FGS-TFI: Tunable Filter Imager

The FGS-TFI is a versatile, general-use camera that enables the JWST to image a 2.2 arcmin × 2.2 arcmin field of view through narrow-band (R~100) filters. The central wavelengths of the filters can be selected by an observer over a wide range of near-infrared wavelengths. Although it is packaged with the Fine Guidance Sensor, the FGS-TFI is completely independent of the guide camera and plays no role in acquisition and tracking functions.

The novel capability of "tuning" filters to user-specified wavelengths is provided by a Fabry-Perot etalon and a suite of order-blocking filters. The optical coatings on the etalon plates permit the FGS-TFI to operate between 1.5 and 5 microns, with a small gap in coverage between 2.6 and 3.1 microns. Motions of the etalon are controlled by six piezoelectric actuators, which provide a high degree of stability and repeatability. The detector is a 2048 × 2048 HgCdTe array manufactured by Rockwell Scientific with 18 micron pixels and a composition tuned to provide a long-wavelength cutoff at approximately 5.3 microns. The FGS-TFI can be used in either direct imaging or coronagraphic imaging mode, with full-frame or sub-array readout.

The FGS-TFI complements other JWST science instruments by providing images with the quality of NIRCam through filters with wavelength resolution and sensitivity comparable to the low-resolution mode of NIRSpec. It also provides the observatory with imaging capability over the wavelength gap caused by the dichroic beamsplitter in NIRCam. The ability of the TFI to scan in wavelength while keeping all other components of the instrument and observatory fixed provides vastly superior suppression of the "speckles" caused by mid-frequency distortions in the primary mirrors. Thus, the TFI is expected to be the instrument of choice for demanding coronagraphic observations in the near-infrared

The FGS-TFI is being developed for the JWST project by the Canadian Space Agency, who have selected ComDev to be the prime contractor. René Doyon (Université de Montréal) is the Principal Investigator and leader of the Science Team overseeing the construction of the FGS-TFI.

Figure 1: Schematic illustration of the optical path of the FGS-TFI. (Click on the figure to enlarge.)