The data acquired during one Observing Period by the Wide Field Cameras
(WFC1 and WFC2) of the SAX satellite will be received on so-called
Final Observation Tapes (FOT) from the Science Data Centre (SDC).
The specifications of the FOT is laid down in the "Mission Support
Component FOT layout Document" (TPZ/GO.SDC-100/FOT-TN/SAX).
This document describes the first stage of the FOT data-processing for the
WFCs. This includes the extraction of all science data and housekeeping
data plus some general satellite information data into a set of FITS files.
Using WFC instrument and satellite HK parameters filtering and cleaning of
the event data is performed. This stage preceedes the next stage where
the Scientific Analysis is done. (See document T.B.D.)
In the following it is assumed that there exists a top directory WFC_TOP
that holds all relevant subdirectories. Some of the subdirectories may
also a subdirectory of the FTOOLS structure (e.g. saxwfc). That is
accomplished by the use of symbolic links. This dual approach is done
to enable the flexible location of directories.
The flightdata from the WFC instruments are telemetered to the
ground and stored in the Science Data Centre (SDC). The telemetry
consists of a stream of packets containing the data as produced
by the instrument.
The layout and contents of the packets for the WFCs are fully described
in the SAX Wide Field Camera User Manual (SRON document SRU/SAX/PR-15
or ALENIA document SX-MA-AI-019 SAX USER'S MANUAL Volume 11)
At the SDC this data is processed and results in the production of
Final Observation Tapes (FOT) which are shipped to the requesters of
the data either for the WFCs or for the Narrow Filed Cameras (NFI)
Not only the data received when the satellite is locked on the target
(Normal Observation) but also during the time it is slewing from the
previous target (Initial Slew) and the time it is slewing to the next
target (Final Slew) is stored on the FOT.
Here we only discuss the FOT of the WFCs which contains all information
of the Wide Field Camera (WFCs) recorded during one Observing Period (OP).
Although during such a period a fixed part of the sky will be observed
it is very likely to be split up in several Observations mostly resulting
from instrument mode changes by commanding. This may occur as a reaction
on earth eclipses and passes through radiation belts or otherwise.
Each such Observation results in a specific set of data files. Such a set
consists always of the following files:
It is possible that a FOT may comprise of more than one tape. As the
distribution tapes may store 2 Megabytes splitting of FOTs is higly
unlikely to occur.
For a complete specification of the FOT refer to the "Mission Support
Component FOT layout Document" (TPZ/GO.SDC-100/FOT-TN/SAX)
All FITS file formats produced during the Stage-I processing are
described in document T.B.D.
In the past SDC has distributed FOT files both on tape and on DISK.
In the later case the a naming scheme has been followied that is
incompatible with the scheme that is used by the WFC software.
For the purpose of name conversion a script is available to
make symbolic linkfiles. This script is called 'w_vaxlink'.
It will ask a for the FOTID string that has to be used and the
number of the observing period of the normal pointing part
"Users' Guide to FTOOLS" (HEASARC issue of Ftools)
2.0 Stand-alone scripts
To facilitate the processing of FOTs or groups of files belonging to one
or more FOTs a number of scripts are available. These scripts invoke
a chain of programs and take care of passing the correct filenames.
They are also intended for standard batch processing of FOTs.
For locating those scripts a separate 'etc' directory is available
Scripts for running tools in a predefined way are available for UNIX
based operating systems. Currently all scripts are using the C shell
environment by invoking '/bin/csh'.
All scripts recognize the following global shell variables or
a subset if them. If not specified defaults are used.
WFC_TOP topdirectory where the wfc software is located
WFC_ETC directory where scripts and special input files are located.
WFCCAL directory where calibration files are located
FOT_TOP directory where the FOT files are located
TAPE device name of the tape unit
A detailed description of all switches and argument can be obtained by
executing the relevant script with switch '-h'.
w_proc
This is the most general script to do complete or partial StageI
processing. Typical switches are: '-fot' to copy files from tape
to disk; '-fits' to process FOT data into FITS files; '-clean' to
produce filtered files, '-shift' to do event positioning procesing,
'-plot' to produce survey plots (in postscript).
If reading from tape it invokes the 'dd' utility to copy the
data from the FOT onto disk.
This script is most suitable to be used when dispatching one of
the following scripts
w_fits
Extract data from FOT or DISK by executing program 'wfotfits'
Can easier be invoked via w_proc.
w_clean
Create cleaned event and housekeeping files by invoking the
programs 'whkproc', 'wmakgti' and 'wgtiflt'
In addition rate peaks are detectded and the attitude will
be analyzed by programs 'watproc' and 'wattana'
Can easier be invoked via w_proc.
w_shift
Produce positioned corrected event lists by invoking program
'wshift'
Can easier be invoked via w_proc.
w_plot
Produce survey plots by invoking program 'wqplot'
Can easier be invoked via w_proc.
w_slew
Produces a file of accumulated detector counts per timebin.
