Observing with GTC

MEGARA: (updated August 2024)

Important NOTE: For Semester 2025A MEGARA is available only in integral-field Unit (IFU) mode. Further information about the instrument can be found in the MEGARA web page.

HiPERCAM: (updated August 2024)

The HiPERCAM high-speed, multi-band optical imager is a GTC visitor instrument (P.I. Vik Dhillon) located in the Folded-Cass G focus. HiPERCAM is able to image simultaneously in 5 channels (u', g', r', i', z') and to frame at (windowed) rates of well over 1 kHz in a FOV of 2.8 x 1.4 arcmin with a 0.081 arcsec/pix plate scale.

Important NOTE: HiPERCAM is offered during Semester 2025A including the new COMPO subsystem, which offers autoguiding capabilities and increases the FOV available to look for suitable reference stars for differential photometry. Further information about the instrument can be found in the HiPERCAM web page.

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Applying for time

Observing time on GTC is shared by different groups. Fixed fractions of time are available for the Spanish community, University of Florida and Mexico. In addition there exists the possibility to apply for collaborative time under agreements between Spain and Mexico. Finally, GTC participates in the CCI International Time Programs (ITP).

Approximately 6 months before the start of each semester an Announcement of Opportunity is issued by the observatory where specific details may be found on for instance the instruments and observing modes offered. ITP time is announced separately, once per year. For ITP proposals different rules and limitations apply; for details we refer to the ITP Announcement of Opportunity.

Regular proposals

All regular observing proposals are processed through the on-line application system hosted at the Instituto de Astrofísica de Canarias (IAC): CAT. This is referred to as Phase-1 of the proposal handling procedure. It is important that applicants apply to the appropriate time fraction (Spanish CAT, Florida, Mexico, or the collaborative Spain-Mexico time).

GTC Large programmes (for Spanish time only)

For Semester 2025A, Large Programmes are offered at GTC.

Up to a maximum of 100 hours per semester of the CAT observing time at the GTC can be allocated to GTC Large Programmes (GTC-LP). A GTC-LP is defined as a programme with the potential to lead to a major advance in a specific field of astrophysics, requiring a minimum of 100 hours of GTC time which can be distributed during 1 to 4 semesters. In addition to a strong scientific justification, proposers should have the necessary manpower and tools for a quick and comprehensive analysis of the data and publication of the results. From Semester 2025A there are 75 h available per semester to be used either with MEGARA (IFU mode), OSIRIS+ (imaging and longslit spectroscopy modes), and EMIR (imaging and longslit spectroscopy modes). A single instrument or a combination of two/three instruments can be requested. Please also note that ToO proposals are not accepted for GTC-LP..

As the observing time is deduced from the CAT share, only PIs from Spanish institutions can apply for GTC-LPs. There are no rules or restrictions about co-applicants. Proposals for GTC-LPs will be presented at the same semester deadlines of the CAT general call for proposals, using the same application tool by filling an adapted application form.

GTC Director's Discretionary Time (GTC-DDT)

From September 2014, there is Director's Discretionary Time available at the GTC (GTC-DDT). Details on how to proceed with this kind of proposals can be found in the Spanish CAT web pages.

The historic balance of use of observing time can be seen in the adjacent graph, showing the actual use of observing time for each class of time allocation. Note the decreases produced in Semester 2020A (due to COVID-19 pandemic) and in Semester 2021B (due to stand down caused by the volcano eruption in La Palma), that were recovered from 2022 onwards.

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Observing semesters

Observing semesters at the GTC are as follows: the A-semester runs from March through to August, while the B-semester runs from September until February the following year. Regular proposals normally remain active during one semester only. However, the Time Allocation Committee may decide to extend the duration of proposals. CCI-International Time Programs (ITP) normally cover a one-year period, starting in the B-semester and running through the following A-semester.

