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Attached Parallels
While the three NICMOS cameras are no longer at a common focus, under many circumstances it is desirable to obtain data simultaneously in multiple cameras. Generally, Cameras 1 and 2 will be used simultaneously while Camera 3 will be used by itself.
Since some programs by their nature do not require more than one camera (e.g., studies of isolated compact objects), to make the most of the limited lifetime of NICMOS, observers are encouraged to add exposures to their proposals to obtain the maximum amount of NICMOS data consistent with efficiently accomplishing their primary science program. Detailed instructions for this process will be included in the Phase II proposal instructions. Internal NICMOS parallel observations obtained under this policy will be known as attached parallels and will be delivered to the prime program's observer and will have the usual proprietary period.
This section applies only to Phase II proposals-you need not worry about this for
your Phase I proposal.
The recommendations attached below are intended for General Observers (GOs) who do not establish a scientific rationale for observations with the non-prime NICMOS camera(s) in their Phase I submission to the TAC. They are subject to revision.
Table 3.5: Attached Parallel Recommendations
Pointings are defined as:
The Infrared Background
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 microns there are a number of deep molecular absorption bands in the atmosphere (top panel of Figure 3.5), 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 3.5). Unfortunately, outside the absorption features there is also considerable background emission in both lines and continuum. Most of the background between 1 and 2 microns comes from OH and O2 emission produced in a layer of the atmosphere at an altitude ~ 87 km (bottom panel of Figure 3.5).
Figure 3.5: Atmospheric Absorption and Emission Line Spectrum
in NICMOS Operational Range
The location of HST above the atmosphere removes these terrestrial effects from the background. Now, 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 will be 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 the shorter wavelengths, sensitivities will be affected by the zodiacal background which is, of course, strongly spatially dependent (see Table 3.6). Observations by the COBE satellite have implied that at positions 45 degrees out of the ecliptic the zodiacal background can be approximated as:
5.0x108/
1.09 + 6x10-8B(,T) photons cm-2 µm-1 steradian-1
Where is the wavelength in µm and B is the blackbody function for the zodiacal dust temperature T (roughly 265 K).
At wavelengths longer than 1.6 microns the thermal background of the telescope rises and may have to be removed by obtaining off-source images. By using filtering techniques such as median filtering any contaminating sources in these offset fields can be removed in a composite background frame which can then be subtracted from the data.
Figure 3.6 shows the HST background for each of the three cameras (the solid line represents Camera 1, the dotted line represents Camera 2, the dashed line represents Camera 3) 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, and the NICMOS optics, based on-orbit data and the transmission of all the NICMOS fore-optics. It does not include the transmission of any filter, nor the response of the detectors.
Figure 3.6: HST Background for Each Camera.
We are presently obtaining direct observations to measure and establish the stability of the thermal contribution to the background during SMOV and early in Cycle 7.
On-orbit measurements now indicate that the prelaunch estimates for the HST thermal emission were overly conservative. The emissivity of the OTA appears to be close to 4% leading to a significant reduction in the expected thermal background for all filters longward of 1.5 microns. (In addition, it appears that the contribution to the background from the zodiacal dust was overestimated by a factor of two, a change that affects all filters.) These changes result in significant improvements in the sensitivity of NICMOS at the longest wavelengths where the background count rates are reduced by the largest amount.
Table 3.7 compares the predicted background rates in several filters in NIC2 before SMOV with the revised rates resulting from the first on orbit test. The Exposure Time Calculator tool on the STScI NICMOS WWW page has been updated to reflect the newly measured background rates. Background count rates for any filter/camera combination may be obtained by selecting Input Info on the Exposure Time Calculator results page.
Table 3.7: Background count rates for selected filters in NIC2
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 a target will always be rather close to the Earth's limb, and so will always see an elevated background (at the shorter wavelengths where zodiacal emission would ordinarily dominate). For targets faint enough that the background level is expected to be much brighter than the target, the observer is recommended to specify the LOW-SKY option. This will increase the minimum allowed Earth avoidance angle, requiring scheduling during a time for which the zodiacal background is no greater than 30% above the minimum achievable level, at the cost of a slight decrease of the available observing (visibility) time during each orbit. Note that this restriction is only helpful when observations are background limited.
Conversion Between Fluxes and Magnitudes
Throughout the NICMOS documentation we will frequently use flux units of Janskys (Jy). A detailed discussion of the conversion between various units and Janskys is given in Chapter 12. Here we summarize the central wavelengths and zero-point fluxes for the more commonly encountered photometric bands in Table 3.8, using the most commonly encountered photometric systems, the CIT and the UKIRT systems.
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Last updated: 07/24/97 15:17:04