It may be used to produce flux curves when the instrument
is in slew mode.
w_spv
Process the diagnstic modes (DIR004 and DIR005) by the analysis
software written for the Sensor Performance Verification.
2.1 Usage
The general format of the runline is like:
::: script_name [-switchname [argument]] ..... [-switchname [argument]]
Arguments always have to include a specification of the FOT.
For this the '-f FOTID' is used where the FOTID is the identifying string.
Only in case the files are retrieved from the tape in the same call to
w_proc will by default the identifying string extracted from the first
line of the Tape Directory.
Arguments not recognized by w_proc will be passed on to subsequently
invoked scripts.
Some general used switches used by all scripts are:
-h Display a concise help on the available switches of the script.
It will not show the switches of possible subsequently invoked
scripts.
-vv # Regulates the output volume. If 0 a minimal output will occur.
If 1 it shows the programs that were active and some info on
files produced. Larger value will pruduce more debugging info.
-t @ Defines the directory where the input disk files are located
In case of files first copied from
-d @ defines the destination directory where all subsequent result
files will be stored.
examples:
Copy a FOT onto disk in directory 'FOTDIR' from default tape-unit
::: setenv FOT_TOP /disk2/fotdir
::: w_proc -fot
Process a FOT from tape completely with standard options and
save the output in a logfile
::: w_proc -fot -all -t FOTDIR > logfile
To re-process a selected range (obs 3 thru 5) of observations and store
results in different destination directory
::: w_proc -f FOT_ID -clean -t FOTDIR -d DESTDIR -n1 3 -n2 5
To process all relevant files for SPV test.
So all observations with modes DIR001, DIR004 and DIR005
::: w_spv -f FOT_ID -t FOTDIR
In the future these scripts will be made available under the
HEAD system. This enables a window based invoking of the processing.
2.2 General overview of programs
- Program 'wfotfits' is used to read either directly from the FOT
or from a copy of it on disk. It will extract all the relevant
data to produce a number of FITS format files for further processing.
- The data from the housekeeping files are cleaned and converted to
physical values by program 'hkproc'. Besides that a dead-time
correction file is produced derived from ratemeters in the
science housekeeping, and in addition a ratepeak detection
algorithm is performed and produces a file with bursts that are
been found.
- 'wmakgti' is a general program to create GTI files form any data
in a FITS file. It is used to make GTI files from cleaned
housekeeping data and ephmerides data. It uses input selectyion
files that contain the conditions to mark the accepted times.
- with program 'wmrggti' a group of GTI files can be merged into one
- From the eventdata files (normal mode or high time res. mode)
by program 'wgtiflt' the events not belonging to the GTIs are
filtered. Also it filters out and stored the events coming from
the Inflight Calibration sources. The latter data is used for
the gain correction calculation.
- The attitude data is updated with additional data about observation
mode and auxilliary information about earth centre position by
program 'watproc' The resulting file will be used for analizing
the camera attitude.
- The attitude data is analyzed by program 'wattana' in order to
produce clusters with one Nominal Pointing to be used later for
events repositioning.
- By program 'wshift' the detector positions of Normal Mode eventdata
is shifted to correct for misalignments from the Nominal Pointing.
It produces a NPI fiel to assign the evens to a Nominal Pointing.
- The gain of the detector is analyzed by program 'wgain' from
the events emiited from the In-Flight Calibration Sources.
- Program 'wqplot' produces a general plot per observation showing
the eventrates, the gain and some instrument meters. Additionally
it can produce listings of issued commands and specific status flags.
In order to enable the tracing back of the data flow the parameters that
control the processing options will be stored in all resulting files.
These parameters will be stored as keywords into the Primary Header
of an FITS output file.
The following general parameters are the same for all of these programs.
a. verbsty=integer
<![CDATA[
The number controls the volume of the output to STDOUT
The higher number the more information about the
processing is given. If 0 there will be minimal output;
only signalling the starting end the stopping of the
program and warnings will be output plus additional
information on fatel errors that may occur.]]>
b. title=string<![CDATA[
Option to insert user defined text as a parameter
keyword into the fileheader]]>
c. blockio=boolean<![CDATA[
If 'no' use is made of block reads or writes to or from
the file. The advantage is much faster I/O times
than when using the standard ftools/fitsio routines.
