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 Service Mode runs with Paranal Instruments must be made with p2. For (designated) Visitor Mode observation preparation, please follow dedicated Visitor Mode Guidelines.

Please refer to the Phase 2 step-by-step preparation with p2 page 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. Usuers who do require contigous observations might use concatenation scheduling container (see definition below) by follwing the rules applied to this case.
  • Multi-mode, multi-configuration OBs are normally not permitted in Service Mode. Although multiple configurations (e.g. combining imaging and spectroscopy) within one OB may sometimes reduce overheads, scheduling and calibrating such OBs is inefficient and can increase the calibration load to an unsustainable level. 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. 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 a user intends to observe time-critical events or monitor a target at specific time windows, this must be indicated 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 (most relaxed) possible Constraint Set values. OBs that can be executed under a broad range of conditions are easier to schedule. In particular, for photometric calibration 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.
  • Nested scheduling containers (see definition below) should be used only if strictly necessary as they increase complexity for observations and scheduling.

Time-linking of OBs

Some OBs must be executed within precise time windows for scientific reasons, rather than any time when the external conditions (moon, seeing, transparency...) would allow the execution. The following types of time-dependencies can be recognized:

  • Absolute time constraints, meaning that an OB must be executed at specific dates that can be predetermined. An example is the observation of a binary star at a precise phase of its period or a planetary transit observation.
  • Relative time links, implying that an OB must be executed within a time interval after the execution of a previous OB, but not necessarily at a fixed date. Examples of this are monitoring observations of a variable source at some pre-defined intervals.

Both types of time-dependency are implemented within p2. Whereas absolute time constraints are available at the level of single OBs, the relative time links are implemented within the new "Time Link" container.

Within a Time Link container, the user can define a series of OBs, having the earliest and latest time when a given OB in the series must be executed with respect to the preceding OB. The time-related information is stored in a database, from where it is retrieved by scheduling tools available to the operator on the mountain in order to build up a short-term schedule that properly takes these constraints into account.

If an OB with absolute time constraint or time-linked OB that acquired an absolute time constraint following execution of a previous OB in sequence (i.e. OB to be observed after earliest from and before latest from time of the previous OB in the sequence) is not successfully completed within the specified time interval, it will expire and get status F(ailed). Such an OB is not observable any more and policy for time-critical OB execution applies.

If the time-linked OB expired in the middle of a time-link sequence, the sequence execution continues as follows:

  • If the failed OB is not the very first OB of the time link, it had the absolute time window corresponding to delay from the previously executed OB. After it expired the next OB acquires an absolute time window by adding the relative minimum and maximum time delays to an assumed hypothetical execution for the failed OB in the middle of its constraint window. 
  • If the failed OB is the very first OB of the time link, the failure can only occur if this OB has one or more absolute time constraints defined and all of them have expired. In this case the next OB acquires an absolute time window by adding its relative minimum and maximum time delays to an assumed hypothetical execution of the failed OB at the end of its last absolute time interval. 

It should be noticed that, depending on the length of the relative time intervals, and the delays between them, a failure of an OB in a sequence may result in a cascade of failing OBs.

Concatenation of OBs

In some cases it may be desired to execute the OBs consecutively, with no other observations in between. This has been implemented in p2 within the "Concatenation" container. The Concatenation container consists of two or more OBs that must be executed "back-to-back" without breaks. The sequence of the execution of OBs in a Concatenation typically follows the sequence as they are listed in the p2 window. 

Definition of groups of OBs

Groups of OBs are used to express the preference to complete observations of a given group of OBs before continuing with other OBs (or groups of OBs) within the same observing run. This is the most loose scheduling container concept, and the priority for execution of the group with respect to other groups within the same run is defined though group priority that has values 1-10 (1 top, 10 lowest priority) as for the user priority for loose OBs. The priority for execution of OBs within the given group is regulated through the OB group contribution. 

If OBs within the group, whose observation started, are not observable (constraints are not fulfilled), it is possible to start observations of another group. After that group score defines which groupo will be given priority in case both groups of OBs are observable again. 

