Present Calibration Uncertainties

Related Memos: Chandra Absolute Astrometric Accuracy

Related Memos:2005 Calibration Workshop - An Improved HRC-I Degapping Correction

Focal Length, Plate Scale, and Relative Chip Positions

Since the release of CALDB 3.3 in Dec. 2006 there now exist a complete set of cti-corrected products for all 10 chips at a focal plane temperature of -120 C (the operating ACIS temperature since Jan. 2000). Using CIAO 3.4 or later, acis_process_events applies the appropriate time-dependent gain and cti-corrections by default. BI chips are corrected for both parallel and serial CTI. Based on the calibration products released in CALDB 3.3, the rms residuals in the gain are 0.3%. These residuals are nearly energy independent and spatially uniform across all 10 chips (see Fig. Residual Gain Errors and the 2005 calibration workshop report listed below).

Related Memos: 2005 Calibration Workshop - CTI and Time Dependent Gain Corrections for ACIS

July 30, 2004 - Corrections for Time Dependence of ACIS Gains

2003 Calibration Workshop - Time-dependent Gain for ACIS

LETG - An updated version of the HRC-S de-gap coefficients table was released in CALDB 3.2.0 which has empirical wavelength corrections derived from multiple LETG/HRC-S observations of Capella. These observations were taken at a range of off-sets to cover a large portion of the HRC-S with bright emission lines of known energies. Based on these observations, the rms deviation between the known and computed wavelengths is 0.006 A on the central HRC-S plate, and 0.01 A across the entire HRC-S detector (see Fig. LETG dispersion)

Related Memos: 2005 Calibration Workshop - HRC-S/LETG: Degap and Wavelength Corrections

2004 Calibration Workshop - Characterizing Non-linearities in the Chandra LETG+HRC-S Dispersion Relation

2003 SPIE - Characterizing the Non-Linearities in the Chandra LETG/HRC-S Dispersion Relation

2003 Calibration Workshop - The Dispersion Relation of the LETGS

2003 Calibration Workshop - Characterizing Non-linearities in the Chandra LETG+HRC-S Dispersion Relation

2003 Calibration Workshop - Degap Corrections for HRC-S Grating Observations

2003 Calibration Workshop - Amplifier Mis-match as a Possible Source of HRC Event

HETG - Based on a systematic analysis of multiple HETG/ACIS-S observations of Capella, the rms residual between the known and computed wavelengths for both MEG and HEG spectra is δ λ / λ = 1.0 X 10^--4 (see Fig. HETG dispersion).

Related Memos: 2005 Measuring the Accuracy of Chandra/HETGS Wavelength Scale with Capella Data

HRMA (On-Axis Effective Area) - During ground-based testing at the XRCF, the absolute effective area of the mirrors was measured with emission line sources and continuum sources and with two different detectors (a gas proportional counter and a solid state detector). Extensive analyses of these data show that the systematic uncertainties in the absolute effective area of the HRMA are less than 3% at all energies (see Fig. On-Axis Area ). An updated version of the HRMA effective area was released in CALDB 3.2.1 that included a 22A layer of hydrocarbon contaminant on the mirrors. The contaminant increases the reflectivity of the mirrors near the Ir-M edge (2.2 keV) and corrects a problem with earlier versions of the HRMA effective area (see Fig. Ir-M Edge Residuals Eliminated).

HRMA (Off-Axis Vignetting) - The supernova remnant G21.5-09 was observed at 6 angles off-axis to test the predictions of the raytrace code regarding the vignetting of the mirrors. A comparison between the raytrace code and the G21.5-09 observations showed that the SAOSAC predictions are within 5% at energies below 6 keV and within 10 arcminutes of the optical axis. At higher energies and larger angles off-axis, the uncertainties can be as large as 10% (see the memo listed below).

