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XMM-Newton Science Analysis System Page: 1
binned att
July 9, 2007
Abstract
binned att is a library which contains the essential routines for constructing,
writing and reading a binned-attitude time series.
version (from VERSION ): 1.3.1
1 Description
X-ray or optical photons are detected as `events' on the CCD cameras of XMM-Newton. Before one
can do some science with these events, it is necessary to determine where in the sky each event comes
from. This task is complicated by the fact the the attitude (ie, pointing direction and roll angle) of
the XMM-Newton spacecraft does not stay exactly constant during the course of an exposure. Given a
suфciently detailed record of this wandering of the attitude it is of course possible to compute the sky
origin of each individual event, but it is much more eфcient in computational terms to approximate this
notional continuous record by a discrete series of N attitude samples. Each sample is separated from
its neighbours by time boundaries: it is sensible to choose the boundaries such that the attitude wander
within a given pair of boundaries is restricted to within a preset acceptable range. Events falling between
boundaries i and i + 1 are therefore assumed to be associated with attitude i + 1, and their sky position
determined accordingly.
Matters become a little more complicated if it is desired to create maps in sky coordinates which derive
from things other than the simple occurrence of events. For example, a map of the relative exposure
of the camera at each point in the sky. For this one needs additionally to associate a weight with each
attitude sample. One can therefore perform N transforms, one per attitude sample, of the exposure map
from CCD coordinates to sky coordinates, weight them accordingly and then simply sum them together.
The present library therefore denes the concept of a series of N attitude samples, with optional weights
associated, separated by N 1 time boundaries. The library provides routines for calculating the series
of attitude samples from a supplied ne-detail record of the attitude wander, for calculating weights from
GTIs, and also for reading and writing such an attitude series to or from a FITS table.
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The attitude series is implemented in f90 as a variable type with the following structure:
type :: attBinType
type(SpacecraftAttitudeType) ::& ! type defined in module caloaldefs
att
real(double) ::&
binStartTime,&
gtiDuration
logical ::&
isGood
end type attBinType
Some points:
 A series of attitude samples is represented within this library as a vector of type `attBinType'.
In this situation the value of `binStartTime' of the rst element of the vector is ignored by
convention.
 The name `gtiDuration' for the weight is just because of historical inertia and may be
changed to the more general term at some later time.
 The eld `isGood' is included so as to be able to label bad attitudes independently of a GTI
schema.
The FITS implementation of the series of N attitude samples is a little dierent, in that only elements
2 to N are stored in a table, the attitude and weight (not the start time, which is undened) of the rst
element being stored in keywords of the table header. The details of the convention are as follows. The
table should contain the following header keywords:
 (mandatory)(double) RA FIRST, DECFIRST and PA FIRST: respectively RA, dec and position
angle (all in decimal degrees) of the rst attitude in the series.
 (mandatory)(boolean) FIRST OK: indicates whether the rst attitude is `good' or not.
 (optional)(double) WGTFIRST: the weight of the rst attitude sample. If this keyword is
present, the corresponding column in the table must also be present; if the keyword is not
present, any WEIGHTS column is ignored and all weights are assumed to be 1.
 (mandatory)(double) RA PNT, DEC PNT and PA PNT: average or nominal attitude, in decimal
degrees.
 (mandatory)(string) AVRG PNT: meaning of the ` PNT' values, eg mean or median.
 (mandatory)(double) RA DELTA, DEC DELTA, PA DELTA: the attitude-wander limits discussed
in section ?? of the documentation for task attbin. These are in decimal degrees.
The required structure of the table itself is best described by a chunk of code from the library:
time => real64Data(addColumn(binnedAttTab, 'TSTART', REAL64, 's'))
raDeg => real64Data(addColumn(binnedAttTab, 'RA', REAL64, 'deg'))
decDeg => real64Data(addColumn(binnedAttTab, 'DEC', REAL64, 'deg'))
paDeg => real64Data(addColumn(binnedAttTab, 'PA', REAL64, 'deg'))
isGood => boolData( addColumn(binnedAttTab, 'IS_GOOD', BOOLEAN))
weights => real64Data(addColumn(binnedAttTab, 'WEIGHTS', REAL64))
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Of these, only the column WEIGHTS is optional (if the keyword WGTFIRST is not present in the table
header). If it is not present, all weights are assumed to equal 1.
It should be noted that this binned-attitude `time series' does NOT conform to OGIP recommendations.
However in my view this is a deciency in OGIP, which should be expanded to allow the edges of time
bins to be recorded, not just the bin centre times.
1.1 binUpAttitudeFromOdf
The algorithm for constructing the series of attitude samples is to sample the attitude at a number of
relatively nely-spaced times, and keep only those samples for which the dierence from the last `kept'
sample exceeds preset limits. The present subroutine accepts a time sample and the prescribed limits
from the caller, obtains (and stores) the spacecraft attitude at that time value, and returns this to the
caller if its displacement from the last `kept' sample exceeds the limits. The latter circumstance signals
the start of a new attitude `bin'. A new bin should also start if the sampled attitude is not `good', or is
again `good' after a period of `bad' attitudes.
subroutine binUpAttitudeFromOdf(time, maxDelta, attInBin, attIsGood&
, attIsNew)
real(double), intent(in) :: time
type(SpacecraftAttitudeType), intent(in) :: maxDelta
type(SpacecraftAttitudeType), intent(out) :: attInBin
logical, intent(out) :: attIsGood, attIsNew
end subroutine
1.2 sampleBinnedAttitude
subroutine sampleBinnedAttitude(binnedAtt, time, outAtt, attIsGood&
, attIsNew)
type(attBinType), intent(in) :: binnedAtt(:)
real(double), intent(in) :: time
type(SpacecraftAttitudeType), intent(out) :: outAtt
logical, intent(out) :: attIsGood, attIsNew
end subroutine
1.3 writeBinnedAttitude
subroutine writeBinnedAttitude(binnedAttSetName, binnedAttTabName&
, binnedAtt, pntDeg, maxDelta, pntStyle, writeWeightColumn)
character(*), intent(in) :: binnedAttSetName,&
binnedAttTabName
type(attBinType), intent(in) :: binnedAtt(:)
type(SpacecraftAttitudeType),&
intent(in) :: pntDeg, maxDelta
character(*), intent(in) :: pntStyle
logical, intent(in), optional :: writeWeightColumn
end subroutine
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1.4 readBinnedAttitude
subroutine readBinnedAttitude(binnedAttSetName, binnedAttTabName&
, binnedAtt, noWeightColumn)
character(*), intent(in) :: binnedAttSetName,&
binnedAttTabName
type(attBinType), pointer :: binnedAtt(:)
logical, intent(out), optional :: noWeightColumn
end subroutine
1.5 gtiWeightBinnedAttitude
This calculates weights from the amount of GTI within each attitude bin.
subroutine gtiWeightBinnedAttitude(gtis, binnedAtt)
type(IntervalT), intent(in) :: gtis(:)
type(attBinType), intent(inout) :: binnedAtt(:)
end subroutine
References
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