The Integrated Science Instrument Module (ISIM) for the James Webb Space Telescope (JWST) has successfully
completed its final cryogenic performance verification tests. The performance of the newly upgraded Fine Guidance
Sensor (FGS) / Near Infrared Imager and Slitless Spectrometer (NIRISS) was evaluated in these tests. We describe
some of the key guider performance results which have been obtained and compare them to previous results with an
older generation of H2RG infrared detector arrays. The identification mode sensitivity improvement is described along
with noise equivalent angle (NEA) sensitivity performance improvements in tracking and fine guiding modes. Tracking
mode allows the Observatory line of sight to settle in advance of the fine guidance mode and also facilitates moving
target observing. The NEA of the FGS-Guiders will in part determine the ultimate image quality of the JWST
The flight model Fine Guidance Sensor (FGS) on the James Webb Space Telescope (JWST) has successfully completed
its performance verification tests. The FGS cryogenic test is described along with some of the key guider performance
results which have been obtained. In particular we describe the noise equivalent angle (NEA) performance as a function
of guide star magnitude for the guider tracking mode. Tracking mode must be able to follow a guide star moving across
the field of view of either guider, primarily to allow the Observatory line of sight to settle in advance of the fine
guidance mode. FGS tracking mode will also be used for JWST’s moving target observing mode. The track testing
made use of the two movable sources within our JWST telescope simulator. The NEA of the FGS-Guiders will in part
determine the ultimate image quality of the JWST Observatory.
The Fine Guidance Sensor (FGS) on the James Webb Space Telescope (JWST) has a science observing capability that
was to be provided by a tunable Fabry-Pérot etalon incorporating dielectric coated etalon plates with a small vacuum gap
and piezoelectric actuators (PZTs). The JWST etalon was more challenging than our existing ground-based operational
systems due to the low-order gap, the extremely wide waveband and the environmental specifications. Difficulties were
encountered in providing the required performance due to variability in the mechanical gap after exposure to the
vibration, shock and cryogenic cycling environments required for the JWST mission. The risks associated with
operating the flight model etalon in the space environment, along with changes in scientific priorities, resulted in the
etalon being replaced by the grism-based Near-Infrared Imager and Slitless Spectrograph (NIRISS). We describe here
the performance of the etalon system and the unresolved risks that contributed to the decision to change the flight
The Tunable Filter Imager of the James Webb Space Telescope will be based on blocking filters and a tunable Fabry-
Perot etalon with an average resolution of about 100. It will operate in two wavelength bands from 1.6 μm to 2.5 μm and
from 3.1 μm to 4.9 μm at a cryogenic temperature of about 35K. It will respectively be used to study the First Light and
re-ionization of the universe by surveying Lyman-alpha sources and to provide an in-depth study of proto-planetary
systems as well as giant planets of nearby stars.
The Tunable Filter Imager (TFI) is designed to image a sky field of view of 2.2' by 2.2' (magnified to 4.6 deg. x 4.6 deg.
at the etalon). Its tunable etalon has an aperture of 56 mm. It operates at low orders 1 and 3 for the two wavelength bands
which reduces the number of blocking filters to a number of eight. The etalon gap tuning between 2.5 μm and 5.5 μm is
provided by piezoelectric actuators and will be servo controlled by using capacitive displacement sensors.
In this paper, we present the etalon's opto-mechanical design that allows us to achieve the stringent requirements in
terms of resolution over a wide infrared wavelength band, and operation at low gap at cryogenic temperature. Cryogenic
test results will be shared as well.
Previous publications for the JWST-FGS-TFI instrument described the design and fabrication of mirror coatings for
scanning Fabry-Perot etalons. Since that time, we have extended the fabrication process using ellipsometry analysis over
the full operational bandwidth from 1.0 to 5.0 microns for both mirror and anti-reflection coatings. This paper will
present single and multiple layer ellipsometry analysis of the a-Si/SiO2 optical properties. Analysis improvement came
from a-Si/SiO2 interface consideration and simultaneous use of ellipsometric data from Woollam V-VASE and IRVASE
instruments. Simulations of reflectance and transmittance based on the ellipsometric analysis results will also be
compared to spectrophotometric measurements for witness pieces.
The Fine Guidance Sensor (FGS) on the James Webb Space Telescope (JWST) has a science observing capability
provided by the Tunable Filter Imager (TFI). The TFI incorporates dielectric coated Fabry-Perot etalon plates with a
small vacuum gap. The separation of the plates is controlled by the Etalon Control Electronics (ECE) board, using
piezoelectric actuators (PZTs) and capacitive displacement sensors (CDS). The TFI measures over the wavelength range
of 1.6 to 4.9 microns with a spectral resolution of R~100. We present the key components of the etalon system and the
approach for characterizing and testing the system. Initial results from assembly-level testing are also presented.
The Fine Guidance Sensor (FGS) of the James Webb Space Telescope (JWST) features a tunable filter imager (TFI)
module covering the wavelength range from 1.5 to 5.0 μm at a resolving power of ~100 over a field of view of
2.2'×2.2'. TFI also features a set of occulting spots and a non-redundant mask for high-contrast imaging. This paper
presents the current status of the TFI development. The instrument is currently under its final integration and test phase.