The drawback is that files created in that way may
not be compatible with those on different com[puter
systems and that adding columns in a file needs
modifying all the programs that read or write such
a file.]]>
d. calfile1=string<![CDATA[
Most programs make use of instrument dependent values.]]>
calfile2=string <![CDATA[
For each camera these values are stored and grouped in
a separate Fits calibration file.]]>
2.3 Parameter passing by the ftools convention
A program is invoked in the following form
=> program-name par-value1 ... par-value# [option1] [option2] ... [option#]
where option can either be [option-name] or [option-name=value]
where values may be either a string (with or without quotes), an integer,
a real, a boolean (only yes or no are accepted)
Parameters are position dependent, but can be placed anywhere in the runline
by the [parname=par-value] construct.
resulting filename conventions
layout: ffffff_%%%%w$@zz$.xxxx (= one letter per field)
where:
<![CDATA[
ffffff = Any alpha-numeric string, but normally the FOTnumber of
the tape consisting of 6 letters <br>
%%%% = observation sequence number as unique within one FOT.
'0000' for general FOT data <br>
w# = wfc camera id (either w1 or w2)
'__' for general FOT data <br>
@ = observation mode (either s = slew or p = pointing)<br>
No distinction for 'initial' or 'final' slew and not
relevant for generakl FOT data.<br>
zz$ = instrument/derived contents
hs$ = housekeeping science
hi$ = housekeeping instrument
rn$ = rate normal
rh$ = rate high
ba$ = background analysis
dr$ = diagnostic reduced
de$ = diagnostic extended
rp$ = pseudo-events
ib$ = packet block info
rc$ = calibration source events
pk$ = peak rates
dt$ = dead times
gn$ = gain values
for general files '%%%%w#@zz$' will be any of the following
0000__eph0 = ephemeris data
0000__att0 = attitude data
0000__utc0 = OBT tp UTC converison date
0000__obs0 = observation survey file
0000w#hit0 = IT housekeeping data
additional files
rn$_gti = GTIs for valid NM event data
rh$_gti = GTIs for valid HTR event data
hi0_gti = GTIs for valid Ins. HK event data
hs0_gti = GTIs for valid Sci. HK event data
]]>
It is not advisable to run Stage-II processes on raw data file ($ = 0)
However it is possible to pass cleaned and filtered files to it ($ = 1)
as well as position corrected files ($ = 2)
A full description of all files, their naming and contents can be fiound
in document (TBD) "Description of SAX/WFC files produced by Stage-I"
Programs and I/O
For an exhaustive description of parameters use the ftools help
function. E.g. fhelp wfotfits.
wfotfits:
<![CDATA[
- reads FOT files from disk
- optionally reads files directly from tape
- produces data files in FITS format
- make observations table indicating mode and start/stop times
- converts all times to UTC according to OBT_UTC data
- produce Good Time Intervals of DIR001 and DIR002 data
These GTIs only reflect the time that data was actually received.
- produces info packet blocks
- optionally store pseudo-events separately
In Normal Mode every eight seconds a set of these occur.
(tape/disk)
FOT
|
V
--------
wfotfits
--------
|
|--> *__att0* [attitude data]
|--> *__eph0* [ephemerides data]
|--> *__utc0* [OBT-UTC table]
|--> *__obs0* [observations table]
|--> *w#hit0* [IT diagnostic ITHKD000]
|--> *w#@hi0* [instrument housekeeping HKD000]
|--> *w#@hs0* [science housekeeping ENG000]
|--> *w#@rn0* [normal rate data DIR001]
|--> *w#@rh0* [high rate data DIR002]
|--> *w#@ba0* [background analysis data DIR003]
|--> *w#@dr0* [reduced diagnostic data DIR004]
|--> *w#@de0* [extended diagnostic data DIR005]
|--> *w#@gti* [Good Time Interval files]
|--> *w#@ib0* [block info]
|--> *w#@rp0* [pseudo-event data] (optional)
|--> *w#@rn0.gti [GTI file]
|--> *w#@rh0.gti [GTI file]
|--> *w#obs0.txt (observation survey text]
-
- During one observation no mode change should take place.
