1.3 The IR Channel

The IR detector employs a 1024 × 1024 Teledyne (formerly Rockwell Scientific) low noise, high quantum efficiency (QE) HgCdTe detector array with ~0.13" pixels, covering a nominal 136 × 123" field of view. Only the central 1014 × 1014 pixels are useful for imaging. The outer rind, 5 pixels in width, contains light-insensitive pixels that are used as reference. The HgCdTe array is actively cooled by a six-stage TEC that maintains 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 allow accumulated signal to be read out non-destructively multiple times without affecting other pixels (a mode referred to as MULTIACCUM). This capability can be exploited to significantly reduce the effective read-out noise, enable recovery of sources that saturate during the integration time, and enable recovery of pixels affected by cosmic rays (CR), since CRs 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 (see Chapter 8). IR detectors do not show long-term on-orbit charge transfer efficiency (CTE) degradation, because they do not employ the charge-transfer mechanism used in CCDs. However, they are intrinsically non-linear, although 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 aluminum 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. The WFC3 IR channel does not have a mechanical shutter, thus when the channel is not in use, the Blank is moved into the beam to block light from entering the detector, and the detector itself is continuously reset at the fastest possible rate (i.e. read out as fast as possible) for the full frame (about 2.93 seconds.)

Figure 1.4 shows a schematic of the IR channel aperture projected onto the sky with respect to the U2/U3 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 1.4: The IR Aperture Diagram illustrates the fiducial points of the full-detector apertures (IR and IR-FIX), and the outlines of the concentric subarray apertures (512 × 512, 256 × 256, 128 × 128, and 64 × 64). The IR-FIX aperture has its fiducial point at the geometric center of the IR detector and the IR aperture at an optimum point near center of IR detector. Although the POSition TARGet (POSTARG) coordinate systems for the other apertures are not illustrated, they are oriented the same, but have origins at each aperture's fiducial point. (U2 = –V2 and U3 = –V3).