Chandra X-Ray Observatory


TOO Trigger Form
DDT Form
Cost Proposal Form


Read and time triggering appropriately:

Triggering TOO and DDT requests sends texts and alarms to CXC staff. Please read and time your triggering appropriately. Submission of TOO and DDT requests with response windows less than 30 days trigger high-priority alerts for selected CXC personnel. Please be sure that your program requires rapid turn-around before submitting requests with response windows starting at 5 days or less. To the extent possible, please submit all requests during normal EDT/EST working hours.

Chandra Pitch Angle Restrictions:

Due to thermal limitations, restrictions on the satellite pitch angle are in place. These restrictions may make a proposed observation impossible to schedule.

BEFORE SUBMITTING A REQUEST for either a TOO or DDT observation:

However note that the Mission Planning team will make the final decision regarding the feasibility of an observation.

TOO Periods:

For TOOs approved in July of a given year for observation in the next Cycle, we allow early triggering if there are no competing programs for the Cycle already underway. Untriggered TOOs in the Cycle underway are normally cancelled at the Dec 31/Jan 1 transition, except for approved followups of triggered TOOs

Targets of Opportunity (TOO)

There are two types of TOO:

  1. Peer-reviewed TOO are TOO proposals that were submitted and accepted as a result of the Chandra Call for Proposals (CfP). The observer must demonstrate that the peer-reviewed trigger criteria have been met. The observer must also supply target coordinates if these were not indicated in advance. The TOO observer is encouraged to use the Remarks box to report any changes in observing instructions, any special observing instructions, instrument parameters, and/or other alerts that will aid in obtaining a successful observation.

    To activate a peer-reviewed TOO please use the TOO Trigger Form.

  2. Unanticipated TOO are charged to the Director's Discretionary Time (DDT). These will not have not been previously accepted through the Chandra Call for Proposals (CfP). For these types of TOO, a compelling scientific case must be made in order to justify their insertion into the mission timeline. Additionally, all pointing and instrument parameters must be specified. To request an unanticipated TOO, please see below.

    Please note that the decision whether to observe a TOO is made by the Observatory Director or his/her designated alternate. TOO fundamentally interrupt the observing timeline (which is planned at least 72 hours in advance), so considerable work must be done to alter Chandra's schedule.

Director's Discretionary Time (DDT)

Please Note: The separate scientific justification upload is not required as stated in the Cycle 19 CfP. For specifics, please read the CfP errata dated Feb 9, 2017.

Up to 1 Msec of Chandra observing time is reserved every cycle for CXC Director's Discretionary Time (DDT) observations. This allocation of time shall include unanticipated, non-peer reviewed TOO (see above).

Proposals for use of DDT should be confined to unique scientific opportunities for which the scientific return might be compromised by waiting to propose until the next Chandra Cycle. Proposals that could otherwise reasonably go through the typical peer review process generally will not be favorably received. A DDT request will be assessed by the CXC director in consultation with the NASA Project Scientist.

Proposals should be submitted using the DDT Form. A modest proprietary period of up to 3 months may be granted by the CXC Director upon request.

Funding may also be provided for such observations if deemed necessary to facilitate maximal scientific return, following a simple, ad-hoc review. If you wish to request funding, submit a cost proposal using the cost proposal form. It should not be submitted until after the proposal has been approved and is due after the observation is performed. US-based personnel only are eligible for funding. Please refer to the cost proposal help page for assistance. Your budget allocation can be obtained from the CXC by contacting our helpdesk.

For more information, please see:

Smithsonian Institute Smithsonian Institute

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email: Smithsonian Institution, Copyright © 1998-2017. All rights reserved.