In case this might occur the remainder of the file is skipped'
]]>
whkproc:<![CDATA[
- reads housekeeping files and discards invalid records
- converts temperature monitors to physical values
- converts voltage monitors to physical values
- Does a burstpeak hunt on science ratemeters
- Produces deadtime values per second from science ratemeters
*w#@hi0* --->|
*w#@hs0* --->|
V
-------
whkproc
-------
|
|---> *w#@hi1* [cleaned + converted Ins-HK data]
|---> *w#@hs1* [cleaned Sci-HK data]
|---> *w#@dt1* [deadtime data]
|---> *w#@pk1* [ratepeak data]
-
]]>
wmakgti:
<![CDATA[
- produces Good-Time-Interval tables from selection criteria (GTI)
*w#@hi1* --->|
*w#@hs1* --->|
ihk_sel ---->|
shk_sel ---->|
eph_sel ---->|
V
-------
wmakgti
-------
|
|---> *w#@hi1.gti TEMPORARY
|---> *w#@hs1.gti TEMPORARY
-
]]>
wmrggti:
<![CDATA[
- merges several GTI files into one
- The reulting GTI files will be used to calculate the integrated
time (ONTIME) that cleaned data was received.
*__eph0_gti* --->|
*w#@rn0_gti* --->|
*w#@rh0_gti* --->|
*w#@hi1_gti* --->|
*w#@hs1_gti* --->|
V
-------
wmrggti
-------
|
|---> *w#@gti*
-
]]>
wgtiflt:
<![CDATA[
- filters events files according the GTI file
- removes pseudo-events
- extracts events belonging to the In-Flight Calibration sources (IFC)
from Normal Mode events.
*w#@rn0* ------>|
*w#@rh0* ------>|
*w#@rn0.gti --->|
*w#@rh0.gti --->|
V
-------
wgtiflt
-------
|
|---> *w#@rn1* [filtered Normal Rate events]
|---> *w#@rc1* [IFC events]
|---> *w#@rh1* [filtered High Rate events]
-
]]>
watproc:
<![CDATA[
- Make separate attitude file for each camera
- Add Earth Centre in WFC coordinates (x,y,z)
- Add observation mode information to attitude data
*__att0* --->|
*__eph0* --->|
*__obs0* --->|
V
-------
watproc
-------
|
|---> *w1att1* [augmented attitude data]
|---> *w2att1* [augmented attitude data]
-
]]>
wattana:
<![CDATA[
- Analyzes one or more attitude files
- Produce one list of nominal-pointing sets (NP) from actual pointings
*w#att1* --->|
V
-------
wattana
--------
|
|---> *__pnt1* [pointings table]
-
]]>
wshift:
<![CDATA[
- Shift the event positions according to the NP-sets
- Group the events per NP
*w#@rn1* --->|
*__pnt1* --->|
V
-------
wshift
-------
|
|---> *w#@rn2* [event file for stage-II]
-
]]>
wgain:
<![CDATA[
- Compute
*w#@rc1* --->|
*__obs0* --->|
V
-------
watproc
-------
|
|---> *w#@gn2* [augmented attitude data]
-
]]>
wslew:
<![CDATA[
- produce per time bin number of events and actual pointing
*w#srn1* --->|
*w#srh1* --->|
*w#att1* --->|
V
-------
wslew
-------
|
|--> *w#ssn2* [event file for stage-II]
|--> *w#ssh2* [event file for stage-II]
-
]]>
wqplot:
<![CDATA[
- Produce plots with instrument/eventrate/attitude behaviour
*w#@hi1* --->|
*w#@hs1* --->|
*w#@gn2* --->|
*w#pnt2* --->|
*__obs0* --->|
V
-------
wqplot
-------
|
|---> *w#@plt.ps [PostScript plotfile]
-
]]>
Transfer of files to STAGE-II processing
<![CDATA[
- Produce sky images and light-curves
*w#@rn2* --->|
*w#@rh2* --->|
*w#@dt1* --->|
*w#@pk1* --->|
*w#@gn1* --->|
*w#att1* ---<|
*__obs0* --->|
V
--------
Stage-II
--------
]]>
3. General topics
<![CDATA[
This chapter addresses a number of issues and methods used
Of mentioned program only the generallfunction is described.
For details on the options for running these programs refer
to the individual help texts
]]>
3.1 TIME ASSIGNMENT
<![CDATA[
When the instrument sends a packet to the telemetry it will always
contain a time stamp. This timestamp is derived from the spacecraft
clock pulse. The resolution of the spacecraft clock is 2^-16 seconds.
(= 15 microsec.) In the WFC's, however, the highest used resolution
is 2^-12 (1/4096) seconds. The time resolutions are:
DIR001 Normal mode 2^-11 = 488 microsec.
DIR002 High Time Resolution mode 2^-12 = 244 microsec.
DIR003 Background Analysis mode 2^-10 = 977 microsec.
DIR004 Diagnostic Reduced mode 2^-12 = 244 microsec.
DIR005 Diagnostic Extended mode 2^-12 = 244 microsec.