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The time allocation process

Observing time is made available to astronomers from institutes in Spain, Mexico and University of Florida. Besides these three partners, once per year opportunities are opened for CCI International Time Programs (ITP). Since semester 2015B Spain offers observing time for GTC Large Programmes (GTC-LP). For all these regular observing opportunities a call for proposals is issued to which eligible astronomers must respond in order to be considered for observing time. Proposals are first assessed and ranked on their scientific merit by the respective Time Allocation Committees (TACs). Following that, a technical assessment takes place by GRANTECAN personnel. The TACs decide on which proposals will enter the observing schedule. Execution of the observations in the queue is primarily carried out in order of scientific priority, as decided by the TACs.

Apart from the scientific priority, also the required observing conditions play an important role in the correct planning of the observations. In queue-scheduled observing mode the observatory will attempt to match the required observing conditions to the actual observing conditions on-the-night. This method of dynamic scheduling implies that normally observations are not given a fixed observing slot. This allows for a higher chance of success for the most highly ranked proposals, and it allows the best possible exploitation of the prevailing observing conditions.

Based on estimates of the typical observing conditions throughout the semester, each proposal that has passed the TAC's scrutiny will be classified in one of four bands:

1. A-band proposals have very high priority and all possible efforts will be made to execute all of the required/approved observations in the requested semester. If for any reason, A-band proposals remain unfinished by the end of the corresponding semester, the existing GTC Phase 2 will be granted an extended life-time of two additional semesters, or until their completion, whichever comes first. Unfinished A-band proposals do not have to be re-submitted in the following two semesters because of their carry-over status.
2. B-band proposals will be executed on a best-effort basis during the requested semester, and do not have a carry-over status. Approximately three quarters (according to the TACs ranking) of the total amount of GTC available observing time falls in this band.
3. C-band proposals will be executed in extraordinary cases when no A- or B-band objects are available but the good weather conditions permit observations during the requested semester. C-band proposals do have very little opportunity of execution until completion of the project, yet they are included in the GTC observing queue. It should be noted that the total queue is over-subscribed, which will affect proposals in C-band.
4. D-band proposals (or fillers) require very relaxed observing conditions and cover a wide range of coordinates so that they can therefore be carried out essentially any time during the semester. Observations in this band will only be executed if the observing conditions do not permit observations of proposals in A-, B-, or C-bands. The scientific ranking will of course be a decisive factor in the final scheduling also for these proposals.

As a secondary consideration GTC will give priority to completing observing programs in order to maximize the scientific return.

Given this method of scheduling observations, potential users of the telescope are advised not to overstate their requirements on the observing conditions as this will generally increase the pressure on the most favorable conditions and hence reduce the chance of successful completion. Also, the final execution success of an specific program is strongly related with the visibility distribution of targets along the semester. Objects that lie in an overpopulated range of coordinates would decrease their chance of execution unless they were classified as a top ranked proposal. Following graph shows the R.A. distribution in the last observing semesters at GTC (Semesters 2024A+2024B).

Observations may also be executed in classical observing mode. This implies that the observations are scheduled on specific nights and are executed in the (local or remote) presence of the applicant and her/his team. If for whatever reason (weather of technical) the observations cannot be carried out, then the opportunity is lost. However, note that observing efficiency in classical observing mode is notably larger than in queue observations, and GRANTECAN offers some flexibility in the scheduling of this mode to increase the chance of success of the proposal .

If you would prefer to carry out your observations in visitor mode but cannot be present on the night scheduled for your observations then the GRANTECAN astronomer can carry out the observations for you in service mode. Or alternatively, on the observing night remote contact through eves dropping during the observations is also an option. In this way the PI can remotely assess the data quality and adjust the observing program without the need to be physically present in the control room. The PI should contact her/his support astronomer well ahead of time if she/he will direct the observations remotely.

Interested prospective applicants should request "classical" observing on their application form. Once granted time by the TAC, the PI will be informed as to whether the observations will indeed be carried out in classical visitors mode, and on which dates, or in queue-scheduled mode. We note that only programs in the top of the TACs priority list (A or B bands) can enter as visiting observer programs . Programs of lower priority (C band) that requested visitor mode will automatically be scheduled in the observing queue with their corresponding priority.

All regular observing proposals have a life span of one semester. However, the Time Allocation Committee may decide to extend the duration of the observing window over more semesters.

