Astronomy from Space презентация

Skeleton
Why space astronomy? Atmospheric windows and distortion
Visible range. Atmospheric seeing and transmissivity. Objects and processes to be observed. Examples: HST, Gaia, Kepler, JWST. Asteroids for navigation in the solar system
UV. HST and IUE. Objects and processes.
IR. Effects of water vapour. Objects and processes. IRAS, SIRTF
Radio. Few examples. Big size, doable from the ground. COBE. Pulsars for attitude
X-rays. Atmosphere opaque. Chandra, ROSAT
Gamma rays. Atmosphere opaque. Objects and processes. Integral, Compton-GRO

Why space astronomy?
The atmosphere filters and distorts the incoming light:
Some parts of the EM spectrum are completely blocked by the atmosphere
Others are only partially so
In the visible spectrum, the atmosphere severely distorts de wave fronts (seeing)
An analogue phenomenon happens at radio wavelengths due to the ionosphere

Atmospheric windows

Visible

Atmospheric distortion due to turbulence. To a higher or lesser extent, this distortion (seeing) is
always present.

CCD cameras
Charge Coupled Devices are based on the photoelectric effect. Resulting electrons are stored in capacitors
These capacitors are disposed in 2D, and are able to hand over their charge to the one in the neighbouring line
CCD chips have efficiencies of ~70%, and sport a linear behaviour
They have a noise due to finite temperature (dark current)
Ionizing particles generate similar signals as photons.

CCD cameras. Pre-processing

Flat field: Caused by inhomogeneity in the illumination
Sources: dust, defects on the CCD chip, contamination

Dark field: Caused by thermal noise. The longer the exposure,
the higher the electron counts. Extra problem: hot pixels
Solution: low temperature

Quantum efficiency: fraction of detected photons as a
function of wavelength

Filters
CCD cameras are monochromatic.
To obtain colour information, there are sets of standard filters.

Adaptive optics
Adaptive optics are a family of methods to improve the resolution of ground based telescopes
The basis is to “deform” the main mirror of the telescope in order to keep the images of stars as point-like as possible
Sometimes, the observatory creates an “artificial star” by means of a laser. This artificial star is used as a reference for the adaptive optics system

Adaptive optics allow
a significant reduction of
distortion at the cost of an
increase in complexity

Gaia – ESA cornerstone mission L1
Gaia is gathering information (position, parallax, brightness, and spectra) of 1.7 billion objects
It will generate 53 TB of data
The Payload Data Handling System pre-processes the information gathered on-board to reduce the amount of data relayed to the ground
The mission will last 5 years (up to 2019)
Launched with a Soyuz-Fregat from Kourou

Map of the sky generated with Gaia’s DR2 (includes 1.7 billion objects)
Gaia DR2

Catalogues for Star Trackers
So far, the catalogues used for star trackers have included a few million stars with different errors both in astrometric coordinates as well as in brightness
With the advent of data from Gaia, both sources of errors can be considered negligible
Errors in coordinates ≈ 0.05 mas
Error in brightness ≈ 1 – 20 millimag
The catalogue can be made as large as required with even sky distribution

Often, there is a prior on the pointing that allows a very significant
reduction on the amount of calculations

The James Webb Space Telescope
It is the successor of the Hubble Space Telescope
The diameter of its main (segmented) mirror is 6.5 m
Its development has been plagued with difficulties, and it is behind schedule, as well as above budget
Launch is expected in 2020 with an Ariane 5 from Kourou

Silly ideas also have an effect on Astronomy

Ultraviolet

Infrared

Infrared telescopes

There are several windows in the IR, and so observations can be
made from the ground provided the atmospheric water vapour
content is low
Near IR

IRAS all-sky map

ISO (2.5 – 240 μm)

Infrared detectors
The CCDs we have analysed for visible range have a fair QE in the NIR
Other detectors are the bolometers, temperature-sensitive resistance materials can be used to determine the amount of electromagnetic radiation falling on them

Radio

X rays

The Crab Nebula as imaged by ROSAT (left) and Chandra (right). Credit: S. L. Snowden USRA, NASA/GSFC (ROSAT), NASA/CXC/SAO/F. Seward et al. (Chandra)

