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  SNfactory Collaboration
Role of CfCP

The CfCP is involved in the high precision calibrations of the SNfctory data.

On the software side, the CfCP is responsible for determining day-to-day changes in the atmospheric extinction. Next to the SNIFS spectrograph, a five-filter imager (built at LBNL) records calibration stars in the vicinity of each SN Ia under study; day-to-day changes in the apparent brightness of these calibration stars is used to measure changes in the atmospheric extinction. The challenge in this calibration is that atmospheric changes are measured in filter bandpasses roughly 1000 Angstroms wide; from these data, we need to extract atmospheric changes as a function of wavelength in bins of a few Angstroms.

On the hardware side, the CfCP is building a system to monitor the relative spectral output of the flat-field lamp at the U. Hawaii telescope. This lamp will be used to calibrate the spectral response of the spectrogragh on non-photometric nights (when cirrus clouds and other atmospheric effects cause fluctuations in the brightness of astronomical sources). This extra calibration is necessary since SNe Ia are bright for only a short period, and therefore to study the entire light-curve we will need to take data on non-photometric nights. The lamp-calibration is useful only if its output spectrum is constant; an independent instrument is therefore under contruction at Chicago to check the lamp spectrum, and to make adjustments if changes are observed. Note that a 1% change in the lamp temperature (which is about 3000 K, half the temperature of the sun) results in a 7% change in the relative intensity between 3000 and 10,000 Angstroms. The Chicago monitor system, which will be mounted behind the secondary mirror, uses PIN diodes and filters to measure the relative Blue/Red ratio.