1.6 Instrument Heritage

ACIS was first selected for definition as a result of a proposal in response to AO-OSSA-3-83, on 5 March 1985. Prof. Gordon Garmire of Penn State University led a team of 17 scientists from MIT, JPL and Penn State, whose submission was selected as one of four proposals for definition (Phase A/B).

In 1989 Prof. Garmire submitted an expanded proposal based on the definition phase studies, backed by a revised and expanded scientific team. The MIT team was led by Deputy IPI, Dr. George Ricker and included the development of CCDs at Lincoln Lab, under the direction of Dr. Richard Savoy, and the scientific development of CCD technology by four other MIT scientists (Dr. Mark Bautz, Prof. George Clark, Dr. John Doty, and Prof. Saul Rappaport). The PSU team, lead by Lead Co-I, Dr. John Nousek, and four other PSU scientists (Dr. David Burrows, Prof. Eric Feigelson, Dr. Richard Griffiths, and Prof. Daniel Weedman), concentrated on ground software development and calibration efforts. The JPL/CalTech team was lead by Mr. S.A. Collins, assisted by four others (Dr. Alan Metzger, Dr. Steve Pravdo, Dr. Guenter Riegler, and Dr. Wallace Sargent). This team concentrated on CCD chip evaluation and scientific applications. Performing the critical role of ACIS Project Manager was Mr. Phil Gray of MIT.

As a result of the Phase C/D review, the ACIS was selected for development for launch, and deemed a `core' instrument of the AXAF mission. Initiation of a new start for AXAF was delayed for two years, postponing the start of AXAF Phase C/D activities until 1991.

In 1992-3 major revisions to the mission plan were incorporated to respond to budgetary pressure on the program. AXAF was split into two spacecraft. The first, AXAF-I, was to be launched by the Space Shuttle and then into a high orbit ($\sim$10,000 $\times$140,000 km) by a solid rocket insertion motor. The second mission, AXAF-S, was to be launched by a Delta or Atlas expendable launch vehicle into a polar sun synchronous orbit. ACIS was assigned to the AXAF-I spacecraft, for which the weight had to be substantially reduced from the original design. AXAF-I contained the original glass optic design, but used only four of the planned six mirror shells. The increased observing efficiency available in high Earth orbit partially compensated for the loss of mirror collecting area.

In 1993 the AXAF program was further reduced by cancelation of the partner AXAF-S spacecraft, with a relegation of the prime instrument to flight aboard the proposed Japanese Astro-E mission.

An important precursor to the ACIS program was the launch of the Japanese-U.S. ASCA satellite (Astro-D) on February 20, 1993. This satellite carried two Silicon Imaging Spectrometers (SIS), consisting of arrays of Lincoln Lab CCD chips, operated in a very similar manner to that planned for AXAF. The SIS were built by a collaboration headed by George Ricker and included many of the members of the ACIS team (notably Mark Bautz, Gordon Garmire and John Nousek).

Although the SIS carried X-ray CCDs similar to the ACIS chips into orbit for the first time, and has made many important discoveries, the unmatched imaging optics (more than 200 times higher angular resolution) of the AXAF HRMA will open an entirely new era of discovery. The SIS experience has also enabled us to make important improvements in ACIS to avoid, eliminate or decrease problems such as flickering pixels, dark frame errors, light leaks and radiation damage, which have reduced the SIS CCD performance.

In 1994 and 1995 the personnel working on ACIS substantially increased as detailed design and fabrication commenced. Notable additions were Dr. Peter Ford, in charge of flight software, and Mr. Jim Francis. Dr. Bill Mayer replaced Phil Gray as ACIS Project Manager.

The ACIS flight instrument passed through thermal vacuum testing at Lincoln Laboratories in March and April, 1997. The tested and fully functional flight instrument was delivered to the Marshall Space Flight Center on April 14, 1997. It underwent end-to-end calibration testing with the HRMA until April 26, 1997, and after the HRMA was removed for assembly into the AXAF spacecraft body, the ACIS instrument received additional detector level calibration at the XRCF through May 17, 1997.