Run ChaRT
Introducing ChaRT v2
Several changes to the options and inputs required to run ChaRT have been introduced in the latest version—allowing for simulations compatible with sub-pixel analysis, in particular—including input source spectrum format or inclusion of observation-specific aspect solution files. Details are described in the Preparing to Run ChaRT thread.
ChaRT is a web interface to the SAOsac raytrace code which was developed by the CXC for calibration purposes. A raytrace matching the user inputs is run through the Chandra optics to produce a collection of rays. An email is sent to the user when the raytrace data is available for download.
This web interface handles one simulation at a time (i.e. you can't submit multiple sources at once), but users can submit multiple jobs one after another to simulate multiple sources.
For detailed instructions on obtaining ChaRT inputs, follow the Preparing to Run ChaRT thread.
Explanation of ChaRT inputs
For detailed instructions on obtaining ChaRT inputs, follow the Preparing to Run ChaRT thread.
- Email Address
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ChaRT sends an email when your job has finished running and the data is available on the FTP site. The CXC may also contact you at this email address if there is a problem running the ChaRT job.
- R.A.
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The right ascension of the source position. The value may be supplied in decimal degrees or in sexagesimal format. Values are required to be J2000.
- Dec
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The declination of the source position. The value may be supplied in decimal degrees or in sexagesimal format. Values are required to be J2000.
- Off-axis Angle
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The off-axis angle of the source, θ, in arcminutes.
- Azimuthal Angle
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The azimuthal angle of the source, ϕ, in degrees.
- Spectrum File
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The definition of the spectrum at the entrance aperture of the HRMA for which you would like a PSF to be simulated.
The spectrum file must contain 3, and only 3 columns in the following order: lower energy limit (keV), upper energy limit (keV), and photon flux photon/cm2/sec. Both FITS and various ASCII formats are supported.
This file may be created with the save_source function in Sherpa, as shown in the Preparing to Run ChaRT thread.
- Monochromatic Energy
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A single monochromatic energy in keV for the simulation. The allowed energy range is 0.2 - 10 keV.
- Photon Flux
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The photon flux, photon/cm2/sec, at the specified monochromatic energy. The value should be between 1.0e-9 and 1.0e-2 photon/cm2/sec.
- OBS_ID
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The OBS_ID of the observation being analyzed. The aspect solution file can be retrieved from the Chandra archived automatically; saving the user from having to upload the file.
Be sure to double check the list of reasons why users not use this option in the Preparing to Run ChaRT thread.
- OBI_NUM
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There are a very small number of multi-obi observations: observations separated in time that share the same OBS_ID value. These observations need to be processed separately; the OBI_NUM value in the header is used to distinguish them.
This box will be disabled unless the user enters the a value for one of the mutli-obi observations into OBS_ID field.
- Upload aspect solution file
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Users analyzing proprietary dataset or who have made modifications to the aspect solution file must upload the aspect solution file to ChaRT.
- R.A.
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The right ascension of the telescope pointing. The value may be supplied in decimal degrees or in sexagesimal format. Values are required to be J2000.
Users doing analysis with Chandra detector coordinates can set this value to 0.
- Dec.
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The declination of the telescope pointing. The value may be supplied in decimal degrees or in sexagesimal format. Values are required to be J2000.
Users doing analysis with Chandra detector coordinates can set this value to 0.
- Roll
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The roll angle of the pointing relative to North. The value must be between 0 and 360 degrees.
Users doing analysis with Chandra detector coordinates can set this value to 0.
- Exposure Time
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The duration of the simulation in ksec. The maximum exposure time is 200 ksec.
This value is usually chosen to match the exposure time of an observation, which is stored in the EXPOSURE header keyword of the event file.
- Number of iterations
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The number of simulations to perform. The same source parameters are used for each iterations, each done with a different random seed.
Users may request between 1 and 50 iterations.
- Number of rays
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Rather than specifying the number of iterations, users can specify the number of simulated rays that they require. ChaRT will repeat the simulation multiple times using the same parameters (with a different random seed) until it generates at least the number of specified rays. Since the number of simulated rays is not the same as the number of simulated events once the ray file has been processed through MARX or psf_project_ray, ChaRT increases the number of rays by 40%.
Users may request between 1 and 1000000 rays. However, the total number of iterations cannot exceed 10000.
- Random Seed
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A random seed that allows the ray generator to create a thoroughly random and statistically appropriate sampling of rays. The default value, -1, indicates that the system's clock time will be used.
To recreate a previous run of ChaRT, you must enter the same seed value.
The random seed is incremented by 1 for each realization after the first.
Coordinates
Chart is used to simulate a point source PSF at a specific location. The user can choose to enter coordinates in either Celestial or Chandra Detector coordinates.
Source Spectrum
The spectrum of the source can be supplied as either a single monochromatic energy or by uploading a file containing the source spectrum.
Pointing Information
Users need to supply information about the Chandra pointing. Most Chandra observations are taken while the spacecraft pointing dithers around some nominal location. This has a blurring effect on the PSF as discussed in the Preparing to Run ChaRT thread.
The pointing information is supplied via the aspect solution file or by supplying a single pointing.
Realizations
Multiple simulations using the same parameters may be necessary to obtain sufficient number of counts to overcome the statistical noise.