Telescopes Light Colletors

Op#cal-infrared telescopes Light colletors

Atualização: 4/5/18 AGA5802, Jorge Meléndez Ask students to mount telescope & discuss: - Diameter of the objec#ve - Angular resolu#on - Focal length (or focal distance) - Focal ra#o f/# - Angular size vs. Physical size - Field of view of CCDs - Problems with lenses and mirrors - Diﬀerent loca#ons for focal plane - Mounts - Coa#ng Bibliography

Kitchin – Astrophysical Techniques (5th Ed, 2009), Chap. 1 (par)

Léna – Observational Astrophysics (2nd Ed, 1998), Chap. 4 (par)

Birney – Observational Astronomy (2nd Ed, 2008), Chap.6

Smith – Observational Astrophysics (1995) Chap. 2

Walker – Astronomical Observations (1995), Chap. 2 e 3

www.das.inpe.br/~claudia.rodrigues/ www.if.ufrgs.br/~santiago/ www.astro.caltech.edu/~george/ ngala.as.arizona.edu/dennis/ Bibliography

Roy & Clarke – Astronomy: Principles and Practice (4th Ed, 2003)

Roth – Handbook of Practical Astronomy (2009), Chap. 4 Schroeder – Astronomical Optics (2nd Ed, 2000), several chaps. Lecture notes, Prof. Antonio Mário Magalhães Main characteris#cs

collects energy ( ∝ collector area)

image forma#on: ang ular resolu#on

● Increase in collec#ng area à

detec#on of fainter object s

beUer angular resolu#on Angular resolu#on

Is the capacity to resolve ﬁne details. The resolu#on is the smallest angle that could be discerned Angular resolu#on: diﬀrac#on of a slit Light diffraction imposes a limit on angular resolution The diffraction pattern of a slit of width d is:

-3λ/d -2λ/d -λ/d λ/d 2λ/d 3λ/d Angular resolu#on: diﬀrac#on of a circular aperture Diffraction pattern

Lens

Primeiro zero da função de Bessel Airy disk: central disk defined by the first Roy & Clarke minimum Angular resolu#on: circular aperture (c) Kitchin Airy function sets the resolution limit

First zero of Bessel func0on Note: r = d/2 Roy & Clarke Angular resolu#on: Rayleigh criterium The resolu#on limit is when the maximum of one Airy disk is over-imposed on the 1st minimum of the other image

The first minimum occurs at a radius α [rad]: α = 1.22 λ /d

HST (2.4m) ~ 0,05”

(c) Kitchin Rayleigh vs. Dawes criteria Rayleigh criterion: α = 1.220 λ / d Empirical Dawes criterion ~10% lower FWHM = 1.029 λ / d

In prac#ce both limits are diﬃcult to reach due to imperfec#ons of the instrument and the perturba#ons of the terrestrial atmosphere Roth Angular resolu#on: Rayleigh criterium

α [rad] = 1,22 λ /d For V = 540nm: α[”] = 0,136 /d[m] A telescope with d = 13,6 cm reaches in the op#cal an angular resolu4on of 1”, about the same than the limit imposed by seeing (~ 1”) Angular resolu#on: Rayleigh criterium

α [rad] = 1,22 λ /d For V = 540nm: α[”] = 0,136 /d[m] For K = 2,2μm: α[”] = 0,55 /d[m] In the K band (infrared), a telescope with d = 1 m reaches an angular resolu4on of 0,55” Focus, focal plane, focal distance F: focal distance, D: aperture of the lens/mirror

Lens axis

Principal focus Lens (a.k.a. focal point) Focalplane

Principal axis D

focal length is the distance to image an object at infinity

www.antonine-education.co.uk/Physics_A2 www.antonine-education.co.uk/Physics_A2 F Focal length Focal ra4o f/# F: focal distance, D: aperture, focal ra#o f/# Lens axis

Lens Principal focus Focalplane

Principal axis D F www.antonine-education.co.uk/Physics_A2 www.antonine-education.co.uk/Physics_A2 Focal length Focal ratio ≡ f-ratio ≡ f-number ≡ f/# = F/D Short (low) f/ (e.g. f/5): faster: brighter: wider ﬁeld of view www.antonine-education.co.uk/Physics_A2 Response: F/D = 10, D=1,6m the Cassegrain focus. Find the focal distance F Example: the 1,6m telescope at OPD has f/10 at F: focal distance Focal ra4o D