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Reserved Objects

The instrument science teams are granted guaranteed observing time in compensation for their investments. The coordinates and observing modes selected for these programs are for the exclusive use of these teams on GTC for the duration of their program. Therefore PIs wishing to apply for time on GTC are kindly requested to avoid combinations of reserved targets and observing modes. Details of the targets and observing modes may be found on the following web pages for OSIRIS/OSIRIS+, MEGARA, and EMIR, respectively.

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Preparing for the observations

Some weeks before the start of the observing semester the Principal Investigator (PI) will be asked to complete the second phase of the preparation for the observations. This Phase-2 process aims to define the individual steps of the observing program in sufficient detail for the observations to be carried out by observatory staff astronomers, and to automate the observing process. Details such as instrument setup, target coordinates, and exposure times are fixed in this part of the process. At the end of this phase the full observing proposal will be described in a coherent and structured fashion, broken down into Observing Blocks (OBs), very similar to what is customary at other large telescopes.

This important part of the observing preparation is carried out through a Phase-2 web interface. PIs will receive instructions on how to enter this process, well ahead of the observations. Moreover, a GTC staff astronomer will be associated with each scheduled observing proposal and may be called upon to assist the PI and her/his team in planning the observations.

In the case of queue-scheduled observations the final definition of the observations must match the observing time and observing conditions granted by the time allocation committee. The total observing time specified in the Observing Blocks includes all the typical observing overheads. Standard calibrations taken by observatory personnel during the day or (preferably) during twilight (e.g. bias frames, twilight flat fields, spectral lamp exposures, and photometric or spectro-photometric standards) will be shared between users and not be accounted to the science projects (as explained in the previous section, OSIRIS+ tunable filter observations are an exception to this). However, any additional calibrations that the PI may want to obtain (e.g. nightly wavelength calibrations, specific calibration stars) must be described and where necessary defined as observing blocks and will account towards the total available observing time.

Observing blocks are treated like atoms of an observing program and are normally not split in time. Long observing blocks are more difficult to schedule and more sensitive to unstable observing conditions (both atmospheric and technical). The observatory only guarantees the required observing conditions for observing blocks up to one hour and recommends using this duration as the maximum total length of any observing blocks.

In the case of classically-scheduled observations the observations will also have to be defined through the normal Phase-2 definition process, preferably well in advance. The Observing Blocks will allow automation of the observations and the data handling, facilitate error checking, and will make the observing process more efficient. Clearly the selection of objects and observing modes must always correspond to the observing program as agreed by the time allocation committee.

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Observing conditions

For observations that are scheduled as part of the observing queue the observatory aims to execute the observation when the sky conditions are at least as good as the conditions requested in the proposal and approved by the time allocation committee. For an overview of the sky conditions at the Roque de los Muchachos Observatory we refer the reader to the IAC site testing webpage and to this brochure, that, although now somewhat out of date, contains a summary of the general characteristics of the site.

Note that to assess the seeing or image quality conditions we use the FWHM of point sources in the actual science images. These measurements of seeing cannot simply be equated to standard seeing measurements such as those delivered by a differential image motion monitor or to data that is normally used in statistical studies. Dependencies on wavelength, airmass and telescope aperture play a significant role. For instance, the actual seeing becomes worse as airmass increases or when observing at shorter wavelengths, and hence the chance of meeting a specific seeing requirement diminishes. PIs should take this into account when setting the seeing limit in their observing proposal.

Extinction measurements are not always available in real time and hence the assessment of the cloud cover and dust extinction during the night may be imprecise. Observatory personnel will do their best to assess the transparency of the sky based on their experience. This may affect observations that require photometric sky conditions.

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Observing overheads

The definition of observations in terms of Observing Blocks must take into account the unavoidable observing overheads.

As a guideline, in the case of OSIRIS+ currently the main overheads are for target acquisition, 10 minutes in both imaging and spectroscopy mode (not including the time for acquisition and throughslit images), instrument preparation (30 seconds), and CCD readout (21 seconds in the standard observing mode). In case of the use of the tunable filter an additional 10 minutes is accounted for wavelength calibration, and, as explained above, night-time calibration observations must be specified as part of the program. For MEGARA, as in OSIRIS+, 10 min are assumed for target acquisition in IFU mode. For HiPERCAM and , the same overheads than OSIRIS+/MEGARA (10 minutes per observing block) will be considered for configuring the instrument.

