Observing with GTC



icon Introduction

The Gran Telescopio Canarias (GTC) started its operational phase with its first scientific observations on March 16th, 2009. Gradually, as the telescope and the instrumentation suit develop, more time is being dedicated to scientific activities. During Semester 2018A about 80% of the time will be dedicated to science observations. The remaining time is primarily being dedicated to extending and improving the functionality of the telescope and the commissioning of new instrumentation and observing modes.

Potential applicants for observing time are referred to the formal Announcement of Opportunity for each observing semester that will be posted on the GTC web pages and where details can be found on the instruments and observing modes that are offered.

Observations are being carried out either in queue-scheduled service mode, or in classical observing mode with the observations scheduled on specific nights. Applicants who wish to carry out the observations themselves according to a fixed time table should indicate this on the application form by selecting the "classical" observing mode option. The observatory will take this into account when drawing up the observing schedule, but no guarantee can be given beforehand that the request can be honored. In particular programs of low priority will likely be carried out in queue scheduled mode.

For those PIs who will come to observe at GTC we present the following practical information in english and spanish. We urge visitors to carefully read these instructions.

GTC external

During Semester S18A the telescope will host three instruments: OSIRIS, EMIR and HiPERCAM. OSIRIS is permanently installed in the telescope´s Nasmyth-B focal station and offers the following observing modes: broadband imaging, narrowband imaging with tunable filters, long-slit and multi-object spectroscopy (MOS) in the optical wavelength range, and frame-transfer readout modes and fast-photometry modes. Frame-transfer and fast-photometry readout modes will only be available for observations in classical visitor mode, in the presence of the PI.

Also for MOS observations a number of restrictions apply: MOS observations require significant preparations by the science team as well as by the observatory. These preparations require time and therefore lead to certain scheduling restrictions. OSIRIS MOS observations are only admitted for programs that fall in the top half of the TACs ranked list. Furthermore, to design and manufacture MOS masks the science team must have available either accurate equatorial coordinates for all their science targets as well as for the alignment stars, or they must have access to an OSIRIS Sloan r´ band image that can be used to design a mask. In case an OSIRIS pre-image must be obtained, no guarantee can be given that the full cycle of obtaining a pre-image, mask design, mask manufacturing and preparation, and observation can be completed within one semester. Hence when requesting MOS observations in classical visitor mode, the science team must already have an adequate OSIRIS Sloan r´ image of their field available before the start of the semester. If not, the observations cannot be scheduled. Alternatively, OSIRIS MOS observations can be scheduled in queue mode. But as before, in case the science team requires pre-images of the MOS fields as part of the program, it is required that the program is given an execution period beyond one semester by the Time Allocation Committee (i.e. the ones that fall within the first quartile of time allocated by the Spanish CAT), otherwise the observation cannot be accommodated.

More details on the observing modes for this instrument can be found here.

EMIR is now permanently installed in the telescope´s Nasmyth-A focal station, and it's offered in imaging and longslit spectroscopy mode for the Near Infrared wavelength range. Details on the instrument can be found here. Finally, the optical high-speed, 5-band HiPERCAM visiting instrument will be located in one of the Folded Cassegrain focus and will be offered on shared-risk basis at the beginning of the semester (March-May 2018).

OSIRIS: (updated February 2015)

When preparing observing proposals it is important to make realistic estimates of the observing overheads at the time of writing a proposal. As a guideline, for simple observations in both imaging and spectroscopy mode a total overhead of 10 minutes per Observing Block should be accounted for, and for tunable filter imaging to 20 minutes. Note that these numbers serve as a guide only and may vary accordin g to the detailed definition of the OB (for example, they don't include the time for acquisition and throughslit images, if required, as they depend on the exposure time needed) (details can be found in the Phase-2 manual).

Observing overheads with the tunable filters are large due to the complexity of the observing process and the necessity to regularly re-calibrate the filter. These overheads, to first approximation, have been included in the Phase-2 definition tool. When using the tunable filter, for reasons of overall efficiency and since these calibrations are specific for each observing program, we require that applicants define their night-time tunable-filter calibrations also as Observing Blocks. The time necessary for these calibrations will be charged to the observing program.

