Astrophysics (SPAT0033-1) PDF

Astrophysics (SPAT0033-1) Michaël De Becker Master in Space Sciences Master in Aerospace Engineering  Part I: A multi-wavelength view of the Universe Ch. 1: Overview of the Universe's content Ch. 2: Observational tools for astrophysics  Chapter 2: Observational tools for...

Astrophysics (SPAT0033-1) Michaël De Becker Master in Space Sciences Master in Aerospace Engineering  Part I: A multi-wavelength view of the Universe Ch. 1: Overview of the Universe's content Ch. 2: Observational tools for astrophysics  Chapter 2: Observational tools for astrophysics  Various instrumental designs are needed to measure light at different energies.  The specificity of the different spectral domains is at the origin of several subdivisions in astrophysics.  However, their complementarity constitutes a excellent incentive to exploit them in combination.  This chapter is designed to provide an overview of these. tools. However, for a more detailed or specific description, one will refer to other courses.  Chapter 2: Observational tools for astrophysics Main points addressed in this chapter...  Overview of observatories across the electromagnetic spectrum (a non-exhaustive list...)  Overview of some techniques used to collect astronomical data. . How to access astronomical observation facilities?  Ch. 2: Observational tools for astrophysics Overview of observatories used across the electromagnetic spectrum (a non-exhaustive list)  Chapter 2: Observational tools for astrophysics Arecibo, Puerto Rico (300 m) (destroyed in 2020) FAST, China (500 m) © Arecibo Observatory/NSF  Chapter 2: Observational tools for astrophysics Effelsberg, Germany (100 m) Green bank, USA (100 m)  Chapter 2: Observational tools for astrophysics VLA, USA (27 X 25 m) © NRAO  Chapter 2: Observational tools for astrophysics © NCRA-TIFR GMRT, India (30 X 45 m)  Chapter 2: Observational tools for astrophysics Mills Cross, Australia (450 m) UTMOST, Australia (780 m) © CSIRO © Swinburne University of Technology/UTMOST  Chapter 2: Observational tools for astrophysics LOFAR, The Netherlands © ASTRON Chapter 2: Observational tools for astrophysics LOFAR, The Netherlands © ASTRON Chapter 2: Observational tools for astrophysics © © JIVE  Chapter 2: Observational tools for astrophysics Spitzer (© NASA) (2003 – 2020) Herschel (© ESA) (2009 – 2013)  Chapter 2: Observational tools for astrophysics James Webb Space Telescope, NASA (with participation of ESA) Launched on 25 December 2021 Largest optical telescope in space (6.5 m) Cost: 10 Billion USD ! © NASA  Chapter 2: Observational tools for astrophysics Hubble, NASA (with participation of ESA),: (1990 –...) Optical flaw due to miscalibrated equipment during the mirror's manufacture Before and after adding a correction glass Repair mission in 1993 © NASA  Chapter 2: Observational tools for astrophysics ELT (© ESO, to be built in Chile), 39 m VLT (© ESO, Chile), 8.2 m  Chapter 2: Observational tools for astrophysics Subaru, (Hawai, USA), 8.2 m CFHT (Hawai, USA), 3.6 m © NAOJ © CFHT Corporation Chapter 2: Observational tools for astrophysics DOT (Devasthal, India), 3.6 m © AMOS Chapter 2: Observational tools for astrophysics ILMT (Devasthal, India), 4 m (inauguration in March 2023)  Chapter 2: Observational tools for astrophysics - Not so many dedicated telescopes to date in the UV - A few instruments exist on-board other satellites Galex (© NASA) (2003 – 2013)  Chapter 2: Observational tools for astrophysics XMM-Newton (© ESA) Chandra (© NASA) (1999 –...) (1999 –...) Suzaku (© JAXA) (2005 – 2015)  Chapter 2: Observational tools for astrophysics INTEGRAL (© ESA) (2002 –...) Fermi (© NASA) (2008 –...) Chapter 2: Observational tools for astrophysics HESS (Namibia) Why not using the atmosphere as a detecting device for very high energy photons?  Chapter 2: Observational tools for astrophysics CTA (Chile) Southern component of the Cherenkov Telescope Array, expected to be in operation in 2025 (?) (Computer generated image, © ESO)  Ch. 2: Observational tools for astrophysics Overview of some techniques used to collect astronomical data  Chapter 2: Observational tools for astrophysics Various techniques have been developed for the purpose of astronomical observations. New techniques are still being developed and refined by engineers, in tight collaboration with astrophysicists who express their needs, motivated by specific science questions. These techniques are specific to some spectral domains and they deserve special attention for astrophysicists working in these fields. More detailed descriptions can be found in specific courses. Knowing about these techniques is required to be smart users of observation tools. Technical specifications tell the users what are the strengths and limitations of a specific tool. Being conscious of that is a requirement to make a proper use of observation facilities.  Chapter 2: Observational tools for astrophysics What are the main components of observation facilities? Basically, one needs - a system that will collect light in a way or another (the telescope...) - a system that will measure light in a way or another (the instrument...) For instance, for a ground-based...or, for a ground-based facility or space-borne facility working working in the radio domain... in the visible domain...  Collecting device: antennas  Collecting device: reflecting that collect some radio telescope focusing light to a waves specific focal plane  Instrument: receivers that  Instrument: for instance, a measure the signal as a camera that records the current/potential difference image of the sky in the conducting material  Chapter 2: Observational tools for astrophysics Main telescope designs... A first fundamental difference must be made between two kinds of telescope (typically in the visible domain). Refracting telescopes (light goes through a lens) The passage of rays of light through the lens affects the propagation direction : they are concentrated thanks to the principle of refraction Rays of light are typically sent to another lens behind which the detection device is located. This second lens makes light rays parallel again. Concave lens Convex lens Marion, J.B., 1981, Physics of the modern world, 2nd Ed.  