Lens , D: aperture, focal ra#o Lens axis Principal focus Principal F f/# = F /

à F = 16m Focal plane D

f/# Focal plane & focal length (or focal distance) study.com Focal plane & focal length

mirror Light gathering power (LGP) LGP is the area of the objec#ve LGP = π (D/2)2

Young person in

darkness: de ~7mm Ex. Telescopes rela0ve to the eye: 10 cm: 200 0mes ! 8m: 1.3x106 0mes ! Signal-to-noise ra#o (S/N) S/N = sinal/ruído

S/N = 2 S/N = 4 S/N = 8 S/N = 16 simula#on of stars observed with a signal- to-noise ra#o ranging from 2:1 to 16:1 Integra4on 4me Joining the 4 ESO/VLT telescope (8m each) we can have a super-VLT with a diameter equivalent to 16m. If we can observe a galaxy in 1h with the super-VLT, how long would it take to achieve the same S/N if the observa#on is done with the 1,6m telescope at OPD?

Rpta: 100 hours. Considering 2h of observa#on at OPD (near the meridian) by night, the observa#on may take 50 nights … 2 t1 = (D2/D1) t2

t1 , t2 : observing #me with telescopes of diametersD1 , D2 Physical size of the image F Focal distance

Angular size of Image height image in radians Lens s D θ

ircamera.as.arizona.edu Physical size s of image θ on the detector: s = F tanθ ~ F θ Example: if F = 8m, estimate s for θ = 1” 1 rad = 206265” à s = 8m*1/206265 = 3,9 x 10-5 m = 39 µm How large is the pixel of a CCD?

m m m m m Physical size of the image, plate scale F Focal ra0o = F / D Focal distance

Angular size of Image height image in radians Lens s D θ

ircamera.as.arizona.edu

Physical size s of image θ: s = F tanθ ~ F θ Plate scale p = dθ/ds = 1/F = 206265”/ F IAG: f/13.5; D=61cm. What is the plate scale? F = 13.5 x 61cm = 8235 mm p = 206265”/8235mm = 25”/mm = 0.5”/20µm (1mm=103 µm) Field of view F Focal distance

Angular size of Image height image in radians Lens s D θ

ircamera.as.arizona.edu Plate scale p = dθ/ds = 1/ F = 206265 / F Using the same CCD chip on two telescopes with different focal lengths affects the field of view and image scale, but not the display size (because the number of pixels is the same). CCD: 9-micron pixels, 765x510 array Telescope A: 500mm focal length A B Telescope B: 1000mm focal length Refractor telescopes: disadvantages ● Mechanical Lens must be supported on the edges. As there is no central support, it may get distorted ● Size of the tube In order to diminish distortions (chromatic/spherical) the focal distance must be large, increasing the cost $$$. Ex: huge domes ● Absorption by the lens Absorption of UV light ● Imperfections in the lens or air bubbles Longest refractor (1m) at Yerkes observatory Refractor telescopes: chroma#c aberra#on

Refraction index n(λ) à focal distance f(λ)

Chromatic aberration Refraction index n(λ) à focal distance f(λ) Correction with achromatic lens” by fusing lenses (doublets, triplets, etc): but correction is not perfect Disadvantages of refractors: coma Coma can affect also reflector telescopes Coma affects parabolic mirrors. The rays parallel to the axis of the parabola are OK, but if the rays strike © Kitchin: The severity of the coma at at an angle, then a given angular distance from the there is coma op4cal axis is: 1/(f/#)2

In Newtonian reﬂectors a focal ra#o of f8 or larger gives acceptable coma for most purposes. At f3, coma will limit the useful ﬁeld Roy & Clarke of view to about 1 of arc Disadvantages of refractors: as#gma#sm Astigmatism Rays that pass through the tangential (optical axis) & sagittal plane do not focus on the same point Can affect also reflectors

Roy & Clarke Other aberra4ons

● Image curvature: ﬂat object shows distorted image, i.e. focus is not on a plane

● Distortion: varia#on of the magniﬁca#on on the image plane Reﬂector telescopes I “Invented” by Newton (1668) At least the first working reflector

●Objec#ve: mirror

Does not suﬀer chroma#c aberra#on

Newton’s mirror was spherical, but a parabola is more common, because it focuses at a single point Roy & Clarke Spherical aberra4on Spherical mirror (spherical aberration)