Definition of Nested Containers

For Service Mode observations that use VLT or VLTI instruments on Paranal it is now possible to design more complex observing strategies in the p2 tool with nested scheduling containers. For example a science case that requires time-monitoring of a set of concatenations of science+telluric OBs can be expressed as time-links of concatenations. For the VLTI imaging observations use of groups of pairs of science+calibrator is mandatory, such that the group defines the set of concatenations that contribute to the same image or uv plane. 

Additional Service Mode Requirements for ERIS

Observing Modes and Templates

These ERIS observing modes are offered for Period 113:

AO modes noAO, NGS (Natural Guide Star), LGS (Laser Guide Star), LGS-SE (Laser Guide Star - Seeing Enhancer = LGS without a TipTilt Star)
IFS/NIX targets Sidereal and non-sidereal, including differential tracking
IFS/NIX field tracking all modes except NIX APP, FPC, and SAM observations
IFS/NIX pupil tracking only NIX/APP and NIX/SAM
SPIFFIER all gratings and plate scales
NIX imaging 13mas and 27mas cameras, all filters
NIX windowing use of a sub-array of the detector
NIX APP Apodizing Phase Plate.  13mas camera, narowband K- and L-band filters only, pupil tracking only
NIX SAM Sparse Aprture Masking.  All SAM masks, pupil tracking only.
NIX FPC Focal Plane Coronagraphy.  13mas, L and M-band filters only, pupil tracking only
NIX LSS Long Slit Spectroscopy. L-band low resolution spectroscopy with only one setup
NIX Fast and slow readout modes
IFS/NIX non-sidereal/differential tracking Observations of moving targets
IFS/NIX pupil tracking Tracking the pupil rather than the field.  Note this is offered for NIX APP observations.
Rapid response mode Automatic triggering of observations of transient phenomena requiring almost immediate follow-up.

These ERIS observing modes are NOT offered for Period 113:

AO spatial filtering The mechanism is fixed at the largest diameter aperture
Pupil tracking Except for NIX APP, FPC, and SAM observations
NIX pupil wheel filters Blocking and Spider are not offered.

Additional modes may be offered in Period 113 depending on the progress of commissioning activities.

Magnitude limits for Natural Guide Star (NGS) and Tip Tilt (TT) stars

Mode GAIA BP GAIA RP Off-Axis range Airmass range
NGS <= 19 <= 11 0-60" (<30" recommended) <= 1.9
LGS <= 19 7-18 0-60" <= 1.9

Note: LGS with TT stars between 16-18 can only be performed when the fraction of lunar illumination (FLI) is 0.5 or less. See the constraints page and user manual for further details.

You must not attempt to circumvent these limits in p2. NGS and TT star magnitudes will be verified using the coordinates provided. OBs with guide stars outside of these limits will be rejected.

Angular Size of Guide Star (NGS only)

Performance when guiding on extended sources has not yet been tested. The NGS source must be point-like.

Target and Guide Star Coordinates in P2 – IFS/NIX

You must ensure that the coordinates for the NGS or TT star are at the same epoch and using the same equinox as the target. This is done automatically by the ObsPrep tool when selecting any object from a catalog (assuming the object has a proper motion). If you manually enter these coordinates, you must be very careful when using coordinates from different catalogues; these must be manually precessed to the same epoch (ideally the middle of the requested period) and equinox (ideally the ICRS frame). Accurate and precise coordinates for both the NGS/TT star and the target star in the same reference frame will enable a rapid acquisition as the offset between the AO stage and IFS is known precisely.

Acquisition Position – NIX

The target will always be acquired at following detector pixel coordinates:

  • nixIMG
    • windowF: (960, 1050) (laser location on NIX detector)
    • window2: (1024, 1536) or (1024, 512) in the sub-array
    • window4: (1024, 1536) of (1024, 256) in thew sub-array
  • nixAPP (coordinate of the central calibration spot), nixSAM:
    • windowF: (800, 1536)
    • window2: (800, 1536) or (800, 512) in the sub-array
    • window4: (800, 1536) or (800, 256) in the sub-array

You must not request the target to be centred elsewhere in the field of view during acquisition. Instead, use an offset to move the target to the desired position in the first science exposure.

