For sources near the optical axis of the telescope, the size of the point spread function is smaller than the size of the ACIS pixels. Applying the energy-dependent sub-pixel event-repositioning (EDSER) algorithm can improve the image quality of ACIS data for such sources.

This document provides guidance on how to reprocess the data with the EDSER algorithm and analyse the results. Please read the technical details available in the ACIS Sub-Pixel Event Repositioning why topic as well.

About the PSF Calibration

The CXC believes that the sub-pixel event-resolution (SER) algorithm does improve the ability to separate small scale structure, but we have not yet calibrated ACIS on sub-pixel scales, so obviously the potential for artifacts exists. As reported last year to the Chandra Users' Committee (Report, PDF), we have a plan to enhance our simulation tools to represent the SER-enhanced PSF. This is a significant calibration, modelling, and development effort since we need to model the grade distribution and have a much more detailed internal model of the instrument. Refer to the Chandra Ray Trace caveats for some other limitations on the PSF modelling tools.

In principle, sub-pixel analysis should not affect the HRMA ray tracing. However, at the sub-arcsecond scale, HRMA calibration is still on-going (e.g. Probing higher resolution: an asymmetry in the Chandra PSF). Therefore, there may be uncalibrated mirror-related effects at the subarcsecond scale in addition to the unmodelled ACIS detector pixel effects.

1. Reprocessing the Data

The first step in sub-pixel data analysis is to ensure that the data has been reprocessed with pix_adj=EDSER. EDSER became the default parameter value in CIAO 4.3, and has been the default in standard data processing (SDP) since version DS 8.4 (28 June 2011).

Before the use of EDSER, the coordinates of ACIS events were randomized by +/- 0.5 pixel to avoid possible aliasing affects associated with the CCD pixel grids. Information on that correction is available in the pixel randomization why topic.

The chandra_repro reprocessing script runs all the data processing threads for ACIS imaging data. The steps are also described in detail in the ACIS Data Preparation analysis guide.

Note that it is not possible to apply the sub-pixel event-repositioning algorithm EDSER to CC33_FAINT or CC33_GRADED mode data.

2. Run ChaRT to create a PSF

The Chandra Ray Tracer (ChaRT) simulates the best available point spread function (PSF) for a point source at any off-axis angle and for any energy or spectrum. Technical details are available from the About ChaRT page.

It is important to read the caveats before running ChaRT and using the resulting PSF simulations in analysis.

3. Create a Sub-Pixel PSF Image

Thread: Creating an Image of the PSF

The projected event file created with ChaRT and Marx contains X and Y values without any pixel quantization, allowing a 2-D histogram image can be made at any desired sub-pixel scale.

To create a sub-pixelated image with dmcopy, specify a binning factor that is less than 1. This example uses 0.1, and the limits have been changed to +/- 25.6 pixels to create a 512 square pixel image:

unix% dmcopy \
      "marx_psf.fits[bin x=4678.41:4729.61:0.1,y=3674.33:3725.53:0.1]" \
      psf_subpix.fits

4. Convolve the Data

The sub-pixel PSF image can be used as a convolution kernel for fitting and modelling in Sherpa. It may also be used to convolve the data by running one the CIAO's convolution tools aconvolve.