Frequently Asked Questions


Observation Preparation

  • Do I need to account for overheads in my total exposure time when submitting Service Mode OBs?

    Answer: Yes. Total allocated time = integration time + all standard operational overheads. To estimate your operational overheads, see the individual instrument User's Manuals and/or use the Execution Time Report function in the p2 tool.
  • How many special calibration OBs do I have to submit?

    Answer: You should assume that your science OBs will be executed completely independently of each other, possibly on different nights, and take into account that no calibration OB will be executed more than once. You should submit enough special calibration OBs to cover that situation. Example: if you need to observe and flux-calibrate six targets in a filter that is not supported in the calibration plan of the instrument, you need to provide six special calibration OBs to allow for the case in which each target is observed on a different night. This must be done so even if the same calibration star can be used for all the science targets. For assistance, contact the User Support Department.
  • Can I specify different observing constraints(e.g. seeing, transparency) at Phase 2 than I specified in my Phase 1observing proposal?

    Answer: You can relax your constraints to increase the chances of execution of your OBs (for example, if you specified Seeing = 1.0 or better at Phase 1, you can specify Image Quality that corresponds to V band Seeing at zenith of 1.2 or better at Phase 2). However, more stringent constraints (like Image Quality that corresponds to V band Seeing at zenith of 0.8 at Phase 2, in the previous example) are not allowed, as an essential ingredient of the long-term scheduling of Service Mode programmes over the semester is the constraints that users of approved programmes specified at Phase 1. Allowing more stringent constraints at Phase 2 would thus endanger the completion of even the highest ranked programmes. An allowed exception to this are OBs needed to flux-calibrate observations that can be mostly done under non-photometric conditions, provided that accurate flux calibration is needed for the scientific goals of the programme and that the execution time under photometric conditions does not exceed 20% of the allocated time.The values in the OB constraint sets that are selected (and approved) during Phase 2 preparation (and review) cannot be changed later during the observing period. This is explained in more detail in the Phase 2 Service Mode Guidelines web pages.
  • How do I get my p2 username and password?

    Answer: The p2 username and password correspond to the ESO User Portal username and password of the Principal Investigator (PI), or of the Phase 2 Delegate (in case the PI has delegated Phase 2 access to another User Portal registrant).  If you as PI of a scheduled run, or as a Phase 2 Delegate, have forgotten your User Portal username and/or password please use the appropriate corresponding link(s) on the User Portal login page.

    If you have no accepted programs as PI but wish to learn the use of p2, you can use the p2 demo server set up for this purpose.

  • After reviewing my Phase 1 proposal, I have realized that I can observe a better set of targets than the ones I listed then. Since the scientific goal is the same, can I simply change the list of targets?

    Answer: No. There are multiple reasons. One is to avoid duplication of observations unless scientifically justified and conflict/scooping between different scheduled programs. Furthermore, the allocation of time in Service Mode is made to balance pressure factor on each right ascension interval, derived from the distribution in the sky of the targets that the accepted programmes proposed at Phase 1. The Long Term Schedule that results from the time allocation process would thus be invalidated if changes of target were allowed at Phase 2, this is, after the time allocation has been made.

    It is however possible to accept a limited number of target change requests in cases for which a sound scientific justification exists, such as the existence of new observations that demonstrate that a given object of the original sample had been misclassified and is not relevant to the purpose of the programme any more. Target change requests are reviewed by ESO to ensure the strength of the justification and also that there is no other approved programme that intends to execute observations of the new target in a similar configuration.

    Target change requests must be addressed to the ESO User Support Department via the p2 (Paranal) and p2ls (La Silla) user interfaces. Once logged in, the user should select the Run for which they intend to submit the Change Request. By clicking on the Run, the user will be able to access the Change Request menu on the top right.

  • How do I prepare my Phase 2 proposal for APEX?