In the case of EMIR, an observation in imaging mode a total overhead of 10 minutes per observing block should be accounted for, while for longslit/multi-object spectroscopy this total overhead increases to about 20 minutes, to account for the CSU configuration and acquisition process, except in the case of telluric standard observations where CSU re-configuration is not necessary and only 10 min of overheads are accounted for. In addition to the overheads for target acquisition and instrument/telescope setup, there are also overheads associated to the observing technique. For typical science exposures, open-shutter efficiency for EMIR is about 50% in imaging and 70% in spectroscopy (including dithering, readout overheads, etc.). To optimize the telescope time for a predefined on-source integration time, applicants should make use of the EMIR efficiency calculator available here.

Users should bear in mind that due to the limitation on the OB length in queue mode, shall a continuous observation be too long (longer than 1 hour), such observation must be split into as many OBs as necessary, each OB having its own acquisition overhead and possibly additional standard star observation overhead, since each OB might have to be observed on a different night.

As a general rule, needless to say, it is extremely important not to underestimate your overheads during proposal preparation, otherwise you may end up in a situation where integration time has to be sacrificed in your science targets because the granted time is not sufficient to complete the requested observations.

Further details may be found in the Appendix of Phase-2 help document.

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Targets of Opportunity

GTC accepts Target-of-Opportunity requests. These requests first have to be agreed and granted time by the Time Allocation Committee (TAC) before they can be considered for execution. The TAC may place certain constraints on their execution. GTC´s procedure for triggering Target-of-Opportunity observations can be found here.

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The nightly observing schedule

GTC is mainly scheduled following a dynamic queue of observing proposals. This means that the night-to-night observing plan is dynamic and responds not only to the scientific priority of the proposals, but also to the prevailing weather conditions. In this way the observatory aims to maximize the number of programs that can be completed according to the required sky conditions (seeing, moon phase, cloud cover, and precipitable water vapor), giving highest priority to the proposals that are most highly valued by the time allocation committees. This implies, however, that normally there is no guarantee that an observation will be carried out on a certain night, let alone on a given moment, as the detailed schedule depends on several variable. If the exact scheduling of a program is critical to its success, our advice is to request classical visitor mode since these programs are scheduled on specific nights and are executed under direct supervision of the PI .

Apart from the scientific programs, some nights are devoted to non-scientific activities such as the commissioning of instruments, maintenance activities, etc. GTC´s observing calendar can be found  here.

Occasionally a conflict may occur between scientific programs that require observations on the same moment. The following guidelines apply to resolve such conflicts:

1. Target-of-opportunity override requests only take priority over programs of lower scientific ranking.
2. In case of a direct conflict between two target-of-opportunity requests, GTC staff will strictly follow the well stablished TAC policy.
3. A program scheduled in visitor mode always takes priority, but can be overridden by a target-of-opportunity of higher scientific ranking.
4. Other conflicts are resolved first and foremost according to their scientific ranking, whereby the guaranteed time proposals for instrument builders are always considered of highest priority, followed by DDTs / GTC Large Programmes and finally, TACs regular programmes. However, in order to obtain the best possible use of the telescope time for all parties, aspect such as urgency, sky conditions, number of opportunities, and technical aspects are taken into account, which may include consultation with the PIs.

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Data dissemination / GTC Phase-3

Once the observations have been obtained and are considered to be of the correct quality, the PI will be informed about how to obtain the scientific data files from the FTP server. The data will remain available on the FTP server for a period of at least 50 days.

All observations are also copied to the GTC Public Science Archive, hosted at the Center for Astrobiology in Madrid (https://gtc.sdc.cab.inta-csic.es/gtc), which can also be used to avoid duplications when preparing a proposal. The PI enjoys a one-year proprietary period for her/his data, after which the data will be made publicly available. PIs are also encouraged to contact archive staff at CAB to submit their reduced data once they have been published.

