Space Telescope Science Institute  Chapter 2:  Overview of NICMOS  2.2 Heating, Cooling and Focus

2.1 Instrument Capabilities


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 5 detection in a 3,600 second integration, at an operating temperature of 77.15K, are given in Table 2.2


Table 2.1: Overview NICMOS Capabilities
Mode
NIC1
NIC2
NIC3
Comments
Imaging




  • FOV (arcsec)
11×11
19.2×19.2
51.2×51.2

  • Scale (arcsec/pixel)
0.043
0.075
0.2

  • Sensitivity limit (J, H, K)1
25.2,23.7,-
26.3,24.8,20.1
26.5,25.6,20.7
S/N = 5,
texp = 3600 s
  • Diffraction limit (µm)
1.0
1.75
-

Grism Spectroscopy




  • Type


MOS slitless



200
per pixel
  • (µm)


0.8-1.2
G096



1.1-1.9
G141



1.4-2.5
G206
  • Magnitude limit (Vega H-band) a


20.5,20.4,16.6
A0V, S/N=5,
texp = 3600 s
Polarimetry




  • Filter angles (deg)
0, 120, 240
0, 120, 240


  • (µm)
0.8-1.3
1.9-2.1


Coronagraphy




  • hole radius (arcsec)

0.3

cold mask
1Limiting magnitudes are from the NICMOS ETC (see Chapter 9). Infrared passbands (J,H,K) are defined by Bessell and Brett (1988, PASP, 100, 1134).

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, after 3600 seconds1,2.

Camera

Filter

Bandwidth
(microns)
Limiting Sensitivity
Jansky
Approx. Mag.
NIC1
F110W
0.8-1.35
1.75 × 10-7
J 25.2
NIC1
F160W
1.4-1.8
6.26× 10-7
H 23.7
NIC2
F110W
0.8-1.35
6.73 × 10-8
J 26.3
NIC2
F160W
1.4-1.8
2.21 × 10-7
H 24.8
NIC2
F237M
2.3-2.5
1.71 × 10-5
K 20.1
NIC3
F110W
0.8-1.35
5.49 × 10-8
J 26.5
NIC3
F160W
1.4-1.8
4.75 × 10-8
H 25.6
NIC3
F240M
2.3-2.5
9.83 × 10-6
K 20.7
1S/N calculated for brightest pixel in point source image, using the NICMOS ETC for 77.15K temperature.
2Limiting 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.


Chapter 2:  Overview of NICMOS  2.2 Heating, Cooling and Focus
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