    Answer: APEX does not use p2, but a web-based form. On the said form you will have to provide your email: please make sure that it is exactly the same as the one present in your ESO User Portal profile. The APEX Phase 2 deadline is the same as the VLT deadline. Should you have any questions about your Phase 2 preparation, please contact APEX support via
  • I have an accepted proposal which consists of a pre-imaging run and a follow-up multi-object spectroscopic (MOS) run. May I submit only the pre-imaging OBs now, by the Phase 2 deadline, and the spectroscopic OBs later?

    Answer: Yes, you should submit only the pre-imaging OBs (i.e. no dummy MOS OBs should be submitted at the general Phase 2 deadline). ESO will make every possible effort to execute all pre-imaging as early as possible, and will release pre-images immediately. In effect all pre-imaging OBs will be treated as ``carry-under OBs'', meaning that they will be executed as soon as they are ready, even if that is before the period starts. For the Phase 2 proposers this means that it is important to submit pre-imaging OBs as soon as possible, even long before the deadline. The earlier valid OBs are submitted, the earlier the pre-images will be taken, and the higher the probability that follow-up MOS observations will be completed within the narrow window of opportunity.
  • Can I directly interact with my contact scientist, rather than submitting a request for support?

    Answer: First it should be understood that as of the beginning of July 2021 the e-mail address will be decommissioned. Instead of contacting the ESO User Support Department through e-mails to that address users are kindly asked to use the new web interface at


    Second, usage of that web interface will ensure that your Support Astronomer (or their backup (see below)) is notified of your issue.

    Indeed, support astronomers have other professional and private commitments that may cause them to be away and/or with limited access to e-mail and/or with reduced access to support tools. Through the web interface your issue is communicated to your Support Astronomer directly, or to their backup for times when that is necessary. Hence, your issues are always given prompt attention by an expert on the instrument that your run uses, even in periods when your contact scientist cannot deal with it.

  • What is the accuracy of the UTs pointing and tracking?

    Answer: The UTs have a pointing accuracy of 3 arcsec RMS, driven by the guide star catalogue accuracy. The expected tracking accuracy under nominal wind load is 0.1 arcsec RMS over 30 minutes when field stabilization is active. The UTs also have the capability of tracking targets with additional velocities (e.g. Solar System targets) under full active optics control. Proposers who need this capability should specify the additional velocities in RA and Dec for their targets. Please check here for further details on UTs performances.


  • My run can be executed more efficiently if my OBs are executed one after another, skipping the acquisitions for the latter OBs. Why cannot I ask for such a procedure for service observing?

    Answer: There is no guarantee that user specified conditions will last long enough to complete any given OB sequence. Furthermore, there may be different priorities between programmes that have targets observable under similar RA range. Thus breaking down a sequence of OBs often improves overall operations efficiency by allowing the execution of OBs best matching the external conditions and scientific ranking. Therefore, to maintain the flexibility needed to adapt to changing observing conditions and to maximise operations efficiency, ESO requires that all Service Mode OBs be treated as independent observations with independent acquisitions. If the constraints change during the first hour of observation your OB will be repeated without any penalty or cost to your run. 
  • Having to split my OBs to make them compliant with the rule that no Service Mode OBs could last longer than one hour implies much more execution overhead. Wouldn't it be more efficient to allow longer OBs?

    Answer: Experience has shown that longer OBs make Service Mode observing less efficient, rather than more. The reason is that, the longer an OB, the more likely it is that the external conditions go outside the acceptable range specified in the Constraint Set. Since OBs executed outside constraints must be rescheduled and re-executed, longer OBs imply not only a higher fraction of OBs to be re-executed, but also a larger amount of time wasted in the execution of OBs failed because of the degradation of external conditions.

    Still, some programs may have scientific reasons that require OBs longer than one hour to be scheduled. In such case, a waiver request justifying the need for a longer execution time must be submitted to ESO. When sufficiently justified, these requests are accepted under the condition that the OB will be considered as executed within constraints even if the conditions degrade after the first hour of execution.