All other modes have a time resolution of 1 second only.
In the FITS files produced by Stage-I times can occur as keyworods in
headers and as columns in the rows of the data part.
As a general rule in Stage-I all times of the eventfiles are the
clock times in 2^-12 ticks. This does not reflect the actual time
resolution. All times in the headers are in MJD
]]>
3.1.1 Dates and times in files
<![CDATA[
Times in FITS files may be represented in several ways:
a. Directly as a Modified Julian Day number and fraction (MJD)
It will always be represented in Double Precision (REAL*8) format.
allowing a precision of 15.95 significant decimal digits.
With an MJD daynumber being around 50000.0 this allows a maximam
resolution of about 10^-6 seconds (1 microsecond).
The time column will have the name 'TIME' in this case
b. Indirectly as an integral number of time-units relative to a
certain reference date. In Stage-I processing all of these timea
will be scaled to units of 2^-12 (= 2.44140625E-04) seconds and be
stored in Integer (Integer*4) format. The reference time is given
as in Modified Julian Days as with (a.).
FITS files that have such a time-columns will contain the
following keywords:
MJDREF / MJD of S/C clock = zero
T_SCALE / scale factor of times (seconds)
The time column will be nemaed
WFC_TIME / Space Craft clock time of the WFC
So if the given relative time is TIME_WFC then the actual times in
MJDs will be: MJDREF + (TIME_WFC * T_SCALE / 86400)
c. A combination of a. and b. If only MJDREF is specified in
the priamary header time in MJD will be
MJDREF + TIME
IF T_SCALE is also specified time on MJD will be:
MJDREF + TIME * T_SCALE / 86400.
d. Instrument On-Board time ticks. One tick is defined as 2^-16
second. These ticks will be stored only for the start or end of
observations. Relevant keywords are
SCSEQBEG= spacecraft clock begin (ticks)
SCSEQEND= spacecraft clock end (ticks)
e. As time periods they will be stored in seconds
Used in headers with keywords:
TELAPSE (elapse time from start to end in seconds)
ONTIME (Total uncorrected integrated time ion seconds)
f. In Calendar days and times are stored in character string keywords
and have the form 'dd/mm/yy' (all numerics) and 'hh:mm:ss' or
'hh:mm:ss.sss' Used keywords are DATE-OBS, TIME-OBS, DATE-END and
TIME-END.
]]>
3.1.2 External time referencing
<![CDATA[
Neither the science data nor the housekeeping data coming from the
instrument contains actual date and time stamps. Only the On-Board Time
in units of 2^-16 ticks are available and stored in the telemetry packets.
In order to enable the matching of these tick-times to actual dates
a special reference file will be present on each FOT containing an
OBT to UTC conversion table. This file is used to calculate a
reference time (MJDREF) for each new observation.
To obtain the reference date the instrument time is matched with
the latest OBT that is preceeding the instrument time (ticks) so that
the reference can be calculated as follows:
if SCSEQBEG contains the ticks of the first event and
OBT is the closest time in the OBT_UTC files, then
MJDREF = UTC - OBT / ( 2^-16 * 86400)
where UTC is the time corresponding to the OBT value
The following problems are provided for:
a. In case the time is falling outside the provided conversion OBTs
a warning is issued. Then depending which is closest to the
instrumemnt time either the first or the last OBT is taken to
calculate the reference data (MJDREF).
b. The OBT counter may scroll though zero during the observation.
This will happen every 2^16 (65535) seconds or 18.2 hours.
This can be easily solved as during one Observing Period the
starttimes within one observation must not occur before the endtimes
of previous observations. Only the first one will take the
earliest OBT match.
]]>
3.2 DATA CLEANING
<![CDATA[
Cleaning of data files is used here a very broad meaning.
During this cleaning stage the following functions are performed:
- conversion of instrument housekeeping values to physical units
for all temeratures, voltages.
- Testing and rejecting all data that have obvious defects like
a-sequential times, monitoring meters out of limits or unstable
High Voltage after instrument state switching.
- extracting calibration (IFC events) data from science data.
- filtering instrument monitoring data (Pseudo-events) from science data.
The Housekeeping files are cleaned by program 'whkproc' which does the
value conversions and the bad time rejections. The resulting data files
are used for making Good Time Interval files by program 'wmakgti', which
uses a selection criteria file containing the conditions for accepting
good data. Presently on three different data streams GTI files are
selected.
a. Selection criteria on Instrument Housekeeping which are laid down
in an expression file (wfc_sel_ins) containing one single line
b. Selection criteria on Science Housekeeping which are laid down
in an expression file (wfc_sel_sci) containing one single line
c. Selection criteria on Ephemeris which are laid down in an expression
file (wfc_sel_eph) containing one single line.
