Abstract
In 2009, the Wide Field Camera 3 (WFC3) was installed in the Hubble Space Telescope during SM4. WFC3 is a panchromatic instrument, covering the wavelength range between 0.2 to 1.7 microns with a UVIS and an IR channel. Observing modes available are direct imaging and slitless spectroscopy, with options to 'stare' and 'scan' during exposures. We have established the photometric stability to be ~0.5%, and the relative photometry to approximately 1 –5% depending on wavelength. However, the absolute flux calibration has larger, unknown uncertainties. Thus, the goal of the Photometric Flux Calibration Ladder is to provide increased accuracy of the zeropoint measurements, improve characterization of photometric uncertainties, provide high accuracy color corrections as well as monitor sensitivity trends in both the UVIS and NIR channels. Our aim is to develop a calibration ladder from the brightest standard star(s), e.g Vega, to the faintest to improve the absolute photometric calibration and cross-calibration of the observing modes now available for grism spectroscopy and direct imaging. The source list consists of stars observed with STIS, ACS, NICMOS, SPITZER/IRAC, and some of the proposed JWST calibration standards. Brightness spans the range from V = 0 -17 mag, and J=0-15 mag. Stellar spectral types include white dwarfs, A, G, as well as K and M stars. I will discuss our efforts to provide an absolute, above the atmosphere, calibration of Vega, including the challenges to achieving a 1% absolute calibration of the HST/WFC3.
Building the HST/WFC3 Flux Calibration Ladder
In 2009, the Wide Field Camera 3 (WFC3) was installed in the Hubble Space Telescope during SM4. WFC3 is a panchromatic instrument, covering the wavelength range between 0.2 to 1.7 microns with a UVIS and an IR channel. Observing modes available are direct imaging and slitless spectroscopy, with options to 'stare' and 'scan' during exposures. We have established the photometric stability to be ~0.5%, and the relative photometry to approximately 1 –5% depending on wavelength. However, the absolute flux calibration has larger, unknown uncertainties. Thus, the goal of the Photometric Flux Calibration Ladder is to provide increased accuracy of the zeropoint measurements, improve characterization of photometric uncertainties, provide high accuracy color corrections as well as monitor sensitivity trends in both the UVIS and NIR channels. Our aim is to develop a calibration ladder from the brightest standard star(s), e.g Vega, to the faintest to improve the absolute photometric calibration and cross-calibration of the observing modes now available for grism spectroscopy and direct imaging. The source list consists of stars observed with STIS, ACS, NICMOS, SPITZER/IRAC, and some of the proposed JWST calibration standards. Brightness spans the range from V = 0 -17 mag, and J=0-15 mag. Stellar spectral types include white dwarfs, A, G, as well as K and M stars. I will discuss our efforts to provide an absolute, above the atmosphere, calibration of Vega, including the challenges to achieving a 1% absolute calibration of the HST/WFC3.