HRMA/ACIS calibration

The ACIS team collected calibration data utilizing the HRMA telescope during three data collection intervals. Following the XRCF nomenclature these occured during Phases F, G and H. During Phases F and G the HRMA focussed X-rays were collected on two flight-like CCDs (one front side illuminated (FI), and one back side illuminated (BI)) in a camera utilizing electronics very similar to the MIT CSR lab cameras. This camera, named the ACIS-2C, served as a proxy to enable data collection while the ACIS flight instrument finished the final stages of certification and thermal vacuum testing.

The ACIS-2C camera, using chips of identical design and fabricated in the same lots at Lincoln Lab as the flight ACIS chips, has differing processing electronics and does not hold the chips in the same orientation as the flight unit. Thus these ACIS-2C data are useful for understanding the chip level interaction between the mirror effects and the CCDs, but do not fully calibrate the effect of the flight electronic processing and flight array orientation effects. (It should be noted that the ACIS-2C chips were significantly worse in cosmetic quality than the flight chips. We designed the 2C tests such that only a small portion of the chip area was used, to minimize the effect of the poor portions of the 2C chips.)

Measurements were collected using the flight ACIS instrument and the HRMA at XRCF for eight days during Phase H (see Table 2.1 for the times of ACIS data collection). These data are the most directly relevant data as they use the flight mirrors and detectors, but must still be adjusted for the effects of finite conjugate distance, gravitational loading on the mirrors and a slightly differing CCD temperature operating point than planned for flight (-115 C vs. -120 C).

Following the departure of the HRMA mirrors to meet the schedule of assembly into the spacecraft, the XRCF was used to collect data with ACIS without any focussing optics. These data, collected during Phase I, fit naturally into the sub-assembly lab calibration data, and act as a gross calibration with the beam monitor counter, and are discussed there.

For completeness, it is worth mentioning that data were collected at XRCF during September, 1996, using the ACIS-2C camera and the TMA (Test Mirror Assembly). Data from these tests are still available, and they have not been used in the ACIS calibration. The primary purpose of this testing was to serve as a rehearsal of the true AXAF calibration.
 
 

Table 2.1: XRCF Data Collection Phases Involving ACIS 

Phase	Detector	Mirror	Start Date*	End Date
F	ACIS-2C	HRMA	12:11 22 Feb 1997	17:52 10 Mar 1997
G	ACIS-2C	HRMA	9:57 10 Apr 1997	8:41 12 Apr 1997
H	Flight	HRMA	23:56 17 Apr 1997	6:00 26 Apr 1997
I	Flight	none	6:45 7 May 1997	0:04 18 May 1997
R	ACIS-2C	TMA	7:00 30 Sep 1996	13:28 5 Oct 1996

^* Times are given in local (Central) time. Note these refer to the start and end of ACIS data collection intervals, not necessarily to the start and end of the XRCF Phase.