Cycle 26 Grating RMFs/ARFs

Caveats and Notes

We have prepared sample grating RMFs and corresponding ARFs for proposal planning purposes. As in the CY25 release, all of the GRMF files below were built with mkgrmf, using the latest LSF parameter data for the HETG/ACIS-S, LETG/ACIS-S, and LETG/HRC-S configurations. In all cases, the grating pixel randomization, RAND_TG is set to 0 pixels (turned off).

The ACIS-related gARFs were built with the CIAO script fullgarf, using PHA2 files from specific archival observations and the appropriate GRMF energy grid as the seed energy grid. They therefore include dither, and do not have exactly the nominal aimpoints as their source positions. The configuration information, including SIM_Z position and ObsID numbers, are included in the GARF headers.

The ACIS-S configuration effective areas (*.garf or *.arf) were derived using CIAO 4.16, CALDB 4.11.0, including the ACIS contamination model file acisD1999-08-13contamN0015.fits, with the time delay value set at 863654469.184s, corresponding to the UTC date 2025-05-15T00:00:00, or half-way through the CY26 observation period. For the HRC-S configurations, all effective areas were derived from CIAO as well, using the latest QE and QE uniformity calibrations.

The HRC-S configuration GARFs were built with the HRC-S QE (N0016) and QEU (N0015) released with CalDB 4.9.6 on 2021-09-24. Relative to the CY25 GARFs, the CY26 GARFs are increased by a flat ~7.1% over the energy range 0.06-.630 keV. Above that range, the difference decreases exponentially from ~7.1% down to ~1.5%.

To download individual files, <Shift>-click on the filename. The complete set of responses is available as a gzipped tarfile located on the Chandra Archive server, under the directory:
https://cxc.cfa.harvard.edu/cdaftp/arcftp/ChandraCalDB/proposal/CY26/.

**Table 1: Cycle 26 Grating Responses.**
Configuration	Grating Type	Order	gRMF Filename	gARF Filename	Source ZOFFSET, YOFFSET (arcmin)
HETG/ACIS-S	HETG	0	aciss_hetg0_cy26.rmf*	aciss_hetg0_cy26.arf	-1.02389, -0.33 gRMFs were built with mkgrmf *Zeroth-order RMF is imaging rmf built with mkacisrmf.
	HEG	1 -1	aciss_heg1_cy26.grmf aciss_heg-1_cy26.grmf	aciss_heg1_cy26.garf aciss_heg-1_cy26.garf
		2 -2	aciss_heg2_cy26.grmf aciss_heg-2_cy26.grmf	aciss_heg2_cy26.garf aciss_heg-2_cy26.garf
		3 -3	aciss_heg3_cy26.grmf aciss_heg-3_cy26.grmf	aciss_heg3_cy26.garf aciss_heg-3_cy26.garf
	MEG	1 -1	aciss_meg1_cy26.grmf aciss_meg-1_cy26.grmf	aciss_meg1_cy26.garf aciss_meg-1_cy26.garf
		2 -2	aciss_meg2_cy26.grmf aciss_meg-2_cy26.grmf	aciss_meg2_cy26.garf aciss_meg-2_cy26.garf
		3 -3	aciss_meg3_cy26.grmf aciss_meg-3_cy26.grmf	aciss_meg3_cy26.garf aciss_meg-3_cy26.garf
LETG/ACIS-S	LETG	0	aciss_letg0_cy26.rmf*	aciss_letg0_cy26.arf	0.0, 0.0 (set manually in model OBS_ID) gRMFs were built with mkgrmf *Zeroth-order RMF is imaging rmf built with mkacisrmf.
	LEG	1 -1	aciss_leg1_cy26.grmf aciss_leg-1_cy26.grmf	aciss_leg1_cy26.garf aciss_leg-1_cy26.garf
		2 -2	aciss_leg2_cy26.grmf aciss_leg-2_cy26.grmf	aciss_leg2_cy26.garf aciss_leg-2_cy26.garf
		3 -3	aciss_leg3_cy26.grmf aciss_leg-3_cy26.grmf	aciss_leg3_cy26.garf aciss_leg-3_cy26.garf
LETG/HRC-S	LEG	0	hrcs_letg0_cy26.rmf	hrcs_letg0_cy26.arf	0.0, 0.0 gRMFs were built with mkgrmf NOTE: The *.garf files for HRC-S/LETG have been built using the CIAO 4.16 mkgarf routine. The encircled energy fraction correction is included in the GRMFs, and not the GARFs. We have also scaled the wavelength ranges in the GARFs and GRMFs in order to cover the detector array appropriately, and include the geometry of plate gaps and edges. See item 3 in Caveats and Notes below.
		1 -1	hrcs_leg1_cy26.grmf hrcs_leg-1_cy26.grmf	hrcs_leg1_cy26.garf hrcs_leg-1_cy26.garf
		2 -2	hrcs_leg2_cy26.grmf hrcs_leg-2_cy26.grmf	hrcs_leg2_cy26.garf hrcs_leg-2_cy26.garf
		3 -3	hrcs_leg3_cy26.grmf hrcs_leg-3_cy26.grmf	hrcs_leg3_cy26.garf hrcs_leg-3_cy26.garf
		4 -4	hrcs_leg4_cy26.grmf hrcs_leg-4_cy26.grmf	hrcs_leg4_cy26.garf hrcs_leg-4_cy26.garf
		5 -5	hrcs_leg5_cy26.grmf hrcs_leg-5_cy26.grmf	hrcs_leg5_cy26.garf hrcs_leg-5_cy26.garf
		6 -6	hrcs_leg6_cy26.grmf hrcs_leg-6_cy26.grmf	hrcs_leg6_cy26.garf hrcs_leg-6_cy26.garf

Caveats and Notes

These gARFs and gRMFs are for proposal planning purposes only. They should not be used for analysis of any real Chandra data. The Grating Spectroscopy threads give instructions for building an observation-specific gRMF and gARF.
LETG/HRC-S proposal planners: the above gRMFs and gARFs for the LETG/HRC-S configuration must be used in gRMF/gARF pairs for appropriate response settings to be employed. The LETGS group also has convolved gRMFs and gARFs, known as "gRSP" files, available for download on the LETGS Effective Area webpage. That webpage explains the proper use and interpretation of these files.
A Note concerning the LETGS Encircled Energy Fraction: The LETG/HRC-S configuration requires that an encircled energy fraction, or EEARF, be considered in order to apply the appropriate response for these grating data. With the release of CalDB 4.6.9 in August 2015, an improved encircled energy fraction is included in the GRMF, and hence should not be included in the GARF. Therefore, the LETG/HRC-S GARFs (all orders) for LETG/HRC-S have been built so that the encircled energy fraction is not applied to the GARF. HOWEVER, in order for the users results to agree with the results of PIMMS for this configuration, one must either use the GRMF and GARF in tandem (in XSPEC, Sherpa, or similar fitting environment), or one must apply the encircled energy fraction to the GARF. The GARFs may be modified using the script factor_rsp, which is described in the documentation file factor_rsp.txt. The script may be [shift-click] downloaded for use in any shell with isis-script in one's path. ISIS is available from links at the ISIS Home Page, but is not included with CIAO 4.16.