nassp masters 5003f - computational astronomy - 2009 lecture 18 x-ray instrumentation and...

NASSP Masters 5003F - Computational Astronomy - 2009

Lecture 18

• X-ray instrumentation and calibration (mostly XMM).

• XMM users’ guide:– http://xmm.esac.esa.int/external/xmm_user_

support/documentation/uhb/index.html


XMM-Newton• 3 x-ray EPIC

telescopes– European Photon

Imaging Cameras.– 2 of these have:

• MOS detectors• Reflection Grating

Arrays (RGAs)

– The 3rd has• a pn detector• No RGA.

• Mirrors all the same– nested Wolter– f ~ 7 m. Schematic of the satellite


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.


Sky frame or basis.

x at RA=0

y at RA=6 hr

z at dec=90°

Unless you have good reasonnot to, always construct aCartesian basis according tothe Right-Hand Rule:

xy screws toward z;yz screws toward x;zx screws toward y.

“Direction cosines” really justmeans Cartesian coordinates.


Boresights• Ideally, each instrument on a satellite

would be perfectly aligned with the spacecraft coordinate axes.

• In real life, there is always some misalignment. This is called the boresight of the instrument (I think it is an old artillery term).

• Define a set of Cartesian axes for each instrument.

• Components of these axes in the spacecraft reference system (basis) form the boresight matrix for that instrument.


Boresights• With these matrices, conversion between

coordinate systems is easy. Suppose we have an x-ray detection which has a position vector v 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 = u). This is simple:

• Then all we need to do is calculate

• QED.

vBAu 11

222atan and 2atan yxzxy uu,udec,uuRA


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.


Other coordinate systems:

• 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).

• ‘Dithering’ can avoid this.


Moiré example:

Original binned data: bin widths are 2 units.

Re-binned data: bin widths are 3.5 units. Moiré effect causes a dipevery 4th bin (since 4 is the smallest integer n such that nx3.5 is exactlydivisible by 2).


EPIC telescope schematic(not to scale)

MOS

pn

Mirror assemblies

Optic axis

Optic axis

ReflectionGrating Array

Reflection GratingSpectrometer

Filter wheel

Mask

CCDs


Mirror effects: PSF• No mirror system of finite aperture can

produce a perfectly sharp image.• Rather, each point source is smeared out

(convolved) by a Point Spread Function (PSF).

• More usual, high F-number optics produce a PSF which is reasonably independent of off-axis angle;

• This isn’t true for x-ray grazing-incidence optics.– For both XMM and Chandra, the PSF varies

markedly with off-axis angle.


Mirror effects: PSF


Mirror effects: PSF• XMM PSF is

complicated.– Asymmetrical core.– Inner ‘star’.– Outer wings with

shadows from the mirror ‘spider’.

– RGA streak.

• The average radial profile is best described by a King function:

– r0, α depend on energy.

2

0

King1 rr

ArS


Mirror effects: vignetting.

• The mirror assemblies have a small ‘acceptance angle’ – transmitted flux drops by a factor of 2 to 3 (it’s energy dependent!) from optic axis to outside of field of view (FOV).

• The ratio of transmittance at any position on the detector plane to that at the optic axis is called the vignetting function.


X-ray interaction with matter• Can break it into continuum and resonant.• Both sorts generate ions.

– ‘Continuum’ absorption scales with• Density• 1/E.

– Resonant absorption:• electron is kicked out from an inner orbital.

X-ray

Atom

+

e-

+

KLM


Resonant absorption continued:• Because it is an inner orbital, doesn’t much matter

if atom is in a gas or a solid. The inner orbitals are pretty well insulated from the outside world.

• X-ray must have energy >= the amount needed to just ionize the electron.

– Hence: absorption edges located at energies characteristic of that orbital (labelled eg K or L) and that element.

X-ray energy

Absorption


EPIC cameras• MOS:

– “Front-illuminated” – means that the charge detection and movement electronics are on the illuminated surface (same as the retina).

– This means that• pixels can be smaller (1.1”);• the MOS cameras are not very sensitive to soft x-rays (because

these are absorbed in the electronics before reaching the detection substrate);

• they’re not very sensitive to hard x-rays either (because the substrate is too thin to absorb many).

– 7 chips (each 600x600 pixels square) in a hexagonal array, staggered in height to (very roughly) follow the curved focal surface.

