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IRIS Exposure Time Calculator User Instructions


This page has a description of IRIS that details the instrument focal plane, the layout of imaging detectors and where the pick off mirrors for the IFU sample the focal plane. Observing modes are detailed, specifically the filters available for the imager and the combinations of filter, grating and spatial sampling for the IFU.

The input fields for the Exposure Time Calculator (ETC) are described and there are tables of filter and grating characteristics so that appropriate filters and gratings can be chosen for a science program.

The instrument description and description of input fields for the IRIS ETC were developed by Nils Rundquist and Arun Surya of the CASS OIR Lab at University of California, San Diego.

Instrument description

IRIS Modes

The InfraRed Imaging Spectrograph (IRIS) is a first-light instrument for the Thirty Meter Telescope which consists of an imager and Integral Field Spectrograph (IFS) which will sample the corrected adaptive optics field by the Narrow Field InfraRed Adaptive Optics System (NFIRAOS) in the near infrared (0.84 - 2.4 μm). The imager will provide a 34” x 34” field of view at TMT’s diffraction limited plate scale of 4 mas.  IRIS’s IFS offers two spatial sampling methods at varying plate scales and fields of view. A summary of the modes provided by IRIS is shown in Table 1 below.

Capability mode




Field of View









4 mas

34 x 34

Set by filter


37 filters

Variety of bandpasses

Slicer IFS

88x45 Spaxels

50 mas

25 mas

4.4 x 2.25

2.2 x 1.125

4,000, 8,000



20%, 10%

20%, 10%

Slicer IFS

44x45 Spaxels

50 mas

25 mas

4.2 x 2.25

1.1 x 1.125

4,000 - 10,000



20%, 10%, H+K

Lenslet IFS

112x128 Spaxels

9 mas

4 mas

1.01 x 1.15

0.45 x 0.51






Lenslet IFS

16x128 Spaxels

9 mas

4 mas

0.144 x 1.15

0.064 x 0.51

4,000 - 10,000



20%, H+K

20%, H+K

Table 1. Summary of IRIS Modes.


The IRIS imager will consist of 4 Hawaii 4RG detectors situated surrounding the IFS pick-off mirror. This will allow for simultaneous use of the IFS and imager modes, and for on-detector guide window capability using the imager detectors, as illustrated in Figure 1. The design of the imager and IFU combination is 'serial', i.e. light passes first through the imager optics and filter before passing to the IFU. A filter is used before the IFU to limit the wavelength range entering the IFU.



IRIS Focal Plane for ETC instruction page

Figure 1. Illustration of the IRIS focal plane in operation with NFIRAOS. The green squares represent the 4 imager detectors (4mas/pixel). The small rectangles and squares in the center represent the 4 field of view options for the IFS. The patrol regions for the probe arms of the On-Instrument-Wave-Front-Sensor (OIWFS) are shown that pickoff natural guide stars for the AO system.

IRIS instrument configuration options

Figure 2. IRIS instrument configuration options: The beam to the IRIS imager and IFS passes through a user selected filter. A small on-axis fixed mirror sends the central beam to the lenslet optics, two reimaging lenses allow selection between 4 and 9 mas spatial scale. A selectable mirror can send the on-axis beam to the image slicer and two reimaging lenses allow selection between 25 and 50 mas spatial scale.

IRIS Filters and Gratings

IRIS has multiple filters and gratings available to the user. Filters are used with imaging mode and in conjunction with gratings in IFU mode. These modes span the full wavelength range from 0.8 - 2.4 microns with varying filter width and transmission profiles.  The approximate transmission curves for individual filters are shown in Figures 3 and 4 for the standard and specialty filters, respectively, both in terms of transmission ratio and flux per micron. Tables detailing the characteristics of each filter and the characteristics of each grating are below.

IRIS standard filters

Figure 3. Transmission and flux curves of IRIS’s standard wavelength filter set.


IRIS speciality filters

Figure 4. Transmission and flux curves of IRIS’s small-bandwidth specialty filters.

ETC Mode Descriptions

This section provides descriptions of the different field options available to the online ETC segmented between the Imager and IFS modes.


Scale: The IRIS Imager operates at a 4mas/pixel plate scale (sampling the 7mas@1 micron diffraction limited spatial resolution). 
Field of View: The IRIS Imager provides a 34” x 34” field of view.
Filter: This field selects the filter bandpass through which the signal-to-noise ratio or exposure time will be calculated. 


Mode: Slicer or Lenslet arrays, depending on the desired field of view, plate scale, and spectral resolution.
Grating: Select the grating desired for the desired wavelength range and resolution.
Plate Scale: Select the plate scale desired for the given mode.
Field of View: Select the desired field of view for the given mode.
Spatial Elements: The number of spatial elements in the observation as defined by the mode and grating selected.
Filter: Select the desired bandpass for the simulation.

Source Properties

Point Source: This option indicates that the simulated source should be point-source like in nature, with flux distributed according to the Point Spread Function associated with that spatial position and wavelength range.
Extended: This option indicates that the flux should be distributed over the observed spatial field, according to some profile and not point-source like. 
Flux: Flux density of simulated source in ergs per second per cm2 per angstrom.
Spectrum: Enter the desired spectrum shape for the simulated observation.


Signal-To-Noise Ratio: Enter the desired peak signal to noise ratio of the observation if calculating the required exposure time.
Exposure Time: Enter the exposure time of the observation if calculating the resulting signal-to-noise ratio.
Number of Frames: Enter the number of frames taken in the observation of the provided exposure time if calculating the resulting signal-to-noise ratio.
Total Integration Time: This returns the total integration time needed for an observation with the given signal-to-noise ratio or the provided exposure time and number of frames.


Zenith Angle: Select the zenith angle for your observation, options are (in degrees): 0, 30, 45.
Good / Average / Bad Conditions: Select the atmospheric conditions for the simulated observation.  “Good” is 75% seeing from the Cn2 profile, “Average” is 50%, and “Bad” is 25%. Appropriate PSFs are used for the selected conditions.
PSF Location: Select the location across the detector field of view for the simulated point source. If using the IFS, the location is defined to be on-axis, at the location (0”,0”).

IRIS focal plane PSF locations

Figure 5. Grid of PSF locations across the field of view of a single detector of the IRIS imager which are available for selection by user for a point source simulation. The on-axis location corresponds to the position of the IFU.

ETC Walk Through

This section will eventually provide a basic walk through of several basic ETC use cases. (2018/10/12)

The IRIS Filter Set

Table of Characteristics of IRIS Filters


The IRIS Grating Set

Table of Characteristics of IRIS Gratings