During the observing night the observatory will carry out a series of standard calibrations that for OSIRIS include bias frames, flat fields, spectral lamp exposures, and (spectro-) photometric standard stars. This will be done to the best of our ability, taking into account the science programs that are scheduled during the night, the weather conditions and possible problems with the equipment that may occur. Since these standard calibrations will not be specific to any one science program the time required for this will not be charged to any program but taken from the general observing overheads. An exception to this philosophy are OSIRIS tunable filter observations for which, as explained above, calibrations will have to be explicitly specified and charged to each program. Given the wide range of combinations of the instrument setup, the standard calibrations are restricted to 2x2 binning of the CCD, unwindowed readout of the CCD, and only for a readout speed of 200 kHz. (Note that from semester 2013B onwards the standard readout mode will be 200 kHz for all OSIRIS observing modes). For more technical details see the OSIRIS instrument page. Observing programs that require a different setup will have to define their calibrations as observing blocks (or in the README file, where appropriate) for which the time used at night (e.g. for observing standard stars) will be charged to that program.

EMIR: (updated February 2016)

For semester 2018A the instrument will be made available for imaging and longslit spectroscopy. For observing overheads when using EMIR we refer to the following section of the description of the instrument: EMIR observing strategy and Phase 2 and the Phase-2 manual. As a rough estimate, for typical deep exposures in imaging mode, open-shutter efficiency can be of the order of 50% (including dithering, readout overheads, etc.).

During the EMIR operation the user should select the appropriate telluric standard and prepare the corresponding observing block, so the time at night will be charged to the program. As in any other NIR instrument, sky background variations dictate the use of dithering in all the scientific observations. EMIR manages this in a direct way by only defining the total on-source time required. The system internally will divide this in an optimal combination of a series of different exposures by taking into account the coherence time variation in the desidered filter/grism. For more technical details please read the EMIR user manual and the Phase-2 manual.

HiPERCAM: (updated February 2016)

The HiPERCAM high-speed, multi-band optical imager is a new GTC visitor instrument (P.I. Vik Dhillon) that will be located in the Folded-Cass E focus. HiPERCAM will be 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 3.9 arcmin with a 0.113 arcsec/pix plate scale.

HiPERCAM is expected to be received at the telescope by the end of 2017 (October-November 2017), and is offered during semester S18A on a "shared risk" basis. Further information about the instrument can be found in the HiPERCAM web page.

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icon 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)

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. For S18A onwards there are 100 h available per semester to be used with EMIR.

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.

timeUse graph

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icon 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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icon 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 (TAC). 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.

osiris picture

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 three bands:

  1. Band-A proposals are considered projects of highest scientific priority for a give set of observing conditions. These proposals should enjoy a high chance of being completed successfully.
  2. Band-B proposals are backup proposals with a reasonable chance of getting data. It should be noted that the total queue is over-subscribed, which will affect proposals in Band-B the most.
  3. Band-C 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 be executed when no Band A or B proposals can be carried out for reasons of poor weather. 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 current observing semester at GTC (Semester 2017B).

RAdist graph

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.

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-half of the TACs priority list can enter as visiting observer programs . Programs of lower priority 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.

An overview of the time allocations can be found through this link by selecting a semester and the option show time allocations for semester.

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icon 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, EMIR and HiPERCAM, respectively.

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icon 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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icon 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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icon 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 HiPERCAM, the same overheads than OSIRIS (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 spectroscopy this total overhead increases to about 15 minutes, to account for the CSU configuration and acquisition process. 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% (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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icon 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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icon 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 then these are dealt with on a first-come-first-serve basis.
  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 ESO-GTC proposals are considered of highest priority within the Spanish time, followed by the guaranteed time proposals for instrument builders. 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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icon Data dissemination

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. The PI enjoys a one-year proprietary period for her/his data, after which the data will be made publicly available.

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.

After the one-year proprietary period the raw data is copied to the GTC Science Archive, hosted at Center for Astrobiology in Madrid (http://gtc.sdc.cab.inta-csic.es/gtc). PIs are also encouraged to contact archive staff at CAB to submit their reduced data once they have been published.

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icon 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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icon 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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icon 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), instaled in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma."

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."

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.

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: 08 September 2017