Chapter 2: Observational tools for astrophysics Main telescope designs... A first fundamental difference must be made between two kinds of telescope (typically in the visible domain). Refracting telescopes (light goes through a lens) For instance, the small telescope built by Galileo in 1609 was a refracting telescope. Main disadvantage: small field of view, and requirement to build a very long telescope to increase the magnification (objective-to-eyepiece focal length ratio) at the expense of FoV © Encyclopaedia Britannica  Chapter 2: Observational tools for astrophysics Main telescope designs... A first fundamental difference must be made between two kinds of telescope (typically in the visible domain). Reflecting telescopes Light from a cosmic source is reflected by a (light undergoes a reflection concave (primary) mirror, and depending on the on a mirror) optics design it may undergo additional reflections before being sent to the detection device. Marion, J.B., 1981, Physics of the modern world, 2nd Ed.  Chapter 2: Observational tools for astrophysics Main telescope designs... What about modern telescopes? Only small telescopes used as optical monitors make use of the refractive design. Most telescopes with a size greater than a few tens of cm are reflectors. The most obvious advantage of reflecting telescopes: Bigger size telescopes can be manufactured : greater collecting area and better angular resolution Photons are collected by the primary The capability to resolve spatially mirror. The greater its size, the more details on the sky is limited by the photons collected in a given time diffraction. The limit is of the order of interval. This improves the sensitivity, the ratio λ/D, where λ is the and thus the capability to detect faint wavelength and D is the diameter of sources. the primary mirror. Bigger mirrors lead to a more detailed imaging.  Chapter 2: Observational tools for astrophysics Main telescope designs... Various reflecting telescope designs exist... Risk of significant obstruction in front of the primary mirror if the instrument at the focus is quite large. Parabolic primary mirror A quite long focal length can be achieved without the need to build very long telescopes (improved magnification, but smaller FoV).  Chapter 2: Observational tools for astrophysics Main telescope designs... Various reflecting telescope designs exist... Large instruments are not blocking any more the FoV. A massive instrument will however exert a torque that may lead to stability issues. Parabolic primary mirror Also called Nasmyth design. Similar to Newtonian, but the location of the instrument closer to the primary reduces the issue of the torque. The instrument is typically located in a room beside the telescope.  Chapter 2: Observational tools for astrophysics Main telescope designs... Various reflecting telescope designs exist... Distortion of off-axis sources Ritchey-Chrétien telescope: Variant of the Cassegrain telescope, with hyperbolic shape primary and secondary mirrors → good off-axis optical performance (suppression of the coma aberration) → most frequent design for modern professional telescopes © Photographylife.com © Tamasflex  Chapter 2: Observational tools for astrophysics Main telescope designs... Some disadvantages have to be considered...  The telescope is open to the air → it may need frequent cleaning  Particular attention must be paid to the alignment of the optic  Prime focus instrument, secondary mirrors, and the support structures (spiders) produce diffraction features that affect the quality of imaging. These imaging artifacts produce notably the well-known cross features on some astronomical imaging... © C. S. Baird  Chapter 2: Observational tools for astrophysics Primary mirrors of reflecting telescopes... Monolithic mirror : Mirror made from a single piece of glass. Size limitation quite severe (~ 8 m). Segmented mirror: Array of mirrors designed to be used as a single curved mirror. Preferred design for larger mirrors. © Cmglee  Chapter 2: Observational tools for astrophysics Primary mirrors of reflecting telescopes... Main advantages of large primary mirrors (beyond the limit of monolithic manufacturing) 1. Large collecting area: The larger the telescope is, the greater the number of photons collected → better sensitivity and better signal-to-noise (S/N) ratio of measurements 2. Improved angular resolution: The typical angular separation (in rad) between two separate sources on the image is Θ ~ λ / D (where λ is the wavelength, and D is the diameter of the primary mirror) → for a given λ, larger telescopes lead to imaging with better details  Chapter 2: Observational tools for astrophysics Active optics vs adaptive optics... The quality of images can be The quality of images can also be affected by distortions of mirrors due affected by flucutations of the density of to gravity and thermal changes. the Earth atmosphere. This leads to time variable and position variable A correction can be provided by refractive index, resulting in a actuators. These distortions can be deformation of the wavefront on rather monitored by wavefront sensors. short time scales. They occur at a rather low frequency (~1 Hz), and the corrections have to These fluctuations can be monitored by be made at the appropriate wavefront sensors., but at a high cadence. The amplitude of the frequency (100 Hz). The amplitude of distortion is significantly high. the distortion is rather small. Low frequency correction of mirror High frequency correction of secondary shapes to mitigate these effects is optics elements to mitigate these referred to as active optics. atmospheric effects is referred to as adaptive optics. Both active and adaptive optics systems are required for modern optical telescopes  Chapter 2: Observational tools for astrophysics Telescope mounts... The equatorial mount: One axis points to the Celestial North Pole (its rotation is modifying the Right Ascension of the aim point of the telescope), and the other is perpendicular to it (it adjusts the Declination). Advantage: It naturally tracks celestial objects along the diurnal motion by rotating only about one axis at constant speed. Disadvantage: Both axes are tilted, and this may cause some issues for the stability of the pointing due to gravity. © Telescopeguides.com  Chapter 2: Observational tools for astrophysics Telescope mounts... The altazimuth mount: One axis points to the Zenith (its rotation is changing the azimuth direction), and the other is perpendicular to it (it adjusts the height above the horizon). Advantage: The pointing is more stable than in the case of tilted axes.. More compact and more simple, and thus less expensive. Disadvantage: Both axes need to be rotated in a concerted way to follow the diurnal motion of celestial objects. → this can easily be handled by computer assisted tracking → modern telescopes adopt © Telescopeguides.com preferentially altazimuth mounts  Chapter 2: Observational tools for astrophysics Examples of modern reflecting telescope... Keck telescope : Located in Mauna Kea (Hawai). Two telescopes with a diameter of 10m. Segmented primary mirror. Rithchey-Chrétien design. Both Coudé/Nasmyth and Cassegrain foci are equipped with instruments.  Chapter 2: Observational tools for astrophysics Examples of modern reflecting telescope... Devasthal Optical Telescope (DOT): Indo-Belgian telescope in the North of India. Largest steerable optical telescope in India. Ritchey-Chrétien design, with 3.6m primary mirror. Built by the AMOS company (Belgium). © AMOS  Chapter 2: Observational tools for astrophysics Examples of modern reflecting telescope... James Webb Space Telescope : Primary made of 18 segments, for a total diameter of 6.5 m. Diffraction spikes both due to the hexagonal segmentation and the support spider. © NASA © Cmglee © NASA  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Switching from single aperture to interferometry Reminder: the angular resolution of a single aperture telescope of diameter D at a given wavelength λ is Θ~λ /D For an interferometer, the angular resolution is expressed by Θ~λ /B where B is the baseline, i.e. the distance between two apertures in the interferometric array For a given wavelength, the angular resolution of any interferometer is improved by a factor B/D as compared to that of individual apertures in the array.  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Basic principle of interferometry  Two apertures pointing to the same direction will be hit by a plane wave, but the path length is different for both apertures: the path length difference is δ  Aperture 1 will be hit after aperture 2, with a time delay : Δt = δ / c, where c is the speed of light  This difference in optical path is at the origin of a phase shift between the signals collected by the two apertures  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Basic principle of interferometry Basically, the signals from appertures 1 and 2 are combined:  If δ is equivalent to an integer number of λ, the signals add up: the interference is constructive  If δ is equivalent to an odd number of λ/2, the two signals cancel out: the interference is destructive  The alternate presence of bright and dark components leads to a fringe pattern : it can be seen as some kind of instrumental response of the interferometer  Depending on the position of the source on the sky, it might be measured or “turned off”  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Basic principle of interferometry  The combination of the signals occurs in a so-called interferometric lab (away from the individual telescopes).  In order to get a constructive interference from a specific direction, the optical paths lengths must be adjusted  Use of delay lines → path length modified → modification of the phase shift → adjustment to make the source of interest coincident with a bright fringe  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Basic principle of interferometry For instance, the CHARA array (USA), with 6 apertures A two-aperture inteferometer:  only 1 baseline  fringe pattern oriented along only one direction → incomplete information A N-aperture interferometer  N (N-1) / 2 baselines  More complex fringe pattern that probes several directions projected on the sky plane → enhanced imaging capability Note: this applies too when you have a drink with friends! © CHARA  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Basic principle of interferometry Some difficulties about interferometric imaging:  Imaging results from the inversion of interferometric measurements, whose sampling of spatial data is not complete → the image that you get is not the real image  Image reconstruction is a heavy task → it may be highly demanding in terms of CPU and data storage  The imaging processing leads to unreal features, artifacts, unexpected structures → dealing with that is not straightforward  The reconstructed image is strongly affected by bright sources in the field → faint sources may not be analysed properly  The reconstucted image is a so-called “dirty image” → the use of adequate cleaning algorithms is needed  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Basic principle of interferometry Its main advantages is however worth the effort.. Interferometric arrays give access to angular resolution scales completely out of reach of single telescope → such arrays, to some extent, behave as a huge telescope whose aperture is that of the physical extension of the array on the ground  in the optical domain, baselines of a few 100 meters lead to an angular resolution of the order of a few milli-arcseconds (mas)  in the radio domain, imaging at the mas scale is possible, provided baselines of thousands of km are used (Very Long Baseline Interferometry, VLBI)  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Examples of interferometric arrays The Very Large Telescope Interferometer (VLTI, Chile): Near IR © ESO  4 Unit Telescopes (UT): individual aperture of 8.2 m  4 Auxiliary Telescopes (AT, built by AMOS in Liège): individual aperture of 1.8 m; can be moved along rails to adjust the baselines depending on the angular scales one intends to measure (up to about 200 m)  Chapter 2: Observational tools for astrophysics Beyond the limitations of single aperture angular resolution... Examples of interferometric arrays The Jansky Very Large