration Plan. Their execution time is then charged to your time allocation.
User-supplied OBs for calibration are only executed once. You should assume that your science OBs will be executed completely independently of each other, possibly on different nights, and take into account that no user-supplied OB (for a science or a calibration target) will be executed more than once. You should submit enough OBs to cover that situation. For example, if you need to observe and flux-calibrate six targets in a filter that is not supported by the calibration plan of the instrument, you need to provide six OBs for photometric calibration to allow for the case in which each target is observed on a different night. This must be done even if the same calibration star can be used for all the science targets.
Please indicate in the User Comments fields of science OBs when a user-provided calibration must be executed contiguously.
Since P90 'concatenation containers' can be used within P2PP version 3 to link science and calibration observations that must be executed contiguously.
Please note that there are two separate kinds of calibrations in operational terms:
- Calibrations that observe a reference celestial source, like a spectrophotometric standard star or a nearly-featureless star for the measurement of telluric lines. The OBs prepared for this kind of calibration are generically considered as normal OBs (to be prepared within an "OB" in P2PP3), and their construction is the same as for other science OBs. However, depending of the instrument they use either templates whose name contains the indicator calib, or flags in the templates that indicate that the observation is intended for calibration.
- Calibrations with no celestial source associated, like flat fields taken with a flat field lamp or dark exposures. OBs for this kind of calibration should be prepared within the "CB" (Calibration Block) in P2PP, as no acquisition template needs to be specified in this case. Calibrations to be executed in daytime are the most common examples in this category.
ISAAC Calibration Plan
The ISAAC instrument is calibrated by the Science Operations staff in the morning after the science observations. (Almost) all instrument setups used in the science observations are calibrated automatically. Full details can be found in the User Manual, which is available on the ISAAC Documentation page.
Due to non-reproducibility effects involving the grating and the slit, there are usually slight differences in the position of the grating and the slit when flats and arcs are taken, which is usually the following day.This can limit the accuracy at which spectroscopic data can be flatfielded and wavelength calibrated. To circumvent this, special templates have been created to allow flatfields or arcs to be taken at the end of the spectroscopic templates (ISAACLW_spec_cal_NightCalib for the Aladdin arm, ISAACSW_spec_cal_NightCalibfor the Hawaii arm). If used, these templates must be attached at the end of spectroscopic OBs.
It is believed that the flat field non-reproducibility problem only affects observations at high signal-to-noise ratio (S/N >100). It is therefore recommended that users use the night time calibration templates to take flats immediately after their observations if they want high signal-to-noise data. One should also take night time flats for the telluric standards. As this is not part of the ISAAC calibration plan users will have to provide the appropriate OBs. To get the highest signal-to-noise ratio one should set the nod throws of the telluric standard and the science target to be the same.
Observers who do not wish to obtain such high signal-to-noise data (this applies to most observations done with ISAAC) can safely ignore the night-time flat field calibrations.
Requesting night time arcs is usually not necessary for observations below 2.2 microns, since the OH lines provide an in situ wavelength calibration. Above 2.2 microns there are few OH lines, so we recommend that users requiring accurate wavelength calibration in the SWS1-MR mode above 2.2 microns use the night time calibration templates. Alternatively one can use the many telluric features as an in situ wavelength calibration, or as a means of determining the wavelength offset for observations calibrated with the daytime arcs.
Little experience has been obtained with similar problems in the Aladdin arm. However, the same template has been created for the Aladdin arm, in case users feel they need accurate flat fields or arcs. Note that the arcs should usually not be necessary, since the sky leaves plenty of telluric features for wavelength calibration. In the M band, it is not possible to do accurate wavelength calibration with the arc spectra. Although we will provide an arc that is taken with the grating in third order, the telluric features should prove to be more accurate.