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Telescope Astronomy Physics

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Notes On Telescope - Astronomy Physics

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    Introduction to Astrophysics (Astronomical Telescopes)
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    Telescope Communications antenna Control and pointing systems Instrument and system bays Largest telescopes ?? Optical : GTC, HEC, KECKI, KECK2 Radio: FAST, Arecibo, RATAN-600 Light Solar array Explore mytripx:oi-n Explore My Trip
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    Why build bigger and bigger telescopes? Light gathering power: To allow collection of photons over a larger area that helps to detect fainter and distant objects with greater accuracy. Aperature of the telescope: The larger the aperture, the more light the telescope collects and brings to focus, and the brighter the final image. The largest telescopes are built at the highest altitudes, in order to avoid as much of the Earth's atmosphere as possible (to avoid seeing effect).
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    Why build bigger and bigger telescopes? Resolution: how well you can distinguish two objects, or detail in a single object. It is an angle, usually in arc seconds. High resolution is good, means sharp image. Poor resolution means blurry. How to enhance resolution ? Determined by telescope magnification and diffraction. With increasing resolution, the "blob" on the left is seen to be a cluster of thousands of stars (right) aaau Astronomical Telescopes
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    Two main functions of telescopes: 1. To allow collection of photons over a larger area that helps to detect fainter objects with greater accuracy. 2. To allow higher angular resolution that helps to resolve and study spatial information of extended objects. Magnification: Its ability to enlarge an image, depends on the combination of lenses used. The eyepiece performs the magnification. Since any magnification can be achieved by almost any telescope by using different eyepieces, aperture is a more important feature than magnification. Eyepiece 'Allow you to change the telescope's magnification in order to produce sharp image. ' Provide comfortable eye relief (the distance between your eye and the eyepiece when the image is in focus)
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    Eyepiece Telescope's field of view: The field of view is the amount of sky you can see, whether with your unaided vision, binoculars, or a telescope. Field of view is measured as an angle, in degrees, minutes, and seconds of arc . Apparent field of view: Apparent Field True Field How much of the sky, in degrees, is seen edge-to-edge through the eyepiece alone (specified on the eyepiece). Eyepiece apparent fields range from narrow (250 - 300) to extra-wide angle (800 or more). The true field (or real field) Of view: This is the angle of sky seen through the eyepiece when it's attached to the telescope. Telescope Focal Length Magnification = Eyepiece Focal Length True Field = Apparent Field + Magnification Example: A telescope with a 2000mm focal length, and a 20mm eyepiece has a 500 apparent field. The magnification would be 2000mm + 20mm = IOOx.
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    The true field would be 50 + 100 = 0.50 . Resolving power of Telescopes The resolving power of a telescope may be defined as the ability of a telescope to separate objects with a small angle between them. Limitations: Aberrations due to the optical design or flaws in the manufacture and alignment of the optical components can degrade the resolving power, as well as the seeing effect of the Earth's turbulent atmosphere. However, even if a telescope is optically perfect and is operated in a vacuum, there is still a fundamental lower limit to the resolving power it can achieve. This is known as the theoretical resolving power.
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    . infinite distance aperture image of star Figure shows that the telescope brings the planeparallel waves incident upon the aperture into focus, forming an image of the star in the focal plane. It does this by inducing a phase change on the wavefront which varies across the telescope aperture. However, since the aperture does not cover the entire wavefront radiated by the star, but only a very small portion of it, a diffraction pattern is produced. Resolving Power of Telescopes Airy disc focal plane aperture
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    The diffraction pattern produced when imaging a point source with a lens-based telescope. The image appears as a spot surrounded by concentric rings which decrease in brightness with increasing distance from the centre. The bright central spot, known as the Airy disc after the British astronomer who first studied it, is theoretically predicted to contain 84% of the light, and the first ring contains less than 2%. The size of the Airy disc puts limit on the resolving power of a telescope. a is the angle between the centre of the Airy disc and the first minimum and denotes the theoretical resolving power of the telescope. Resolving Power of Telescopes According to Rayleighls criterion for resolution, two point sources are said to be just resolved when the centre of one Airy disc falls on the first minimum of the other diffraction pattern. This results in a 20% drop in intensity between the maxima. An expression for Rayleighls criterion can be obtained from theory by calculating the positions of minima of intensity in a point-source diffraction pattern. For the first minimum, this gives 9 = 1.22 /D (radians) = 206265 X 1.22 R/D (arcsec) This is known as theoretical angular resolving power of the telescope. ( X wavelength of light and D is the diameter of the lens' aperture)
