ACIS Calibration Information

This page contains current and historical documents regarding calibration of Chandra's Advanced CCD Imaging Spectrometer (ACIS). Items are organized by topic (see menu on left) and are ordered by date for each topic, with most recent items at the top. For topics with on-going monitoring or calibration (e.g. background, gain correction) the status is summarized at the top of the section. Please see also these additional resources:

Background

Status Blank sky datasets and ACIS "stowed" datasets are available in the CALDB.

Old blank sky datasets and "stowed" datasets and tools for modeling the quiescent background, created by Maxim Markevitch, can be found at Maxim's ACIS background page. [Apr 2009; deprecated]

New Background Rates [updated 2023 Oct]:

Total telemetered background rates (including all grades and the upper event cutoff at 15 keV) for chips S2 (FI) and S3 (BI) as a function of time. Vertical dashed lines are year boundaries. Data are plotted through late-August 2017. Beginning in Cycle 11 (late 2009) the default high energy limit was changed from 15 keV to 13 keV. Subsequent 15 keV rate estimates were scaled from 13 keV rates using 13 kev/15 keV conversion factors of 0.82 for S2 and and 0.68 for S3. These factors were determined from observations (April-July 2009) when the rates were fairly constant, and with the onboard high energy limit set to either 15 keV or to 13 keV.
[Click on the figure for a PDF version.]

Typical total quiescent background rates (cts/s/chip; 2019, averaged April through early October), including all grades that are telemetered (not just standard ASCA grades), by chip type and upper energy cutoff. These values can be used to estimate the probability of telemetry saturation. Note that the rates have been increasing rapidly in 2016; users should take into account the likelihood of further increases during this cycle. Beginning in Cycle 11 (late 2009) the default high energy limit was changed from 15 keV to 13 keV. Subsequent 15 keV rate estimates were scaled from 13 keV rates using 13 kev/15 keV conversion factors of 0.82 for S2 and and 0.68 for S3. These factors were determined from observations (April-July 2009) when the rates were fairly constant, and with the onboard high energy limit set to either 15 keV or to 13 keV.

Period (keV)	Aug 1999	2000-2003	2009	2023 (January - mid October)
Upper E cutoff	15 keV	15 keV	15 keV	15 keV^†	13 keV	12 keV	10 keV
Chip S2 (FI)	10	6.3	10.7	5.4	4.5	4.4	4.0
Chip S3 (BI)	11	7.7	14.7	8.9	6.0	5.5	4.0

^†Scaled from 13 keV rates

Approximate on-orbit standard grade background counting rates (2019, averaged July to mid-September). The rates are cts/s/chip, using only ASCA grades 02346, no VF filtering, excluding background flares and bad pixels/columns and celestial sources identifiable by eye. These values can be used for sensitivity calculations

Energy Band (keV)	I0	I1	I2	I3	S1	S2	S3	S4
0.3-10.0	0.35	0.36	0.35	0.39	1.93	0.37	1.00	0.36
0.5-2.0	0.06	0.06	0.06	0.07	0.17	0.07	0.14	0.08
0.5-7.0	0.17	0.20	0.20	0.23	0.52	0.21	0.63	0.19
5.0-10.0	0.19	0.20	0.18	0.21	1.40	0.20	0.63	0.19
10.0-12.0	0.12	0.12	0.11	0.12	1.10	0.12	0.79	0.12

Memos and papers:
- Chandra ACIS detector background uniformity from ACIS "stowed" backgrounds T. Gaetz, 24 Feb 2014
- Particle background observations: dark Moon and ACIS stowed (PDF) M. Markevitch, 27 Dec 2005
- Chandra Spectra of the Soft X-Ray Diffuse Background (PDF), M. Markevitch, et al., 2003, ApJ 583, 70
- Reducing ACIS quiescent background using Very Faint mode, A. Vikhlinin, 22 Oct 2002
- Using Chandra Level 0 Event Histogram Files to Characterize the High-Energy Particle Background (PDF), B. Biller, P. Plucinsky & R. Edgar, 22 Jan 2002
- The Temporal Characteristics of the Chandra X-ray Observatory High Energy Particle Background (PDF), C.E. Grant, M.W. Bautz & S.N. Virani, 2002, ASP Conference Proceedings, 262, 401
- General discussion of the quiescent and flare components of the ACIS background, M. Markevitch, 28 Sep 2001
- ACIS background measurement prior to opening the mirror cover (PDF), F. Baganoff, 1 Sep 1999

ACIS Calibration Information

Background

Bias

CC Mode Calibration

Cross Calibration

Charge Transfer Inefficiency (CTI)

Detector Geometry

Energy Resolution

Event Grades

Gain

Hot/Bad Pixels

Miscellaneous

FEP0 Problem

Ground Calibration

Transfer Streak Calibration

Optical Blocking Filter and Contamination

Pileup

Spatial Resolution, Encircled Energy and PSF

Quantum Efficiency and QE Uniformity

Response Matrices

Subpixel Event Repositioning