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CalFUSE keeps the data in the form of a
time-tagged photon event list until spectral extraction. For histogram data, a photon event list
is also created but all events are tagged with the same time value. This event list
is stored in the Intermediate Data File (IDF; see also Section 220.127.116.11).
IDFs are FITS files with four Header Data Units (HDU). Below is a description of the contents of each HDU.
HDU1 only contains a full header for TTAG data.
However, for HIST data, HDU1 will also contain the SIA table used for
HDU2 contains several arrays. Their dimensions are
determined by either the total number of events detected (TTAG data), or
the number of pixels containing an event (HIST data). The specific arrays
- For TTAG data, these are the arrival times copied
directly from the raw data file. For HIST
data, the arrival times are all equal and set to the mid-point of the
- XRAW and YRAW:
- For TTAG data, these are copied
directly from the raw data file. For HIST data, each non-zero pixel of a
HIST image is converted into a single entry in the IDF, with XRAW and YRAW
equal to the mean pixel coordinates of the binned image. This results in
a "de-binned" image that exhibits a striped pattern since most rows in the
de-binned raw image are empty.
- This is the Pulse Height Amplitude. For TTAG data, the
pulse height of each photon event is copied from the raw file to the IDF.
Values range from 0 to 31. A typical pulse-height
distribution has a peak at low values due to the intrinsic detector
background, a Gaussian-like peak near the middle of the range due to
real photons, and a tail of high pulse-height events. One can improve
the signal-to-noise ratio of faint targets
by rejecting photon events with extreme pulse-height values. Pulse-height
limits are defined for each detector segment and saved in the main IDF FITS
header as PHALOW and PHAHIGH. The photon events with pulse heights outside
of the nominal range are flagged by setting the appropriate bit in the
LOC_FLGS array (below). None of this information is available for HIST data.
Thus, for HIST data each entry of the PHA is arbitrarily set to 20.
- The photon weight array for TTAG data is initialized
to unity. Photons whose
X and Y coordinates place them outside of the
active region of the detector are flagged and their weights are set to 0.
For HIST data, the WEIGHT array for a pixel is initialized to the number of
photon events in the pixel. The weights are later scaled to correct for detector
dead time (Dixon et al., 2007).
- XFARF and YFARF:
- These are the coordinate arrays corrected to
the FARF (the Flight Alignment Reference Frame), which represent the
output of an ideal detector, corrected for geometric distortions.
- X and Y:
- These are the FARF coordinates corrected for mirror,
grating, and spacecraft motions. These give the coordinates of an ideal
detector, at rest with respect to the object.
- This array lists the numerical code for the channel
assigned to the photon (see Table 4.10). The
assignment is done on the basis of the location of the event on the
detector and of the boundaries of the detectors defined by the
active image masks.
- The screening routines use information from the timeline table (HDU4
below) to identify photons that violate pulse-height limits, limb-angle
constraints, etc., Bad photons are not deleted from the IDF, but
merely flagged. Flags are stored as single bits in an 8-bit byte (see
Table 4.11). This set of flags indicates
time-dependent (orbital) effects. For each bit, a value of 0 indicates that the
photon is good, except for the day/night flag, for which 0 = night and
1 = day. It is possible to modify these flags in the IDF file without
re-running the pipeline. For example, one could exclude day-time photons
or include data taken close to the earth limb.
- This set of flags indicates detector-location dependent
effects (see Table 4.12). They are similar to the TIMEFLGS.
- The LAMBDA array contains the heliocentric wavelength given to each
photon assigned by CalFUSE.
- The ERGCM2 array records energy density in units of erg cm-2.
ERGCM2 = WEIGHT × hc / LAMBDA / Aeff(λ),
To convert an extracted spectrum to units of flux, one must divide by the
exposure time and the width of an output spectral bin.
X and Y coordinates are written to the IDF as arrays of 8-bit integers using the
FITS TZERO and TSCALE keywords. This process effectively rounds each
element of XFARF and
X to the nearest 0.25 of a detector pixel and each element of YFARF and Y to the nearest 0.1 of a detector pixel.
HDU3 gives the good-time intervals (GTIs), stored as
two arrays, START and STOP. For TTAG data, the initial values
are copied from the raw data file, but they are modified by CalFUSE by
various screening routines. By convention, the START value of each
GTI corresponds to the arrival time of the first photon in that interval.
The STOP value is the arrival time of the last photon in that
interval plus one second. The length of the GTI is thus STOP —
START. For HIST data, a single GTI is generated with START =
0 and STOP = the exposure time.
HDU4 is called the timeline table. It contains several
arrays composed of status flags and spacecraft and detector parameters used
by the pipeline. The size of the arrays are set by the length of the
exposure, with an entry created by CalFUSE for each second of the exposure.
- For TTAG data, the first entry of this array
corresponds to the time of the first photon event, and the final entry to
the time of the final photon event plus one second. (Should an exposure's
photon-arrival times exceed 55 ks, timeline entries are only created for
each second in the good-time intervals.)
For histogram data, the first element of this
array is set to zero and the final element to the exposure duration
computed by OPUS +1. Because exposure time is required to be equal to both
Σ (STOP — START) summed over all entries in the GTI table, and
the number of good times in the timeline table, the final
second of each GTI is flagged as bad. No photons are associated with the STOP time of
- Only the day/night and OPUS flags of this array are populated
when the IDF is created; the other flags are set by the various CalFUSE
- TIME_SUNRISE, TIME_SUNSET, LIMB_ANGLE, LONGITUDE, LATITUDE,
- These arrays are computed by CalFUSE from the orbital
elements of the spacecraft.
- This array is populated with values from the
time-engineering (housekeeping) file (Section 5.2).
- LIF_CNT_RATE, SIC_CNT_RATE:
- For TTAG data, these arrays
give the count rates within the target aperture, excluding regions
contaminated by airglow. For HIST data, they are the dead time corrected
values of the LiF and SiC counter arrays derived from the fields I_DETC[SIC][A]
in the housekeeping files. The bracketed quantities
can be 1 or 2, SIC or LIF, and A or B, respectively, corresponding to the
- gives the Fast Event Counter (FEC) Rates.
These are derived from the
fields I_DETCFE[A], given in the housekeeping file. The bracketed
quantities can be 1 or 2, A or B, respectively.
- gives the Active Image
Counter (AIC) count rates. These are the dead time corrected values of the
fields I_DETCAI[A] in the housekeeping file. The bracketed quantities
can be 1 or 2, A or B, respectively.
- For TTAG data, this array is
the count rate in pre-defined background regions of the detector,
excluding airglow features. The array is not populated for HIST data.
- YCENT_LIF, YCENT_SIC:
- For TTAG data, these arrays trace
the centroid of the target spectra with time
before motion corrections are applied. These two arrays are not
used by the pipeline, and they are not populated for HIST data.
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