Acquisition Position – IFS

The target indicated in the finding chart will always be acquired at the center of the selected field of view specified in the acquisition template. An offset can be used to re-center the object in the science exposure if the field of view of the first science template is different from the acquisition template, if precise centering is required.
You must not request the target to be centered elsewhere in the field of view during acquisition. Instead, use an offset to move the target to the desired position for the first exposure of the science template.

Offset Conventions

Note that the detector orientation is not consistent between the different ERIS modes, the direction of East is flipped with the NIX 13mas-JHK and 13mas-LM cameras:

  • IFS, NIX 27mas-JHK camera: North up, East left at PA=0
  • NIX 13mas-JHK, 13mas-LM: North up, East right at PA=0

All offsets are given in arcseconds and all offsets are telescope offsets. A “SKY” offset moves the telescope by the corresponding amount in RA/Dec, a “DETECTOR” offset moves the telescope by the corresponding amount in X/Y, i.e. will move the target in -X/-Y. For example:

  • A “SKY” offset of +1” in RA with the 27mas-JHK camera with PA=0 will move the telescope +1” in RA, moving the target +1” in X on the detector (i.e. to the right).
  • A “SKY” offset of +1” in RA with the 13mas-JHK camera with PA=0 will move the telescope +1” in RA, moving the target -1” in X on the detector (i.e. to the left).
  • A “DETECTOR” offset of +1” in X with the 27mas-JHK camera with PA=0 will move the telescope -1” in RA, moving the target -1” in X on the detector i.e. to the left).
  • A “DETECTOR” offset of +1" in X with the 13mas-JHK camera with PA=0 will move the telescope +1” in RA, moving the target -1” in X on the detector (i.e. to the left).

See the User Manual for a graphical depiction of the offset convention.

Detector cosmetics – NIX

Consult the ERIS User Manual  and ObsPrep to see the regions of the NIX detector that contain significant clusters of bad pixels. Dither patterns should be carefully designed to avoid placing your target within these regions.

Selected DIT Values

If you are using a DIT of more than 150s for either IFS or NIX observations it must be either 300, 600, or 900 seconds to reduce daytime calibration activities. OBs with DITs of other duration longer than 150s will not be accepted under any circumstances.

Detector Saturation – IFS

The count level must not exceed 50% of the detector well depth. This corresponds to a limit of 40k e- (20k ADU).

The table below gives the brightest magnitude permitted for each grating and plate scale configuration. These limits were calculated using the ERIS ETC assuming:

  • DIT=1.6, NDIT=1
  • An A0V spectrum
  • NGS on-axis, 50% turbulence category
  • Airmass of 1.2, FLI of 0.5, and PWV of 2.5mm
  25mas 100mas 250mas
J low-res J = 4.3 J = 6.9 J = 7.1
H low-res H = 4.5 H = 6.8 H = 7.1
K low-res K = 4.0 K = 6.3 K = 6.5
J short/middle/long J = 3.2 J = 5.8 J = 6.0
H short/middle/long H = 3.4 H = 5.8 H = 6.1
K short/middle/long K = 3.2 K = 5.4 K = 5.7

These limits are relaxed by 1 magnitude in visitor mode, but you should consider the effect of persistence when planning your observing run.

Detector Saturation – NIX

Detector saturation is permitted up to ten times the saturation level of the detector. This corresponds to a limit of 850k e- (163k ADU in SLOW mode, 327k ADU in FAST mode).