Program 'wmakgti' can read the selection criteria either as a parameter
argument or as a line contained ia a textfile.
Merging of all or a set of GTI files is done by program 'wmrggti'.
Here is flow of the cleaning with standard processing:
FOT
|
V
==========
wfotfits
==========
|
|
|-->FOTID1_%%%%w#@hi0 ---------------------------------------
| |
|-->FOTID1_%%%%w#@hs0 --------------- |
| | |
|-->FOTID1_0000__eph0 V V
| | ========= =========
| | whkproc whkproc
| | ========= =========
| | | |
| | V V
| | FOTID1_%%%%w#hi1 FOTID1_%%%%w#hs1
| | | |
| V V V
| ========= ========= =========
| wmakgti wmakgti wmakgti
| ========= ========= =========
| | | |
| V V V
| FOTID1_0000__eph0gti FOTID1_%%%%w#hi1gti FOTID1_%%%%w#hs1gti
| | | |
| |--------------------- | -------------------
| | | | |
| | V V V
| V =========
| V wmrggti
| V =========
| |
| V
| FOTID1_%%%%w#gti
| |
|-->FOTID1_%%%%w#@r@0 ----------- |
| | |
| V V
V =========
V wgtiflt
V =========
| \
| ---->FOTID1_%%%%w#@ic1 -----
V |
FOTID1_%%%%w#@r@1 V
| =============
V gain
============ calculation
shift =============
processing
============
(where FOTID_%%%%w#@r$0 = Any camera (w1 or w2) and either normal or slew
observing mode (pr or sr) and either Normal mode (rn) or Hight Time
Resolution mode (rh).
]]>
3.3 ATTITUDE PROCESSING
<![CDATA[
Reconstruction of the sky from the detector image can only be done if
during the recording of the photons the pointing of the instrument is
within narrow bounds. The satellite is guaranteed to maintain its
pointing stability within an error of 1 arcminute and the absolute
pointing error is guaranteed to be exact within 3 arcminutes.
For the WFC having a field of view of about 40 degrees the error
in absolute pointing is not very critical. In order to prepare for
larger instability than expected or allowed the following three
programs have been implemented:
a. Program 'watproc' copies the raw attitude file for each instrument
separately and adds information about Observing Mode (Normal/Slew)
plus the position of the Earth w.r.t. the camera. From the Ephemeris
data that gives the location of the satellite is calculated the centre
of the Earth in the camara coordinates km in X, Y and Z).
b. Program 'wattana' analyzes the attitude data and establishes a number
of clusters so that actual pointings belonging to one cluster are within
the permitted maximum limits from the center or nominal pointing of
that group. Such a nominal pointing is characterized by Right Ascension,
Declination and North Angle. For each of these a maximum range can
be specified, both for Normal Pointing or Slewing Operational Mode.
The result will be a file that contaions the list of all calculated
Nominal Pointings. In the case of ideal stability it shall of course
have only one. It is envisaged to run this analysis standardly over the
complete Observing Period (one FOT).
c. Program 'wshift' will shift the position of each individual photon event
over the detector to the position that is would have had if the detector
had positioned to a reference pointing. The reference pointing will be the
closest from a set of Nominal Pointings as calculated by 'wattana'. As the
shift is done of a part of the Observation Period only a subset of the
Nominal Pointings will be used.
A file containing the Time Intervals per Nominal Pointing is produced in
order to enable the separate processing the of photon events of one Nominal
Pointing at a time as is required by the correlation programs of Stage-II
proceswsing which can operate only on one accumulated detector image.
The layout of such a Nominal Pointing Interval (NPI) file is identical to
those of the Good Time Interval (GTI) files, but with an extra column
containing the number of the Nominal Pointing.
If only one NP is calculated and the range of the photon detector
positions is very small it is possible to skip the shifting of the events
altogether and pass the event datafile directly to Stage-II processing.
