WFOS has been designed to allow a future addition of an Integral Field Unit as an upgrade path for the instrument. When using the IFU observing mode, the IFU would be installed behind the telescope focal surface and above the collimator system and the camera system as shown below. In the other observing modes (slit spectroscopy and direct imaging), it is retracted from the light path.
The IFU would divide the FoV into 18 slices with an image slicer. The large FoV (20 x 27 arcsec2) is a unique capability of the large-aperture of TMT as shown in the figure below.
An IFU optical design concept has been developed based on the two-flat-mirror system adopted in KCWI on the Keck telescope and SINFONI on the VLT. Relay optics reimage the field of view on the slicer with a certain magnification factor. The slicer slices the image and delivers each sliced image to the corresponding pupil mirror. The pupil mirror reflects the light to the WFOS optics. In this concept, virtual pseudo slit images are created at the telescope focal surface.
The table below shows current parameters of the IFU. In the current concept, the IFU has two relay optics with different magnification factors and two slicers with different slice widths, and these slicers and relay optics are separately switchable. Their combination provides four different field-of-view (FOV) options with different slice widths on sky.
In the current IFU concept, the narrower slice width modes achieves 0.2- and 0.4-arcsec widths on sky. But the physical widths in those modes are the same as those of 0.75- and 1.5-arcsec-width modes, and therefore the spectrum resolutions are also the same. Some science cases demand physically narrower width to achieve higher spectral resolution. One example adapting the demand is shown in the table below; note that the finest scale slicer would provide full wavelength coverage from 0.31-1.0 microns with R~4100, an attractive option for diagnostic-quality observations of single targets. This case requires 4 different slicers but only one set of relay optics.