Array (JVLA, USA, New Mexico): radio 27 antennas in a Y-shape configuration Each antenna has a diameter of 25 m The length of every arm can go up to 21 km The array is used in various configurations, from the largest (A configuration) to the most compact one (D configuration), depending on the angular scale intended to be studied. © NRAO (→ more examples in SPAT0069-1 Radio astrophysics)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Collecting and processing of light by telescopes is one thing, but it is not enough to proceed with the astronomical data analysis. In the case of interferometers, one can consider the beam combiner as the instrument, leading to the raw data that will be processed by the users. For individual telescopes, light sent to one of the foci has to be processed in a way or another, depending on the scientific needs → instruments mounted on telescopes constitute crucial parts of astronomical tools; they deserve to be at least briefly reviewed  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imagers The output is an image, a 2D spatial distribution of light in a given spectral range. This is used to identify the existence of new sources, measure the integrated amount of light in a given waveband, investigate the morphology of sources or the spatial distribution of some of its emitted light, measure the position of sources... Spectrometers The output is a distribution of the amount of light coming from a given source as a function of wavelength. This gives a more detailed insight into the composition of the source and into some of its physical properties... Spectro-imagers The output is a data cube, including the spectral dispersion of light in every pixel of the image... Polarimeters The output is an information about the polarization of the light coming from a specific celestial source. One talks about spectropolarimetry when polarization is measured as a function of wavelength...  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Before the advent of electronic devices, imaging detectors were photographic plates Glass plate covered with light-sensitive emulsion of silver salt → exposure to the light collected by a telescope → recording of an astronomical image Only imaging technique for astronomy over the 19th Century, until about 1990 when electronic cameras were made available © Harvard, Harvard College Observatory  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Before the advent of electronic devices, imaging detectors were photographic plates Advantages: - Possibility of long integration time (much better than the fraction of second offered by the eye) - Possibility to record images (→ careful analysis of images could be considered well after the measurement) Disadvantages: - Very low efficiency (only of few percent of the light is actually recorded) - Non-linearity of the response (intensity of the signal not proportional to the amount of light) - Poor reproducibility (same exposures on the same field do not necessarily lead to the exact same result)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Pixel : element of spatial resolution Array of pixels, with charge transfer capability synchronized with a control clock. Operation in 3 steps: 1. Integration (charge generation through photoelectric effect and collection on electrodes) 2. Charge transfer (by varying electrode potentials) 3. Read-out (with analog-to- digital conversion) © N. Alfaraj  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Advantages: - Recording of digital images (→ post-processing and analysis much more versatile) - Linearity of the response (→ very adequate for quantitative measurements) - High quantum efficiency (about 90%, about a factor 30 better than photographic plates) Disadvantages: - Limitation in their size (although mosaics of CCDs can be used for wider field imaging) - Sensitive to impact by cosmic rays (in practice, limitation on the integration time and requirement for careful correction of images)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) QE curve for a modern CCD used for astronomy Quantum efficiency: Fraction of incident photons leading to an electron-hole pair with successful read out by the device Not uniform as a function of wavelength of incident photons. © Finger Lakes Instrumentation  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Linearity ad saturation: Ne The number of measured free ~105 electrons (Ne) is linearly proportional to the number of incident photons (Nγ) up to a limit where the detector is saturated. This is due to a less efficient attraction of free electrons when the electrode potential well is nearly full. 0 ~105 Nγ  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Front illuminated (FI) vs thin back illuminated (BI) CCD: Penetration depth of photons increases as a function of wavelength FI → loss of efficiency for higher energy photons stopped by the electrodes (especially in the UV) for FI detectors → thin BI CCD offer an interesting alternative with a better sensitivity especially at shorter wavelengths (a bit more difficult to build) Silicon BI Isolating layer Electrode  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) A perfect CCD should offer perfectly identical pixels, with exactly the same collecting area, the same efficiency in converting light to charges, it should be uniformly exposed and it should provide no signal when not exposed to light and be only sensitive to light. Of course, actual CCDs are not perfect → adequate strategies of use must be adopted  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Actual CCDs are not made of perfectly identical pixels, with the same collecting area! → even in case of a uniform exposure, the probability to be hit by a photon is slightly different from one pixel to the other → the response to a uniform exposure is not a uniform distribution of the signal across the image → Solution: flat fielding (see a bit later)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Actual CCDs are not uniformly exposed! Some dust on the lens of the camera is likely to alter the exposure The design of the camera itself may lead to a non-uniform exposure Unexpected manufacturing defects can also