|Space Telescope Science Institute|
NICMOS, the Near Infrared Camera and Multi-Object Spectrometer, is an HST axial instrument, containing three cameras designed for simultaneous operation. The NICMOS optics offer three adjacent but not spatially contiguous fields-of-view of different image scales. The instrument covers the wavelength range from 0.8 to 2.5 microns, and contains a variety of filters, grisms, and polarizers. Each camera carries a complement of 19 optical elements, selected through independent filter wheel mechanisms, one per camera. In order to allow operation of the NICMOS detectors and to minimize the thermal background of the instrument, NICMOS needs to be cooled to cryogenic temperatures. The basic capabilities of the instrument are presented in Table 2.1.IR imaging: NICMOS provides its highest sensitivity from 1.1 to ~2 microns, where it is superior to an 8m class telescope. Chapter 4 discusses the overall throughput of NICMOS and the optical elements available in each camera. The low background which HST offers between 0.8 and 2 microns allows deep photometry. Our estimates of limiting sensitivities per pixel for a 5σ detection in a 3,600 second integration, at an operating temperature of 77.15K, are given in Table 2.2. Users should note that an integration time of 3600 sec is not practical and is for illustrative purposes.Table 2.1: Overview NICMOS Capabilities
11×11 19.2×19.2 51.2×51.2 texp = 3600 s
• Diffraction limit (μm)
• Δλ (μm) texp = 3600 s
• Δλ (μm)
Limiting magnitudes are from the NICMOS ETC (see Chapter 9). Infrared passbands (J,H,K) are defined by Bessell and Brett (1988, PASP, 100, 1134).
• Grism Spectroscopy: Camera 3 has three grisms which provide a multi-object spectroscopic capability with a resolving power of R~200 per pixel over the full field of view of the camera. Their wavelength ranges are 0.8 to 1.2 microns, 1.1 to 1.9 microns, and 1.4 to 2.5 microns. Because the grisms are slitless, the spectra of spatially resolved objects are confused and multiple objects can overlap.
• Imaging Polarimetry: Three polarizing filters with pass directions of 0, 120, and 240 degrees are provided for the wavebands 0.8–1.2 microns in Camera 1 and 1.9–2.1 microns in Camera 2.
• Coronagraphy: A 0.3 arcsec radius occulting hole and cold mask, in the intermediate resolution Camera 2, provide a coronagraphic imaging capability.Chapter 5 discusses these three special capabilities in more detail.
Table 2.2: Limiting Sensitivities in Janskys for S/N = 5 detection of a point source in a standard aperture of diameter 0.5", 0.5", and 1" for NIC1, NIC2, and NIC3, respectively, for a 3600 sec exposure.,. 1.75 × 10-7 J ≈ 25.2 6.26 × 10-7 H ≈ 23.7 6.73 × 10-8 J ≈ 26.3 2.21 × 10-7 H ≈ 24.8 1.71 × 10-5 K ≈ 20.1 5.49 × 10-8 J ≈ 26.5 4.75 × 10-8 H ≈ 25.6 9.83 × 10-6 K ≈ 20.7
S/N calculated for brightest pixel in point source image, using the NICMOS ETC for 77.15K temperature.
Limiting magnitudes are from the NICMOS ETC (Chapter 9). Infrared passbands (J,H,K) are defined by Bessell and Brett (1988, PASP, 100, 1134). A0V spectrum assumed to convert between NICMOS passband flux (in Jy) and conventional, Vega-normalized JHK magnitudes.