• This causes slight shadowing of the edges of the central chip by the others.

– Readout time is ~2 seconds (full window imaging mode).


EPIC cameras

• pn:– “Back-illuminated” – the charge detection and

movement electronics are on the rear. X-rays strike the detection substrate first.

– This means that• pixels have to be larger (4.1”);• the pn camera is sensitive to x-rays over a much

wider bandwidth than MOS.

– 9 ‘chips’, 200x64 rectangles, but all on the same rigid squarish block of silicon.

– Readout time (in normal imaging mode) is ~ 0.07 seconds (much faster than MOS).


X-rays to events.• It isn’t as simple as 1 CCD pixel per

incident x-ray.– Each x-ray creates a charge cloud of

electrons, with a certain radius.– The charge cloud can overlap more than 1

pixel.– Thus patterns of excited pixels which

correspond to a single x-ray have to be identified;

• then all charge from that set of pixels must be added up total energy of the x-ray.

– Each recognized pattern is called an event.– What XMM calls patterns, Chandra calls

grades.


Example MOS patterns


X-rays to events.

• Complications:– X-rays are not the only things which can

cause ionization in the chips: can also have cosmic rays.

• However, these tend, on average, to produce elongated electron clouds.

– These patterns are easy to filter out.– What can’t be avoided however is a slight loss of

detection capability – where a cosmic ray has struck, an x-ray can’t also be detected (for that frame). See later discussion of exposure.

– Dead or ‘hot’ CCD pixels.– Chip edges.


Out Of Time Events (OOTEs)• As said last lecture, CCDs (at least in

imaging mode) are operated in a cyclic fashion.

• Each cycle (called a frame) is composed of an integration interval followed by a readout interval.

• But! X-ray cameras don’t have shutters! So even during the readout part of the frame, as the rows are being shunted towards the base of the CCD, x-rays are being absorbed.– This results in a vertical smearing of all the x-

rays absorbed during this time.


OOTEs continued• The MOS chips use a more complicated

readout strategy:– Each chip has in fact twice as many pixels as

‘advertised’.– The extra pixels (which can be made much

smaller, since they don’t have to detect x-rays, just hold charge) are located behind an x-ray absorbing shield.

– The readout phase is divided into 2 parts:• a quick phase during which all the exposed rows are

shunted into this frame store;• a slow phase during which the frame store is read out

to the ADC.

• Result: MOS have far fewer OOTEs.


OOTEs continued

Bright pn OOTEs Faint MOS OOTEs


Pileup

• Earlier it was said that, in order to preserve the relation between charge size and x-ray energy, the frame time had to be short enough for the probability of 2 x-rays landing on the same pixel, same frame to be small.

• It does happen, however... and obviously the brighter the source, the more likely it is.

• The phenomenon is known as pileup.


Pileup• Because of patterns, interaction between 2

events is difficult to calculate (but has been done however).

• Broadly speaking, 2 piled-up photons look like a single photon of the sum of their energies.– This mucks up the spectrum of the source.

• Many piled-up events generate ‘cosmic ray-like’ patterns and are thus discarded.– MOS diagonal doubles are a good diagnostic.

• Heavy pileup leads to the event energy being greater than the accepted cutoff – these events are then also discarded.– The result is that ‘holes’ are seen at the centres of

very bright sources.


Other modes of operating the CCDs• So far what has been described is full-

window imaging mode.• But there are at least 2 other modes:

– Small-window imaging mode.• If we’re prepared to sacrifice some imaging area,

we can have a shorter frame time.– A way to image very bright sources while avoiding

pileup.

• See timing diagram next slide...

– Timing mode. In this mode, the CCD is read out continually much finer time resolution. This only works where the x-ray flux is dominated by a single bright source.

• The pn has an additional ‘burst mode’ which can give time resolution down to 7 μs.


Shift and discard rows 1 to 250 (quick)

Shift and discard rows 351 to 600 (quick)

Shift and read rows 251 to 350 (slow)

Shift and discard pixels 1 to 250 (quick)

Shift and discard pixels 351 to 600 (quick)

Shift to ADC pixels 251 to 350 (slow)Integrate

100x100 MOS small window mode example:


Windowed imaging examples

nassp masters 5003f - computational astronomy - 2009 lecture 18 x-ray instrumentation and...

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