Overview

The Observatory has two radioactive sources solely dedicated to ACIS calibration^4.10. The first of these is the Internal Contamination Montior (ICM), located on the underside of the ACIS door. The flux from this tighly collimated source falls primarily on S2 Quad D and S3 Quads A-C. The purpose of the ICM is to allow an accurate measurement of line ratios immediately after launch. The post-launch ratio is then compared to the ratios determined during pre-flight testing. Discrepancies between the pre- and post-launch measurements would indicate the presence of contaminants on the ACIS detectors. Once the door is open, the ICM will not be visible again. The second radioactive source is the External Calibration Source (ExtCalSrc), viewable by ACIS when the HRC-S is in the Observatory focus. The ExtCalSrc consists of three seperate sources that produce a number of K$\alpha$lines and illuminate the entire ACIS focal plane with near uniformity. The ExtCalSource will allow determination of chip gain and monitoring of other performance parameters during the entire lifetime of AXAF.