Parabolic mirror

Types of focus: prime focus Smith

Jesse P200 Palomar Greenstein at Prime focus: f/3,3 prime focus cage Anglo Australian Telescope Image © 1997, text © 2010, Australian Astronomical Observatory, photograph by David Malin (himself inside the prime focus cage) Prime focus of Subaru (8m) Smith f/1,8

Subaru http://oir.asiaa.sinica.edu.tw/subaru/pfs.php Prime focus of Blanco telescope (4m at CTIO)

Installa#on of DECAM DECAM installed (September 2013) 2,2° FOV (62 CCDs) DECAM + 12 CCDs (guiagem e foco) (Dark Energy Camera)

f/2,7 primary

153 pix

4096 pix pix 2048

hUp://www.noao.edu/mee#ngs/decam/media/DECam_Data_Handbook.pdf DECam Fornax mosaic

© Dark Energy Survey Collabora#on NGC 1365 is a barred spiral galaxy around 60 million light years from Earth, located in the Fornax galac#c cluster. © Dark Energy Survey Collabora#on Reﬂectors: Cassegrain focus adequate position to put instruments compact telescope: secondary increases the focal distance of the telescope

Secondary: Primary: hyperbolic parabolic

Kitchin Reﬂectors: Cassegrain focus

Secondary: hyperbolic

Primary: parabolic Palomar 5m: Last P200 big telescope of Palomar Cassegrain type Reﬂectors: Ritchey-Chré#en telescope Variation of Cassegrain with hyperbolic primary: better image quality Secondary: hyperbolic

Kitchin

George Ritchey's 24- inch (60cm) reflecting telescope, first RCT Primary: constructed in 1927 hyperbolic Reﬂectors: Ritchey-Chré#en telescope HST: 2.4m, fsys=57.6m, f/24 Cut through (comprimento do telescópio HST é de 13m)

secondário hiperbólico

primário hiperbólico SOAR: 4.1m, f/16

LNA: 1.6m, f/10 Nasa Reﬂetors: focos Nasmyth & Coudé Nasmyth telescope with alta-azimutal mount flat mirror at the altitude axis position of the focus is fixed: independent of the movement of the telescope adequate for heavy instruments

Keck

Big telescopes have Nasmyth focus for big instruments, except Gemini … Reﬂectors: Coudé focus Coudé: similar to Nasmyth but for equatorial mount Example: Coudé spectrograph at OPD

Coudé focus Kitchin Smith Reﬂetors: problems similar to those of refractor telescopes ● Spherical ● Coma ● Astigmatism ● Image curvature ● Distortion

● Vignetting (is not a fault of mirror or lens. It arises because of uneven illumination on the image plane (obstruction of the light path by parts of the instrument) Mounts: Equatorial

Birney Mounts: Equatorial / German

Roy & Clarke Mounts: Equatorial / Fork (Garfo)

Roth Fork equatorial 2,2 m telescope at La Silla Fork mount

Old large telescopes (<=5m) have horseshoe mount

Palomar 200 (replica) Mounts: Alta-azimutal common at all large telescopes Zénite Altura

Gemini North Azimute Subaru has Prime + Nasmyth + Cassegrain focus Smith

Gemini has only Cassegrain focus

Subaru The poin#ng and guiding are not perfect (in any mount) mechanical ﬂexure errors in the gears (“engrenagens”) Atmospheric refrac#on

Correc#on of the guiding is made by guiders & auto- guiders à needed for long integra#ons © To the Sky, Measure Other focus: Gregorian

Ellipsoidal secondary Chromey

Parab. Parab.

Hiperb. Ellipsoid.

RC: primary and secondary hyperbolic

Parabol. primary © To the Sky, Measure Other focus: Gregorian

Ellipsoidal secondary Magellan 6.5m telescopes Chromey

Parabol. primary Schmidt Camera (catadioptric telescope)

Parab. Parab.

Hiperb. Ellipsoid. Proposed by Bernhard Schmidt (1879 – 1935) in 1929 telescópio híbrido: lens+mirror

- espelho primário esférico (sem coma do parabólico)

- lente antes do primário corrige aberração esférica

- única aberração: curvatura de campo Schmidt Camera (catadioptric telescope) Proposed by Bernhard Schmidt in 1929

Corrector lens

Photographic film

Spherical primary (spherical aberration) Spherical primary + lens eliminates aberration

Spherical primary Figures from Smiley et al. 1936, PA 44, 415 © Kitchin Palomar Observatory Sky Survey - POSS The Survey utilized 14-inch square photographic plates, covering about 6° of The 48 inch (1,22m) Oschin Schmidt sky per side (approximately 36 square Telescope at the Palomar Observatory degrees per plate). November 11, 1949 to December 10, 1958.

