NIRcam Update

Marcia Rieke, mrieke@gmail.com, & Massimo Robberto, robberto@stsci.edu

Figure 1

NIRCam (Near Infrared Camera) is the main imaging instrument onboard James Webb Space Telescope (JWST) for the wavelength range 0.6–5 microns. In addition to serving as a powerful imager for the JWST core science goals, which range from cosmology and galaxy formation to star formation and exo-planets, NIRCam also plays a key role as a facility instrument. NIRCam will perform the wave-front sensing (WFS) functions needed to keep the primary-mirror segments—and in general, the entire telescope optical train (OTA, optical telescope assembly)—properly aligned. The optimal focusing of all JWST instruments depends critically on this instrument.

A team at University of Arizona and Lockheed Martin Advanced Technology Center (LM-ATC), under the direction of Principal Investigator Marcia Rieke, is building NIRCam.

Due to its vital role for the mission, NIRCam is designed as a double instrument, which maximizes redundancy while doubling the field of view for science observations. It is composed of two modules, and built on two separated optical benches that are mounted back-to-back (Figure 1). The modules are functionally identical, having matching optical and focal-plane components,1 and are mirror images of each other.

 

NIRCam design

Figure 2To illustrate the NIRCam layout, it is sufficient to refer to one of the two modules (Figure 2).   First, a pick-off mirror that can be moved for focus and pupil alignment reflects the light coming from the OTA. The beam, folded by a slightly powered mirror, enters the collimator, a sophisticated, three-element lens group. Next, a dichroic beam-splitter reflects the wavelengths shorter than 2.3 microns and transmits wavelengths longer than 2.4 microns, producing the short-wavelength (0.6–2.3 microns) and long-wavelength (2.4–5 microns) channels of NIRCam. The layouts of the two channels are similar: the beam passes first through a filter-wheel assembly, comprising a filter wheel and a pupil wheel, and then through a second lens group, which finally relays the image onto the detector.

The fields imaged by the short- and long-wavelength channels are the same. Nevertheless, the image-relay groups deliver very different image scales on each channel. In the short-wavelength channel the scale is 32 mas/pixel, which corresponds to Nyquist sampling—five pixels subtend the diameter of the first Airy null—at 2.0 microns. In the long-wavelength channel the scale is 65 mas/pix, which corresponds to Nyquist sampling at 4.0 microns. On the HgCdTe Hawaii-IIRG 2k × 2k detectors in this channel, which have a 5-micron cutoff, the field of view is 2.2 × 2.2 arcmin. To cover this field of view on the short-wavelength channel, NIRCam utilizes a focal plane assembly (FPA) with four similar HgCdTe detectors (with 2.5-micron cutoff) arranged in a 2 × 2 configuration. This FPA is equivalent to a 4k × 4k chip with a central gap between quadrants of about 50 pixels (1.6 arcsec). With 40 megapixels, NIRCam has more detector real estate than all of the optical and IR detectors ever flown on Hubble.

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1 There are, however, differences in the ordering of the coronagraphic focal-plane masks.