QC0 OB grading

Basic OB grading policy

In this page, an overview of the Quality Control level 0 (QC0) OB grading process is provided.

Service mode observations are graded in near real-time according to the high-level policy published in the Call for Proposals:

    "ESO will consider an OB as successfully executed if all the conditions in the constraint set are fulfilled. OBs executed under conditions marginally outside constraints by no more than 10% of the specified value will not be scheduled for re-execution."

The following guidelines are adopted in the implementation of the grading policy and related to the phase 2 of OB preparation:

  • The Basic Requested Constraints set (BRC) defines the oberving conditions and consists for most instruments of the following parameters: Image Quality (IQ), Airmass, Sky Transparency, Fractional Moon Illumination, Moon Distance, Precipital Water Vapour, Twilight Allowance. Parameter definitions are detailed here.
    • NB1: The BRC for SPHERE and the interferometric instruments provide Turbulence Category (TC), instead of IQ.
    • NB2: The BRC for CRIRES and MUSE include both TC and IQ as the instruments provide both full-AO and noAO/GLAO modes.
  • Each constraint set parameter is evaluated individually against the PI requested value and receives a grade "A", "B", or "C". Grade "A" means that a constraint parameter is fullfilled. Grade "B" means that a constraint parameter is not fulfilled but within 10% of the requested value. Grade "C" means that a parameter constraint is violated by more than 10%.
  • Parameters are evaluated during the first 60 minutes of the OB execution, minus the time it takes for the acquistion of the source (as this does not effect the data delived by the science template). E.g., if the acquistion template takes 10 minutes, then the parameters are evaluated over an "evaluation period" of 50 minutes.
  • The final OB grade is an aggregation of the individual parameter grades, where the worst parameter grade determines the overall final OB grade.

Basic Requested Constraints set and their values

A description of the assessment for each of the constraint parameters is provided. Details on the parameter definitions are given here.

ASM parameters

The measurements for grading parameters related to ambient conditions are retrieved from the Atmospheric Site Monitor (ASM) database. The ASM provides a typical sampling of 1.5-2 minutes.

For OB grading, the ASM values are averaged over the duration of only the science exposure of the OB. Variability of the ASM parameter measurements  during the science exposures can be due to rapid fluctuations of the atmospheric conditions or because of ASM measurement uncertainties which are inconsequential for the quality of exposure. The OB grading is therefore done using the ASM values smoothed by a 10 minute rolling average.

The measurements are publically available at the tables from these links:

The BRC parameter turbulence category is a combination of coherence time and DIMM seeing. Each of these two parameters contributes individually to the final, aggregated, OB grade.  

Image Quality

The seeing indicates the atmospheric turbulence at 500 nm at zenith. Image quality (IQ) is the FWHM of the resulting point spread function on the detector. Users specify this IQ as a constraint in their OBs. Whenever possible, the IQ is measured directly on the science detector (for imagers), on the instrument's acquistion camera, on a reconstructed image (IFU), or at a reference wavelength in harmony with the wavelengths used in the ETC and p2 (for spectrographs), see the table below. IQ is therefore a QC0 parameter.

To estimate the image quality in the absence of a direct measurement possibility (for example for fibre spectrographs or target fields with no bright point sources) it is computed from the seeing taking into account the airmass, the wavelength setup of the observation, a turbulence model and the telescope and instrument transfer functions. See the ESO ETC pages for details on the implemented formulae.


IQ reference wavelength
600 nm
700 nm
at the central wavelength of the bluest setting
Others at bluest wavelength covered by the setup

FLI and moon distance

The provided moon constraint parameters are assessed as maximum allowed values. If violated by more than 10%, the observation is graded "C".


The provided airmass constraint parameters are assessed as maximum allowed values. If violated by more than 10%, the observation is graded "C".


The sky transparency is defined by the weather officer for CLR, THN, and THK conditions. Photometric conditions (in the optical passbands) are declared by means of FORS2 or OMEGACAM zeropoint measurements.

  • Photometric: No visible clouds, transparency variations under 2%, only assessable by the analysis of photometric standard stars.
  • Clear: Less than 10% of the sky (above 30 degrees elevation) covered in clouds, transparency variations under 10%.
  • Thin cirrus: Transparency variations above 10%.

The transparency of the mid-IR sky is probed and assessed by computing the sensitivity reached by mid-IR standard star observations.

Internal parameters

In addition to the user provided constraints of the BRC parameter set, the QC0 process foresees in real-time instrument performance and execution monitoring to make sure that the highest achievable data quality is reached. Depending on the instrument and data type, different parameters are verified and may trigger follow up investigation or verification of their effect on the data quality weighted by their science purpose. PIs can be contacted by USD for data verification.

The QC0 grading process takes the following additional parameters into account, depending on the data type produced by the instrument. These parameters are referred to as "QC0 internal parameters" as they are more closely related to instrument+telescope performance, rather than ambient observing conditions.

  • Ellipticity for imaging instrument modes
  • Signal-to-noise ratio for spectrographic instrument modes
  • Saturation
  • RTC (SPARTA) performance for MUSE NFM
  • Low wind effect for SPHERE
  • Mid-infrared sensitivity for VISIR imaging and spectroscopy
  • Conversion factor for VISIR imaging
  • Fringe quality and transfer function (piston stability) for interferometric instruments
  • Fringe tracker "lock ratio" for Gravity and GRA4MAT



Special cases

Long OBs (transits)

Very long OBs, much longer than 1h, can be accomodated via waiver request, e.g., for exoplanet transits. Such OBs are subject to special classification rules as the weather stability cannot be guarenteed over such long stretches of time. Therefore, if a first execution attempt of a long transit OB has to be terminated (e.g., due to either weather degradation or technical problems) leading to scientific uselessness, the long OB will be queued for repetition. However a second failed execution will result in a "D" classification, meaning it will be taken out of the queue and not scheduled for repetition.

Concatenations longer than 1 hour

Concatenations are mostly used to frame science OBs with calibration OBs. Changing weather conditions may invalidate the usefulness of the calibrators. For this reason each OB in a concatenation with a calibrator, is graded individually on the 1 hour rule. This applies in particular to calibrators observed more than 1 hour after the start of the concatenation, in case a waiver has been granted for longer duration of a concatenation. An OB graded C within the concatenation invalidates the entire concatenation.