Here is flow of the standard attitude processing of one single FOT:
FOT
|
V
==========
wfotfits
==========
|
|
|-->FOTID1_0000__att0 ------
| |
|-->FOTID1_0000__obs0 ---- |
| | |
|-->FOTID1_0000__eph0 -- | |
| | | |
| V V V
| =========
| watproc
| =========
| |
| |
| |-->FOTID1_0000w1att1 ----------->>>
| |
| |-->FOTID1_0000w2att1 ----+------>>>
| | | |
| - | V
| | =========
| | wattana
| | =========
| { |
| | V
| | FOTID1_0000w2pnt1
|-->FOTID1_####w2prn0 -- | |
| | | |
| | | ----
| V ----- |
| ============ | |
V filter | |
V processing | |
V ============ | |
| V V
V ========
FOTID1_%%%%w2prn1 -------> wshift
| ========
| | |
| | ---------------
| | |
| V V
| FOTID1_%%%%w2prn2 FOTID1_%%%%w2npi2
| | |
| | |
| V |
| ============ |
-------------> Stage-II <-----------
processing
============
Here is flow of the attitude processing of several FOTs (having same target):
FOT
|
V
==========
wfotfits
==========
|
| | |
|-->FOTID1_0000__att0 ------ | |
| | | |
|-->FOTID1_0000__obs0 ---- | | |
| | | | V
|-->FOTID1_0000__eph0 -- | | | FOTID3_0000w#att1
| | | | V |
| V V V FOTID2_0000w#att1 |
| ========= | |
| watproc | |
| ========= | |
| | -------------- |
| | | |
| |-->FOTID1_0000w1att1 --->>> | |
| | | |
| |-->FOTID1_0000w2att1 -------- | |
| | | | | |
| - | V V V
| | =========
| | wattana
| | =========
| | |
| | V
| | MERGED_0000__pnt1
|-->FOTID1_####w2prn0 -- | |
| | | |
| | | -----------
| V ----- |
| ============ | |
V filter | |
V processing | |
V ============ | |
| V V
V ========
FOTID1_%%%%w2prn1 ------> wshift
| ========
| | |
| | ---------------
| | |
| V V
| FOTID1_%%%%w2prn2 FOTID1_%%%%w2npi2
| | |
| | |
| V |
| ============ |
-----------> Stage-II <----------
processing
============
]]>
3.3.1 SATELLITE AND INSTRUMENT AXES
<![CDATA[
The orientation of the satellite determines the orientation of each
individual camera with respect to the sky. The sky coordinate of the
sighting axis (Z-axis) of an instrument is called its pointing.
Use is made of the following right-handed coordinate system:
+Y +X
| |
| |
|_____+X === |_____+Y
/ /
/ /
+Z <= towards eye of reader => -Z
In this system the North-Angle is defined as the the angle to turn
anti-clockwise the +X onto the plane through the North Pole and the
Z while having the +Z axis (= sky pointing) in your back.
Note, that in this definition the NA will be undefined if +Z points
exactly at any of the two poles.
For the instruments the following definitions are in force:
for each camera:
Z = the heart axis through the instrument with the detector at -Z
and +Z at the mask side pointing to the observed sky
Y = the side of the detector perpendicular to the groundplate with
the groundplate at -Y and the other side of the instrument at +Y
X = the side of the detector parallel at and closest to the
instrument groundplate with +X going left when looking from
outside the instrument to the backside of the detector
WFC-1: X = - SAT Z
Y = - SAT X
Z = - SAT Y = instrument pointing
WFC-2: X = SAT Z
Y = - SAT X
Z = + SAT Y = instrument pointing
(checked 19/11/1996)
As each axis is given by its Right Ascension and its Declination
the following pointing relations exist:
WFC1: SAX:
----- -----
ra_x = RA_Z
dec_x = DEC_Z
ra_y = 180 + RA_X
dec_y = - DEC_X
ra_z = 180 + RA_Y = RA_PNT
dec_z = - RAS_Y = DEC_PNT
and
WFC2: SAX:
----- -----
ra_x = 180 + RA_Z
dec_x = - DEC_Z
ra_y = 180 + RA_X
dec_y = - DEC_X
ra_z = RA_Y = RA_PNT
dec_z = DEC_Y = DEC_PNT
It is envisaged that the instrument alingment on the satellite is not
exact. So the difference between the nominal and the actual pointing
of each instrument is defined by the following three parameters:
THETA the angle between the nominal Y-axis and the actual Z-axis
minus 90. (nomally 0 degrees)
PHI the angle between the plane going through nominal Y-axis
and the nominal Z-axis and the one going through the nominal
Y-axis and the actual Z-axis (nomally 0 degrees) turning
clockwise with +Y in the back
ROLL the angle between the plane going through nominal Z-axis
and the nominal Y-axis and the one going through the nominal
Z-axis and the actual Y-axis (nomally 0 degrees)
These values will be stored and updated in the detector calibration file
(wfc#.det) as part of the data extension named 'align'.
]]>
SAX/WFC ORIENTATION from INSTRUMENT DIRECTORY on FOT
<![CDATA[
R.A., Dec. and Roll are taken from file 'w2instdir_000000_00_0.dat'
WFC2 pointing axis is the same as the Satellite +Y axis.