  • Is it possible to monitor the progress of my program?

    Answer: Yes. Both the Run Progress Reports page and the Science Archive query form allow you to access updated information on the progress of your observations. In p2 it is now also possible to see the current status of OBs. More information about OB statuses visible in p2 is available here.
  • If my programme cannot be completed by the end of the Period, can I ask to have it carried over to the next one?

    Answer:If your programme obtained a Priority Class A and has not been completed by the end of the Period, it will be considered as a candidate for carryover to the next Period without you having to request it. In this case you will get a notification from User Support Department about the possibile carryover at the time the Call for Proposals for the next observing period is issued. This is not possible at present with Priority Class B or C programmes, which are terminated at the end of the Period regardless of their status of completion. Please see our page on the philosophy and scheduling of Service Mode programmes for more information. If you had a Class B or C Programme and you see that the end of the Period is approaching without it being near completion, you are strongly encouraged to resubmit it as a new observing proposal.
  • How can I know precisely on what dates my Service Mode observations will be executed?

    Answer: A key feature of the flexible scheduling approach followed at the ESO Observatories is that Service Mode Programmes do not have definite dates assigned to them. Rather, they are executed according to the external observing conditions, some of which are unpredictable, like the sky transparency or the seeing. Only in this way it is possible to ensure that each of the many programmes approved every semester in Service Mode is executed under the conditions that are necessary for its scientific goals.
    The Principle Investigators and their Phase 2 and/or data delgate(s) can subscribe to receive e-mail notification whenever one of their OBs has been executed. To subscribe login to the (Garching Night Log Tool) gNLT overview page using your User Portal credentials.
  • ESO has informed me of the allocation of time to my run, but only in class C. Is it worth it to prepare any Phase 2 material at all?

    Answer:Yes! ESO selects class C programmes from those that did not get a high enough rating to be above the time allocation cutoff line, but whose constraints made them schedulable under a very wide range of conditions (i.e., in intervals of bad seeing, with moon, or under poor sky transparency conditions). Higher rated runs normally have more stringent constraints and, when the conditions are below average, only class C runs may be executable. Due to the high pressure factor at ESO telescopes, the scientific quality of class C runs is normally still very high, and experience shows that Service Mode class C programmes, which would not have been scheduled in classical Visitor Mode, have produced very valuable scientific results.

Data Distribution


Frequently Asked Questions related to ESPRESSO

  • How can I choose between the singleHR and singleUHR modes?

    Answer:The median measured resolving power of the singleHR mode is 140,000 while for singleUHR it is 190,000. The singleUHR mode should be used if the highest ESPRESSO spectral resolution is required for your science case. However, the UHR, due to its smaller fibre, gives a lower S/N and a lower RV precision compared to the HR mode. As a result, if a high S/N or high RV precision is important for your science case and the highest spectral resolution is not needed, the singleHR mode should be preferred. Detailed calculations can be performed with the ESPRESSO ETC available at The RV precision of the UHR mode is expected to be of the order of 1m/s.
  • Which of the two pixel binning modes, 1x1_FAST or 2x1_SLOW, should I use for the singleHR mode?

    Answer: Generally, the 1x1_FAST mode is used for relatively bright targets (V<10-12), while the 2x1_SLOW mode is used for fainter targets (V>10-12). Detailed calculations can be performed with the ESPRESSO ETC available via High-precision RV observations may preferentially better be executed with the 1x1_FAST setting. Overheads are very similar for the two modes (the 2_1_SLOW mode reads the detector slower, but it has half the pixels).
  • Which of the options should I use for 'Source on fiber B'?

