Last modified: 29 February 2024


Grating Spectra

Filename Description Content ICD SDP Pipeline
acis_pha2.fits ACIS L2 grating spectra SPECTRUM v1.4 TG2
hrc_pha2.fits HRC L2 grating spectra SPECTRUM v1.4 TG2

The creation process for ACIS and HRC gratings spectra is the same. A brief overview is given here; refer to the Analysis Guide for Chandra High Resolution Spectroscopy for complete details. Some the content presented here is from the Guide.


Transmission grating spectra for a Chandra observation are provided in the form of PHA files: 1-D counts histograms. (The name "PHA" is historical, referring to "Pulse Height Analyzer" or "Pulse Height Amplitude".) PHA are actually available in two formats, referred to as "Type I" and "Type II". Type I PHA are the traditional format of PHA, with sequential channels stored in sequential rows. Type II PHA files may contain many Type I PHA, with each Type I PHA stored as a row of the overall file. Thus, for any given row of a Type II PHA, the counts histogram is stored as an array. Since Chandra grating observations have at least two orders (for LETGS; 12 orders for HETGS), the Type II PHA is the standard method of storing grating observation data.

Among other things, the pipeline processing will find the grating arms using tgdetect, and assign each event to an order with tg_resolve_events. Finally tgextract is used to create the pha2.fits file.

To begin analysis on the pha2 file, see the Examining PHA2 Files thread.

Creator pipeline: TG2

tgextract - tgextract is the final step of a series of tools designed to process chandra grating data. See the Grating Spectroscopy threads page for info on reprocessing this data.

Useful links:

SPECTRUM-specific columns:

Column Name Units Description
SPEC_NUM - Spectrum Number (each row indicates a different PHA Type I)
TG_M - Diffraction order (m)
TG_PART - Spectral component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background)
TG_SRCID - Source ID, output by detect
X pixel X sky coord of source
Y pixel Y sky coord of source
CHANNEL[8192] - Vector of spectral bin numbers
COUNTS[8192] count Counts array (a spectrum)
STAT_ERR[8192] count Statistical uncertainty (error) on counts column
BACKGROUND_UP[8192] count Background count vector
BACKGROUND_DOWN[8192] count Background count vector
BIN_LO[8192] angstrom Bin boundry, left edge
BIN_HI[8192] angstrom Bin boundry, right edge

The Type II PHA files will also have a second block called "REGION" attached. This block spatially defines the regions used to extract the source and background spectra for the observation. For example, an HETG/ACIS observation has a source, upper background, and lower background defined for each of the six orders of the HEG and MEG spectra. This creates a total of 36 regions.

REGION-specific columns:

Column Name Units Description
SPEC_NUM - Spectrum Number (each row indicates a different PHA Type I)
ROWID - Source or a background region?
SHAPE - Shape of region
TG_LAM angstrom Dispersion coordinate vector for SHAPE
TG_D degrees Cross-dispersion coordinate vector for SHAPE
R[2] (angstrom , degrees) Radius vector for SHAPE
ROTANG degrees Rotation angle for SHAPE
TG_PART - Grating part index (HEG=1, MEG=2, LEG=3)
TG_SRCID - Source identification number
TG_M - Diffraction order