Service Mode Rules and Recommendations for Observation Blocks

Preparing Observation Blocks

Observations at all ESO telescopes are carried out by executing Observation Blocks (OBs) provided by the users. OBs for all Service Mode programmes using Paranal instruments are constructed and submitted to ESO using the P2PP version 3 (P2PP3) Tool. For (designated) Visitor Mode observation preparation, please follow dedicated Visitor Mode Guidelines.

Please refer to the P2PP3 User Manual and to the User Manuals of the different instruments for more specific information on the structure and content of OBs, and how to build OBs for different instruments. A number of tutorials describing step-by-step the construction of OBs for different instruments is available.

Service Mode OBs: rules and advices

It is important to keep in mind the Service Mode policies and the following rules and guidelines when designing a Service Mode programme or when preparing a Phase 2 package:

  • Some observing strategies cannot be supported in Service Mode; in particular, real-time decisions about the sequencing of OBs, complex OB sequencing, or decisions based on the outcome of previously executed OBs (like adjustment of integration times or execution of some OBs instead of others).
  • OBs are only executed once. If you want to repeat an identical observation multiple times, you must submit multiple OBs. This requirement applies to standard stars as well.
  • OBs are normally executed non-contiguously. Since efficient Service Mode operations require continuous flexibility to best match the OB constraints with actual observing conditions, OBs for a given programme may be scheduled non-contiguously. Therefore, users should not expect their OBs to be executed in a specific sequence or in a linked way, unless a sound scientific justification (indicated in the README file and approved with a Phase 2 Waiver in case of a contiguous execution lasting longer than 1 hr) exists. Approved OB sequences should then be prepared as concatenations. Exceptions to this rule are cases in which one OB observing a calibration source needs to be executed contiguously to a science OB. In such a case place both OBs into a concatenation scheduling container to enforce their contiguous execution.
  • Multi-mode, multi-configuration OBs are normally not permitted in Service Mode. Although multiple configurations within one OB may sometimes reduce overheads, scheduling and calibrating such OBs is extremely inefficient and can increase the calibration load to an unsustainable level. Examples of such multi-configuration OBs are those combining imaging and spectroscopy in a single OB, spectroscopy with multiple grisms or central wavelength settings, or imaging with a large number of filters (although most imagers allow multiple broadband filters in one OB). Multi-configuration OBs are accepted only if duly justified and authorized by means of a Phase 2 Waiver Request.
  • OB execution times must be below 1 hour. Long OBs are more difficult to schedule and execute within the specified constraints because of the unpredictable evolution of the observing conditions. For this reason, OBs taking more than one hour to execute are accepted by ESO only in exceptional cases and provided that a Phase 2 Waiver Request is submitted and approved. In such cases, ESO will consider the OB successfully executed if the constraints were fulfilled during the first hour of execution, even if conditions degrade after that time.
  • Concatenation scheduling container execution time must be below 1 hour and exceptionally for CRIRES instrument science+telluric standard concatenation must be below 1.5h. Only in exceptional cases, and provided that a Phase 2 Waiver Request is submitted and approved, longer concatenations may be submitted. In such cases, ESO will consider the concatenated OBs successfully executed if the constraints were fulfilled during the first hour of execution, even if conditions degrade after that time.
  • User-provided calibration OBs that need to be executed contiguously with science OBs need to be specified via concatenation scheduling containers.
  • Time constraints must be indicated in the OBs. If you intend to observe time-critical events or monitor a target at specific time windows, you need to indicate this under the Time Intervals tab of the OBs. Please note that absolute (UT) time constraints refer to the interval in which the OB can be started, whereas for Local Sidereal Time (LST) time intervals, the time interval refers to the entire duration of the OB. For monitoring observations it is often more appropriate to put OBs in a time-link container. Specifying time windows as broad as possible will reduce the possibilities that your OBs are not executed because of higher priority programmes or because the observing conditions did not allow the observations during the interval that you specified.  Usage of absolute time intervals must be scientifically justified in the README file. Please read carefully the time-critial OB execution policy.
  • Specify the weakest possible Constraint Set values. OBs that can be executed under a broad range of conditions are easier to schedule. In particular, if photometry is needed of a field, it is normally sufficient to obtain a short integration under photometric conditions (transparency = PHO) and carry out the rest of the integration with OBs having a transparency = CLR constraint.

Additional Service Mode Requirements for CRIRES

Observing Modes and Templates

CRIRES is currently offered in the following two basic modes:

NGS  with adaptive optics using a Natural Guide Star 
NoAO  without adaptive optics

The preparation of Obervation Blocks (OBs) is simple, as the number of relevant templates is small: for each of the 2 above-mentioned observing modes, there are 2 acquisition templates: one for 'normal' observations and, since period 84, one for differential tracking. Moreover, there are 3 science observing templates and 4 calibration templates.

A detailed description of the templates and their parameters is provided in the CRIRES User Manual as well as a complete list of central wavelengths.

