Clinton Evans

The JWST fine guidance sensor

The science instrumentation for the James Webb Space Telescope (JWST) has concluded its Phase A definition stage. We have developed a concept for the JWST Fine Guidance Sensor (FGS), which will form the Canadian contribution to the mission. As part of the JWST re-plan in early 2003, the FGS design was recast to incorporate a narrow-band (R~100) science-imaging mode. This capability was previously resident in the NIRCam instrument. This FGS science mode makes use of tunable filters and filter wheels containing blocking filters, calibration sources and aperture masks. The science function of the FGS Tunable Filters (FGS-TF) remains complementary to the NIRCam science goals. Narrow-band FGS-TF imaging will be employed during many of the JWST deep imaging surveys to take advantage of the sensitivity to emission line objects. The FGS-TF will also provide a coronagraphic capability for the characterization of host galaxies of active galactic nuclei and for the characterization of extra solar planets. The primary function of the FGS remains to provide the sensor data for the JWST Observatory line-of-sight stabilization system. We report here on the overall configuration of the FGS and we indicate how the concept meets the performance and interface requirements.

The James Webb Space Telescope science instrument suite: an overview of optical designs

The James Webb Space Telescope (JWST) Observatory, the follow-on mission to the Hubble Space Telescope and to the Spitzer Space Facility, will yield astounding breakthroughs in the realms of infrared space science. The science instrument suite for this Observatory will consist of a Near-Infrared Camera, a Near-Infrared Spectrograph, a Mid-Infrared Instrument with imager, coronagraph and integral field spectroscopy modes, and a Fine Guider System Instrument with both a Guider module and a Tunable Filter Module. In this paper we present an overview of the optical designs of the telescope and instruments.

Tunable filters for JWST Fine Guidance Sensor

The Canadian contribution to the James Webb Space Telescope (JWST) mission will be the Fine Guidance Sensor (FGS), incorporating a science-observing mode using tunable filters. We describe here the requirements, the opto-mechanical design concept and bread-board test results for the JWST FGS tunable filters. The FGS requires two continuously tunable filters over the wavelength ranges 1.2 - 2.4 microns and 2.4 - 4.8 microns each having a spectral resolution in the range of R~70 to 200. The selected implementation uses dielectric coated Fabry-Perot etalon plates with a small air gaps. The design finesse is ~30 and the filters are used in 3rd order. The operating temperature is ~35K. Current coating designs provide implementations that require only five blocking filters in each wavelength range to suppress unwanted orders. The filters will be scanned via the use of low voltage piezo-electric transducers. We present results from cryogenic tests of coating samples, PZT actuators and a structural model. The PZT actuators were found have a displacement of ~3.3 microns at 30K with an applied voltage of 125V, more than sufficient for the required scan of the Fabry-Perot plate spacing. The prototype etalon coating was found to be very stable cryogenically, having a measured change of transmission of only ~1% at 77K. The same coating on a 12.7 mm thick substrate, similar to that planned for the filter, was found to have a 18 nm peak-to-valley surface figure change when cooled to 30K. These results demonstrate that the development of tunable filters for the JWST FGS is on track to meet the technology readiness requirements of the program.

Cryogenic pupil alignment test architecture for the James Webb Space Telescope integrated science instrument module

The James Webb Space Telescope (JWST) is a space-based, infrared observatory designed to study the early stages of galaxy formation in the Universe. It is currently scheduled to be launched in 2013 and will go into orbit about the second Lagrange point of the Sun-Earth system and passively cooled to 30-50 K to enable astronomical observations from 0.6 to 28 μm. The JWST observatory consists of three primary elements: the spacecraft, the optical telescope element (OTE) and the integrated science instrument module (ISIM). The ISIM Element primarily consists of a mechanical metering structure, three science instruments and a fine guidance sensor with significant scientific capability. One of the critical opto-mechanical alignments for mission success is the co-registration of the OTE exit pupil with the entrance pupils of the ISIM instruments. To verify that the ISIM Element will be properly aligned with the nominal OTE exit pupil when the two elements come together, we have developed a cryogenic pupil measurement test architecture to measure three of the most critical pupil degrees-of-freedom during optical testing of the ISIM Element. The pupil measurement scheme makes use of: specularly reflective pupil alignment references located inside of the JWST instruments; ground support equipment that contains a pupil imaging module; an OTE simulator; and pupil viewing channels in two of the JWST flight instruments. Current modeling and analysis activities indicate this measurement approach will be able to verify pupil shear to an accuracy of 0.5-1%.

