Monitoring the Health and Safety of the ACIS Instrument On-Board the Chandra X-ray Observatory

Abstract

The Chandra X-ray Observatory (CXO), NASA's latest "Great Observatory", was launched on July 23, 1999 and reached its final orbit on August 7, 1999. The CXO is in a highly elliptical orbit, approximately 140,000 km x 10,000 km, and has a period of approximately 63.5 hours (≈ 2.65 days). Communication with the CXO nominally consists of 1-hour contacts spaced 8-hours apart. Thus, once a communication link has been established, it is very important that the health and safety status of the scientific instruments as well as the Observatory itself be determined as quickly as possible.In this paper, we focus exclusively on the automated health and safety monitoring scripts developed for the Advanced CCD Imaging Spectrometer (ACIS) to use during those 1-hour contacts. ACIS is one of the two focal plane instruments on-board the CXO. We present an overview of the real-time ACIS Engineering Data Web Page and the alert schemes developed for monitoring the instrument status during each communication contact. A suite of HTML and PERL scripts monitors the instrument hardware house-keeping electronics (i.e., voltages and currents) and temperatures during each contact. If a particular instrument component is performing either above or below pre-established operating parameters, a sequence of email and alert pages are spawned to the Science Operations Team of the Chandra X-ray Observatory Center so that the anomaly can be quickly investigated and corrective actions taken if necessary. We also briefly discuss the tools used to monitor the real-time science telemetry reported by the ACIS flight software.The authors acknowledge support for this research from NASA contract NAS8-39073.The CXO's operational orbit has an apogee of approximately 140,000 km and a perigee of nearly 10,000 km, with a 28.5 • initial inclination. The CXO's highly elliptical orbit, with an orbital period of approximately 2.65 days, results in high observing efficiency. Moreover, the fraction of the sky occulted by the Earth is small over most of the orbital period, as is the fraction of time when the detector backgrounds are high as the CXO dips into the Earth's radiation belts. Consequently, approximately 85% of Chandra's orbit is available for observing. In fact, uninterrupted observations lasting as long as 2.3 days are possible. 1The CXO carries two focal plane science instruments: the High Resolution Camera (HRC) and the Advanced CCD Imaging Spectrometer (ACIS). The Observatory also possesses two objective transmission gratings: a Low Energy Transmission Grating (LETG) that is primarily used with the HRC, and the High Energy TransmissionThe High Resolution Camera, HRC, is a microchannel plate (MCP) instrument. It is comprised of two detector elements, a ∼ 100 mm square optimized for imaging (HRC-I) and a ∼ 20 x 300 mm rectangular device optimized for the Low Energy Transmission Grating (LETG) Spectrometer readout (HRC-S).The HRC has the highest spatial resolution imaging on Chandra -≤ 0.5 arcsec (FWHM) -matching the High Resolution Mirror Assembly (HRMA) p...