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 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 multiple 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.
Some OBs must be executed within precise time windows, 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.
- 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 roughly constant 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.
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 follows the sequence as they are listed in the p2 window. However, please notice that this sequence is not strictly enforced during execution.
In P2PP v2 it was possible to assign an execution priority to each OB, so that the operator is aware of the ones that have a higher scientific importance at the time of deciding on observations to execute for a given programme.
It has been recognized nevertheless that such simple priority scheme is sometimes insufficient to deal with programmes containing large number of OBs, and especially for surveys containing large numbers of target fields observed in a number of instrumental setups. In such cases the need for a prioritization scheme above the individual OB level, which can take into account the past execution history of the programme, becomes clear. One can consider for instance the case of a survey of several target fields to be observed through several different filters, with each field and filter specified in a single OBs. Depending on the science goals of the programme it may be desirable to complete the observations of a given field in all filters before proceeding to the next field, or conversely to observe all the fields in a given filter before proceeding to the next filter, or even ensure that contiguous coverage among the fields takes priority.
The approach adopted to deal with such cases is the definition of Groups of OBs, in which internal priorities within each group are reflected in the form of a contribution of each OB to the total group score. The short-term scheduling tools available on the mountain will take into account the current scores of each group of OBs, and will then apply a number of rules in order to prioritize the possible OBs to be executed according to them. Such rules will for instance give the highest execution priority to those OBs that set a new maximum of the score among the existing groups; and among those, the highest priority will be given in turn to those that produce the largest increase in group score. By assigning to the OBs the appropriate contributions to the scores of their respective groups, the users can make sure that the progress in the execution of the programme will take place in a way that is consistent with the scientific priorities of the observations. In addition, it will be possible to assign different priorities to each group.
The Survey Area Definition Tool (SADT) is a utility developed by the VISTA consortium that allows users to define areas to be covered by surveys executed with either VIRCAM at VISTA or OmegaCam at the VST according to a number of criteria. The SADT determines the central coordinates of the different pointings required to cover the field according to the specifications, as well as ancillary guide star information to allow acquisition and guiding. The output produced is a file to be ingested into P2PP version 3 containing all the target information needed for the preparation of the OBs with which the survey will be executed.
The latest distribution of the tool can be downloaded from the SADT web page, where also the SADT Cookbook with step-by-step instructions and examples can be found.
Additional Service Mode Requirements for HAWK-I
(Note: the same rules apply for non-AO and AO observations)
Saturation limits and persistence
Like many other infrared detectors the HAWK-I detectors show a persistence effect if the observed sources are too bright. In Service Mode, this problem would seriously compromise subsequent observations of other programmes, therefore the following rules apply:
- When using DITs smaller than 30 secs, persistence effects can be neglected.
- When using larger DITs > 30s the maximum accepted saturation is 7 times the saturation level.
Because the saturation level on a given object also depends on the sky conditions, users are recommended to check carefully their fields against saturation using the HAWKI Exposure Time Calculator during Phase II.
The magnitude of the brightest object in the field, including standard stars, must be indicated in the "Instrument comments" field in each OB.
Requests for imaging observations not compliant with these limits must be submitted as a Phase 2 Waiver Request, which will be evaluated on individual case basis. If judged acceptable, ESO will try to devise operational strategies (e.g. observations at the end of the night, scheduling other BB imaging OBs after the observations in question).
Minimum Time between offsets
Rapid offsets of the telescope lead to a degradation of image quality and to excessive overheads (>> 100%). Therefore, the minimum time allowed between telescope offsets is one (1) minute. The integration time parameters (DIT, NDIT, NEXP) should be defined so as to ensure that this rule is strictly followed. Please consider that with large sky offsets and DIT * NDIT * NEXP = 60s, the overheads are already on the order of 100%.
In order to prevent daytime calibrations to run over an unreasonable execution time, the DIT values for long exposure times are restricted. In Service Mode it is therefore mandatory to select one of the following DIT values in case the DIT exceeds 120 seconds: 150, 180, 240, 300, 600, 900 seconds. For observations with broad-band filters please remember to define short DIT values,i.e. lower than 15 seconds, in order to avoid saturation on the sky background.
HAWK-I follows the standard astronomical offset conventions and definitions: North is up and East to the left.
All offsets are given as telescope offsets in arcseconds: if the telescope moves towards N-E the target moves in the opposite direction on the detector FOV, i.e. towards S-W.
Detector gaps and target acquisition
Mind the gaps! The HAWK-I field consists of 4 detectors with gaps of 15" in between them. If you don't want your target to fall onto the center of the gaps, you should define an offset in the acquisition template. This is done by using the following two entries in the HAWKI_img_acq_Preset template (or the HAWKI_img_acq_LGS_Preset template):
- TEL.TARG.OFFSETALPHA "Alpha offset for the target (arcsec)"
- TEL.TARG.OFFSETDELTA "Delta offset for the target (arcsec)"
The figure below shows the offset conventions (TEL.TARG.OFFSETALPHA, TEL.TARG.OFFSETDELTA) to place the target in the center of one of the four quadrants, assuming that your target coordinates match the OB coordinates (i.e. at the beginning of the acquisition your target is in the middle of the gap).
For instance, to place the target in the center of Q1, which is the lower left quadrant (S-E), the telescope must move towards N-W, hence you must use the following offsets in the P2PP:
- TEL.TARG.OFFSETALPHA "Alpha offset for the target (arcsec)" : -115"
- TEL.TARG.OFFSETDELTA "Delta offset for the target (arcsec)" : +115"
The positive position angle is defined from North to East. As shown in the figures below, if for PA=0 your target falls in the center of Q1 and you want to move it in Q4 you can either change the parameters TEL.TARG.OFFSETALPHA, TEL.TARG.OFFSETDELTA (see previous section) or just rotate the PA by +90 degrees.
FastPhot mode observations
FastPhot mode (FastJitter observations) is offered both in VM and in SM. However, in the case of lunar occultations, only disappearances are offered in SM. VM must be requested in the case of appearances.
The following constraints on the detector window parameters are imposed:
- Only contiguous windows that span entirely the width of the detectors are offered. This implies that "Number of columns for each window stripe"=128 and "First column of window within a stripe"=1. Therefore, the total size of the output file along the X axis is always 128 x 32 = 4096 pixels.
- Only 3 values for the window height are allowed, hence "Number of rows for each window stripe" can be set to 32, 64 or 128 pixels. There are no restrictions on where the windows are located along the Y axis, the users are free to select any possible value, from 1 to 2047, as the "First row of window within a stripe".