Related Memos: 2005 Calibration Workshop - Improvements to the HRMA Effective Area

2003 SPIE - Chandra Observatory X-ray Mirror Effective Area

2003 Calibration Workshop - Chandra X-ray Observatory Mirror Effective Area

Measurement of Telescope Vignetting from G21.5-0.9 Off-axis Observations

HRC - The present version of the HRC-I QE is derived by fitting in-flight observations of G21.5-09, Cas A, and HZ43 to a model of the HRC-I QE derived from ground-based measurements. The blue shaded region in Fig. HRC-I QE shows the 1 σ uncertainty in the QE as a function of energy. The combined 1 σ uncertainties in the HRMA/HRC-I effective area are less than 7%. The uncertainties in the HRC-S QE are discussed in the gratings section.

Related Memos: 2004 Calibration Workshop - Status of the HRC

A New Flight Model of the HRC-I MCP Quantum Efficiency

An Update to the HRC-I MCP Quantum Efficiency Model

ACIS - The uncertainty in the absolute effective area of ACIS is a combination of the uncertainties in the ACIS QE near the read out, the QE uniformity (QEU) map, the depth of the contaminant on ACIS and the HRMA effective area. Uncertainties in the depth of the contaminant only affect the QE below approximately 2.0 keV. The contaminant is essentially transparent at higher energies. The ACIS QE at a given location on a given chip is computed from the ACIS QE at the read-out, which is unaffected by CTI, plus the spatial corrections contained in the QE map uniformity (QEU) map. The present version of the QEU map accounts for column-by-column variations in the QE due to the effects of CTI. The 1 σ uncertainties in the QEU are approximately 1% (see High-resolution QEU maps for ACIS-I and S1,2,3 chips). The cross-calibration uncertainties between the QEs of the FI and BI chips are less than 2% (see Absolute QE of ACIS S1, S2 and S3 from XRCF data at selected energies).

Periodic observations of the ACIS external calibration source and several astronomical objects show that there has been a continuous degradation in the low energy QE since launch due to the build-up of contaminant on the ACIS filters. The contaminant is thicker near the edges of the two ACIS detectors where the filters are colder ( see ACIS QE Degradation). This reference, along with those cited below, give detailed information regarding the chemical composition of the material, the time dependence of the contamination build-up and known spatial variations in the depth of the contaminant. A small correction was added in CALDB 3.3 to the QE of both BI and FI chips near the Si-K absorption edge, to account for the Si-K escape peak effect. This adjustment consists of a narrow 4% correction at 1800 eV.

Combining the 3% uncertainty in the absolute effective area of the HRMA along with the uncertainties in the ACIS QE, QEU and depth of the contaminant on the filter gives a 5% uncertainty in the absolute effective area of the ACIS detectors near the aim points (i.e., excluding the top and bottom 100 rows on ACIS-S and beyond a 6 arcminute radius on ACIS-I). Due to additional uncertainties in the depth of the contaminate near the edges of the detectors and mirror vignetting, the uncertainties at larger radii are approximately 10%.

Related Memos:Composition of the Chandra ACIS Contaminant

2004 Calibration Workshop - Improved High Spatial Resolution Calibration of ACIS QEU

2004 High-resolution QEU maps for ACIS-I and S1,2,3 chips

2004 Absolute QE of ACIS S1, S2 and S3 from XRCF data at selected energies

2003 Calibration Workshop - Absolute ACIS Quantum Efficiency

2003 Calibration Workshop - Time Dependence of the ACIS Contamination

2003 Calibration Workshop - Spatial Distribution of the Contaminant on the ACIS Camera

2003 Calibration Workshop - Composition of the Chandra ACIS Contaminant

Related Memos: LETG+HRC-S Effective Area memo

2004 Calibration Workshop - Improvements to the HRC-S QE uniformity and LETGS effective area

Chandra LETG Higher Order Diffraction Efficiencies

2002 SPIE - In-Flight Effective Area Calibration of the Chandra LETG

HETG - With the release of CALDB 3.2.1, the cross-calibration uncertainties between the HEG and MEG are less that 5% at all energies (see the memo listed below). With the latest calibration files, the uncertainties in the HETG/ACIS-S 1st order absolute effective area are less than 8% at all energies. This includes uncertatinties in the absolute HRMA effective area, the ACIS QE, QEU and the 1st order HETG transmission efficiency.

