Astro 500 1
Find values for WIYN & SALT instr.: Detector gain, read-noise, system efficiency WIYN Ø WHIRC Ø Bench Spectrograph Ø MiniMo Ø OPTIC What did you find? Ø ODI SALT Assignment: Ø SALTCAM v Work in pairs Ø RSS v Tabulate information v Assess availability, ease of access v Bring to class to present 2
Homeworks Show your work Cite your sources Make your write-up neat and organized Ø I can t grade it properly if I don t know what s going on. Wikipedia is not a peer-refereed source. Ø If you use a result from wikipedia, show the derivation make sure wikipedia is correct! 3
Telescopes & Optics Outline Defining the telescope & observatory Mounts Foci Optical designs Geometric optics Aberrations Conceptually separate Critical for understanding telescope and instrument capabilities and preliminary instrument design Essential for detailed instrument design and for evaluating data. 4
Telescopes as Facilities The Observatory: far more than telescope Ø Opto-mechanical and thermal control Ø Acquisition & guiding Ø Telemetry and sensing Ø Instrumentation and instrument interfaces (ports) Ø Software for telescope and instrument control Ø Technical support and maintenance Ø Data storage and transfer Ø Software pipelines for data reduction and analysis Ø Environment for observer and operator Ø Personnel management, technical and scientific leadership 5
Telescopes: broader relevance Telescopes are examples of optical systems serving as cameras, i.e., producing images from sources at infinity (collimated light). Spectrographs and re-imaging systems essentially use variants of these systems as collimators and cameras. 6
Telescopes What parameters define telescopes? Ø Collecting Area (A) Ø FOV (Ω) Ø Image Quality Ø Cost Ø Throughput Ø Spectral range Ø Plate scale/magnification Ø Pointing/tracking highly coupled We will focus on optical/ir telescopes; Most of this parameter space is relevant for light collection at other wavelengths. Telescopes as cameras Telescopes 7
Examples of Telescopes Ø Refracting o Galilean (1609) o Keplerian (1611) Ø Reflecting (4x harder to make!) o Newtonian (1672) o Gregorian (1673) o Cassegrain (1668) o Ritchey-Cretien (1927) Ø Catadioptric o Schmidt (1931) o Maksutov (1944) Why? Ø Multi-mirror o MMT (1977) o Keck (1992) o HET (1996), SALT (2005) o all ELT s (2014+) Trends to larger collecting area, but more compact in mounting and focal-ratio (speed). 8
ELT s Hale Keck 1&2 SALT VLT1-4 Herschel Parsons Yerkes Newton Galileo Credit: R. Racine 9
Telescope Mounts 3.5-4.5m 6.5-10m Ø Equatorial* o German (Yerkes) o English Yoke or English Cross o Fork (Shane 3m) o Horseshoe (CTIO & KPNO 4m) Ø Alt-Az (Altitude-Azimuth) o ARC, WIYN, ARC, SOAR o Keck, VLT, Subaru, LBT, MMT Megallan Ø Transit o Arecibo (radio) o LMT - liquid mirror telescope Ø Transit-Azimuth o HET, SALT First: 1977 Easiest for tracking Best for flexure, but pointing limitations Best compromise Most compact but requires computer control; has field rotation Cheapest but limited sky coverage (40%) Cheap and up to 70% sky coverage *https://en.wikipedia.org/wiki/equatorial_mount 10
Telescope Foci Ø Prime Ø Newtonian Ø Cassegrain What s the difference? Ø Folded Cassegrain Ø Nasmyth Ø Coude Where s prime focus? 11
