TGCat Processing

TGCat processes all data within an automatic system to allow the best access to newly public gratings data ready for analysis, and to allow the tgcat team to provide the best extractions of gratings data with ease and little delay. All TGCat processing is done by ISIS/S-Lang functions which set up and call CIAO tools.

For an overview of the TGCat pipelines see: Processing Definitions
For full details of CIAO tools, see http://cxc.harvard.edu/ciao/intro/tools.html
For information about ISIS, see http://space.mit.edu/cxc/isis

Gratings And Modes

grating/detector/mode combination supported: Modes of ACIS:

TE: Timed Exposure
CC: Continuous Clocking

grating detector modes:

Detector	Grating	Mode
HETG	ACIS-S	TE
HETG	ACIS-S	CC
LETG	ACIS-S	TE
LETG	ACIS-S	CC
LETG	HRC-S	-

A quick guide to the extractions

Chandra data are identified by a "sequence number" and by an "observation identifier", or OBSID. The sequence number generally defines a single scientific observation, but due to operational constraints, any given sequence number for a specific target may be broken into several intervals, and each is assigned a unique OBSID. We extract spectra by OBSID. During analysis, these may be combined. In the summary plots, we show combined data in order to provide the best overview of the quality of the observation. The data products themselves maintain the individual components of the spectra (different orders, gratings).

- In a later release, we intend to also provide data combined over the obsids of each sequence number

Very briefly, the extraction process consists of the following steps:

perform event processing (geometric and pulse-height corrections; filtering on good events, good times);
determine the zeroth order's centroid (this is the origin of the wavelength scale);
define a spatial mask which encloses spectral regions;
filter on the spatial mask and transform spatial to spectral coordinates (wavelength, and cross-dispersion angle); if the detctor is ACIS, use the CCD energy to resolve orders.
bin the spectra, sorted by order and grating, filtering on the cross-dispersion region; from the region adjacent to the source spectrum, bin a background spectrum (most important for LETG/HRC, rarely important for HETG/ACIS).
Generate response files (effective area, or "ARF", and line-spread-function, or "RMF");
compute a light curve from the diffracted photons;
compute some summary spectral quantities (counts and fluxes in bands);
generate summary plots.

- For an in-depth look at the pipelines for each Grating/Detector/Mode combination see Processing Definitions

Finding the Zeroth Order

TGCat uses one of two automatic methods for determining the zero order position:

the standard CIAO tool, tgdetect
and for cases in which the zeroth order is blocked or heavily distorted by pileup, we use findzo, an ISIS program which intersects the MEG trace with the ACIS readout streak.

The methods used are checked during the Validation and Verification ( V&V ). If the method used failed to center on the zero order accurately, the alternate automatic method will be used or the V&V scientist will provide either exact coordinates of the zero order centroid or a region to act as an initial guess ( for cases where the target source is displaced from the proposers TARGET coordinates )

V&V process

All the extractions were inspected by a person familiar with the instruments and spectra. We check that the zeroth order position is accurate, that the spectral extraction region is appropriate, check for avoidable source confusion, and that all the products exist. The V&V procedure is guided by the same plots as presented in the preview pages. Comments and warnings are available in the results table ("v" link), and as download products. Comments provide information about the source which may be relevant for analysis. Warnings indicate problems or features which may complicate analysis. Some common comments and warnings are:

COMMENTS:
- Comment: Bright, hard spectrum; extracted with narrow sky masks to improve HEG flux at very short wavelengths (1.2-1.7A) (but at the expense of background regions).
- Comment: Mildly extended source <~size arcsec>
- Comment: Complex source:
- Comment: Multiple sources; extraction is of
WARNINGS:
- Warning: Extended source, large extraction region used to include extended emission.
- Warning: Very extended source; special analysis required.
- Warning: Background important
- Warning: Spectra only valid in wavelength range [lo-hi] (energy range [lo-hi]) due to detector masking.
- Warning: Multiple sources in the field may cause contamination of the primary extraction at some wavelengths.
- Warning: CC-mode order-sorting with "osip=none" (a "flat" order sorting PHA region) accepts some zeroth order scattering at short wavelengths (<2A).
- Warning: ACIS T = -110C, no CTI correction available.
- Warning: Very bright: spectra may be piled.
- Warning: heavy "streaking" on S4 not rejected; (custom application of "destreak" might mitigate).

VV process - Obsids Not in Catalog

There were some processing errors which caused us to reject extraction until custom processing is done ( these are not included in the catalog):

Error: Multiple sources - background contaminated. Need to extract only backgrounds from one side. [Mainly relevant for LETG/HRC-S; default backgrounds are very wide.]
Error: Blocking of some orders/regions due to HRC-S shutters (requires re-extraction of <+/-> side). (some Crab observations)
Error: Extended source - requires wider cross-dispersion extraction region.
Error: far off-axis source - requires wider cross-dispersion extraction region.

Finally, there were obsids which could not be processed due to inherent limitations (e.g., HETG used with just the ACIS-S3 (acis-7) readout, or LETG used with HRC-I); the list of these obsids can be viewed here .