Related Memos: 2005 Calibration Workshop - HETGS Effective Area Updates

2004 Calibration Workshop - Overview of the HETGS Calibration

High Energy Transmission Grating Spectrometer Calibration

Energy Resolution

Related Memos: 2005 ACIS Energy Response Performance with the External Calibration Source

2003 Calibration Workshop - A New Paradigm for the Generation of ACIS Response Matrices

HETG -The LSFs of the HETG and LETG are generated by fitting parametric functions to simulated MARX spectra. The modeled LSFs are then validated with gratings observations of Capella taken at several angles off-axis. The HEG and MEG LSFs are fit with two gaussians and two Lorentzians at each energy. A recent study showed that the 1 σ uncertainties in the FWHM of the lines are 0.00064A for the MEG and 0.00035A for the HEG, with little energy dependence between 0.8 and 6.0 keV. The minimum FWHM for MEG spectra is 0.017A and occurs between 5-12A. The minimum FHWM for HEG spectra is 0.008A and occurs between 3-8A (see Fig. FWHM of HETG Spectra). The relative uncertainties in the FWHM are thus less than 4% for MEG spectra and less than 5% for HEG spectra.

Related Memos: LSF Verification: Capella HETG/ACIS-S

Exercising the Chandra Grating Line Response Functions

LETG - Similar to the HETG, the LSFs for LETG/HRC-S spectra are generated by fitting parametric functions to simulated MARX spectra. A recent comparison of these models with a set of Capella observations taken at different angles off-axis shows that the uncertainties in the FWHM are less than 20% at all energies. The uncertainties in the FWHM of LETG/HRC-S spectra are most likely dominated by uncertainties in the HRC-S de-gap map at the 1 to 2 pixel level. More detailed information about HRC-S spatial nonlinearities can be found in the memos listed under the LETG absolute energy section above.

Related Memos: 2003 Calibration Workshop - Absolute ACIS Quantum Efficiency

2003 Calibration Workshop - Spatial Distribution of the Contaminant on the ACIS Camera

HRC - Spatial variations in the QE of the MCPs were measured with ground-based flat field tests. These data sets were then used to derive the HRC-I and HRC-S QE maps. A series of 40 observations of Capella were taken at different locations on the HRC-I from 2006 through 2007. These observations were used to update the de-gap coefficients of the HRC-I and improve image reconstruction. Using the present version of the HRC-I QE map and accounting for intrinsic variations in the X-ray flux of Capella during these observations, the rms scatter in corrected count rates is approximately 1% accross the HRC-I.

Related Memos: 2007 Calibration Workshop - Verifying the HRC-I QE and QE Uniformity

HRC Flat Field Maps

LETG - The in-flight effective area of the LETG/HRC-S has been derived from observations of Sirius B, HZ43, PKS2155-304, and 3C273. The two white dwarfs strongly constrain the LETG/HRC-S effective area below 277 eV (44A), while the two AGN constrain the shape of the effective area at higher energies. The rms scatter in the residuals on the scales of 0.05A between the modeled spectra of these four objects and the data is approximately 6%.

Related Memos: 2004 - Improvements to the HRC-S QE uniformity and LETGS effective area

2002 SPIE - In-Flight Effective Area Calibration of the Chandra LETG

HETG - The in-flight effective area of the HETG/ACIS-S has been derived using the same sources as that used for the LETG/HRC-S. At present, all residuals between the modeled spectra and data for these sources are less than 5% at all energies.