Types of Telescopes Category Primary Secondary Corrector Name Principal Aberrations Singlet lens Spherical refractor spherical + chromatic Singlet mirror Paraboloid mirror Newtonian coma + astigmatism Doublet mirrors Paraboloid Hyperbaloid Cassegrain coma Hyperbaloid Hyperbaloid Ritchey-Chertien (RC) astigmatism (twice Cassegrain field) Parabaloid Ellipsoid Gregorian field curvature Ellipsoid Spherical Dall-Kirkham Ellipsoid Ellipsoid Aplonatic Gregorian (AG) Multiplets Spherical Aspheric lens or Schmidt achromate doublet Spherical Hyperbaloid Aspheric lens Schmidt-Cassegrain best images but large obstruction v. wide field Spherical Spherical Spherical meniscus lens Maksutov Spherical 4-mirror asphere HET, SALT Low cost 12
Singlets 13
Doublets* *with a confusing mixture of labels for telescope types and foci 14
Multiplets: Catadioptrics Spherical Primary 15
Why different designs? Trade-offs primarily between cost, desired field of view (at given image-quality) and collecting area. Image-quality and field-of-view concern aberrations. 6 principal aberrations: Off-axis Radial Ø Chromatic Ø Spherical Ø Coma Ø Astigmatism Ø Distortion Ø Field curvature Lenses only The Big 3 Not strictly aberrations 16
Where we re headed Geometric Optics Ø Reflection Ø Refraction Ø Thin lens Ø Spherical optics Ø Conics Ø Stops Ø Pupils Ø Chief rays Ø Marginal ray Ø System design Optics Aberrations Chromatic Spherical Coma Astigmatism Distortion Field curvature Ø Telescope summary 17
Reading Walker - ok place to start Schroeder - ferocious, but very rigorous and remarkably general and complete McLean - fairly complete, but missing basics Ø Recommend: 1. Start with Walker to get an overview 2. Pick up McLean and a basic optics book (e.g., Meyer-Arendt, Introduction to Classical and Modern Optics ) and work out quoted results; return to Walker to reconcile results and nomenclature. 3. Then hit Schroeder s text running to go to expert level. 18
Some Everyday Concepts Specular and Diffuse Reflection Refraction Specular Diffuse Refraction 19
The Thin Lens s parallel ray & marginal ray 1 f = 1 s + 1 s' m = s' s = h' h = magnification f ratio = f /D, D = clear aperture h focal ray chief ray f Image Object f h 1 f P = optical power s 20
The Thin Lens h s D 1 f = 1 s + 1 s' m = s' s = h' h = magnification f ratio = f /D, D = clear aperture f Image Object Object at f h s 21
Spherical Optics Focal Length Defined Definition 1 f = 1 s + 1 s' = 2 R Object at Infinity s C s 1 = s' 1 f 1 = s' f = R/2 is referred to as the paraxial focal-length 2 R R f 22
Snell s Law n and n depend on λ Index = n θ Q θ P Index = n B C 1 V = (n F n C ) /(n D 1) = dispersive power F,D,C: solar-spectrum Fraunhofer lines (486, 589, 656 nm). V vs n is called a glass table. Fermat s principle: least time 23
An Aside: Implications of Snell s Law α (n 1)θ z (n 1)t n % d t sinθ 1 cosθ ( ' * & n cosθ' ) t θ α z n n n θ θ d Impact of wedges and and plane parallel plates on an optical beam If beam is converging, astigmatism introduced. Can be eliminated with wedge. 24
The Thick Lens s parallel ray & marginal ray h focal ray chief ray n 1 n 2 f 2 Image Object f 1 h n 2 = n n 2 1 f 2 R = refractive power f 2 /n 2 = reduced focal length R = radius of curvature s 25
Optical Power, P Two surfaces separation d index n n R 1 P 1 f = P 1 + P 2 d n P 1 P 2 R 2 Lensmaker s formula: f Two-surface spherical lens in air or vacuum R 1, R 2 radii of curvature R>0: center of curvature behind lens P = 1 f = (n 1) # % 1 1 + $ R 1 R 2 units : dioptor (m 1 ) d(n 1) nr 1 R 2 & ( ' 26