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Session 14 - Real Calibrations.
Display session, Monday, June 08
Atlas Ballroom,

[14.04] Optical Flux limits for observing with the ACIS Instrument on AXAF

G. P. Garmire, G. Chartas, L. Townsley, A. B. Garmire (Penn State U.), M. Bautz (MIT)

The AXAF CCD Imaging Spectrometer instrument on the Advanced X-ray Astrophysics Facility employs CCD detectors as the sensing elements. A thin polyimide film coated with aluminum forms a barrier against the exposure of the CCDs to visible light photons. The aluminum coating over the imaging array of CCDs is thicker than the coating over the spectroscopic array of CCDs. These filters block about 10^-6 of the light entering the spectroscopic array and about 10^-7 of the light entering the imaging array. Results of calculations for various spectral models of stellar temperatures to determine how bright stellar objects can be before they begin to produce photoelectrons in the images formed by the X-ray focusing mirrors will be presented. On the imaging array preliminary estimates are that stars brighter than about 1.4 magnitudes for G stars 1.1 for B stars will add to the noise. For the spectroscopic array the limits are 3.7 for G stars and 3.1 for B stars. By inserting the gratings when performing spectroscopic observations, the extra attenuation afforded by the gratings adds 3.8 magnitudes to the filter attenuation, i.e. stars as bright as -0.1 for G stars and -0.7 for B stars can be observed before extra photoelectrons are added to the noise of the CCD in the zero order image. This work was supported by NASA Contracts NAS8-38252 and NAS8-37716.

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