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    Types of Telescope Focusing light with a lens Object Focusing light with a mirror Object Image 1. Refractor Telescopes: Use glass lenses to focus light rays. Galilean Telescope (1609) Keplerian Telescope (1611) 2. Reflector Telescopes: Use mirrors instead of lenses to focus light rays. Image From the very beginning, refractors suffered from a problem caused by refraction of light. Reflecting and Refracting types of Telescopes Refracting type telescope:
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    Advantages: 2. As there optics are permanently Disadvantages: 1. Heavy- larger aperture telescope 1. Because of their simple design — fixed and aligned — more reliable 2. Longer body— impact on Single lens Achromatic lens are Blue image Red image Yellow image Red and yellow images Blue image transportation and storage 3. Expensive — high quality lenses costly. Cheaper- aberration Reflecting type telescope: 1. Compact and Portable. Advantages: 2. More affordable as mirrors are cheaper than lenses Disadvantages: 1. The mirror needs regular re-alignment 2. Susceptible to spherical aberration
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    Yerkes Observatory Reflecting Telescopes Most modern telescopes use mirrors, they are 'reflecting telescopes"
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    Chromatic Aberrations eliminated Fabrication techniques continue to improve Mirrors may be supported from behind Mirrors may be light-weighted Mirrors may be made much larger than refractive lenses It is a design that allows for very large diameter objectives. Several designs of refelecting telescopes are there. 15 Gregorian Telescope The Gregorian telescope, described by Scotish astronomer and mathematician
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    James Gregory in his 1663 Gregorian Telescope The Gregorian telescope consists of two concave mirrors; the primary mirror collects the light and brings it to a focus before the secondary mirror where it is reflected back through a hole in the centre of the primary. This design of telescope renders an upright image, making it useful for terrestrial observations. There are several large modern telescopes that use a Gregorian configuration such as the Vatican Advanced Technology Telescope, the Magellan telescope, the Large Binocular Telescope, and the Giant Magellan Telescope.
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    Newtonian The Newtonian telescope was the first successful reflecting telescope, completed by Isaac Newton in 1668. eyepiece The primary image from the parabolic mirror is directed at 45 degree from the principal axis by a plane (flat) mirror into an eyepiece located at the side of the telescope. It is one of the simplest and least expensive designs for a given size of primary, and is popular with amateur telescope makers as a home-build project.
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    secondary mirror (convex) Cassegrain eyepiece The Cassegrain telescope was designed by Laurent Cassegrain. primary mirror(parabolic) In the Cassegrain reflector light from the primary mirror is reflected backwards from a secondary, convex mirror and directed through the middle of the mirror. Here the image is further magnified by an eyepiece. The Cassegrain has an advantage over a Newtonian by having a large f-number (ratio of focal length to primary mirror diameter). This allows much greater magnification to be attained. One other important advantage is the length of the telescope. Cassegrains are much shorter.
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    coerectoe plate secondary mArror (convex) Schmidt cassegrain telescope eyep•ecø prima"' mirror(oarabobc) Along with the other advantages of a Cassegrain, a SCT reduces spherical aberration to a minimum. it does this using a 'corrector plate'. This is a specially designed lens, having properties of both convex and concave lenses.
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    Camera Hyperbolic secondary mirror Ritchey • Chrétien (RCT) Hyperbolic primary mirror Ritchey— Chrétien telescope Ritchey—Chrétien telescope was invented in the early 1910s by American astronomer George Willis Ritchey and French astronomer Henri Chretien. A Ritchey—Chrétien telescope (RCT or simply RC) is a specialized variant of the Cassegrain telescope that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors (coma). The RCT has a wider field of view, free of optical errors compared to a more traditional reflecting telescope configuration. Since the mid 20th century, a majority of large professional research
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    telescopes have been Ritchey—Chrétien configurations; some well-known examples are the Hubble Space Telescope, the Keck telescope and the ESO Very large telescope. Nasmyth and Coude Telescope The Nasmyth design is similar to the Cassegrain except the light is not directed through a hole in the primary mirror; instead, a third mirror reflects the light to the side of the telescope to allow for the mounting of heavy instruments. This is a very common design in large research telescopes