Related Memos: 2005 Calibration Workshop - HETGS Effective Area Updates

On-Axis PSF - A high fidelity raytrace code (SAOSAC) that simulates the performance of the Chandra mirrors (HRMA) has been in development since long before launch. The raytrace code was fine tuned before launch based on extensive measurements of the imaging properties of the HRMA at the XRCF at MSFC. A recent comparison between SAOSAC simulations and a long HRC-I observation of Ar-Lac shows that the residuals in the on-axis PSF between the raytrace code and the data are less than 10% within 2.0" of the image centroid (see Fig. On-Axis HRC PSF ). The residuals between SAOSAC simulations and point sources observed with ACIS are slightly larger (see Fig. On-Axis ACIS PSF ). Also, the residuals with the ACIS PSF are not azimuthally symmetric. There appears to be an elongation in the ACIS PSF toward the read-out direction (see Fig. ACIS Imaged Point Source). The larger residuals in the ACIS PSF are most likely due to the imaging properties of ACIS which is presently under investigation. The same version of the raytrace code was used to generate the HRMA PSF library available in CIAO (see the CALDB products listed below) so users should expect the same accuracy in the PSF library. There is also a web based interface ( ChaRT ) for running SAOSAC raytrace simulations.

PSF Wings - There is so little power in the HRMA PSF scattered beyond 10 arcseconds, that it is not practical to populate the wings of the PSF with a sufficient number of photons when generating the PSF library. To determine the wings of the PSF, we observed Her X-1 for 50 ksec with ACIS-S and found that the wings are well represented by analytic functions (see the memos below). Fig. PSF-wings shows an example of the best-fit analytic function to the wings of the HRMA PSF in two different energy bands.

Off-Axis PSF - The Chandra PSF varies considerable with source position and energy. Off-Axis images are roughly elliptical with position angles and ellipticities that not only depend on the off-axis angle of the source, but also the azimuthal angle of the source (see Fig. Off-Axis Images for some examples). We have found that the off-axis PSF can be parameterized by the ECF (Enclosed Count Fraction), i.e., the fraction of the total counts within a given ellipse. The ECF is analogous to the encircled energy for azimuthally symmetric images. The ECF has been calibration based on a large suite of raytrace simulations of point sources at different angles-off axis, azimuths, and photon energies. Tables have been generated that give the best-fit values of the semi-major axis, ellipticity, and position angle of an ellipse that encloses a certain fraction of the total counts as a function of source position and energy. The residuals between the tabulated ECF values and a sample of HRC-I observations of point sources are less than 5% (see Fig. ECF Comparison)

Related Memos: 2003 SPIE - Calibration of Chandra's Near On-Axis Performance.

2003 SPIE - A Parameterization of the Chandra Point Spread Function

2003 Calibration Workshop - Calibration of Chandra's Near On-axis PSF

2003 Calibration Workshop - Calibrating the Wings of the Chandra PSF

2003 Calibration Workshop - A Parameterization of the Chandra Point Spread Function

2004 Analysis of PSF Wings

2003 Calibration Workshop - Clock Correlation Contribution to Chandra Timing Accuracy

Related Memos: 2003 Calibration Workshop - Astronomical Calibration of the Chandra Clock

Discovery of X-Ray Emission From the Crab Pulsar at Pulse Minimum

ACIS-CC The absolute time accuracy in ACIS-CC mode observations is primarily limited by the absolute accuracy of the aspect solution. A one pixel (0.5 arcsecond) uncertainty in the aspect solution of a photon corresponds to an uncertainty of 2.85 msec in the absolute arrival time of that photon in CC mode read-out. A script has been written which is available through the memo references, that interpolates the event times in CC mode. Applying this script to an ACIS-CC mode observation of the Crap pulsar yielded errors of 1.5 msec.

Related Memos: ACIS CC mode absolute time

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.    Email: cxcweb@head.cfa.harvard.edu Smithsonian Institution, Copyright © 1998-2004. All rights reserved.