nassp masters 5003f - computational astronomy - 2009 lecture 22 xmm instrumentation and calibration....

NASSP Masters 5003F - Computational Astronomy - 2009

Lecture 22

• XMM instrumentation and calibration.– Mirror effects

• PSF• Vignetting

– EPIC cameras• EPIC background• Event lists and selection


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


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.– PSF caused by scattering, not diffraction– 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.


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


Flux calibration• It is difficult to find x-ray sources which

stay constant over time.• One can calibrate on the ground, but the

ride to orbit shakes things up.• The Crab nebula (remnant of a supernova

in AD 1054) is the best compromise.– The nebula itself is pretty stable but there is a

pulsar (with x-ray pulses) in the centre.– Crab is also very bright – saturates modern,

sensitive detectors like XMM.

• ‘Crab units’ commonly used as a measure of x-ray flux.


Software packages• It seems common practice in the HE community to

develop a new package for each new mission.– EG

• ROSAT: EXSAS• XMM: SAS• Chandra: ciao

– Not ideal either from software engineering principles, nor efficient use of resources.

– Packages tend to be non-portable, non-generic• There’s also small incentive to document things properly!

– Ciao is the best in my view. The USA tends to do software better than Europe – they spend more money on it for one thing. (But big is not always beautiful...)

– SAS: some good generic FITS stuff there – but difficult to get at.

• I’d like to do something about that!


Background• Background from instrumental noise

– Worse at low energies & higher chip temperatures.

• X-ray background– Cosmic– Fluorescence

• Si of course, but also Al and Cu from support structure.

• Particle background– Hard, penetrating – “cosmic rays”.

• Fairly constant in time;• Fairly isotropic.

– Soft protons (~100 eV).• Flaring time behaviour.• Funnelled by the mirrors.• These weren’t suspected before launch! A major headache,

because too strong a flare can damage the CCDs.


Background examplesNote the background in the masked areas.

Mostly from flares.

Cu fluorescence. Instrumental noise at low energy.

(Masking here is done via software.) (Masking here is done via software.)

MOSpn

pn pn

dec

RA


Background – what to do with it

• Significance of background depends on what you want to do.– Spectra: obviously one needs to know the

spectrum of the background as well as possible.

– Images, in particular source detection and flux measurement: spatial properties of the background are important.

• Cosmic ray, x-ray and flares all have different spatial behaviour – so working out the proportions is important.

– Time series:• Soft proton flares dominate the problem.


Other mainly spatial problems with EPICs:

Optical loading from a bright visible-lightsource (filters minimize this)

Single-reflection arcs from far-fieldsources


Event lists• In high-energy astronomy, we deal not

with voltages or brightnesses (essentially floating-point quantities) but with lists of events – 1 event per photon.

• Each event comes with the following data:– Its pixel position on the CCD.– If necessary, the number of the CCD.– Its frame number.– Its energy. (XMM: the column is called PI.)

• Maybe also: a quality flag, event pattern, etc.

• In XMM output the events are stored in a table in a FITS file.


Event selection

• The aim is to separate ‘interesting’ events from ‘boring’ events – eg divide the events into those which probably come from a source and those which don’t.

All events

Good Bad

r

E

t

Define a selectionvolume

•Limits in defining volume shapes.•Problems integrating over overlapping volumes.•FITS format for storing selections: Data SubSpace (DSS)


Diagnostic plots:It’s helpful to plot 2 of the eventcoordinates – here energy vs time.

PN

Cu fluorescenceline

Al fluorescenceline

Time

Pho

ton

ener

gy

‘So

ft p

roto

n’

bu

rsts


Diagnostic plots:MOS 1

Al fluorescenceline

‘Gatti’ events


V

Gatti process – a kind of dithering.

Histogramof events withvoltage V.

ADC levels are analog- thus not evenly spaced.

Distorted digitizedhistogram.

+

V

t ADC

-

Undistortedhistogram.

V

t

=

nassp masters 5003f - computational astronomy - 2009 lecture 22 xmm instrumentation and calibration....

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