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    Focal ratio or f-ratio Focal length (F) is the distance between the light collecting aperture and the primary image or the focal plane. A plane through focal point and at right angles to the optic axis is called the focal plane. Focal ratio, f is defined as the ratio of the focal length F and the diameter D, of an aperture Collecting aperture F (focal length) Optic axis Primary image Focal ratio or f-ratio What does f/ratio mean to the observer? f = FID Focal plane
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    1). Telescope Size / Portability: A 12 inch f/5 is movable by one person comfortably; a 12 inch f/ 15 is a massive instrument suitable only for a permanent observatory. 2). Typical Magnification Range: A telescope of a given aperture (say, 6") and small f/ratio will give lower magnification with a given eyepiece than one with a larger f/ratio. For example: a 120mm f/8.3 refractor with a 20mm eyepiece gives a magnification of 50x; a 120mm f/5 refractor would give a magnification of 30x with the same eyepiece. Thus, short f/ratio telescopes are most easily used for wide field viewing; f/ratio telescopes for high-powered observations. 3). Optical Distortion: Shorter f/ratio telescopes suffer more from certain optical distortions (aberrations) than those with longer f/ratios. In general, f/ratios under f/6 are considered fairly short, f/6 to f/10 medium, and over f/ 10 long. The range f/6 to f/ 10 is a fairly good one for general purpose telescopes. Focal ratio or f-ratio HOME WORK Long
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    A safer comparison would be comparing two telescopes of the same aperture, but with different f/ratios. 1. Which would provide a brighter image an 8 inch f/ 10 or an 8 inch f/6 ? 2. Which would provide more magnification, a 6 inch f/8 or a 6 inch f/5 ? 3. Which would provide a wider field of view a 4 inch f/6 or a 4 inch f/4 ? Plate Scale of a Telescope optical axis apertu re focal plane
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    Light from two distant sources arrives at the telescope. As all astronomical sources are effectively at an infinite distance compared to the dimensions of a telescope, the rays from each source can be assumed to be parallel. The light is collected by an aperture of diameter D which forms an image of the two sources at the focal plane. The line through the centre of the aperture and the focal plane, and orthogonal to them, defines the optical axis of the system. Since the sources are at infinity, the focal length, F, of the telescope is defined by the distance between the focal plane and the aperture. In astronomical telescopes, the aperture is usually a lens or a mirror. Plate Scale of a Telescope The plate scale (p) of a telescope can be described as the number of degrees, or arcminutes or arcseconds, corresponding to a number of inches, or centimeters, or millimeters (etc.) at the focal plane of a telescope. The relationship between the size of the image in the focal plane , 'S' and its angular size on the sky, 6 is given by s=F9 (9 is in radian) Astronomers usually refer to the plate scale (p) in units of arcseconds per mm.
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    o/s = e/(9F) = 1/F=206265/F (arcsecond/mm) P=206265/(fD), Where f is the focal ratio of the telescope. Telescope Mounts a = Latitude Polar Axis )eclination Axis The equatorial mount is Altazimuth - The simplest type of mount with two motions, altitude (up and down/vertical) and azimuth (side to side/horizontal). Good altazimuth mounts will have slow-motion knobs to make precise adjustments. These type mounts are good for terrestrial observing and for scanning the sky at lower power but not for deep sky photography. popular because it simplifies the tracking of celestial objects. For any given location on the Earth, the polar axis is adjusted to align with the Earth's rotational axis, thus properly compensating for Earth rotation at the observer's Latitude. With this adjustment, the polar Altazimuth Mount
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    axis can be aligned with the Earth's spin axis, regardless of observer location (unlike the Altazimuth mount). Astronomical Detectors •The purpose of any detector is to record the light collected by the telescope. •All detectors transform the incident radiation into a some other form to create a permanent record, such as particles (photographic plates), photoreceptor molecules (eyes), or electrons (CCDs). •There are eight important properties by which to gauge the utility of a detector: 1) Quantum Efficiency and Spectral Response 2) Temporal Response and Resolution 3) Dynamic Range 4) Linearity and Stability 5) Noise 6) Spatial Resolution and Field of View
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    7) Ease of Conversion to Digital Signal Quantum Efficiency (QE) and Spectral Response How much light are you wasting? • Quantum efficiency is defined as the percentage (or fraction) of incident photons that are detected [QE = # photons detected / # incident photons]. •A detection can be defined in any number of ways, such as crystals formed (photographic emulsion) or charge-coupled device (CCD). The point is that the photon is recorded. • Ideally you want QE=IOO%. The best we can get at optical wavelength is 90%. • No detector is efficient at all wavelengths, but rather has a QE that varies with wavelength. The spectral response is the dependence of QE upon wavelength. Typical peak values: Eye: 1-2%, Photographic plate: 1-2%, photomultiplier tube: 20-30%, CCD: 70-90%, IR array (HgCdTe): 30-50%