occur → Solution: flat fielding (see a bit later)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Actual CCDs are not made of pixels with perfectly identical quantum efficiency! Every pixel constitutes an individual sub-unit of the array One cannot reject the possibility that some minor differences may exist in their capability to convert a photon impact into a free electron Once an electron if freed, minor differences in the electrodes may lead to pixel- to-pixel differences → Solution: flat fielding (see a bit later)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Actual CCDs are not displaying a null signal when not exposed! Even in the absence of light, some electrons are freed in the semi-conductor. This phenomenon is amplified as the temperature increases → a non-zero signal is measured → when the camera is exposed, that dark signal adds to the actual signal from the sky field → Solution: cooling of the camera and measurement of the dark level (see a bit later)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Actual CCDs are not only sensitive to photons! High energy charged particles called cosmic rays are permanently crossing our environment → they can hit detectors at any place, at any time → they will therefore free electron that will generate some undesirable signal → Solution: short exposures and manual/automated correction (see a bit later)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Typical observation procedure: Science data:  Exposure on the sky → raw image Calibration data:  Exposure with closed shutter → dark or bias image  Exposure on a uniform field → flat field image (it might be several exposures, averaged and normalized) Basic principle: convert a raw image into a reduced image making appropriate use of calibration data  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Most frequent detector used in optical astronomy nowadays: → Charge Coupled Device (CCD) Typical observation procedure: Details on the procedure are specific of every instrument, observatory policy, or specific need for a given observation  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes (Bessel M.S. 2005, ARA&A, 4, 293) Imaging Photometric bands: Beside broad band imaging, astronomical images can be obtained in specific narrower bands. Narrower bands are useful to analyse rough spectral properties of astronomical sources. Photometry is the technique that allows one to measure the amount of light measured in a specific band. Various photometric systems exist, some of them are represented here. Curves represent the response of these bands, and their identifiers are also specified.  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Imaging Photometric bands: Imagers can be equipped with several filters compliant with some standard photometric bands. Typically, filters can be mounted on a rotating wheel allowing users to switch easily from one band to the other, using the same telescope and the same CCD camera. The number of filters on the wheel Filter wheel of the MIRI camera, mounted can vary between a few, and a on the JWST. It contains 18 elements. quite large number. © Hensoldt  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy What is the interest to apply spectroscopy to astronomical observations? Imaging already provides a wealth of information, and multi-band imaging gives access to information spread over narrower spectral domains. However, multi-band photometry is not enough to access more detailed information at the scale of individual spectral lines → strong requirement for instruments capable to resolve small details in the light collected by telescopes → dispersive elements constitute key elements in spectroscopic instruments  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Spectrometers include a dispersive device, that allows to separate light into various spectral components → prisms, gratings... Light from the telescope is sent through the dispersive element, before being recorded (for instance on a CCD camera). Prism © R.W. O'Connell  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Most appropriate tool to investigate spectral lines of various origins: Absorption lines and emission lines A source provides some Some atoms/ions are continuum radiation and some excited, i.e. higher energy material along the line of sight levels are occupied by can absorb some of the electrons that can relax by radiation at some specific reaching lower levels, wavelength corresponding releasing the equivalent → the background spectrum energy as photons presents some deficit of flux → some excess emission at some wavelengths appears at some specific → energy transferred from wavelengths the radiation field to the → energy transferred from matter matter to the radiation field  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Basic principle of line formation © NASA  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Some more specific cases… Fluorescences lines : emission lines that follow immediately the absorption of a photon by a given atom/ion/molecule Forbidden lines : emission lines characterized by a low transition probability, that are said to be forbidden as they can’t be observed in laboratory in usual conditions (typically, excited species are de-excited because of collision before the radiative transition occurs). However, in very low density environments (such as some interstellar or extragalactic regions…), the density is so low that collisions are very rare → so-called forbidden transitions have the time to occur before collision relaxation happens → these lines are observed  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes No spectral overlap Spectroscopy © Pryiam Study Centre Atomic hydrogen, an important example n : principal quantum number Ly : transition down to n = 1 H : transition down to n = 2 Pa : transition down to n = 3 Br : transition down to n = 4 Pf : transition down to n = 5 For greater level differences, use of the upper n: → H8 : Balmer transition from n=8 !! Energy level differences not to scale!!  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Atomic hydrogen, an important example  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Atomic hydrogen, an important example  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Atomic hydrogen, an important example Valid for atomic H !  