Overview

Instrumental Factors

Optics

The filter transmissions as functions of wavelength were measured in the laboratory, and the resulting curves are presented in Chapter 11. Some filters may have minor leaks outside the primary filter bandpass; the reality of these has not yet been established, and we assume here for the purposes of sensitivity calculations that the transmission is zero outside the primary bandpass.

NICMOS contains a total of seven mirrors external to the dewar, each of which reduces the signal received at the detector. The mirrors have protected silver coatings (except for the field divider assembly which has a gold coating) for maximum reflectivity, and have 98.5% reflectivity. The dewar window has a transmission of roughly 93%. Therefore, the combination of optical elements is expected to transmit ~84% of the incoming signal from the OTA.

The sensitivity will obviously be affected by the pixel field of view. The smaller the angular size of a pixel, the smaller the fraction of a given source that will illuminate the pixel. Finally, the optical efficiency will be degraded further by the reflectivities of the aluminum with MgF₂ overcoated HST primary and secondary mirrors. These are given as exactly one minus the emissivities.

Background Radiation

For the purposes of sensitivity calculations, we used the values listed in Tables 6.1 and 6.2 and assumed that the effects of debris on the mirrors can be ignored.

Table 6.1: Optical Efficiency

Optical Element	Efficiency
First bending mirror	0.985
Re-imaging mirror	0.985
Pupil mirror	0.985
Image mirror	0.985
First paraboloid	0.985
Second paraboloid	0.985
Bending mirror	0.985
Dewar window	0.93
Total	0.84

Table 6.2: HST Infrared and Optical Properties

Property	Assumed Value
Primary mirror collecting area	38993 cm2
Primary mirror temperature	291 K
Primary mirror emissivity	0.048
Secondary mirror collecting area	684.4 cm2
Secondary mirror pupil clear fraction	0.76
Secondary mirror temperature	288.5 K
Secondary mirror emissivity	0.048
Focal plane image scale	35.8 arcsec/cm
Back focal distance	640.6 cm

At shorter NICMOS wavelengths, sensitivities will be affected by the zodiacal background which is given by the equation in Chapter 3; the overall expected background is shown in Figure 3.6.

Background radiation will be a slightly worse problem in the case of Multi-Object Spectroscopy (MOS) than in the case of imaging observations. Every pixel on the array will always see the entire background radiation integrated over the grism bandpass. The expected detected background rate per pixel is shown in Table 5.3.