High-contrast imaging performance of the JWST fine guider sensor tunable filter coronagraph

The Fine Guider Sensor (FGS) of the James Webb Space Telescope (JWST) features two tunable filter (R~100) modules covering the 1.2-2.4 μm and 2.4-4.8 μm wavelength ranges, respectively. A set of occulting spots/bars mounted on a small slide located at the edge of the 2.3’x 2.3’ field of view (FOV) along with apodizing masks located in the filter wheel of each channel enable coronagraphic operation. Each coronagraphic field covers a square FOV of 20”x20”. The FGS-TF coronagraph complements the coronagraphic capabilities implemented in NIRCam and MIRI. This paper presents numerical simulations to predict the high-contrast imaging performance of the FGS-TF coronagraph. The combined coronagraphic and differential spectral imaging capabilities of the FGS-TF constitute a powerful tool for detecting and characterizing exoplanets with JWST.

Cryogenic wavefront error measurement for the James Webb Space Telescope fine guidance sensor powered optics

The Fine Guidance Sensor (FGS) is part of the instrument module for the James Webb Space Telescope (JWST). The FGS operates at 37 K and provides feedback to correct motion blur caused by relative motion within the observatory - an issue during long exposures. It also provides a tunable camera for science observations. The FGS powered optics comprises three, Three Mirror Assembly (TMA) - style reflective systems - one with finite conjugates and the remaining two are an infinite/finite conjugate pair. This paper addresses the issues of providing traceable interferometric wave-front error measurements when the test optics are in a cryogenic vacuum chamber. To meet space and time limits, we restrict attention to the finite conjugate device for the purposes of this publication.

JWST fine guidance sensor: guiding performance analysis

The Engineering Test Unit (ETU) of the Fine Guidance Sensor (FGS) for the James Webb Space Telescope (JWST) is currently in fabrication. Extensive modeling of the key FGS-Guider performance parameters has been used throughout the design process and continues to be used to evaluate the expected performance of the as-built instrument. A key parameter of interest is the expected Noise Equivalent Angle (NEA) provided by the FGS. The NEA will, in part, determine the ultimate image quality of the JWST Observatory. In this paper we use updated estimates of the End-oflife impact of contamination to present the current expected NEA performance of the FGS flight model. As component test data becomes available this data will be used as input to the FGS NEA performance model to assess the expected performance of the instrument.

Optimizing dielectric mirrors for scanning Fabry Perot interferometers with constant spectral resolution

Both the spectral resolution and the free spectral range of a scanning Fabry-Perot interferometer at any particular wavelength are functions of the interference order, the reflectance and the phase-dispersion of the cavitys mirrors. It is generally desirable to design the mirror coatings to minimize the phase dispersion. This approach reduces the number of blocking-filters required to suppress multiple orders. It also avoids undesirable order overlap. However, there will always be some phase-dispersion present in the mirror coating design. In an instrument where a constant spectral resolution is required over the tuning range, it is desirable to optimize the combination of reflectance and the remaining phase dispersion to produce the required spectral resolution value. This paper describes a suitable optimization method and provides some numerical results.

Tale of two underwater lenses

The design of two advanced underwater lenses for use with high-resolution, charged coupled device (CCD) still cameras is presented. Some practical aspects of lenses for CCD cameras are discussed and how the customers requirements led to different choices of water-lens interface is demonstrated.

OGSE telescope WFE testing at 30K

The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) gathers the light from astronomical objects and provides it to four scientific instruments and the observatory guider. The Canadian contribution to JWST, the Fine Guidance Sensor (FGS), includes both the FGS-Guider and one of the science instruments, the Tunable Filter Imager (FGS-TFI); both are packaged together and are functionally independent. The FGS OGSE (Optical Ground Support Equipment) is used to simulate the image from the OTE and verify the optical performance of the FGS Guider and TFI during instrument level testing. The OGSE consists of 25 separate telescopes, each of which simulates a point source at a different field location. The OGSE must maintain alignment and image quality at the cryogenic (30-40K) operating temperature of the FGS. This paper presents WFE (wavefront error) testing for one of the telescopes over a temperature range from ambient to cryogenic operating temperatures (30 K). This test made use of a Zygo interferometer with the standard Zygo transmission sphere replaced by a custom-made transmission sphere located in the cryo vacuum chamber. Meanwhile, image position displacements (focus) during cooling down with respect to ambient are also obtained by tracking the position of the transmission sphere. The results show that the WFE degrades from 19 nm (RMS) at ambient to 42 nm (RMS) at 30 K, while the image displaces about 5.6 mm at 30 K with respect to ambient temperature. The reason for the focus displacement is discussed.