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Flux Standards for NICMOS Absolute Calibration
The absolute flux calibration of NICMOS will be calculated from observations of standard stars with known flux distributions F(). The sensitivity as a function of wavelength for the dispersed spectra from the grisms is determined directly from the known F() and the observed response, after any required corrections for flat field response. Sensitivities for the filters are calculated from observations of the standard stars according to the synthetic photometry procedure detailed in Koorneef et al. (1986). Since the pipeline calibration cannot utilize color information, the header of the reduced data will contain the calibration constant for the filter that specifies the equivalent flux for a constant spectral distribution as a function of wavelength. For convenience, this calibration constant appears twice, once in Jansky units and once in erg s-1 cm-2 Å-1 units. Color transformations could be defined for post-pipeline data analysis.
White Dwarf Absolute Standards
The flux distributions of our primary standard stars are defined by models for the four pure hydrogen white dwarfs in Table 15.3 (Bohlin, Colina, & Finley 1995). Bohlin (1996) transfers these calibrations to FOS and IUE data in the UV and to the optical spectra of Oke (1990) to obtain a set of standards with fluxes that are accurate to a few percent from 1150-9200Å. Observations of this set of standard stars produces consistent absolute flux calibrations on the WD scale for the current HST instrument complement. Currently, flux distributions to the limit of NICMOS coverage are only available for four WD (Table 15.3).
Solar Analog Absolute Standards
In order to expand the set of IR standard stars in the appropriate flux range for NICMOS, M. Rieke, R. Thompson, and collaborators at the University of Arizona are making photometric observations of solar-analog stars, which will be used to normalize the solar spectrum to the observed IR magnitudes. This method consists of several steps:
- The solar colors in the photometric system are determined by assuming that the average colors of the solar analogs are equal to those of the Sun (classified as a G2V star).
- The zero point of the absolute flux density in each near-infrared photometric bandpass is calculated from the photometric magnitudes for the Sun and the absolute flux spectrum of the Sun.
- The absolute solar flux density in each photometric band is scaled in proportion to the magnitude of the solar analog star relative to that of the Sun.
The final absolute flux accuracy achieved by the solar-analog method relies on two basic assumptions:
- That the absolutely calibrated reference spectrum of the Sun is known with an uncertainty of a few percent (Colina, Bohlin, & Castelli 1996).
- That the spectra of the solar-analogs are identical to the solar spectrum, i.e., agree within the 2% uncertainty in the shape of the flux distribution at infrared wavelengths.
The solar-analog method was used in the past to determine the absolute calibration of near-infrared photometry (Campins, Rieke, and Lebofsky, 1985) at ground-based observatories. The accuracy in the absolute calibration was at least 5%, and for some bands, 2% to 3% (Campins et al., 1985).
As a check on the solar analog method, the ground-based program includes observations of the WD G191B2B. Furthermore, FOS observations of three prime solar analog candidates listed in Table 15.4 were made during Cycle 6. The FOS red detector was used with the high resolution G780H, G570H, G400H and G270H to obtain high signal to noise spectra of these three candidates over the 2200 to 8500 angstroms range to verify agreement with the solar flux distribution. Additional STIS observations covering the 0.6-1.0 µm wavelength range will be obtained in Cycle 7.
Ground Based Calibrations
The existing near-infrared standard star lists used by ground-based telescopes are inadequate for NICMOS because they are too bright and there is a dearth of suitable observations to rule out low-level Near-IR variability for most stars. As we mentioned previously, to counter this problem, a large program of J, H, K photometry has been conducted by a team lead by Marcia Rieke of Steward Observatory. The star selection is based upon HST Guide Star Availability/Suitability Windows.
In the Northern Hemisphere observations have been performed with a NICMOS2 array on the Mt. Lemmon 60" & Kitt Peak 90" and in the Southern Hemisphere with a NICMOS3 array on the Las Campanas 40".
The primary goals of the program were:
Solar analogs from HST Guide Star Photometric Survey (Lasker and Sturch) form the main flux references to bridge the gaps in atmospheric transmission. Solar analogs were chosen because:
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Last updated: 07/24/97 15:36:32