From the ground, the infrared background is affected by telluric absorption and emission which limits the depth of astronomical imaging. As is well known, between 1 and 2.5
μm there are a number of deep molecular absorption bands in the atmosphere (top panel of
Figure 4.16), and the bandpasses of the conventional near-IR bands of JHK were designed to sit in the gaps between these opaque regions (middle panel of
Figure 4.16). Unfortunately, outside the absorption features there is also considerable background emission in both lines and continuum. Most of the background between 1 and 2
μm comes from OH and O
2 emission produced in a layer of the atmosphere at an altitude ~87 km (bottom panel of
Figure 4.16).
The location of HST above the atmosphere removes these terrestrial effects from the background. Here, the dominant sources of background radiation will be the zodiacal light at short wavelengths and the thermal background emission from the telescope at long wavelengths. The sum of these two components has a minimum at 1.6
microns (roughly the H band). All three NICMOS cameras carry broad-band filters which are centered on this wavelength.
At wavelengths shorter than 1.6 μm, NICMOS reaches the natural background provided by the scattering of sunlight from zodiacal dust, which is, of course, strongly dependent on the ecliptic latitude and longitude.
Table 4.7 gives low, high and average values of the zodiacal background as seen by HST (Stiavelli,
WFC3 ISR-2002-02).
At wavelengths longer than 1.6 microns the HST thermal emission dominates the background seen by NICMOS (
Table 4.8). The thermal emission from the HST is composed of the contributions of the telescope’s primary and secondary mirrors and of the NICMOS fore-optics. The emission of the HST primary and secondary mirrors can be approximated as a blackbody with effective temperature of ~290 K. The emissivity of each mirror is about 3%. The NICMOS fore-optics are approximated by a blackbody with temperature ~270 K.
Figure 4.17 shows the cumulative HST background as a function of wavelength. This background has been calculated assuming a zodiacal light contribution consistent with the mean observed by COBE for an ecliptic latitude of 45°, and also includes thermal emission by the HST primary and secondary mirrors, the NICMOS optics, and the transmission of all the NICMOS fore-optics. It does not include the transmission of any filter, nor the response of the detectors. For comparison, we report in the same figure the J, H, K
s and K band background as observed from Mauna Kea, Hawaii, averaged over one year (J~16.4, H~15.3, K
s~15.3, K~15.0) and normalized to the HST aperture (2.4m).
Monitoring of the changes in the thermal background as a function of time, telescope’s attitude and slews across the sky has shown that the background is stable to better than 5% on orbital timescales and to about 8% (peak-to-peak) over timescales of several months (Daou, NICMOS-
ISR-98-010). In addition, the thermal background is uniform across each detector, except for NIC3 longward of ~1.8 mm. The lack of significant variations within orbits removes the necessity for rapid dithering or chopping when observing in wavebands affected by thermal background (i.e., longward of ~1.7
μm).
Table 4.8 lists the measured background for a representative set of NIC2 filters. The Exposure Time Calculator tool on the STScI NICMOS WWW page also produces background count rates for any filter/camera combination.
For pointings very close to the Earth, the zodiacal background may be exceeded by the earthshine. The brightness of the earthshine falls very rapidly with increasing angle from the Earth’s limb, and for most observations only a few minutes at the beginning and end of the target visibility period will be significantly affected. The major exception to this behavior is a target in the continuous viewing zone (CVZ). Such targets will always be rather close to the Earth’s limb, and so will always see an elevated background, even at shorter wavelengths where zodiacal emission ordinarily dominates. For targets faint enough that the background level is expected to be much brighter than the target, the observer has two options: (1) specify a non-standard Bright Earth Avoidance (BEA) angle, which increases the angle from the Earth’s limb from 20 to 25 degrees, or (2) specify the
LOW-SKY option, which restricts observations to targets more than 40 degrees away from the Earth’s limb and restricts scheduling to times when the zodiacal background is no greater than 30% above the minimum achievable level. The second option decreases the available observing (visibility) time during each orbit and implies scheduling constraints. Both of the options above are available but not supported modes, meaning that the observer must request them through a Contact Scientist during the preparation of the phase II proposal.
Monitoring of the changes in the thermal background as a function of time, telescope’s attitude and slews across the sky has shown that the background is stable to better than 5% on orbital timescales and to about 8% (peak-to-peak) over timescales of several months (Daou, NICMOS-ISR-98-010). In addition, the thermal background is uniform across each detector, except for NIC3 longward of ~1.8 mm. The lack of significant variations within orbits removes the necessity for rapid dithering or chopping when observing in wavebands affected by thermal background (i.e., longward of ~1.7 μm).
