Calibrations for the different HARPS operating modes are described in the HARPS manual, pages 14,15,16,17 and are reported here for easiness.
One OB is made available, which takes care of all the needed calibration for optimum radial velocity measurement. Althougth this OB was designed for the simultaneous ThAr mode, it can also be used by the standard spectroscopy mode of HARPS with almost no redundancy.
The Simultaneous Thorium Reference Method needs a sequence of calibration exposures to be taken before the beginning of the night. No further calibration exposures are required during the night. In order to produce the correct calibration sequence the available observing block "RV Standard Calibration" should be executed - without any changes - before the beginning of the night. It includes:
- 1 bias exposure; the CCD bias is very stable, only one bias is therefore needed by the pipeline. Template: HARPS_ech_cal_bias
- 2 Tungsten lamp exposures where respectively fibre A and fibre B are successively fed by the Tungsten lamp. These exposures are used for order location which is done automatically by the pipeline. The processed products are stored in the calibration database if they pass the quality control of the pipeline and used for the subsequent reduction of the scientific exposures of the following night. Template: HARPS_ech_cal_tun
- A sequence of 5 Tungsten lamp exposures (defined by NREP=5) where both fibres are simultaneously illuminated. This sequence is used by the data reduction pipeline for producing a spectral "master flat-field" which will be stored in the local calibration data base and used for the subsequent reduction of the scientific exposures of the following night. Template: HARPS_ech_cal_tunAB
- 2 (for reasons of redundancy) Thorium exposures in which both fibres are simultaneously fed by light from the Thorium-Argon lamps. The ThAr1, long term reference lamp, illuminates fibre A, the ThAr2 lamp inlluminates fibre B. During the night only the ThAr2 lamp will be used as the reference. The ON time of the ThAr1 lamp is minimized in order to keep it as an absolute reference along the years. Each exposure is used to build a wavelength solution. The instrumental drift with respect to the previous calibration frames is measured (expressed in m/s). If accepted by the built-in quality control, the wavelength solution is stored in the local calibration data base and used for the subsequent reduction of the scientific exposures of the following night. Template: HARPS_ech_cal_thoAB
The user may then repeat a sequence of flat-fields with more than 5 exposures if a SNR higher than 300 is aimed at in later science exposures. The flat field sequence in the "RV Standard Calibration" OB is made of 5 exposures and reaches a Signal to Noise Ratio (SNR) of about SNR=400 at 450nm, 500 at 550nm and SNR=900 at 650nm.
In case the ``RV Standard Calibration'' is not taken, the pipeline will use the closest available calibration data. This might introduce offsets, and possibly have a negative effect on the achievable precision.
The pipeline performs quality checks on each frame. In case one of the frames does not pass the quality check, the closest available calibration data will be used. In this case is however advisable to contact the support astronomer on site, in order to make sure the general health of the instrument is not compromised.
The calibration for the iodine self-calibration method includes a set of flat fields taken with the interposed cell.
All three availablble operation modes of the instrument share the same OB for the calibrations, with the exception of the self-calibration method which moreover requires iodine cell flats. Moreover the proper calibration set has to be taken before the beginning of the scientific observations in order to have the data reduced by the pipeline achieving the highest obtaianble radial velocity accuracy.
The afternoon calibration set as described above is the core of the HARPS calibration plan. Apart from allowing proper online data reduction it also allows the long term monitoring of the instrument. It is strongly advised to the users to run the standard afternoon calibration set.
The pipeline will automatically process the calibration raw data frames and alert the user if any of them does not pass the quality control.
Given that HARPS is operating for a given night essentially as a single mode instrument, that the calibration set is (almost) the same for all instrument modes and that the pipeline asserts the quality of the calibrations taken it seems redunant the use of the CalOb tool, which however is necessary for multimode instruments with different calibration strategies.