ToO Proposals

1. Definition

Target of Opportunity (ToO) proposals are for observations of transient and/or rare phenomena with specific or unspecific objects based on clear observational strategy, such as nearby supernovae, Gamma-Ray Burst follow-up, etc.

2. Scope and Applicability

ToO proposals should be submitted in the same manner as Normal Program proposals, while clearly specifying which Subaru instruments will be used to perform the ToO observations and how much observing time will be required.

The applicability of ToO observations to Intensive Programs is currently under discussion. This mode is not open in S26B.

There are two types of ToO programs.
Non-rapid ToO programs do not require immediate attention and can be executed after 24 hours or more. PIs are required to submit their requests by 9:00 am Hawaii time on the day before their observing night.
Rapid ToO programs require immediate attention and must be executed within 24 hours. PIs may submit their requests within 24 hours. Considering the practical approval procedure, PIs are recommended to trigger the observation several hours in advance.

In the case your proposal includes both rapid and non-rapid ToO, please select "rapid ToO".

3. Requirement

Applicants of ToO proposals must be able to perform observations by their own teams, including remote observations. The Observatory staff is not necessarily required in their team.

4. Gemini ToO Observation

Subaru users who are eligible for Subaru-Gemini time exchange programs may also submit ToO proposals to the time-exchange program with Gemini, which will be executed at queue mode. Please refer to Subaru/Gemini page for more details.

5. Execution of ToO

When a ToO observation request is executed, the Director will make the decision to override scheduled observations. ToO observations are generally conducted in a half-night increments, except during HSC Queue mode, when observations can be undertaken in hourly increments, with the minimum observation time being 2 hours or more. PI or observers should attend the ToO observations. The telescope time of regular observers lost due to the interruption of ToO observations will be compensated by the observatory. Please also refer to the ToO Policy (English/Japanese).

updated on February 3, 2026