Space Instrumentation

Louise K Harra

UCL-Mullard Space Science Laboratory

UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICSMULLARD SPACE SCIENCE LABORATORY

See text book ‘Space Science’ edited by Harra & MasonChapter on space instrumentation by Culhane.

Spacecraft are used for

o Remote observation of the environment beyond Earth o In-situ observation of the extra-terrestrial environment o Observation of Earth’s surface from Space

This lecture will cover: o Why we need to go into space.o Features of space instrumentation - and what conditions they have to

survive.o Use of multilayer coatings to reflect EUV and X-rayso Photon detection by CCDs o Choice of orbits and their restrictionso Future solar instrumentationo Design your own instrument!

Introduction

Wavelength (m) Frequency (Hz) Energy (J)

Radio > 1 x 10-1 <3 x 109 <2 x 10-24

Micro-wave

1 x 10-3 – 1 x 10-1 3 x 109 – 3 x 1011 2 x 10-24 – 2 x 10-22

Infrared 7 x 10-7 – 1 x 10-3 3 x 1011 – 4 x 1014 2 x 10-22 – 3 x 10-19

Optical 4 x 10-7 – 7 x 10-7 4 x 1014 – 7.5 x 1014 3 x 10-19 – 5 x 10-19

UV 1 x 10-8 – 4 x 10-7 7.5 x 1014 – 3 x 1016 5 x 10-19 – 2 x 10-17

X-ray 1 x 10-11 – 1 x 10-8 3 x 1016 – 3 x 1019 2 x 10-17 – 2 x 10-14

-ray < 1 x 10-11 > 3 x 1019 > 2 x 10-14

Electromagnetic Spectrum

Regions of the SpectrumQuantum nature of radiation: E = h = hc/

Radio/Microwave (Frequency/Wavelength) → THz, GHz, MHz, cm, m

Infra-red/Sub-mm (Wavelength) → m, mm

Visible/UV/EUV (Wavelength) → Å, nm

X-ray, -ray (Photon Energy) → eV

Photon absorption by the atmosphere of the Earth

What gets through the Earth’s atmosphere

•X-rays with E>50keV can penetrate to ~30 km above the Earth’s atmosphere - hence can be measured with a balloon flight

•In practical terms you have to go into space to measure these wavelengths

•Even for the optical band (400-1000 nm) this is affected by turbulence - this can be reduced by adaptive optics techniques which have best performance in the IR with <1m

Telescope design

• Grazing incidence: for short wavelengths EM radiation behaves like ordinary light rays if it strikes surfaces at a shallow enough angle.

• Normal incidence: the reflectivity of materials in the soft X-ray/EUV range decreases with wavelength - need multilayers to enhance R.

Wolter type I telescope

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Traditional solution for X-ray/EUV astronomy: grazing incidence telescopes

Chandra X-ray telescopeChandra X-ray telescope

X-rays absorb - how do we measure them?

EUV Multilayer instruments for Solar Physics:

Simplified Schematicof the EIS Instrument

SunFilter

GratingPrimary

CCD Long

CCD Short

Slit

Simplified Schematic of the

EIS Instrument

Solar-B/EIS…Solar-B/EIS…

SOHO, TRACE…SOHO, TRACE…

a-Si

Si substrate

Mo

~70Å

m d θ=2 cos

For a periodic multilayer,Bragg’s law, as in a crystal:

d

Multilayers enable operation at normal incidence.Introduction [a-Si(38Å)/ Mo(33Å)]x40

Measured

Fit, ~7Åσ

Wavelength [Å]

θ=3.0 deg

Reflectance

• Schematic of the TRACE EUV telescope is shown below – a 0.3 m Cassegrain system

• Primary and secondary mirrors are sectored in four quadrants, three of which have Mo2C/Si layers• Quadrant shutter allows one sector at a time to view the Corona and register images on the CCD in the appropriate passband

• Mo2C/Si layers have somewhat enhanced performance compared to Mo/Si

Remote Sensing of Sun

• Information only derived from solar radiation• 4 ‘measurables’: t, x, y, λ• Detectors are 2-dimensional, time obtained by

repeated exposures• Something has to go!

– x, y or λ

Remote Sensing of Sun

• Two approaches

• Filtergrams– Filter out a small portion of the spectrum, and take X-Y images

• Spectroscopy– Throw out one of the spatial dimensions to obtain wavelength resolution

Filtergrams: EIT example

Spectroscopy: CDS example

• 2D images are built up by rastering

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MOSES concept

• We will achieve spectroscopy and imaging SIMULTANEOUSLY over a large field of view

• The MOSES rocket will now be launched on 31st January - first results will be out asap after this.