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    Quantum Efficiency (QE) and Spectral Response CHARGE-COUPLED DEVICE (THINNED) 100 (THINNED CCD) (BULK) z 10 0.1 0.2 (BULK CCD) PHOTOMULTIPLIER TUBE VIDICON PHOTOGRAPH EYE 0.3 0.4 0.5 0.6 0.7 0.8 0.9 WAVELENGTH OF RADIATION (pm) 1.0 1.1
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    Temporal Response How quickly can you take images, and how long do you have to wait? What is the shortest integration possible ? —Time variability ' How long can you integrate? —Faint sources (related to dynamic range and detector noise properties) ' How quickly can you take consecutive images (readout time)? —Time variability —Observing efficiency
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    Dynamic Range Can I look at bright and faint things at the same time? Denotes the ratio between the strongest signal falling on the detector and the weakest measurable signal (or noise level). Camera iXon3 885 EMCCD EM Amplifier with No Gain (dynamic range hüher with EM gain) Luca S EMCCD EM Amplifier with No Gain iXon3 897 EMCCD Conventional Amplifier@ IMHz (dynamic range hüher with alternative pre-amp) Newton 920 CCD 50kHz Readout Rate Pixel Size m2 8 x8 10 x 10 16 x 16 26 x 26 Full Well Capacity e- 30,000 25,000 180,000 510,000 Read Noise 20 (x3 pre- amp) 15 9 (xl pre- amp) 10 (x3 pre- amp) Dynamic Range 1 ,500 1 ,667 20,000 51,000 Decibels dB 86 Bits 11 11 15 16 Dynamic range = largest possible signal / smallest possible signal •The dynamic range is relevant because it tells you for a single image the range of object brightnesses you can observe. •A related quantity for digital detectors is the full well depth (capacity), which tells you how many detections your detector can record before saturation. Example: CCD # of detections is an integer (so min=l), Assume full well depth = 216 (65,536) counts Dynamic range = 65,536/1 = 65536. In magnitudes, this is 12.04 mag of dynamic range.
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    Linearity ' Ideally, you want the response of your detector to be linearly proportional to the number of photons (1 photon •1 detection). ' Human eye and photographic plates highly nonlinear ' CCDs and most other modern detectors are nearly linear —In some cases, must still apply a "linearity correction". 5% non-linear (Saturation ÜöVel) Characteristic density curve c 1.2x105 1 05 8.ox104 4.0x104 2.ox104 o 25 to os 400 500 1.0 20 LP to density •i to — a too — $troØt part to from maximum 23 30 as ig ex904ute 100 CCD 200 300 Time (sec) Photographic Plate
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    Stability Will it count the same tomorrow as today? • How reliable is your photometry ? Is the efficiency of your detector stable in time? —Sensitivity of photographic plates degrades with time, especially in high humidity. —Sensitivity of detectors on satellites can degrade with time due to hard radiation and cosmic rays. Noise It is a sad fact of life that detectors introduce additional noise into observations. Types of noise:
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    1. Poisson noise : Goes as Ni/2, where N is the number of photons. For an ideal detector this is the only source of noise. Two components that contribute to the Poisson noise are (i) source photons, and (ii) background sky photons. 2. Read noise (RN): Some detectors, like CCDs, generate additional noise when the signal is read out 3. Thermal noise (dark noise): thermal agitation of electrons in detector. 4. Noise in electronics: self-explanatory For research instruments, Poisson and read noise are usually the dominant sources of noise Angular Resolution and Field of View Angular resolution: —Recall that the image scale (in arcsec/mm) is determined by the design of the telescope —Two pixels are required to resolve an object.
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    —Consequently, for a detector the angular resolution = image scale • 2 ' pixel size Field of view: FOV = image scale ' Npix ' pixel size Want the resolution to be well-matched to the telescope. —If the detector resolution is worse than the seeing, you're sacrificing performance. —If the detector resolution is much better than the seeing, you're sacrificing field of view Ease of Conversion to Digital Signal For quantitative analysis, you want to convert your data into a digital format. • Eye: out of luck (for now) • Photographic plates: densitometer • CCDs and other modern detectors: direct digital output
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    Table 1.1.1. Classification scheme for types of detector. Sensitive parameter Voltage Resistance Charge Current Electron excitation Electron emission Chemical composition Detector names Photovoltaic cells Thermocouples Pyroelectric detectors Blocked impurity band device (BIB) Bolometer Photoconductive cell Phototransistor Transition edge sensor (TES) Charge-coupled device (CCD) Charge injection device (CID) Superconducting tunnel junction (ST J) Photographic emulsion Photomultiplier Television Image intensifier Eye Class Quantum Thermal Thermal Quantum Thermal Quantum Quantum Thermal Quantum Quantum Quantum Quantum Quantum Quantum Quantum Quantum


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