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33.4 FOS/BL G160L Zero-Order Photometric Calibration

A rough calibration of the FOS/BL G160L zero-order sensitivity for photometry is provided here. No zero-order calibrations for other dispersers are available.

The FOS/BL G160L zero-order was normally readout by the diode array. The spectral passband for the zero-order of this detector and disperser combination ranged from 1150 to approximately 5500 Å. For a flat continuum source, the effective wavelength of the passband was approximately 2400 Å. The signal strength in the zero-order depended on the integral of the target spectral energy distribution and the detector sensitivity over the entire bandpass.

A very approximate calibration of the FOS/BL zero-order sensitivity was originally provided in FOS ISR 091 and improved in Eracleous and Horne, ApJ, 433, 313, 1994. The original calibrations are updated for some errors and are superseded by the sensitivity values, S, given below.

Flux (FOS/BL G160 zero-order) [in mJy] = C / (T x S)

where C is the detected counts per second in the entire profile of the zero-order spectral feature, T is aperture throughput relative to the 4.3 aperture as unity (see Table 32.5 and Table 32.6), and S = 520 (pre-COSTAR) or 620 (post-COSTAR) counts per second per mJ.

Note that since C is defined as the total signal in the zero-order feature, but is somewhat peculiarly expressed in counts/sec, you must be careful about the distinction between total exposure time and exposure per pixel in the output .c5h data products. Therefore, you should calculate C by adding up the zero-order signal in your .c5h file and dividing by NXSTEPS.

This calibration is based upon pre-COSTAR observations of AE Agr and A0620-00, which are both close binary systems with quite red spectra that are dominated at wavelengths longer than 3000 Å by their K-type components. This approximate calibration is estimated to be accurate to only about 50%.

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