• The design is simple with no moving parts. • The reconstruction can be carried out by a simple

subtraction of the orders - this will give you a first ‘quick-look’ of the bulk flows observed.

• For more detailed analysis, algorithms such as SMART are used.

MOSES first results

CDS observation: 28 mins.MOSES spectral image: 20s

Resolution limits

• Spatial resolution is limited by diffraction.• The resolution of a given instrument is proportional to the

diameter of its objective, and inversely proportional to the wavelength of the light being observed.

• So for example, when the spacecraft study of Orbiter determined that the aperture had to be reduced because of thermal problems, this had an impact on the resolution.

Charge coupled devices - behaviour

• Quantum efficiency: Not every photon falling onto a detector will actually be detected and converted into an electrical impulse. The percentage of photons that are actually detected is known as the Quantum Efficiency (QE).

• Wavelength range: CCDs can have a wide wavelength range ranging from about 400nm (blue) to about 1050nm (Infra-red) with a peak sensitivity at around 700nm. However, using a process known as backthinning, it is possible to extend the wavelength range of a CCD down into shorter wavelengths such as the Extreme Ultraviolet and X-ray.

• Dynamic range: the dynamic range of a CCD is usually discussed in terms of the minimum and maximum number of electrons that can be imaged. As more light falls onto the CCD, more and more electrons are collected in a potential well, and eventually no more electrons can be accommodated within the potential well and the pixel is said to be saturated.

CCDs - more on behaviour

• Noise: Dark current - i.e thermally generated noise. At room temperature, the noise performance of a CCD can be as much as thousands of electrons per pixel per second. Consequently, the full well capacity of each pixel will be reached in a few seconds and the CCD will be saturated. Dark current can be massively reduced by cooling.

• Readout noise - the ultimate noise limit of the CCD is the readout noise. The readout noise originates from the conversion of the electrons in each pixel to a voltage on the CCD output node (a typical value would be around 4V per electron). The magnitude of this noise depends on the size of the output node.

a) b)

Challenges in orbit

• Vacuum of space: contaminants can readily move from one part of an instrument to another, high voltage discharge is an issue.

• Sun’s thermal radiation: typically a satellite will be illuminated by the Sun on one side (T~6000K) and the Earth (T~300K) or space (~4K) on the other.

• Ionising radiation - commercial electronics are not suitable as they are not radiation hard

Launching

• The primary driver for cost of the launch is the mass.

• This will then restrict the size of the spacecraft• The instruments will be subject to severe vibration

and acoustic noise from the rocket motors. Mechanical shocks will also be present caused by e.g. the first stage separation.

Vibration testing

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Solar-B going through its paces

Telemetry

• Telemetry is a crucial constraint for solar satellite observations, limiting the cadence of imaging instruments, and reducing the quantity of spectral information produced

• Initially, EIT was allocated 1 kilobits/s (kbs) telemetry rate, corresponding to only 6 full-disk images/day!

• SOHO has a standard science telemetry rate of 40 kbs

Choice of Orbit

• Low Earth Orbit (LEO) - orbit between the atmosphere and the Van-Allen radiation belts. This minimises the damaging effect of high energy particles. They are 200-1200km above the Earth with an orbital period of 90-100 mins. Can be prepared by the space shuttle (e.g. HST, SMM)

• High Earth Orbit (HEO) - is above the radiation belts - more expensive to launch (e.g. XMM-Newton). Apogee > 30,000km.

• Geosynchronous orbit - has the same orbital period as the sidereal period of the Earth. It has an altitude of 42,164km. IUE was operated this way - it meant full time contact.

Leaving Earth Orbit

• The Lagrangian points are the 5 points in IP space where a spacecraft can be stationary relative to the Sun and the Earth (for example).

• SOHO flies at the Lagrangian point of the Earth’s orbit about the Sun, in the same orbit as the Earth, & kept in place by the shepherding effect of the Earth’s gravity.

• Ulysses was launched by the Space Shuttle Discovery in 1990. It headed out to Jupiter, arriving in February 1992 for a gravity-assisted manoeuvre that swung the craft into its unique solar polar orbit.

Where is Ulysses today?

STEREO’s orbit

Solar Orbiter’s orbit

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Take a break! Design your own instrument

• In the 10 min break think about what science question you would like answered and can’t with current/future instrumentation.