The Cygnus Loop Supernova Remnant in X-rays as imaged by three different X-ray telescopes.
From the left: image by early X-ray telescopes mounted on sounding rockets, image by the
ROSAT's High Resolution Imager (HRI) instrument, image by the ROSAT's Position Sensitive
Proportional Counter (PSPC) instrument.
Credits: rocket image: Rappaport et al, ApJ (1979) 227, 285;
ROSAT images: N. Levenson (Johns Hopkins), S. Snowden (USRA/GSFC)

X-ray telescopes
X rays are capable of penetrating matter, and thus are not efficiently reflected by mirrors at large incidence angle.
Giacconi and Rossi developed X-ray telescopes out of a set of designs for X-ray microscopes devised by Wolter
These designs improved the sensitivity by a factor of a 1000 between HEAO-1 and 2 (Einstein), as well as their spatial resolution
Typical metallic materials for XRTs are Au, Ni, and Ir, among others
In the case of multilayer coatings, materials can be Ni/C, W/Si, Ro/Be, among others
Surface roughness is a significant problem

X-ray reflectance

X-ray reflectance

This system generates an annular aperture.
In the case of Chandra, the external paraboloid had a total
surface of 3.2 m2, but the entrance aperture is just 0.047 m2

Solution: co-aligned, confocal, nested mirrors

XMM Newton

Wolter type I telescope (NASA)
Einstein
ROSAT
Chandra
XMM

Wolter type II telescope (NASA)
EUVE

Wolter type III telescope (NASA)

CCDs for X-rays
Semiconductors release electrons when an X-ray photon impacts on them
The number of released electrons range from a few dozens for low energy photons (~0.1 keV) to a few thousands for 10 keV photons
There is a good correlation between the number of electrons released and the energy of the incoming X-ray photon, thus allowing spectroscopic observations (as well as imaging)

Gas proportional counters
In this kind of detectors, a gas is ionized by incoming X-ray photons (but also by high energy particles!). The ion-electron pairs are separated by a weak electric field
The avalanche of electrons from an ionization event is detected (and measured) in the anode
The gas is a combination of inert gas and quenching gas to ensure that the ionization current terminates
Typical mix: 90% Ar, 10% CH4
Usable for X-rays under 20 keV

If the geometry of the detector and the radius of the wire are adequate, the
number of electrons detected are proportional to the energy of the X-ray photon
ion drift region
avalanche region

Scintillators
When an X-ray photon is absorbed by the matter of the detector, produces a visible photon that can be registered
Timescales for scintillation range from ns to several hours (the last case is called afterglow)
Charged particles and X-rays have different pulse profiles, thus allowing a discrimination between both

Microcalorimeters
These devices measure the heat deposited by a single X-ray photon on the detector
To avoid thermal noise, the detector must be cooled to a fraction of a kelvin
The temperature detector uses to be a Si thermistor, whose resistance experiences a dramatic changes even for small increases in temperature, a very low heat capacity, and works at less than 0.1 K
Used in Suzaku and Astro-H, among others

Gamma rays

Fermi LAT gamma ray sky at 1 GeV

Physical processes and detectors
Gamma rays are produced in nuclear processes (X-rays are generated in electronic transition processes)
Scintillators and solid-state detectors work in the same manner than in the case of X-rays
For imaging, gamma ray telescopes rely on Compton effect and pair production

Compton effect detectors
Compton effect accounts for the scattering of high energy photons by charged particles. Work properly at 1 – 30 MeV

Compton effect detectors
Compton effect telescopes scatter the incoming photon on a scintillator, and absorb it on the second layer

Pair telescopes
Useful for gamma ray imaging at energies over 30 MeV

Gravitational wave astronomy

Wrinkles in space time
Violent events involving large masses generate gravitational waves that are within our detection capabilities
Black hole mergers
Neutron star mergers
Very nearby core collapse supernovae
The required accuracy (for LIGO, a ground-based GW observatory) involves determining a longitude of less than 10–3 times the radius of a proton

LISA – Pathfinder
LISA is one of ESA’s cornerstone mission
LISA – Pathfinder is a technological demonstrator to analyse the feasibility of the full mission
In its core, there are two identical masses (2 kg of Au-Pt alloy, with a cubic shape of 46 mm in side) in free fall just subjected to their mutual gravitational fields
The mission was to determine the residual forces acting on the cubes at different frequencies

One of the sources of noise was the residual gas impacting on the cubes

Just something to keep in mind

Asteroid 2015 bz509: an interstellar object in solar orbit