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4 Conclusion

To enact an efficient calibration plan using the two sources described in this paper, a special vacuum chamber has been built. As noted in the introduction, two CCDs are used: the first is a ``reference'' CCD which has been absolutely calibrated against the BESSY synchrotron beam, and the second a ``test'' which is an AXAF flight candidate. Figure 24 displays a schematic of the assembly, indicating the two CCDs on a translation stage which can locate either CCD underneath either the HEXS or tritium source beam. The CCDs are mounted on a liquid nitrogen manifold which controls the temperature to ACIS operating conditions. When not in use, the tritium sources are stored in their own vacuum chamber, thereby minimizing any contamination to the main chamber. A total of three different tritium sources can be used, simply by rotating the magnetically coupled linear transporter and securing the alternate source into the exposure-positioning seat. A system of light emitting diodes (LEDs) and pinhole apertures provides for accurate relative positioning of the two CCDs relative to the x-ray beam.

In conclusion, two x-ray calibration sources have been developed, one based on a commercial electron impact tube, and the other based on a tritium beta-impact source. Theoretical models accurately describe the spectra and fluxs from both sources. The HEXS turns out to be more stable and uniform than the tritium source over long time scales.