We discuss work in progress on a near-infrared tunable bandpass filter for the Goddard baseline wide field camera concept of the Next Generation Space Telescope Integrated Science Instrument Module. This filter, the Demonstration Unit for Low Order Cryogenic Etalon (DULCE), is designed to demonstrate a high efficiency scanning Fabry-Perot etalon operating in interference orders 1 - 4 at 30 K with a high stability DSP based servo control system. DULCE is currently the only available tunable filter for lower order cryogenic operation in the near infrared. In this application, scanning etalons will illuminate the focal plane arrays with a single order of interference to enable wide field lower resolution hyperspectral imaging over a wide range of redshifts. We discuss why tunable filters are an important instrument component in future space-based observatories.
This paper describes the theory of design, operation, and testing of a tunable MWIR Fabry-Perot interferometer operating in low orders. This device is called the agile bandpass tunable filter (ABTF) due to the fact that the spectral bandwidth can be changed by a large factor by changing the order. In first order the system can be tuned over the entire 3.5-5 micrometers spectral region with only a single order sorting filter. We provide a short introduction to tunable filters an then briefly discuss the requirements that low order operation places on the Fabry-Perot dielectric mirrors. Operation in low orders forces one to abandon the classical Fabry-Perot approximation that the mirrors are negligibly thin compared to the plate separation. Rather, one must now account for the phase properties of the dielectric stack mirrors as they produce phase effects comparable to the plate separation. We next address the issue of control of the Fabry-Perot. This is accomplished through a closed-loop system using capacitive sensor on the Fabry-Perot flats to measure the separation of the plates. Additionally we describe how the ABTF is characterized using a FTIR to measure the bandpass shape and position, and we show some examples of measurements made with the ABTF used as a hyperspectral imaging system with a 256 X 256 HgCdTe camera. We conclude with a discussion of potential applications and future work.
The agile bandpass tunable filter (ABTF) is a new instrument with variable resolution and center wavelength that allows MWIR and LWIR infrared cameras to recognize spectral images that are characteristic of specific materials, including gases. Benefits of this new technology include the ability to: (1) Task infrared camera-based sensors to search for specific materials, and (2) Transform infrared camera data into signatures easily understood by untrained users. The application of a working prototype hyperspectral ABTF is described. The prototype is capable of rapidly selecting bandpasses from among several spectral resolutions, and is easily tunable over the MWIR spectral band. The application of the ABTF to signature imaging of gases is shown.
Passive remote determination of the presence and quantity of gaseous chemicals is of interest for a variety of applications, but has been technically difficult to accomplish because of challenging measurement and analysis difficulties. This paper describes progress in the development of infrared plume analysis software. To unravel the atmosphere, background, and thermal contrast relationships it has been found necessary to obtain both spatial and spectral information and to have a set of codes which take these phenomena into account. Measured data were used along with model analyses to develop advanced plume analysis software (APAS) applicable to calculating how accurately amounts of gas ben be determined from overhead with current and possible future sensor technology. The APAS suite of codes incorporates sensor noise, plume contrast and atmospheric effects modules to arrive at a comparative measure of gas plume detection accuracy. The application of the APAS codes to prediction of detection accuracy for several candidate sensors is shown.