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Paper: Analysis of On-Orbit ACIS Squeegee Mode Data
Volume: 262, The High Energy Universe at Sharp Focus: Chandra Science
Page: 409
Authors: Virani, S. N.; Plucinsky, P. P.; Grant, C. E.; Lamarr, B.
Abstract: The Chandra X-ray Observtory Center and the MIT ACIS teams have developed a number of measures to ameliorate the effects of radiation damage suffered by the ACIS front-illuminated CCDs. One of these measures is a novel CCD read-out method called "squeegee mode". A variety of different implementations of the squegee mode have now been tested on the I0 CCD. Using the prominent lines in the external calibration source of Al-K(alpha) (1.5 keV), Ti-K(alpha) (4.5 keV), and Mn-K(alpha) (5.9 keV), we have computed the peak, FWHM, energy resolution, fitted counts in the ASCA g02346 grade set, and grade distribution as a function of row number for the I0 CCD for each squeegee mode variant. Our results for the energy resolution clearly demonstrate that all the squeegee modes provide improved performance in terms of reducing CTI and improving spectral resolution. Our analysis of the detection efficiency shows that the so-called squeegee modes ``Vanilla'' and ``Maximum Observing Efficiency'' provide the same detection efficiency as the standard clocking, once the decay in the intensity of the radioactive source has been taken into account. The squeegee modes which utilize the slow parallel transfer (``Maximum Spectral Resolution'', ``Maximum Angular Resolution'', and ``Subarray'') also show a significantly lower detection efficiency than the standard clocking. The slow parallel transfer squeegee modes exhibit severe grade migration from flight grade 0 to flight grade 64 and a smaller migration into ASCA g7. The latter effect can explain some of the drop in detection efficiency. Since each squeegee mode "flavour" would require lengthy, mode-specific calibration observations, and since an efficacious, ground-based CTI corrector algorithm is now available (see poster by Plucinsky, Townsley, et al at this meeting), a scientific judgement will have to be made to determine which, if any, squeegee modes should be developed and calibrated for use by Chandra observers.
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