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Last modified: 14 December 2023


ACIS Cycle 26 Aimpoint and Off-Axis RMFs/ARFs

We have prepared sample RMFs and corresponding ARFs for proposal planning purposes. These are positioned at the aimpoints of the ACIS-I and ACIS-S arrays, and at selected off-axis points to allow the proposer to estimate the effects of the ACIS CTI on the chips' performance using the current operational mode at focal plane temperature of -120C. Table 1 includes the filenames and corresponding chip and sky coordinates to which they apply.

NOTE: These sample RMFs and ARFs have been calculated using the recently-updated aimpoint positions for ACIS-I and ACIS-S as the reference positions. The readout distances have been updated to include that information.

To download individual files, <Shift>-click on the filename. The complete set of responses is available as a gzipped tarfile, located on the Chandra Data Archive server under the directory link

Table 1: ACIS Cycle 26 Responses.
RMF Filename ARF Filename CCD (CHIPX, CHIPY) Off-Axis Position (arcmin) Distance from Readout (arcmin)
acisi_aimpt_cy26.rmf acisi_aimpt_cy26.arf I3 (953.74, 958.74) 0
(Nearly on-axis)
acisi_2am_cy26.rmf acisi_2am_cy26.arf I3 (953.74, 702.74) 2 5.49
acisi_4am_cy26.rmf acisi_4am_cy26.arf I3 (953.74, 446.74) 4 3.49
acisi_6am_cy26.rmf acisi_6am_cy26.arf I3 (953.74, 190.74) 6 1.49
aciss_aimpt_cy26.rmf aciss_aimpt_cy26.arf S3 (193.74, 520.00) Default pointing position for ACIS-S (Now +10 arcsec from nominal aimpoint) 4.06
aciss_2am_cy26.rmf aciss_2am_cy26.arf S3 (193.74, 264.00) 2
(toward readout)
aciss_-2am_cy26.rmf aciss_-2am_cy26.arf S3 (193.74, 776.00) -2
(away from readout, toward ACIS-I)

These files were built with CIAO 4.16 and CALDB 4.11.0. The RMFs are PI RMFs built with the CIAO phase-2 response tool mkacisrmf. They have 1024 linear energy bins (0.255 keV to 12.0 keV for both ACIS-I and ACIS-S), and 1024 linear PI bins (1-1024). (1-1024). All of the ACIS-I RMF files above are for the CTI-corrected response case, including the CTI corrections for ACIS-S3. That is, the P2_RESP input file used is acisD2000-01-29p2_respN0007.fits, and the CTI-corrected gain file acisD2000-01-29gain_ctiN0007.fits has been applied in mkacisrmf to set the PI energy scale.

The ARFs were built with mkarf, using the appropriate RMF for the energy scale. They are "2-pi" ARFs (no encircled energy correction) and assume a point source. They have been corrected for the time- and spatially-dependent contaminant absorption projected for the date 2025-05-15 (TT=863654469.184s), using the ARDLIB CONTAM file feature in CIAO 4.16, with the current ACIS contamination model, namely acisD1999-08-13contamN0015.fits. The N0015 CONTAM file was introduced in CalDB 4.11.0 on 2023-12-14, and contains a newly developed contamination model being introduced in Cycle 26.


  1. These ARFs and RMFs are for proposal planning purposes only. They should not be used for analysis of any real Chandra data. The Imaging Spectroscopy threads give instructions for building an observation-specific RMF and ARF.

  2. The ACIS-I (FI) RMFs should be accurate to within a few percent between 0.5-6.0 keV. Below 0.5 keV, the effective area and gain for the FI devices is uncertain by 5-10%. Similar uncertainties apply above 6 keV. See the RESPONSE PRODUCTS RELEASE NOTES FOR ACIS for details.

  3. The effective areas were derived for -120C ACIS focal plane temperature, and as such include CTI corrections in quantum efficiency valid for that temperature. These CTI corrections result in an energy-dependent QE gradient, which is visible in the effective area values at different distances from the readouts. Calculated count rates will be reasonably accurate without additional compensation for CTI when using these ARFs.