Mercury-Cadmium-Telluride Focal Plane Array for Warm Alignment of Imaging Spectroscopy Systems
- Tuesday, 01 July 2014
This array minimizes the need to cold-cycle instrumentation, reducing cost and time for integration and testing.
NASA’s Jet Propulsion Laboratory, Pasadena, California
A new technique allows a mercury-cadmium-telluride (MCT) focal plane array (FPA) to operate at room temperature. These results were obtained through experimentation by varying the integration time, frame rate, and bias levels to optimize the output when warm.
Precise focal plane alignment has been a challenge for the integration and testing of imaging spectrometers for the past several years. This alignment process often results in multiple, week-long-duration cold cycles of the instrument, thereby increasing the risk and cost of the project. Instruments using this FPA are normally cooled to temperatures below 150 K for the MCT FPA to properly function. When the FPA is run at higher temperatures, the dark current increases to the point at which the full well is reached, saturating the output.
In a standard alignment scenario, the first few cold cycles allow the FPA to be roughly placed in the optimal position. The later cold cycles fine-tune the positioning for near-perfect alignment. These later cycles take into account the deformation of the opto-mechanical mounts at low temperatures. The ability to run the MCT FPA at room temperature allows the first few cold cycles to be eliminated. The final cold cycle is still necessary for fine-tuning because of the mount deformation caused by the cryogenic temperatures. The ability to run the FPA at room temperature is especially beneficial during these final crucial steps to assure the FPA is adjusted in the right direction and by the correct amount.
By applying modifications, the MCT detector can properly image at 300 K. The sensitivity of the device is rather low, as expected, but simple imaging can easily be achieved.
Results were obtained by imaging a simple scene onto the FPA using a 55-mm f1.8 Computar c-mount lens. The scene is a wristwatch on a black mat (see Figure 1). Because of the low sensitivity of the warm detector, it is necessary to illuminate the scene with a 150-W light. Getting the correct focus was difficult, so the images are a little out of focus. It is apparent in Figure 2 that the dark level on the focal plane while running warm is not very uniform. On average, the dark level is approximately 978 DN, but the standard deviation across the focal plane is 116 DN. This makes dark subtraction an essential component of the imaging process.
Using empirical models and basic laws, the plausibility of warm MCT operation was determined. Further investigation into the detector design reveals opportunities to optimize the MCT detector for use at 300 K. Applying these modifications, warm MCT focal plane operation was achieved with minimal modifications to the drive electronics. Simple imaging with the detector was accomplished in both the visible and near infrared. Verification of the theoretical MCT cutoff wavelength at 300 K was obtained using bandpass filters. Finally, the photon transfer gain was approximated, and the dark current of the warm detector was measured and compared to empirical model prediction. With the use of dark frame subtraction, scientific imaging may be possible.