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Various ways to feed a spectrograph  Long-slit spectroscopy  Fiber-fed spectroscopy  The entrance aperture for the light coming from the telescope is an elongated slit  The slit is positioned on the source in the field of view.  Light admitted in the slit goes through the dispersive element, before being recorded by a CCD  The appropriate position and orientation of the slit allows to improve the contrast, by rejecting nearby sources of light. © G.C. Sloan  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Various ways to feed a spectrograph  Long-slit spectroscopy  Fiber-fed spectroscopy  Individual optic fibers are positioned on specific targets on the field observed by the telescope  In multi-object mode, every fiber conveys light to a dispersive element, and every individual spectrum is recorded  In multi-object mode, this allows to optimize the telescope time (several spectra obtained simultaneously in the same sky field) © Starlink Project  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectroscopy Calibration specific to spectroscopy Dispersion spreads light of the astronomical source on the detector (CCD) along a given direction (science data) → light at various wavelengths is recorded in some pixel columns Measurement of the spectrum of a calibration source (e.g. internal lamp) with specific spectral lines at well known wavelengths (calibration data) → identification of the physical wavelength corresponding to any pixel column on the detector (→ the so-called wavelength solution) → a calibration of the spectrum of the astronomical source is now possible → the output is a wavelength calibrated science spectrum, ready to use for science analysis!  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectro-imaging The purpose of a spectro-imager is to record and image, coupled with the capability to disperse the light of the image → a spectrum is obtained for every subdivision of the image Advantages:  Simultaneous imaging and spectroscopy (morphological and spectral information in one shot)  Optimization of the telescope time (many information in only one exposure) Drawbacks:  Complex instruments for manufacturing (optics with a lot of entangled pieces)  Huge amounts of data (due to the number of spectral resolution elements multiplied by the number of spatial elements)  Expensive instruments  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectro-imaging The purpose of a spectro-imager is to record and image, coupled with the capability to disperse spectrally the light of the image © O. Guyon  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Spectro-imaging Basic principle of an image slicer: Various parts of the initial image are reflected by various components of a slicing mirror, with a gradient of inclinations → multiple slices Every slice is sent to a different pupil mirror and is recorded on different parts of a CCD © O. Guyon  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Polarimetry The polarization of light constitutes another piece of information useful for astrophysics → observational tools able to measure it are welcome ! Electromagnetic (EM) radiation is notably made of an oscillating electric field, and the direction of its oscillation can be represented by a line segment, with arrows to specify the direction of the variation Non-polarized light is made of EM Polarized light is made of EM waves distributed in all possible waves presenting a specific directions. distribution of oscillation direction*. (*linear polarization, in this example)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Polarimetry What is linearly polarized light ? Light polarized in a way that keeps the unique oscillating direction of the electric field steady as the EM wave travels through space. What is circularly polarized light ? Light polarized in a way that leads the oscillating direction of the electric field to change gradually as the EM wave travels through space. Typically, unpolarized light can be seen as the superimposition of two orthogonal linear polarizations. Depending on the properties of the medium traveled through by the wave (that may be birefringent), a phase shift between the two linear polarizations may appear (90° for circular polarization). As a consequence, the resulting electric field follows some kind of helical motion about the Poynting direction.  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Polarimetry Natural light is most of the time unpolarized... However, some polarized light can be measured under specific circumstances. The measurement of a significant polarization tells us about  the nature of the radiative process at work  the geometry of the emitting environment  the medium traveled by the unpolarized light For instance, synchrotron radiation is expected to be polarized. Thus the detection of polarization in the continuum emission from a specific source could be an indicator of the synchrotron emission But, let's be careful! The lack of measured polarization doesn't necessarily reject the synchrotron nature of the measured radiation (→ details in SPAT0069-1 Radio astrophysics)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Polarimetry Natural light is most of the time unpolarized... However, some polarized light can be measured under specific circumstances. The measurement of a significant polarization tells us about  the nature of the radiative process at work  the geometry of the emitting environment  the medium traveled by the unpolarized light Unpolarized light that is “reflected” by material surrounding a source can become linearly polarized. For instance, stellar light re-emitted by circumstellar material will display significant polarization. Note: reflected light will be at least partially linearly polarized in a plane parallel to that of the reflecting plane. This happens for instance with sun light being reflected on the ground surface; reflected light is significantly linearly polarized.  