WFC3 Data Handbook V. 4.0

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1.3	The IR Channel

The IR detector employs a 1024 x 1024 Teledyne (formerly Rockwell Scientific) low-noise, high-QE HgCdTe detector array with ~0.13 arcsecond pixels, covering a nominal 136 x 123 arcsecond field of view. Only the central 1014 x 1014 pixels are useful for imaging. The outer 5-pixels are used as reference pixels. The HgCdTe array is actively cooled by a six-stage TEC that keeps the detector at a nominal operating temperature of 145 K. The spectral response of the IR detector is optimized for imaging at near-IR wavelengths from approximately 800 to 1700 nm.

IR detectors, like the one used in WFC3, show higher dark current and read noise than CCD detectors. In addition, IR detectors allow accumulated signal to be read out non-destructively multiple times, without affecting other pixels. This capability can be exploited to reduce the effective read-out noise significantly and enable recovering pixels affected by cosmic rays (CR), since CR hits may be recognized and removed between adjacent reads.

The WFC3-IR detector is immune to the charge bleeding exhibited by CCDs at high signal levels; however, saturation can still be a problem because pixels subject to the highest signal levels show higher dark-current rates ("image persistence") in subsequent exposures. IR detectors do not show long-term on-orbit CTE degradation, because they do not employ the charge-transfer mechanism used in CCDs. However, they are intrinsically non-linear. Nevertheless, at low and intermediate count levels, the departure from linearity is quite modest and can be well calibrated.

The IR channel has a single filter wheel housing 17 spectral elements covering the near-IR wavelengths: 15 filters and 2 grisms. An 18th slot contains an opaque blocker (called a Blank). For IR observations, the requested element is simply rotated into the light beam. The IR channel operates only in MULTIACCUM mode, identical to NICMOS.

Figure 1.3 shows a schematic of the IR channel aperture projected onto the sky with respect to the V2/V3 reference frame. (For definitions of the coordinate systems in the figure, please refer to Section 6.4.3 of the WFC3 Instrument Handbook) The IR focal plane is tilted 22 degrees with respect to the incoming beam, thus the field of view as projected onto the sky is rectangular, with an aspect ratio of ~0.90. This distortion affects both the photometric accuracy and astrometric precision of the IR images. For a thorough discussion of WFC3 geometric distortion, we refer the reader to Chapter 4

Figure 1.3: Schematic of IR aperture with respect to the V2/V3 reference frame