Wavelength settings

The wavelength setting is usually determined by only one input parameter, the reference wavelength, that corresponds to the wavelength at pixel 512 of the 3rd detector. In service mode, standard wavelength settings should preferably be used, as they correspond to settings that can be best calibrated. In particular, the wavelength calibration of these settings is accurate to within 3 pixels in general. Furthermore, users should be aware of the possible contamination from neighboring orders, even when using a narrow slit (the CRIRES ETC will output the useful spectral range for a given setting).

If more than one wavelength setting is used in the same OB, the overheads for changing the wavelength setting can be minimized by going from large to small encoder values of the grating. Therefore, it is highly recommended to order the wavelength settings within one OB with decreasing encoder value. The encoder values can be found in Tables 4 and 5 of the CRIRES User Manual.

Wavelength settings in the K, L and M band benefit from low values of water vapour in the atmosphere. Therefore, if more than one wavelength setting is used in the same OB, avoid to mix wavelength settings in these bands with those from the J and H band.

OB Comment Fields

Each OB has two Comment Fields: the User Comments and the Instrument Comments. The first field is not mandatory and it is usually used to specify comments relevant to the execution of that specific OB (e.g. it should be followed by its calibration OB).
Instead, it is mandatory to provide some specific information in the Instrument Comments field in P2PP. In the case of CRIRES, this information must include:

  • targeted S/N ratio per pixel for the combined spectrum created by the execution of the template at some reference wavelength
  • R magnitude of the AO guide star, if relevant
  • the magnitudes of the SV guide star and of the science target, in the observing band
  • the spectral type of the telluric star (only in calibration OBs for telluric standard stars) including the range of allowed spectral types

Example for a science OB: S/N=200 @ 2200nm; R_AO=8; K_SV=7.5; K_OBJ=10.
Example for a telluric OB: S/N=200 @ 2200nm; R_AO=8; K_SV=7.5; K_OBJ=10; SpTyp=B2V, range: B1 - B4.

DIT/NDIT and Readout Mode

There are two readout modes available: the default mode FowlerNSamp and (since P84) the FowlerNSampGRstWin mode for bright targets and reduced read-out noise (see Sect. 11.3 of the CRIRES User Manual).

The appropriate choice of DIT and NDIT should be made with the help of the CRIRES ETC.In principle, the choice of the DIT is totally free between the minimum of 1s (for the FowlerNSamp readout mode) and the maximum of 900s. In practice, in order to be able to properly calibrate all the nighttime observations during daytime,

  • observations of bright targets (J,H,K ∼ 8-10 mag) should preferably use one of the following DITs: 1s, 1.5s, 2s, 3s, 5s, 7s, 10s, 15s, 20s, 30s, 45s. In particular, to avoid saturation of the sky, observations in the L or M bands should use a DIT of 10s or less.
  • for faint targets, only the following DITs can be used: 60s, 90s, 120s, 180s, 300s, 600s. For a DIT of 900s a Waiver request must be submitted.
  • minimum DIT values for the FowlerNSampGRstWin readout mode are 0.15s, 0.25s and 0.50s, depending on the windowed area.

Users should instead change the NDIT parameter in order to achieve the desired S/N ratio.

Note that when using the FowlerNSampGRstWin mode and a window size of 64 pixels parts of extended or multiple objects may fall out of the windowed region, and thus suffer flux losses. Therefore, the jitter width must be set to 0, and only one wavelength setting per OB is allowed.

AO and Slit Viewer Guide Stars for CRIRES

The following constraints exist for Adaptive Optics and Slit Viewer guide stars:

  • AO guide stars also called Natural Guide Stars (NGS) cannot be brighter than R=0.2 mag. Stars of R = 17 or fainter will not bring any improvement to the image quality.
  • Slit Viewer Guide Stars (SV GS): reasonable SV guiding on a slit wider than 0.3 arcsec is not possible for objects fainter than J = 14.9, H = 14.5 or K = 14.4 in seeing better than 0.9 arcsec.
  • Note: Guiding on an offset star in ELEV mode is not implemented.


From P90 onwards, P2PP3 has to be used for the preparation of OBs. This offers the opportunity to group OBs into containers, i.e. into concatenations, time-links or groups.

For CRIRES observations, we recommend to put the science-telluric OB pairs into concatenations. That guarantees that both OBs are taken back-to-back. The concatenation of a science and telluric OBs can last up to 1.5hr with no need for waiver.

For time-critical observations of science-telluric OB pairs, we recommend to put only the science OBs into a time-link container, but leave the telluric OBs loose, i.e. do not define the same time sequence for telluric OBs. This avoids the case that science and telluric OBs get out of phase, if for some reason a telluric OB was not observed (for example, the conditions degraded during the execution of the science OB, an observatory telluric was taken instead, etc.). Only if the exact observing date for a time-critical observation is known, you might put a science-telluric OB pair into a concatenation and define for both OBs the same absolute time interval.

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