The table below gives the brightest magnitude permitted for each readout mode, camera, and filter combination. These limits were calculated using the ERIS ETC assuming:

  • DIT=1.6, NDIT=1 for SLOW; DIT=0.034, NDIT=1 for FAST
  • An A0V spectrum
  • NGS on-axis, 50% turbulence category
  • Airmass of 1.2, FLI of 0.5, and PWV of 2.5mm
SLOW (DIT=1.6s)

FAST (DIT=0.034s)


Attenuation (mag)




13mas-JHK/LM 27mas-JHK 13mas-JHK/LM 27mas-JHK ND APP
J J = 10.4 J = 11.6 J = 6.6 J = 7.9 6.00 -
H H = 9.0 H = 10.2 H = 5.0 H = 6.3 5.50 -
K K = 8.7 K = 9.9 K = 4.6 K = 5.9 5.06 -
Ls * n/a L = 3.5 n/a 4.64 -
Lp * n/a L = 3.5 n/a 4.57 -
Mp * n/a  M = 3.0 n/a 4.50 -







Pa-b J = 7.3 J = 8.6 J = 3.2 J = 4.6 5.98 -
Fe-II H=6.0 H = 7.2 H = 1.7 H = 3.0 5.51 -
H2-cont K = 6.5 K = 7.7 K = 2.2 K = 3.5 5.13 2.3
H2-1-0S K = 5.3 K = 6.5 K = 1.0 K = 2.3 5.10 2.2
Br-g K = 5.2 K = 6.4 K = 0.9 K = 2.2 5.06 2.2
K-peak K = 6.9 K = 8.1 K = 2.7 K = 4.0 5.05 2.5
IB-2.42 K = 5.7 K = 6.9 K = 1.5 K = 2.8 4.94 1.3
IB-2.48 K = 5.3 K = 6.5 K = 1.0 K = 2.3 4.91 1.1
Br-a-cont L = 4.0 n/a L = 0.0 n/a 4.57 1.9
Br-a L = 4.0 n/a L = 0.0 n/a 4.56 2.2

* The sky background causes saturation in less than the minimum DIT.

The blocking pupil that can be used in conjunction with short-wavelength (<2.5um) filters has a small attenuation of between 0.05-0.25 magnitudes, larger at longer wavelengths.

Cube Mode – NIX

You must not exceed the maximum NDIT permitted in cube mode for the detector window you are using:

  • windowF: NDIT <= 500
  • window1: NDIT <= 625
  • window2: NDIT <= 1000
  • window3: NDIT <= 2000
  • window4: NDIT <= 2000

Note: an observation reading out the full detector (windowF) in cube mode with NDT=500 will result in a 4.2GB FITS file, with an additional transfer overhead of (NDIT * NROWS)/20460 = 50s. This overhead is included in the p2 execution time calculation.

When using cube mode the maximum number of NDITs permitted within a single OB is limited to the equivalent of:

  • windowF: 15,000 NDITs per hour
  • window1: 20,000 NDITs per hour
  • window2: 30,000 NDITs per hour
  • window3: 60,000 NDITs per hour
  • window4: 60,000 NDITs per hour

These limits are prorated to the total duration of the OB, including overheads. For example, a 30 minute OB using windowF can have at most 7,500 NDITs.

Calibrations (PSF, Standard Star)

These types of calibrations must be added in a concatenation with the corresponding science OB. The instrument settings in the calibration OB must match those of at least one science template in the science OB. 

Standard star observations for telluric calibration of IFS observations must be selected by the user in advance if required (e.g., Telluric star search tool) and placed in a concatenation with the science OB. The observatory will not obtain telluric calibrations for IFS observations unless requested in this manner by the user as a part of their program.

Photometric standards will be observed routinely during PHO and CLR conditions to verify the atmospheric transparency. An OB requesting PHO conditions must have a standard observed both before and after the observation within three hours, otherwise the OB will be repeated. This will be done as a part of the calibration plan, and users do not need to request them in their programs at either Phase 1 or Phase 2.

Dedicated photometric standards can be requested if the highest photometric precision is required (i.e. <2% flux variation). These must be requested by the user, included in the time request for the proposal at Phase 1, and must be included in a concatenation with the science OB at Phase 2.

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