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Polarimetry Natural light is most of the time unpolarized... However, some polarized light can be measured under specific circumstances. The measurement of a significant polarization tells us about  the nature of the radiative process at work  the geometry of the emitting environment  the medium traveled by the unpolarized light Unpolarized light crossing a region filled with dust particles may become significantly polarized A slab of interstellar material populated by dust acts as some kind of polarizer, leading either to linearly or circularly polarized light. (→ see SPAT0020-2 Astrochemistry and SPAT0008-1 Interstellar medium for some applications)  Chapter 2: Observational tools for astrophysics Instruments mounted on telescopes Polarimetry How can we measure the polarization?  Polarizers are used, i.e. filters able to allow light with a specific polarization direction to pass.  To measure polarization in any direction, frequently two perpendicular linear (or opposite circular) polarizers are used.  Circularly polarized light can be converted to linear polarization making use of appropriate optical elements (quarter-wave plate), that introduce a phase shift in one polarization direction (thus removing the circular nature of the polarization). Difficulties:  Various reflections in the optical design will produce/alter the polarization. → an accurate understanding of the optics and of its processing of light waves is required  Measurement of the polarization requires most of the time rather bright sources and/or sensitive instruments  Chapter 2: Observational tools for astrophysics So far, in this chapter…  We achieved an overview of various observatories (ground-based and space- borne) used for astronomical observation  We commented on the need to develop specific technical solutions depending on the spectral domain  We clarified the concepts of collecting systems (telescopes…) and measuring systems (instruments…)  We discussed the main types of measurements that can be made for the purpose of astrophysics studies At this stage, an important question to address is the following: how to access astronomical observation facilities ?  Ch. 2: Observational tools for astrophysics How to access astronomical observation facilities?  Chapter 2: Observational tools for astrophysics General context Two main possibilities National / Institutional / Consortium Observatories open to the observatories : international community :  Immediate access... Competition through telescope  Know the people... time application  Telescope time application, but in a rather 'closed' context (limited competition)  Chapter 2: Observational tools for astrophysics General context Competitive access: “Many will apply, only a few will succeed!” Harsh competition ! → prepare for struggling !  Against competitors Many smart people will also apply and propose relevant projects. Some projects will be resubmitted from a previous call, and will have benefited of some feedback.  Against reviewers Evaluators are all biased Most evaluators will search for reasons to reject your project, and not to support it.  And more important... against yourself ! Proposal writers are all biased too, in a way that is likely not compliant with the bias of evaluators → necessity to step back from our own vision  Chapter 2: Observational tools for astrophysics General context Measurement of the competition level...  Oversubscription factor (OF) : total requested time divided by total available time over a given period  Examples: - VLA : 2 – 3 - GMRT : 2 – 2.5 - VLBA : 4 – 5 - Green Bank Telescope : 3 - EVN : 2 – 3 - Keck : 3 – 5 - Gemini : 2 – 3 - Subaru : 5 - VLT : 3 – 6 (depending on the telescope) - HST : 5 – 6 - XMM-Newton, Chandra : 5 – 6 … 10! - JWST : 1st cycle, 4.1; 2nd cycle, 7.3 - small telescopes : 1 – 2  Chapter 2: Observational tools for astrophysics General context Measurement of the competition level...  Oversubscription factor (OF) : total requested time divided by total available time over a given period  Examples: - VLA : 2 – 3 - GMRT : 2 – 2.5 - VLBA : 4 – 5 Consequence: - Green Bank Telescope : 3 → Most proposals will be - EVN : 2 – 3 rejected - Keck : 3 – 5 → be prepared to work on - Gemini : 2 – 3 something with only weak - Subaru : 5 expectations to get - VLT : 3 – 6 (depending on the telescope) something in return - HST : 5 – 6 - XMM-Newton, Chandra : 5 – 6 … 10! - JWST : 1st cycle, 4.1; 2nd cycle, 7.3 - small telescopes : 1 – 2  Chapter 2: Observational tools for astrophysics General context Why do international observatories work according to a competitive approach?  Observatories LOVE high oversubscription → good argument to say 'our observatory is highly important' → helpful to get funding for operation, maintenance and upgrades; or even to keep it open  It is supposed to warrant on average a high level of science : given the high OF, there is a fundamental need to select some projects, as it is impossible to grant telescope time to all submitted projects  Chapter 2: Observational tools for astrophysics General context Why do international observatories work according to a competitive approach?  Telescope time is expensive → requirement to optimize the observing time (high OF allows to choose among many projects, to do the best science... at least in theory) (Astro2020 APC White Paper)  Chapter 2: Observational tools for astrophysics General context How does it work ?  Calls for proposals are organized regularly, with a periodicity of 4 months, 6 months, 1 year... depending on the observatory  The scientific community is invited to submit proposals, following well- defined guidelines  All submitted proposals are managed by a committee, and are generally distributed to reviewers as a function of the topic  Reviewers are requested to participate in the reviewing process (panel meetings, consultation by e-mail...)  Proposals are ranked according to reviewers opinions  A cut-off is defined according to the available telescope time  Only some proposals are approved, potentially with some different priorities (A, B, C...)  