RA_WFC1 = 180 + R.A. , DEC_WFC1 = -1 * Dec. , NA_WFC1 = 270 - Roll
RA_WFC2 = R.A. , DEC_WFC2 = Dec. , NA_WFC2 = Roll - 90
always valid: NA_WFC1 + NA_WFC2 = 180 modulo 360
]]>
GENERAL DEFINITION NORTH-ANGLE (NA)
<![CDATA[
Valid for an imaging instrument with its own X and Y axes
The angle to turn the +X axis of the SKY-IMAGE anti-clockwise
towards the North Pole, with the observer looking to the sky and
having the detector behind his back.
if Zsat = -Xwfc1 = North then NAwfc1 = 180 and roll = 90
if Xsat = -Ywfc1 = North then NAwfc1 = 270 and roll = 0
if Ysat = -Zwfc1 = North then NAwfc1 = undefined and roll = undefined
if Zsat = Xwfc2 = North then NAwfc2 = 0 and roll = 90
if Xsat = -Ywfc2 = North then NAwfc2 = 270 and roll = 0
if Ysat = Zwfc2 = North then NAwfc2 = undefined and roll = undefined
From the above it can be seen that
NAwfc1 = 270 - ROLL(sat)
NAwfc2 = ROLL(sat) - 90
NOTE:
In a previous definition the X-axis of the instrument was taken to
be the X-axis of the DETECTOR. For orientation of the sky-image,
however, the X-axis of the IMAGE is pointing in the opposite direction.
So, Xwfc(sky) = -Xwfc(det)
]]>
3.4 DEAD-TIME CALCULATION
<![CDATA[
In order to enable the correction of photon fluxes a file is produced
containing the deadtimes per specified period (minimum 1 second).
The calculation is performed by program 'whkproc' from the Science
Housekeeping data.
The MAIN trigger ratemeter counts the number of events which triggers the
low level discriminator of the Main A nnode section of detector.
The LOGIC trigger ratemeter counts the number of events which pass the
first validation check in the Digital Frontend and triggers the Event
Processor to transfer the event data into the Event Processor.
During the transfer process the ratemeter is stopped and also if for
any reason buffers temporarily cannot be sent to the telemetry.
This allows the calculation of the dead-time of the instrumant.
Both meters are reset to zero every second.
Therefore the deadtime = ( RATE_MAIN - RATE_LOGIC ) / RATE_MAIN
and the actual_flux = measured_flux / ( 1 - deadtime )
The deadtime is stored in a FITS file together with the time in Modified
Julian Day plus the accumulation or binning time in seconds
]]>
3.5 RATE PEAK DETECTION
<![CDATA[
Finding peaks in the detector countrate is performed by program 'whkproc'.
Per detected peak a row is stored in a FITS file. The information stored
is risetime and falltime of the peak, the countrate at the peak maximum,
and the total counts of the peak above the running average.
The algorithm for finding a peak is based on the count rate of the
input event processor. This countrate is available every second in the
Science Housekeeping data. The FITS keyword for the column containing
these values is 'RATE_EVENT'. At any point the counts of a fixed number
of seconds (currently 50) are buffered and used to find a peak. A peak
is accepted if the distance to the falltime of a previous peak is at least
a predetermined number of seconds (presently 3) and if for at most a fixed
number of succesive counts (presently 24) counts are above the buffer
average by at least a fixed number (presently 100)
]]>
3.6 GAIN CALCULATION
<![CDATA[
For establishing the detector gain or spectral response use is made of
the events that are produced by the In-Flight Calibration radiation
sources as detected during the Normal Mode operation of the WFCs.
Extracting the IFC events is done by program 'wgtiflt'. There are nine
Fe-55 radiation sources of which eight are emitting about 0.2 photons per
second and 1 producing 10 photons per second plus one Cd-109 soiurce
emitting about 1 photon per second.
The detector positions and boxes from where the photon events are
taken are stored and updated in the detector calibration file (wfc#.det)
as part of the data extension named 'califc'.
Calculating the detector gain is done by program 'wgain'. Per specified
number of seconds the spectrum of the energy bands are accumulated and
analyzed for finding peaks in two different regions covering respectively
the Fe-55 (channels 7 thru 19) and Cd-109 (channels 24-31). The channel
peaks are converted to KeV to compare with the nominal energy peaks at
5.98 and 22.6 KeV respectively.
The channel to energy conversion values are stored and updated in the
detector calibration file (wfc#.det) as part of the data extension
named 'phakev'.
]]>
Questions and comments can be addressed to
helpdesk@sax.sdc.asi.it