Service Mode Rules and Recommendations for Observation Blocks
Preparing Observation Blocks
Both Visitor and Service Mode programmes are carried out at all ESO telescopes by executing Observation Blocks (OBs) provided by the users. OBs are constructed and submitted to ESO using the P2PP version 3 (P2PP3) Tool for all Paranal instruments.
Please refer to the P2PP3 User Manual and to the User Manuals of the different instruments for more specific information on the structure and contents 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 are normally scheduled non-contiguously. Therefore, users should not expect their OBs to be executed on 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 time 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, concatenations longer than 1 hour 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 container.
- 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. 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 external conditions did not allow the observations during the interval that you specified.
- Specify the weakest possible Constraint Set values. OBs that can be executed under a broad range of conditions are easier to schedule, especially if they belong to priority groups B or C. 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 ISAAC
- Overheads and Short Integrations
- Telescope offsets
- Maximum number of filters
- Maximum number of spectral settings
- Bright sources
- Spectroscopy: Target acquisition
- Burst mode observations
In addition to telescope presets, field acquisition and telescope offsets, operational overheads within an observation template are directly related to DIT, the Detector Integration Time (see the ISAAC User Manual). The smaller the DIT, the higher the overheads. Resorting to the minimum DIT of 3.55s (Hawaii array) results in overheads exceeding 100%, which is difficult to accept. Users are requested to use the DITs recommended in the User Manual, and to avoid using the minimum DIT. Use of the minimum DIT must be requested by submitting a Phase 2 Waiver Request. ESO reserves the right to modify the DIT/NDIT values should this justification be missing or weak, or to lower the overall priority of the OB in question.
Rapid offsets of the telescope lead to a degradation of image quality, because the telescope Active Optics needs time to settle and correct the M1. In the course of a template, if Active Optics corrections are not sent to the mirror, aberrations will start piling up, significantly affecting the achievable image quality. Therefore, the minimum time between telescope offsets should be 30 seconds. This can be achieved by an adequate combination of the integration time parameters (DIT, NDIT) depending on which template is used. Taking into account the overheads, this translates approximately into these recommendations: (DIT+4.1) * NDIT > 30 for Hawaii arm imaging observations, (DIT+7.5) * NDIT > 30 for Hawaii arm spectroscopic observations and DIT * NDIT > 30 for Aladdin arm observations, where DIT is in seconds. Please note however, standard star observations (imaging and spectroscopy) may require faster integrations.
Please be reminded that the use of more than 3 broad-band filters in one OB requires the submission of a Phase2 Waiver request. In order to ensure that sufficient sky flatfield data can be provided, the number of narrow band filters in each imaging OB shall never exceed 2.
Since the position of the grating is not exactly stable, it is recommended to not use more than 2 spectral settings in one OB. Otherwise, the calibration accuracy may not be guaranteed. In the case of 2 spectral settings in one OB, a telluric standard will be taken before (with spectral setting 1) and after (with spectral setting 2) the science OB if telescope operations allow this. If this is not possible the science OB will still be classified successful and will not be repeated.
Direct imaging of very bright objects in the Hawaii arm results in residual images that can last up to several hours due to persistence effects in the Hawaii array. In service mode, this problem can affect subsequent observations of other programs. In visitor mode, and provided that the nights are not shared, the potential problems related to the persistence effects are left to the responsibility of the user. The same SW imaging rules applies to the Aladdin detector.The magnitude of the brightest object in the field, including standard stars, must be indicated in the "Instrument comments" field in each OB.
Brightness limits: Imaging
Observations involving fields with objects brighter than 11th magnitude (BB imaging) or 8th magnitude (NB imaging) cannot be guaranteed in Service Mode and in shared visitor nights. ESO reserves the right to lower the overall priority of the OB in question in service, and not to execute the observations in shared visitor nights.
Requests for imaging observations not compliant with these limits must be submitted as a Phase 2 Waiver Request. If judged acceptable, ESO will try to devise operational strategies (e.g. observations at the end of the night, scheduling other imaging OBs after the observations in question).
Brightness limits: Spectroscopy
For spectroscopic acquisition of bright targets, the following filter settings must be used. Note that this applies not only to standard stars, but also to other bright objects in the field of view.
|IR Magnitude||Filters to use|
|>8 and <11||Any Narrow Band filter|
|>6 and <8||Two close Narrow Band Filters on each filter wheel. E.g. NB_2.19 on filter wheel 1, and NB_2.17 on filter wheel 2|
|<6||Two distant Narrow Band Filters on each filter wheel. E.g. NB_2.09 on filter wheel 1, and NB_2.17 on filter wheel 2|
Important note: when the bright object in the field of view is not the science target, the target may become too faint to centre on slit due to the use of the Narrow Band filter(s). In this case (see section 'Target acquisition' below), offsets from a reference star should be used.
Although the Aladdin detector is less sensitive to the effects of bright targets, there are limits to what is acceptable at wavelengths above 2.5 microns. For broad band filters, the limit is 4th magnitude in the IR. For narrow band filters, the limit is 1st magnitude in the IR. Targets brighter than these limits will not be observed. The magnitude of the brightest object in the field, including standard stars, must be indicated in the "Instrument comments" field in each OB.
OBs for which target acquisition cannot be completed within a few minutes of time will not be executed. Acquisition has to rely on either science targets or reference objects brighter than approximately 17-18th magnitude in the IR, when the acquisition is done with Broad Band filters. Exceptions will be tolerated for moving targets, and special situations to be evaluated on a case by case basis.
When the science target is fainter than the above quoted magnitude, the procedure for acquisition should rely on reference objects which are brighter than this limit. These reference objects can either be positioned in the slit together with the target by defining the appropriate position angle on sky (the preferred procedure), or be used for initial centering on slit, followed by a blind offset to move the target into the slit.
These reference objects should be stars or point-like objects. Blind offsets from a reference object should be limited to approximately 1 arcminute. Offsets that are too large could cause the system to select a new guide star, with maybe poorer acquisition accuracy. Experience has shown that the offsets from the reference star are often not accurately defined. For instance, if the offsets are computed from a previously taken ISAAC image, the distortion at the edge of the field can affect the reliability of the offsets if a constant plate scale was assumed. Also, users tend to choose the brightest object in the field, which can then be far from the target, although very often there are fainter reference targets, still bright enough to satisfy the limits mentioned above, that would have been much better choices. Users should try to use reference objects as close as possible to the target, preferably as aligned as possible with the slit, to minimize offset measurement errors, rather than trying to use the brightest reference target.
To the extent this is possible, it is recommended to position a reference object in the slit together with the target, as this allows to control the position of the target on the spectral image and to monitor the flux through the slit, etc.
If you elect to use blind offsets, you must define them in the appropriate MoveToSlit template. The finding chart must clearly indicate the reference and science targets and the position of the slit .
The Burst and FastJitter modes are offered both in VM and in SM. However, in the case of occultations, only disappearances are offered in SM. VM must be requested in the case of appearances.
- The data cube can contain a maximum of 32000 planes (frames), i.e.the maximum NDIT is 16000 in Burst mode and 32000 in Fast Jitter mode.
- The maximum data cube size is 262Mb. Once the window size has been selected, this limits the number of frames/reads and vice versa. The data cube size in b is given by the relation: Xpix*Ypix*4*NDIT. Table 26 of the ISAAC User's Manual reports some conservative upper limits for the data cube size. Datacubes too close in size to the limit of 262 Mb can result in the loss of few frames (NDIT).
Be aware of the naming conventions for the BURST/FastJittermode OBs