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Proceedings Paper

Tunable filters for JWST Fine Guidance Sensor
Author(s): Neil Rowlands; Clinton Evans; Elliot Greenberg; Phil Gregory; Alan Scott; Simon Thibault; Michel Poirier; Rene Doyon; John B. Hutchings; Russ Alexander
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Paper Abstract

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.

Paper Details

Date Published: 12 October 2004
PDF: 12 pages
Proc. SPIE 5487, Optical, Infrared, and Millimeter Space Telescopes, (12 October 2004); doi: 10.1117/12.552040
Show Author Affiliations
Neil Rowlands, EMS Technologies Canada, Ltd. (Canada)
Clinton Evans, EMS Technologies Canada, Ltd. (Canada)
Elliot Greenberg, EMS Technologies Canada, Ltd. (Canada)
Phil Gregory, EMS Technologies Canada, Ltd. (Canada)
Alan Scott, EMS Technologies Canada, Ltd. (Canada)
Simon Thibault, INO (Canada)
Michel Poirier, INO (Canada)
Rene Doyon, Univ. de Montreal (Canada)
John B. Hutchings, Herzberg Institute of Astrophysics, NRCC (Canada)
Russ Alexander, Canadian Space Agency (Canada)

Published in SPIE Proceedings Vol. 5487:
Optical, Infrared, and Millimeter Space Telescopes
John C. Mather, Editor(s)

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