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The InfraRed Telescope (IRT)

The InfraRed Telescope (IRT) on board THESEUS is designed in order to identify, localize and study the transients and especially the afterglows of the GRBs detected by the Soft X-ray Imager (SXI) and the X and Gamma Imaging Spectrometer (XGIS). The telescope associated with the IRT is a Korsch type with a minimal entrance aperture of 0.7 m diameter. The focal plane instrument is composed by a spectral wheel and a filter wheel in which the ZYJH filters, a prism and a volume phase holographic (VPH) grating will be mounted, in order to provide the expected scientific product (imaging, low and high-resolution spectra of GRB afterglows and other transients). The IRT concept block diagram is given below.

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Figure 9. Left: optical design of the IRT instrument and telescope. Right: concept block diagram of the IRT instrument.


Specifications of the entire system are given in Table 3. The mechanical instrument support structure holding the internal mirrors, the electronics, the detectors, and the filter wheel is shown in Figure 10. The thermal hardware is composed by a pulse tube cooling the Detector and FEE electronics and a set of thermal straps extracting the heat from the electronic boxes and camera optics coupled to a radiator located on the spacecraft structure. The overall IRT mass is 51 kg and the total power supply is 81 W.


Telescope type

Off-axis Korsch

Primary & Secondary Size

>600 mm (goal 700 mm) & 214-250 mm

Detector type

Baseline: European ALFA detector (2048x2048 15 mpixels)

Back-up: Teledyne Hawaii 2-RG 2048x2048 18 mpixels

Imaging plate scale

0.6 arcsec/pixel with 18 mm pixel size

Field of view

15x15 arc min in imaging and LRS modes, 5x5 arc min in HRS mode

Rsolution (Dl/l)

20 in LRS mode; 500 in HRS mode

Sensitivity (H band)

20.6 (AB; 300 s) in imaging mode; 18.5 (AB; 300 s) in LRS mode; 17.5 (AB, 1800 s) in HRS mode

Wavelength range

0.7-1.8 min imaging mode; 0.8-1.6 min LRS and HRS modes

Table 3. IRT specifications


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Figure 10. Top: IRT instrument structure assembly. Bottom: Accommodation of the IRT camera and telescope on the satellite optical bench.


IRT observational modes and sequence

The IRT instrument has 3 observation modes:

  • The photometric mode for which spectral filters are used;
  • The low-resolution (LR) mode for which a dispersive prism is used;
  • The high-resolution (HR) mode for which a grism is used.

These components are placed in a filter wheel and are located in the entrance pupil of the instrument. When a gamma-ray burst is identified, the IRT will execure the following observational sequence:

  1. The IRT will observe in imaging mode as soon as the satellite is stabilized within 1 arc sec in the GRB error box. Three initial frames in the ZJH-bands will be taken (10 s each, goal 19 AB 5 σ sensitivity limit in H) to establish the astrometry and determine the detected sources colours.
  2. IRT will enter the spectroscopy mode (Low Resolution Spectra, LRS) for a total integration time of 5 minutes (3 σ sensitivity limit per resolution bin expected for an H 18.5 (AB) source).
  3. Sources with peculiar colours and/or variability (such as GRB afterglows) should have been pinpointed while the low-res spectra were obtained and IRT will take a deeper (20 mag sensitivity limit (AB)) H-band image for a total of 60s. These images will be then added/subtracted on board in order to identify bright variable sources with one of them possibly matching one of the peculiar colour ones. NIR catalogues will also be used in order to exclude known sources from the GRB candidates.
  4. In case a peculiar colour source or/and bright (< 17.5 H (AB)) variable source is found in the imaging sequence, the IRT computes its redshift (a numerical value if 6<z<10 or an upper limit z<6) from the low-resolution spectra obtained at point 1) and determines its position. Both the position and redshift estimate will be sent to ground for follow-up observations, by larger facilities. The derived position will then be used in order to ask the satellite to slew to it so that the source is places in the in the high-resolution part of the detector plane where the slit-less high-resolution mode spectra are acquired. Following the slew, the IRT enters the High-Resolution Spectra (HRS) mode where it shall acquire at least three spectra of the source (for a total exposure time of 1800s) covering the 0.8-1.6 μm range. Then it can go back to imaging mode (H-band) for at least another 1800s.
  5. In case that a faint (> 17.5 H (AB)) variable source is found, IRT computes its redshift from the low-resolution spectra, determines its position and sends both information to the ground. In this case too, the IRT asks for a slew to the platform in order to put the source on axis (optimizing the its sensitivity) and takes data in imaging mode for a 3600s time interval to establish the GRB photometric light curve (covering any possible flaring) and leading the light curve to be known with an accuracy of <5 %.

Note that if at point (2) the burst IR counterpart is not identified, the observation will be interrupted and the satellite will switch back to the main survey mode pointing direction.