Table 4.1: Camera 1 Filters
Name
Central Wavelength (µm)
Bandwidth (µm)
Comment
See also
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Polarizer
Polarizer
Polarizer
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Table 4.1 through Table 4.3 list the available filters and provide an initial general description of each, starting with Camera 1 and working down in spatial resolution to Camera 3. Figure 4.1 through Figure 4.3 show the percentage transmission of each optical element plotted against wavelength. Chapter 11 provides further details and the individual filter transmission curves.
Nomenclature
The name of each optical element starts with a letter or group of letters identifying what kind of element it is: filters start with an "F", grisms with a "G", and polarizers with "POL". Following the initial letter(s) is a number which in the case of filters identifies its approximate central wavelength in microns, e.g., F095N implies a central wavelength of 0.95 microns. A trailing letter identifies the filter width, with "W" for wide, "M" for medium and "N" for narrow. In the case of grisms, the initial "G" is followed by a number which gives the center of the free-spectral range of the element, e.g., G206. For the polarizers, a somewhat different notation is used, with the initial "POL" being followed by a number which gives the PA of the principal axis of the polarizer in degrees, and a trailing letter identifying the wavelength range it can be used in, which is either "S" for short (0.8-1.3 microns) or "L" for long (1.9-2.1 microns).
Figure 4.1: Filters for Camera 1
Figure 4.2: Filters for Camera 2
Figure 4.3: Filters for Camera 3
Filter Sensitivity Curves
Detailed information and transmission curves are provided for each filter (Chapter 11) and for the grisms and polarizers (Chapter 5). Performance information may be obtained from the Exposure Time Calculator (ETC). In this section we explain the formats and outline the use of the Figures from the ETC. For many purposes, these Figures may take the place of detailed calculations and permit observers to quickly determine the feasibility or appropriateness of an observation.
Caution: The Figures in this section and in Chapter 5 are shown as examples and
are based on preliminary estimates of the NICMOS sensitivities, noise characteristics, and backgrounds-observers should rely upon the ETC for their performance predictions.
The Handbook provides the following (see Chapters 11 and 5):
() to permit simple determination of the response of NICMOS to a monochromatic emission line within the filter's useful bandpass. This single parameter encompasses the wavelength dependent DQE, filter transmission, mirror reflectivities (including both the NICMOS fore-optics and the HST primary and secondary mirrors), and dewar window transmission. This parameter is calculated in units of e- s-1 (W m-2)-1 pixel-1. Given a line flux in W m-2, the flux per pixel can be determined using the pixel fraction for point sources or the pixel area for extended sources. Then the () parameter provides the count rate in e- s-1. Additional discussion and examples of this calculation are provided in Chapter 6.
) as a function of time to achieve a signal to noise ratio (S/N) of 10, 25, 50, and 100 (solid, short dash, long dash, and dot-dash lines, respectively). These curves incorporate the expected backgrounds and presently understood noise characteristics of the detectors. To use these plots select a camera and filter combination, locate the flux of your source and read across the figure to determine the exposure time for a given S/N.
) as a function of time. The useful limits of the instrument are shown. Sources falling in the upper right hand region will be saturated (see the discussion of MULTIACCUM mode to observe both bright and fainter sources in the same exposure). Sources falling in the lower left region of the plot will be read noise rather than shot noise limited. Finally, sources to the left of the vertical dashed line at 0.203 seconds must be observed using the BRIGHTOBJ mode.
We have calculated the effects of filter leaks for sources with color temperatures from 700K to 10,000K. At this juncture we note that the reddest of the kinds of sources likely to be observed with NICMOS may have color temperatures lower than 400K, while the bluest sources (probably reflection nebulae) can be significantly bluer than a 10,000K blackbody. We find that significant leaks may occur for nine of the filters. No photometric errors as large as 1% were found for any filters using the hottest (i.e., 10,000K) spectrum (however, as noted above some reflection nebulae may have bluer spectra than this, and we cannot rule out the possibility of errors as large as a few percent in this case, for a few filters). For the reddest source considered here (with a color temperature of 700K), the photometric errors might be as large as an order of magnitude in a few filters. There are still some uncertainties regarding the measured transmission curves, and so the information presented here should be regarded only as cautionary.
On-orbit tests will be performed as part of the Cycle 7 Calibration program and the results posted on the STScI NICMOS WWW pages as they become -available-this is not likely to be before the Cycle 7-NICMOS Phase I deadline.
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