IRIS Exposure Time Calculator

The IRIS Exposure Time Calculator (ETC) is a tool to assist with the development of science cases that involve observations with IRIS (TMT's first light diffration limited near-IR (1-2.4 µm) imager and Integral Field Spectrograph). The IRIS ETC carries out a full decomposition with the expected PSF and may take several seconds to run.

IRIS Exposure Time Calculator User Guide

Enquiries and questions to: instruments@tmt.org

Instrument Setup

Configuration

The IFS is serial behind the imager. Light passes through the imager filter and into the IFS.

Mode

Imager [4mas/pix]

Integral Field Spectrograph

IFS Mode

Grating

Plate Scale

Field of View

Spatial Elements

Exposure

Calculation

Signal-to-noise

Number of Frames

Total Integration time [s]

Signal-to-noise

Exposure time [s] per frame

Source Properties

Point source

Extended Source

Magnitude and flux density are per square arcsecond for extended source.

Magnitude [Vega]

Flux density [erg/s/cm^2/Å]

Integrated flux over bandpass [erg/s/cm^2]

Spectrum

Wavelength [microns]

Line width in velocity [km/s]

Expected Performance and Atmospheric Conditions

Point spread functions (PSF) are defined in each bandpass with varying atmospheric conditions, AO guide star configuration, Zenith angle, and field position across the IRIS focal plane and used to approximate different expected observing conditions.

Atmospheric Conditions

The atmospheric conditions are calculated from turbulence profiles taken during three years of site testing at the TMT site at Mauna Kea 13N. For each profile, the AO wave front error (sum of fitting, bandwidth and isoplanatic error terms) is found and profiles are sorted by this wave front error. The representative profile for a given percentile X% is then defined as the average of all profiles in the range [X-5:X+5]%.

Good - 25% atmospheric conditions

Average - 50% atmospheric conditions

Bad - 75% atmospheric conditions

Point Spread Function Location

The spectrograph PSF is always on-axis. Users can specify field locations across the four imagers.

Zenith Angle [degrees]

The IRIS ETC was developed by members of the IRIS technical team at UC San Diego in collaboration with Caltech/IPAC: Eric Oh (IPAC), Jacob Llamas (IPAC), Nils-Erik Rundquist (UCSD), Arun Surya (UCSD), Greg Walth (Carnegie), and Shelley Wright (UCSD).

ETC was updated at 9:00 PM, November 14th, 2019 UTC