    Answer: While the complete set of options include DARK/ SKY/ THAR/ FPCS, the standard options are SKY or FPCS. The Fabry Perot calibration source (FPCS) is used for relatively bright targets and radial-velocity studies. The SKY is used for fainter targets, for which a sky subtraction is important, and for which a contamination of the FPCS light needs to be avoided. The DARK option can be chosen in case of crowded fields, where the SKY option might introduce contamination from another source in the field. With the DARK option, the entrance to Fibre B is blocked. The THAR source should normally not be used at night. It is available in case of a failure of the FPCS.
  • Which daytime calibration should I associate to Fibre A?

    Answer: The options are THAR or LFC. Since the LFC is still in an experimental phase, we recommend the user to choose the THAR option for the time being. Note that the LFC covers only the 500nm-720nm wavelength range. The pipeline will automatically use ThAr outside this range.
  • Can I use more than one observing template in an OB?

    Answer: This is possible, but normally not necessary. A typical OB consists of one acquisition template and one observing template. A second observing template could however be used to employ two different binning modes or different exposure times within the same instrument mode.
  • Which is the field of view for the ESPRESSO acquisition process?

    Answer: The FoV for acquisition is 17 arcsec. Finding charts should show a field of 30x30 arcsec.
  • As a user, do I have to care about additional calibrations that I need to request?

    Answer: The calibration plan as outlined in the ESPRESSO user manual, should be sufficient for the majority of use cases. Observers interested in highest S/N may request additional flat-field frames. The calibration plan provides flat-field frames with a SNR of about 1000.
  • Which is the constraint set for ESPRESSO and which conditions may be critical?

    Answer: The ESPRESSO constraint set includes the airmass, sky transparency, fractional lunar illumination (FLI), moon angular distance, seeing, and the precipitable water vapor. The ESPRESSO exposure time calculator (ETC) available at can be used to study the effects of these values on your observations.
  • Do I have to worry about precipitable water vapor (PWV) for my ESPRESSO observations?

    Answer: In the ESPRESSO wavelength range, there are three absorbing species: O2, OH, and H2O. Of these, water vapor is the one that affects the largest wavelength range. The effects are not as pronounced as in the (near-)IR, but can have an impact on some scientific objectives. In general, for wavelengths shorter than 700 nm, water absorption signatures can range between about 1-10%, depending on the PWV. The wavelength region of 700-800nm is strongly affected by water absorption, with numerous deep lines that can have an impact on the observations even for low PWV value. If aiming at a high spectral fidelity, you should estimate the impact of water vapor on the lines or wavelength range of interest using the ESPRESSO ETC and the SkyCalc tool available from http:/ If interested in lines in the 700-800nm region, it may be important to set a constraint on the PWV, which is a part of the ESPRESSO constraint set. More information and references are available in the ESPRESSO user manual.
  • Which is the limiting magnitude for acquisition?

    Answer: Acquisition has succeeded with sources of V mag as faint as 20 to 21 in dark sky conditions. Acquisition of targets fainter than a V mag of 18 require approval through a waiver request. Fainter targets can be acquired with a blind offset.
  • How will the quality control be done at Paranal (QC0)?

    Answer: Observations will be graded based on the constraint set. The expected S/N will also be considered for QC0. Please include in the ESPRESSO-specific OB comment field the expected S/N at a wavelength of 550 nm, as reported by the ESPRESSO ETC.
  • Can I use the *snr observing templates in service mode?

    Answer: No. These templates are not offered for service mode observations. In general, the time of all VLT SM observations is accounted with the pre-estimated execution time calculated when submitting the OB.
  • Can I observe moving targets with ESPRESSO?

    Answer: Yes, differential tracking velocities in RA/DEC can be provided as for other instruments at the VLT.
  • Can I select the telescope with which the observations will be carried out?

    Answer: No, ESPRESSO performances are not UT dependent. Your observations in the 1-UT mode will be executed on any of the Unit Telescopes as it fits the overall telescope schedule. Your run is pre-assigned to a certain UT at which it will most likely be executed. However, any OB might be re-scheduled to another UT for operational needs.

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