FITS
Almost all data products (notable exceptions are telemetry files) are
formatted as FITS files.
Each FITS file may contain one or more HDUs, but the first non-empty
HDU is, by definition, the principal HDU. The
properties of the principal HDU are taken to be the properties of the
entire data product.
Note that file names are built up of the following components: instrument, 'f', a number that represents
time or ObsId, version number, optional component, file identifier, and ".fits".
The identity of a product can usually be determined on the basis of the instrument and the file identifier,
if necessary in combination with the optional component.
CONTENT
Each product contains a FITS keyword CONTENT in the header of its
principal HDU that uniquely identifies the type of product.
Processing Level
Beyond the telemetry data (TLM), we distinguish five processing levels
in automated processing: L0, L0.5, L1, L1.5, L2.
Processing Pipelines
Products are created by one of 28 processing pipelines. Not all
pipelines are used to process the data for any particular observation;
the exact choice depends on the characteristics of the observation.
| Pipeline Code | Processing Level | Pipeline |
|---|---|---|
| TP | L0 | Telemetry processing |
| EPHIN0 | L0 | EPHIN extractor |
| ENG0 | L0 | Engineering extractor |
| SIM0 | L0 | SIM extractor (technically, SIM-A and SIM-B) |
| ACIS0 | L0 | ACIS extractor |
| HRC0 | L0 | HRC extractor |
| ACAI | L0 | ACA-I extractor (the "regular" aspect pipeline) |
| ACAC | L0 | ACA-C extractor (for the calibration telemetry) |
| EPHEM0 | L0 | Ephemeris (predictive) |
| ASP0.5 | L0.5 | Aspect |
| OBIDET | L0.5 | Observation Interval Determination |
| SIM0.5 | L0.5 | SIM |
| ACIS0.5 | L0.5 | ACIS |
| HRC0.5 | L0.5 | HRC |
| ACIS1 | L1 | ACIS |
| HRC1 | L1 | HRC |
| ASP1 | L1 | Aspect |
| EPHIN1 | L1 | EPHIN |
| TEL1 | L1 | TEL |
| EPHEM1 | L1 | Ephemeris (definitive) |
| TG1.5 | L1.5 | TGrating |
| TGCC1.5 (TG1.5) | L1.5 | TG ACIS CC |
| ACISCC2 (ACIS2) | L2 | ACIS CC |
| ACISTE2 (ACIS2) | L2 | ACIS TE |
| HRC2 | L2 | HRC |
| TGCC2 (TG2) | L2 | TG ACIS CC |
| TGTE2 (TG2) | L2 | TG ACIS TE |
| TGHRC2 (TG2) | L2 | TG HRC |
Proprietary Data
The information in most data products is public, but in the case of a
proprietary Guest Observer observation, certain products will be
available only to the GO (and authorized individuals in CXC) for the
duration of the proprietary period (nominally one year). The
potentially proprietary products are marked with a "P".
Subsystems
Data products are grouped by the spacecraft subsystem in which the
data originated. These subsystems, or instruments, arelisted in the
following table.
| Subsystem | Description |
|---|---|
| acis | AXAF CCD Imaging Spectrometer subsystem |
| hrc | High Resolution Camera subsystem |
| ephin | Electron-Proton-Helium INstrument subsystem |
| pcad | Pointing Control and Attitude Determination subsystem |
| tel | Telescope subsystem |
| sim | Science Instrument Module subsystem |
| obc | On-Board Computer software subsystem |
| ccdm | Communications Command and Data Management subsystem |
| cpe | CPE hardware and software subsystem |
| eps | Electrical Power Subystem |
| thm | Thermal control subsystem |
| sms | Structure subsystem |
| prop | Propulsion and pyro subsystems |
| misc | Other AXAF signals |
| orbit | Orbit ephemeris |
| solar | Solar ephemeris |
| lunar | Lunar ephemeris |
| angles | Viewing angles |
| clock | Clock measurements |
| axaf | Certain mission-level items |
Engineering Data
The data products may be roughly divided into science-related and
engineering. There are engineering data products for all spacecraft
subsystems and the products come in two versions: TU and Cal.
Measurements such as temparatures and voltages are sent down in
telemetry units (TU), but the more useful version is after conversion
(calibration) to engineering units.
Product Categories
In a functional sense, from the point of view of the user, the
products are divided into three categories:
primary, secondary, and supporting products. An approximate
characterization is that an archival researcher would typically just
want the primary products, while a serious GO would need the secondary
products as well, in order to do a more sophisticated analysis and,
possibly, limited reprocessing and fine-tuning. The supporting
products contain everything else. The standard data distribution to
GOs consists of primary and secondary products. Standard public data
distribution packages consist of primary products, secondary products,
or both. Supporting products may be obtained on an individual basis.
Data Delivery Mechanisms
Eventually, data selected for retrieval may be delivered through one of the
following mechanisms or media:
| Primary products | List A |
| Secondary products | List B |
| Science-related products (includes Primary and Secondary Products) | List C |
| Engineering products | List D |
| Telemetry products | List E |
| Mission products (including OBIDET products) | List F |