NASSP Masters 5003F - Computational Astronomy - 2009 Lecture 21 • 6.4 keV Fe line and the Kerr metric • Satellite observatories – Attitude and boresight • X-ray instrumentation and calibration (mostly XMM). • (XMM users’ guide: – http://xmm.esac.esa.int/external/ xmm_user_ support/documentation/uhb/ index.html)
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NASSP Masters 5003F - Computational Astronomy - 2009 Lecture 21 6.4 keV Fe line and the Kerr metric Satellite observatories –Attitude and boresight X-ray.
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Attitude• The orientation of the spacecraft in the sky is
called its attitude.• It isn’t just the direction it points to, attitude
specifies the roll angle as well.– One way to define this is via a pointing vector and a
parallactic angle.• However, as with any trigonometric system, this has
problems at poles.
– Better is to define a Cartesian coordinate frame of 3 orthogonal vectors.
• Each vector defined by direction cosines in the sky frame.• Attitude is then an attitude matrix A.• This is isotropic (no trouble at poles).• Easy to convert between different reference frames.
– Whatever you do, avoid messing about with Euler angles. Ugh.
Boresights• With these matrices, conversion between
coordinate systems is easy. Suppose we have an x-ray detection which has a position vector v|
inst in the instrument frame. If we want to find where in the sky that x-ray came from, we first have to express this vector in the sky frame Cartesian system (new vector = v|sky). This is simple. Since we have:
Attitude and boresights• NOTE that attitude varies with time as the
spacecraft slews from target to target – but there is ALSO attitude jitter within an observation.– XMM has a star tracker to measure the attitude.
• Attitude samples are available at 10 second intervals.
• So to build up a sky picture from x-ray positions in the instrument frame, one has to change to a new attitude matrix whenever the deviation grows too large.
• Boresights can also change with time, due to flexion of the structure, but this is slow.– Calibration teams measure them from time to time.
• The fundamental spatial coord system is the chip coordinate system – in CCD pixels.
• Note that for time and energy as well as in the spatial coordinates, coordinate values are ultimately pixellized or discrete.– This defines the uncertainty with which they
are known (to ±half the pixel width).– Rebinning can give rise to Moiré effects
(somewhat similar to aliasing in the Fourier world).
• You need to be in space, because the atmosphere efficiently absorbs x-rays.
• Most of the currently interesting results are coming from:– Chandra (good images, so-so spectra)– XMM-Newton (so-so images, good spectra)– SWIFT (looks mostly at GRB afterglows)
although there are several more.
• How do you make an image from x-rays? Don’t they go through everything? So how can you make a reflector?
X-ray Observatories - detectors• Basic substance of the CCD is silicon – but
‘doped’ with impurities which alter its electronic structure.
• There are several sorts, eg:– Metal Oxide Semiconductor (MOS)– pn
• The CCD surface is divided into an array of pixels.
• A photon striking the material ejects some electrons which sit around waiting to be harvested.– The number of ejected electrons is proportional to the
• The readout operation consists of the following steps:
– For each CCD row, starting with that nearest the readout row, move the charges into the next lowest row.– In the readout row, starting at the pixel nearest
the output, shift the charges into the next lowest pixel.
– Convert the analog charge quantity (it is an integer number of electrons, but such a large integer that we can ignore quantum ‘graininess’) to a digital number.– This is done in an Analog to Digital Converter (ADC).
X-ray Observatories - detectors• For x-ray detection, we want to arrange the
frame duration so that we expect no more than 1 x-ray per pixel per frame.
• Why? Because if we can be pretty sure that all the charge per pixel per frame comes from a single x-ray, we can determine the energy of the x-ray. x-ray spectroscopy.
• XMM example:– Spatial resolution is ~1 arcsec (fractional ~10-3).– Spectral resolution is ~100 eV (fractional ~10-2).– Time resolution is ~1 second (fractional ~10-5).