Observe with CHEOPS:
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Observing with CHEOPS



CHEOPS' science programme consists of two major components: The Guest Observers (GO) and Guaranteed Time Observers (GTO) Programmes. 

During the Nominal Mission (NM; from 25 March 2020 until 24 September 2023), the nominal science observing time was split into

  • up to 20% for the ESA-led Guest Observers (GO) Programme, through which the Community can conduct scientific investigations of their own choice 
  • at least 80% for the Guaranteed Time Observing (GTO) Programme, which is under the responsibility of the CHEOPS Science Team

With the start of the Extended Mission 1 (EM1; from 25 September 2023 until 31 December 2026; pending continued support by all member states), the nominal science observing time is split into

  • up to 30% for the ESA-led Guest Observers (GO) Programme
  • at least 70% for the Guaranteed Time Observing (GTO) Programme


Guest Observers (GO) Programme

Of CHEOPS' nominal science observing time, up to 20% in the Nominal Mission (NM) and up to 30% in the Extended Mission 1 (EM1), is available to the worldwide scientific community to conduct investigations of their choice in the form of an ESA-run Guest Observers’ (GO) Programme. ESA solicites proposals for GO Programme observing time primarily through a series of annual Announcements of Opportunity (AOs), filling at least 75% of GO time. Additionally, to allow new targets to be included in the GO programme at any time during the mission, up to 25% of the GO time are allocated as Discretionary Time (DT), as part of the Discretionary Programme (DP).

Proposals are selected and ranked on scientific merit, by an independent CHEOPS Time Allocation Committee, which is appointed by ESA in consultation with the CHEOPS PI. The TAC works independently from the CHEOPS Consortium, and receives technical support from the ESA Project Scientist (as secretary) and the CHEOPS Consortium Project Science Office (PSO). The TAC makes recommendations for CHEOPS observing time to ESA's Director of Science, who then awards the observing time. 

GO Programmes can cover any type of science addressable by CHEOPS' performance capabilities and compatible with the operational, observational and technical constraints of the mission. 

Observations of targets that are on the Reserved Target List (RTL; see below) are not allowed. Updates of the Reserved Target List to include new targets take place on a regular basis. In the NM, the RTL was technically unlimited (in practice constrained to ~150-200 targets). With the start of the EM1, the RTL is limited to 50 targets.

PIs of successful proposals will be invited to submit detailed information relating to the scheduling of their requested observations. This is used to generate the long term schedule for CHEOPS. Observers will be informedof their observation schedules when the relevant weekly activity plan is uploaded to the spacecraft.

ESA supports the community in preparation of proposals via tools and documentation available on this website.


Guaranteed time Observing (GTO) Programme

Of CHEOPS' nominal science observing time, at least 80% in the NM and 70% in the EM1 is dedicated to the GTO Programme. The definition of the GTO programme is under the responsibility of the CHEOPS Science Team, which is chaired by David Ehrenreich and Didier Queloz. The GTO Programme is based on the science case for the CHEOPS mission, which is summarised in the CHEOPS Science Requirements Document and the CHEOPS Definition Study Report (Red Book). Details of the GTO Programme can be found at this link. Details of membership of the Science Team can be found at this link. An overview of the GTO programme as it stands/stood at the time of the opening of the various AOs are available here:

The RTL comprised targets (i.e., lines of sight) from the GTO Programme that cannot be proposed by the GO Programme. The list is frozen at the time of the opening of each AO call. A dedicated Target Checker is available to query the RTL, along with unreserved GTO targets and GTO programmes. 


Questions about CHEOPS or the GO Programme? Please email cheops-support at cosmos.esa.int and we will be happy to help!
This website was last updated on 26 February 2024.