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Discussion of Example 7: ASCA FITS Header

This FITS table contains an event list of the X-rays detected with the SIS1 detector on the ASCA X-ray satellite during a particular pointed observation. Each row contains data for a single photon, and the table contains information for all the events in this particular observation. Eleven different parameters are listed for each X-ray event. The header keywords supply additional information for deriving precise times or positions for each event.

This binary table (XTENSION keyword, BITPIX value of 8, NAXIS value of 2, GCOUNT value of 1) has 22990 rows (NAXIS2 keyword) consisting of 30 bytes (NAXIS1 keyword) distributed over 11 fields (TFIELDS keyword) and no variable length array heap (PCOUNT).

In this example, by contrast with examples 5 and 6, the keywords describing the structure of the table appear immediately after the required header keywords, and the more general information follows. The field titles (TTYPEn) are short, upper-case headings of five characters or fewer; the comment fields of the keyword card image explain the title. TFORMn keywords provide the format, and TUNITn keywords provide the units, where appropriate. All units are lowercase; the case is not significant. The quantity in field 11 is a pure number, and the designers of this table have chosen not to have a TUNIT11 field, a different approach from that of example 6. For many fields, TLMAXn and TLMINn keywords are used, following the HEASARC convention discussed in section 5.6.1.4. The table contains the following fields:

1.
TIME is a double precision floating point number containing the arrival time (in seconds) of the X-ray photon. The time system is described by the Time Related Keywords later in the file.

2.
PI is a 4-byte integer containing the gain-corrected pulse-height energy channel number of the photon. The channel numbers run from 0 to 4095.

3.
X is a 2-byte integer containing the projection of the event sky right ascension onto the X-axis of a rectangular pixel coordinate system; the axis runs from 1 to 1280 (TLMAX3, TLMIN3).

4.
Y is a 2-byte integer containing the projection of the event sky declination onto the Y-axis of a rectangular pixel coordinate system; the axis runs from 1 to 1280.

5.
RAWX is a 2-byte integer containing the CCD X-axis pixel location of the event with no corrections; the axis runs from 6 to 426.

6.
RAWY is a 2-byte integer containing the CCD Y-axis pixel location of the event with no corrections; the axis runs from 1 to 422.

7.
PHA is a 2-byte integer containing the pulse-height energy channel number of the data for this photon, with no gain correction. The channel numbers run from 0 to 4095.

8.
DETX is a 2-byte integer containing the X-axis pixel location of the event in detector coordinates, corrected for instrument distortion; the axis runs from 1 to 1280.

9.
DETY is a 2-byte integer containing the Y-axis pixel location of the event in detector coordinates, corrected for instrument distortion; the axis runs from 1 to 1280.

10.
GRADE is a 2-byte integer describing the shape of the event, used as a quality indicator. Grade numbers run from 0 to 4.

11.
CCDID is a 2-byte integer identifying the CCD number, which runs from 0 to 3.

Each row has nine two-byte integers, one four byte-integer, and one double precision floating point number (eight bytes), for a total of 30 bytes, equal to the value of the NAXIS1 keyword.

The remaining keywords are grouped. Each group is preceded by a centered descriptive heading; the first eight characters are blanks, and thus the headings are treated as comments.

Under ``General Descriptive Keywords'' are keywords that identify the extension and its content. The name, corresponding to a table title, is EVENTS (EXTNAME keyword). The HDUCLASS keyword identifies the authors of the file format and conventions as OGIP, and HDUCLAS1 the type of data as EVENTS. Examples of other types are SPECTRUM and GTI (Good Time Intervals). The usage of these two keywords follows OGIP/HEASARC convention; HDUCLASS identifies the source of the observation as one of NGC 4151 (OBJECT keyword) with the SIS1 instrument (INSTRUME) of the ASCA telescope (TELESCOP), with the principal investigator being I. N. Jones (OBSERVER). The ORIGIN, CREATOR, PROCVER, SEQNUM, and SEQPNUM keywords provide information on the origin of the file and its processing history. The ``Instrument mode keywords'' provide information on the instrument and data configuration.

The ``Time Related Keywords'', except for the standard reserved DATE keyword that identifies the file creation date as March 20, 1996, give the time of observation. The observation started at 57 seconds after 8:51 P. M. on December 5, 1995 (DATE-OBS, TIME-OBS) and ended at 31 seconds after 4:06 A. M. on December 6, 1993 (DATE-END, TIME-END). Of these keywords, DATE-OBS is a reserved FITS keyword; the others are user-defined keywords but with names chosen by analogy with the reserved keyword. In this case, the DATE-OBS keyword refers to the beginning of the observation, but, as noted in section 3.1.1.2, whether the DATE-OBS keyword refers to the beginning, middle, or end of the observation is not specified by the FITS rules and must be described in the file, as it is here. The start time is also specified in Modified Julian Date form using the widely used MJD-OBS, intended as a standard under the World Coordinates proposal. The MJDREF, TIMEREF, TIMESYS, and TIMEUNIT keywords define the time system and scale used for the TIME column of the main table and for expressing the start and stop time of the observations using the TSTART and TSTOP keywords. The ORBITBEG and ORBITEND keywords identify the orbit numbers of the start and stop of the observation. The other keywords provide information on the elapsed times of observation and on defining the time scales.

The ``Position Related Keywords'' describe coordinate systems. The RADECSYS keyword is the World Coordinates convention keyword defined in section 4.3 and identifies the coordinate system used as the FK5, the system in use since the 1976 IAU; EQUINOX identifies the coordinate system as precessed to the year 2000.000000. The next eight keywords describe the direction in which the instrument points. Right ascension and declination are always given in degrees. The RA_PNT and DEC_PNT keywords are specifically listed among the standard HFWG keywords. The next 14 keywords describe the coordinate systems in which the event positions of columns 3-4 and 8-9 are defined, one of the cases defined in section 5.6.2. Keywords similar to the standard coordinate keywords of section 3.1.1.2 but with one intermediate letter omitted and prefaced by T are used. Columns 3 and 4 describe a position in a tangent projection (section 4.2.2.4) of the sky onto a FITS array. For the X-axis, the right ascension at array position 640.5, that is, equivalent to halfway between 640 and 641 (TCRPX3 keyword), is 182.744003 degrees (TCRVL3), and the right ascension changes by $4.4199999\times 10^{-4}$ degrees for a change of 1 in the array position, decreasing with increasing index number (TCDELT3). For the Y-axis, the declination at array position 640.5 (TCRPX4 keyword) is +39.4291992 degrees (TCRVL4), and changes by $4.4199999\times 10^{-4}$degrees for a change of 1 in the array index, increasing with increasing index number (TCDELT4). Columns 8 and 9 provide positions of the events in a linearized focal plane pixel coordinate system obtained by correcting the raw detector coordinates for instrument distortion. FITS array member (640.5, 640.5) corresponds to (0,0) of the pixel coordinate system (TCRPX8, TCRVL8, TCRPX9, TCRVL9), with the scale on both axes 0.027 mm/unit change in index. The OPTICn keywords provide the array position of the optical axis in both linearized detector coordinates and sky coordinates, and the FOV_X_MM and FOV_Y_MM keywords provide the size of the field of view at the detector, 23 mm $\times$ 23 mm. The FITS conventions and a discussion of the ASCA system are available on separate pages at the HEASARC sites at http://heasarc.gsfc.nasa.gov/ and ftp://legacy.gsfc.nasa.gov/.


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