• Come up with science requirements that would define the instrument - spatial, temporal, spectral resolutions, field of view etc. etc.

• We’ll discuss this at the end of the lecture.

Solar-B

• Japan/USA/UK mission• Follow-up to Yohkoh• 3 scientific instruments

– X-ray imager (XRT)– EUV spectrometer (EIS)– Optical telescope (SOT)

• Launch 2006• Mission aim:

– Solar-B will study the connections between fine magnetic field elements in the photosphere and the structure and dynamics of the entire solar atmosphere.

• The ‘Hubble’ of Solar Physics

Solar Optical Telescope

• 0.5 m optical telescope feeds the Focal Plane Package (FPP)– Telescope diffraction-limited to ~0.25 arcsec resolution– Maximum field-of-view: 2.75x2.75 arcmin²

Focal Plane Package

• Spectro-Polarimeter– Based on successful HAO Advanced Stokes Polarimeter– Takes spectra at different polarisations to allow vector magnetic field to be

determined– Raster images produced; pixel size 0.16 arcsec; 164x328 arcsec² field-of-view

• Narrowband Filter Imager– 0.08 arcsec pixels; 164x328 arcsec² field-of-view– Vector magnetograms obtained by rapidly taking images at different wavelengths

and polarizations

• Broadband Filter Imager– 0.05 arcsec pixels; 100x200 arcsec² field-of-view– White light images; velocitygrams; magnetograms

Solar-B – XRT

• X-Ray Telescope• Direct successor to the SXT on Yohkoh• Key features:

– 2 arcsec resolution (1 arcsec pixels)– Greater sensitivity to cool corona (1-2 MK)– 34x34 arcmin² field-of-view (full solar disk)

EUV Imaging Spectrometer

• UK-led instrument (PI: Len Culhane, MSSL)• Spectra in 170-210Å and 250-290Å wavelength ranges• Field-of-view 6 x 8 arcmin²• Spatial scale: 1 arcsec pixels• Spectral scale: 0.02Å pixels

– Line centroids ~3 km/s; line widths ~20 km/s

Solar-B Co-Alignment

• EIS and XRT are physically attached to the SOT• XRT can not be pointed independently of SOT• XRT co-alignment performed by taking white light images• In order to obtain rasters, EIS has internal mirror motions

to change area being observed• Co-alignment of EIS with XRT will be made through cool

XRT channels

STEREO

• Solar-Terrestrial Relations Observatory• Two identical spacecraft leading and following

the Earth• Launch 2005• Four instrument packages

– SECCHI– PLASTIC– SWAVES– IMPACT

• Goal:– Understand the origin and consequences of CMEs

STEREO Mission Phases

• Phase 1 (first 400 days; α50°)– 3-D structure of the corona

• Phase 2 (days 400 to 800; 50°α110°)– Physics of CMEs

• Phase 3 (days 800 to 1100; 110°α180°)– Earth-directed CMEs

• Phase 4 (after day 1100; α>180°)– Global solar evolution and space weather

STEREO – SECCHI

• Instruments– EUVI (EUV imager)– COR1 & COR2 (white light coronagraphs)– HI (heliospheric imager)

• EUVI and CORs are direct follow-ons to EIT and LASCO

SECCHI – EUVI

• Successor to EIT• Image channels: Fe IX 171, Fe XII 195, Fe XIV

211, He II 304• Larger detector (2048x2048 pixels) leads to

– Higher spatial resolution (1.6 arcsec vs. 2.5 arcsec)– Larger field-of-view (1.7 Rsun vs. 1.4 Rsun)

• Higher telemetry ensures higher image cadence

SECCHI – COR1 & COR2

• Two coronagraphs do a similar job to the three coronagraphs of LASCO

• COR1– 1.1-3.0 Rsun; 7.5 arcsec pixels– Measures polarization

• COR2– 2-15 Rsun; 14 arcsec pixels– Higher spatial resolution and time cadence than LASCO C3

Heliospheric Imager (HI)

• UK-led instrument (PI: Richard Harrison, RAL)• Will obtain a new type of solar data: imaging of

CMEs out to 1 a.u.• Images not Sun-centred (unlike coronagraphs)• Two independent telescopes (HI-1, HI-2) with half-

angle fields-of-view of 10º and 35º

Solar Dynamics Observatory

• Launch 2007• Part of International Living with a Star Program• 3 instrument packages selected by NASA:

– AIA (imaging and coronagraph)– HMI (magnetograph)– EVE (extreme ultraviolet irradiance monitoring)

• Geosynchronous orbit– Very high telemetry rate (160 Mbs)

SDO

• HMI (Helioseismic and Magnetic Imager):The Helioseismic and Magnetic Imager will extend the capabilities of the SOHO/MDI instrument with continuous full-disk coverage at higher spatial resolution. PI: Phil Scherrer, PI Institution: Stanford University

• AIA (Atmospheric Imaging Assembly)The Atmospheric Imaging Assembly will image the solar atmosphere in multiple wavelengths to link changes in the surface to interior changes. Data will include images of the Sun in 10 wavelengths every 10 seconds. PI: Alan Title, PI Institution: Lockheed Martin Solar Astrophysics Laboratory.

• EVE (Extreme Ultraviolet Variablity Experiment):The Extreme Ultraviolet Variablity Experiment will measure the solar extreme-ultraviolet (EUV) irradiance with unprecedented spectral resolution, temporal cadence, and precision. Measures the solar extreme ultraviolet (EUV) spectral irradiance to understand variations on the timescales which influence Earth's climate and near-Earth space. PI: Tom Woods, PI Institution: University of Colorado.

Solar Orbiter

• ESA mission to be run jointly with Bepi-Colombo• Launch ~2015-2017• Will get as close as 0.2 a.u.• Both in-situ and remote sensing instrument packages• Remote sensing:

– Visible imager and magnetograph– EUV imager– EUV spectrometer– Coronagraph

Orbit

• Each orbit is around 150 days

• Every 3rd orbit a fly-by of Venus gives an out of the ecliptic kick to the spacecraft

• Orbit reaches latitudes of ~30° during extended mission (> 4 years)

Solar Orbiter – Latitude Range

Getting started with AstroGrid: 3 steps

STEP 1• Launch the AstroGrid workbench from

http://software.astrogrid.org/jnlp/workbench/workbench.jnlp• Search for solar and STP data with HelioScope. (Look under the workbench “Data

Discovery” tab.)• Load solar and STP data from HelioScope with Aladin, TOPCAT, and other tools listed at

http://plastic.sourceforge.net.

STEP 2• Register for an AstroGrid account for access to remote applications and virtual storage.

Email email astrogrid_signup@star.le.ac.uk for a username.• Try out the solar movie maker parameterized workflow (look in “Science Examples” under

the workbench “Workflow” tab).• Look for other solar and STP data analysis, visualization and modelling applications in

the Task Launcher (also under the workbench “Workflow” tab).

STEP 3• Find out more about advanced data searches and applications by clicking the workbench

“Advanced” tab or looking at http://www2.astrogrid.org/documentation/ag-help-docs/.

Solar data with AstroGridThe AstroGrid Helioscope tool finds solar and STP data from virtual

observatories in the UK, US and Europe.How to:

1. Launch the AstroGrid workbench and click “HelioScope”.

2. Enter a start and stop time. Check “time series” and / or “graphic”. Click search.

3. Double click a “time bubble” to select data.

Current Datasets:

• Solar: TRACE; SOHO EIT,

CDS, MDI, and LASCO

• STP: Cluster, ACE, Wind spacecraft,

Ulysses, Polar spacecraft

Coming soon:

• Solar-B, Yohkoh

AstroGrid workbench “Data Discovery” tab. AstroGrid HelioScope solar data search tool.

Solar data analysis with AstroGridThe AstroGrid Task Launcher and Workflow Builder provide access to

remote processing tools, either as stand alone applications or as part of a larger workflow.

How to:1. Launch the AstroGrid workbench and

click the “Workflow” tab.2. Click the “Science Examples” tab to

run a parameterized workflow.3. Or click the “Task Launcher” to run a

single application. 4. Or click the “Workflow Builder” to

construct a complex workflow.

Solar and STP applications:• Solar Movie Maker• Solar Overlay• Solar Data Reductor• CTIP model and movie maker• Plot STP• GNUPlot• ACE

AstroGrid application launcher showing application search on “solar”.

Solar movie maker: example

1. Click “Science Examples”.

2. Log in and select “solar movie maker.”

3. Enter instrument name, start time and stop time and press the OK button.

4. View finished EIT movie: (example)

http://www.mssl.ucl.ac.uk/~eca/Astrogrid/eitmovie.mpg

Future, future instruments

• What are they???