GRANTECAN aims to have the data distributed to the PI as quickly as possible after the data have been obtained, but given the current resource limitations this may take several working days. The PIs are actively informed when their data are available on the GTC FTP server. Moreover, in the Phase-2 tool each PI can see whether her/his observing blocks have been prepared for distribution. In distributing the data priority is given to Target-of-Opportunity datasets.

In order to optimize the scientific data flow, a tool called GTC Phase-3 has been introduced to ease the interaction with users that obtained GTC sientific data. The aim of this tool is to establish a direct connection with GTC staff when any kind of support is required at the different steps of the data inspection, including some help in the data reduction with the most appropriated pipelines for each instrument.

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Observer's toolbox

To assist users of the GTC in preparing their programs and observations we point people first of all to the instrument pages that can be found on this web site. Another helping hand is provided in this observer's toolbox page where links, tools, and useful bits of information can be found.

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Feedback

GRANTECAN welcomes suggestions and comments on your observing experience with GTC. Your feedback will help us to improve our service. If you have received observing time on GTC you can submit your comments through our feedback form at http://gtc-phase2.gtc.iac.es/science/feedback/feedback.php.

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Publications and acknowledgements

Publications that result from data taken with the Gran Telescopio Canarias must credit the use of this telescope in the following way:

"Based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma."

Data from the GTC are also avaible at the GTC Public Archive, where become public once the one-year propietary period is over. Publications that result from data retrieved from the GTC Public Archive must include the following acknowledgement:

"This work is (partly) based on data from the GTC Public Archive at CAB (INTA-CSIC), developed in the framework of the Spanish Virtual Observatory project supported by the Spanish MINECO through grants AYA 2011-24052 and AYA 2014-55216. The system is maintained by the Data Archive Unit of the CAB (INTA-CSIC).

Regarding the instrumentation:

In the case that OSIRIS/OSIRIS+ instrument was used in the publication, the following acknowledgement might be included: "This work is (partly) based on data obtained with the instrument OSIRIS, built by a Consortium led by the Instituto de Astrofísica de Canarias in collaboration with the Instituto de Astronomía of the Universidad Autónoma de México. OSIRIS was funded by GRANTECAN and the National Plan of Astronomy and Astrophysics of the Spanish Government."

Particularly, in the case of SHARDS filters, all the science publications based on the use of these filters should include the following acknowledgement:

"This work is (partly) based on data obtained with the SHARDS filter set, purchased by Universidad Complutense de Madrid (UCM). SHARDS was funded by the Spanish Government through grant AYA2012-31277."

In the case that EMIR instrument was the one used in the publication, the following acknowledgment must be included: "This work is (partly) based on data obtained with the instrument EMIR, built by a Consortium led by the Instituto de Astrofísica de Canarias. EMIR was funded by GRANTECAN and the National Plan of Astronomy and Astrophysics of the Spanish Government."

In the case that MEGARA/MIRADAS instruments were used in the publication, it's also necessary to include the following acknowledgment: "This work is (partly) based on data obtained with MEGARA/MIRADAS instrument, funded by European Regional Development Funds (ERDF), through Programa Operativo Canarias FEDER 2014-2020."

Particularly for CIRCE, all the science publications based on the use of CIRCE will acknowledge both the University of Florida and GRANTECAN: "Development of CIRCE was supported by the University of Florida and the National Science Foundation (grant AST-0352664), in collaboration with IUCAA"

and will include a citation to the CIRCE reference publication: Garner et al. (2014), Proc. of SPIE Vol. 9147, 4.

Finally, in the case that HiPERCAM was used in the publication, it's also necessary to include the following acknowledgment: "This work is (partly) based on data obtained with the instrument HiPERCAM, built by the Universities of Sheffield, Warwick and Durham, the UK Astronomy Technology Centre, and the Instituto de Astrofísica de Canarias. Development of HiPERCAM was funded by the European Research Council, and its operations and enhancements by the Science and Technology Facilities Council."

and will include a citation to the HiPERCAM reference publication: Dhillon et al. (2021), MNRAS, 507, 350.

GRANTECAN would appreciate receiving preprint or references to publications that are based on data obtained with GTC.

For general queries about the observing process at GTC please contact: astro-helpdesk [at] gtc.iac.es

Last modified: 07 October 2024