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Exposure Time Estimation

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Integration Time Calculator (ITC)

The GSAOI Integration Time Calculator (ITC) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions and GSAOI configurations. The ITC is divided into five sections: (1) Spatial profile and brightness of source; (2) Spectral distribution of source; (3) Instrument and telescope configuration; (4) Observing conditions; and (5) Details of observation. In the GSAOI ITC, the Strehl is estimated automatically based on selected filter and image quality, using the Strehl values measured for different filters and seeing conditions reported in the GeMS performance web page (see also Defining the CANOPUS AO WFS stars and the GSAOI OT web pages for details). 

WARNING: A bug has been reported in the GSAOI ITC when performing calculations on extended sources with uniform surface brightness when using the "Photometry in software aperture that gives 'optimum' S/N ratio" aperture size. Until the problem is resolved, please "manually select the software aperture size (under "Analysis method")" when using extended sources with uniform surface brightness.

The Gemini Integration Time Calculator (ITC) consists of a web form in which the user defines the principal aspects of the observation and a Java servlet, resident on a Gemini Observatory computer, performs the calculation and generates the results.

The web form is divided into four sections: (1) the spatial profile, spectral shape and brightness of the astronomical source, (2) the instrument and telescope configuration, (3) the observing conditions and (4) methods of how the observation will be carried out and the results analysed. There are additional help pages in each section that provide further information (e.g. see the more info links within the form).

Within the form, the desired characteristics are defined by selecting radio buttons (e.g. to choose between point or extended source), pull-down menus (e.g. to select the units for source brightness) and text boxes to enter numerical values (e.g. for source brightness). Clicking on the calculate button will post the user-defined values to the servlet and create an output web page containing the results. Unavailable features are indicated by red crosses.

The ITC is built around re-usable modules (e.g. for definition of the astronomical source) and specific elements for individual instruments. There is a separate web form for each instrument. Only the facility instruments listed above are supported; sensitivity tables are available for visiting instruments on the relevant web pages.

caution Caution: the ITCs make some assumptions about telescope and instrument performance and approximations for computational simplicity. Estimated accuracies will be posted as they become available.

ITC performance: imaging calculations typically take 2-3s, spectroscopic calculations take 4-5s, and calculations with multiple IFU elements take slightly longer.

Please report any problems with the ITCs via the Gemini HelpDesk. Please e-mail Bryan Miller ( and Andrew Stephens ( with suggestions (but please use the HelpDesk for questions!).

Exposure Time Estimation | Gemini Observatory


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