Chapter 2: Observational tools for astrophysics General context Timeline... Formal Decision Start of checks on time observations allocation Call for Deadline Start of Results proposals the communicated reviewing to PIs process ~ a few months  Chapter 2: Observational tools for astrophysics General context The different categories of telescope time proposals  Regular proposals Most programmes submitted to observatories...  Large programmes Total amount of observation time above the average, more ambitious programmes  Target of opportunity (TOO) Request to trigger observations at (almost) any time, not in the regular schedule  Director discretionary time (DDT) Exceptional time granted under unusual circumstances  Guaranteed time observations (GTO) Observation time granted to a consortium who provided instruments, or significant contributions to the observatory  Chapter 2: Observational tools for astrophysics General context The typical content of proposals  A scientific justification Description of the context, the main objectives, the methodology, the targets...  An observation sheet Target lists, instrumental setup,...  A technical justification (not always) Description of the motivation to select a given setup, on the basis of technical details, including also some comments for the telescope operators (either separate or in a unique document)  Chapter 2: Observational tools for astrophysics Some guidelines to apply for telescope time The idea...  You need to have an idea... → don't submit something just for the pleasure to participate in the process It may work (by chance !), but do not really count on that...  The idea is a necessary condition, but it is not enough... → you need to develop your idea (do not give ideas to others without giving you a chance to succeed in the application process) → you need to be pedagogical ! (reviewers won't spend hours reading the bibliography of your favorite topic, and they are most of the time not specialists of your specific research topic)  Chapter 2: Observational tools for astrophysics Some guidelines to apply for telescope time Be conscious of who you are talking to !  Panel members are human beings (with their own sensitivity, strengths and weaknesses...)  Panel members have a reasonable scientific culture on the general thematic, but they are generally not specialists of your specific field → do not expect them to fill for you the missing information in your scientific justification  Panel members are not stupid → don't expect them to believe everything you will write in your proposal → do not offend them  Chapter 2: Observational tools for astrophysics Some guidelines to apply for telescope time Comply scrupulously with the guidelines !  First of all, check for the eligibility of your application  Panel members have to make a selection on the basis of many criteria → not respecting the guidelines is a good opportunity for them to downgrade your proposal  Respect the page limit!  Do not use tiny font size to include more content in your pages!  If figures and tables must be counted in the declared size limit, don't try to include them in addition to your text that already fills the allowed space.  Chapter 2: Observational tools for astrophysics Some guidelines to apply for telescope time Be clear, be clear, be clear !  You certainly know what you mean, but reviewers may not... → be prepared for this and take one step back to reconsider what you wrote  Take care of the abstract A significant part of the feeling of reviewers will be influenced by it. An abstract that is not clear will act against you.  Do not neglect the presentation / layout / visual aspect → avoid filled pages without structure, use sometimes bold faces or italics, when justified use enumerations and not only filled paragraphs  Avoid the extensive use of acronyms  When describing the objectives, do not be too long →in one glance the reader should see clearly what you intend to do  Avoid approximate formulations and be direct...  Chapter 2: Observational tools for astrophysics Some guidelines to apply for telescope time Your target...  Make sure that the data you are asking for do not already exist  If the data exist and you are conscious of that, be very clear about the motivation to ask for additional data  Be clear on your motivation to select that / those target(s), and not other ones The observatory / instrument / setup...  Be clear about your motivation to select that observing facility, with that configuration  Be sure you know about the pros/cons about the selected setup  Chapter 2: Observational tools for astrophysics Some guidelines to apply for telescope time In case of resubmission...  Take carefully into account the comments of the reviewers sent to you after your previous (unsuccessful) submission → not doing that will be considered as a mistake, and your project will be severely downgraded  Some panel members will still be there for the next evaluation, and they may remember about your project (to some extent...) → do not expect the panel to restart the evaluation of your project from scratch  Chapter 2: Observational tools for astrophysics Some guidelines to apply for telescope time Avoid as much as possible to be in a rush !  Be sure the required software you need are working well in advance  Be prepared to face unexpected issues  Don't rely on the efficiency of the internet transfer during the last minutes before the deadline AO7 Integral  Chapter 2: Observational tools for astrophysics The access to astronomical observation facilities  Astrophysics research requires (soon or later) the availability of data obtained in observatories (ground-based or space-borne) in any spectral domain.  The access to these data is crucial, but not as straightforward as one could imagine.  Working on telescope time proposal submission can become a significant part of the working time of astrophysicists, at least during some parts of the year when deadlines are close to each other.  This part of the work can be somewhat frustrating, but it is necessary to access the data we need to work on our research topics.  Now that we have achieved an overview of some tools needed for astrophysics and how to access them, it is certainly time to investigate a bit deeper topics of interest in astrophysics. This is the core of Part II (from Chapter 3 to Chapter 5).