!The STScI Digitized Sky Survey http://stdatu.stsci.edu/cgi-bin/dss_form M31 from the Digitized Sky Survey, 60 x 60 arcmin

Reﬂectors : mirror coa0ng Reﬂetores: mirror coa4ng ● Coating in the optical-UV:

aluminum; infrared: silver micro.magnet.fsu.edu/primer/lightandcolor/

Until recently, silver was not much used in big telescopes, because the coating degrades quickly (months). Since 2004 Coating of the 0.6m telescope Gemini uses “protected” of Serra da Piedade at LNA silver Gemini South : First large telescope with protected silver coa#ng Gemini S. primary

Instruments

Coating chamber

Ana (AGA5802 student) in front of Gemini South 100 90 Reflectivity (%) 400 600800 100012001400160018002000 95

Protected silver coating

Al Coating coating of M1 at Gemini Only 50g of for silver less emissivity in emissivity less the IR and Ag: reflectivity better http://www.gemini.edu/node/16 Wavelength (nm) Field rota4on in AltAzimutal telescopes

Left to right: Jorge + AGA5802 students (Patricia, Fernando, Ana, Nathalia, Viviane, Miguel, Marcelo, Andressa) @ NTT 3,6m telescope (La Silla, ESO), April 2012 One of the NTT field de-rotators at a Nasmyth focus © Jorge M., La Silla, April 2012 Extra slides from previous semesters.

Feel free to read them (or not). Who discovered the telescope? - Hans Lippershey (1608): first telescope?

- Patent: "for seeing things far away as if they were nearby" Galileu (1609) Reﬂec#on & refrac#on laws Law Law of reﬂec4on of refrac4on (Snell) α=β sin θ / sin φ = n2/n1 UCIrvine Types of light collector Objective: lens or mirror Lens: refractor telescope

● Historical importance (1608-9)

● Still used in amateur astronomy

Mirror: reflector telescope

● Newton (1668) invented the first useful reflector

● All big telescopes are reflectors The ﬁrst reﬂector?

Eyepieces, magniﬁca4on Lens Focal ratio = F / D

d Magnification m = F / f D Pupil Pupil exit d = D/m eyepiece exit

F f Smith Focal distance of the objective Focal distance of the eyepiece Field of view: examples Plate scale p = dθ/ds = 1/ F = 206265 / F

Using the same telescope with different CCDs affects the field of view and display size, but not the image scale. Telescope: 500mm focal length CCD A: 9-micron pixels, 765x510 array CCD B: 9-micron pixels, 1530x1020 array A B Using different CCDs and different telescopes can lead to any number of results, like for example equivalent fields of view. In this case, a large CCD with a long-focal-length scope gives the same field as a small CCD on a short-focal-length scope, but the image scale is greater on the larger instrument. The display size is also considerably larger in the second image. Telescope A: 400mm focal length CCD A: 7.4-micron pixels, 640x480 array A Telescope B: 1250mm focal length CCD B: 6.8-micron pixels, 2174x1482 array B Reﬂetores: Newtonian focus Smith

Newtonian focus Kitchin ½ degree away from the axis of the image Reﬂectors: image On-axis quality do prime/ image Newtonian Smith

Airy disk

5”

Prime ½ degree away focus from the axis of the image On-axis image Newtonian focus Kitchin ●Transit mount

Meridian circle (astrometry)

→ IAG/USP – Valinhos

Smith Reﬂectors: other focus The 48-inch (1.2m), Schmidt Telescope (f/2.5; covers 70) at Palomar. Map of the whole Northern Hem. (basis for GSC)

Hubble Maksutov (catadioptric telescope)

Parab. Parab.

Hiperb. Ellipsoid. Schmidt-Cassegrain (catadioptric telescope)

Elliptical secondary (or spherical) Chromey

Spherical primary Aluminizing of primary mirror 200-inch Palomar telescope Aluminizado do espelho primário do telescópio P200 (5m) hUp://www.youtube.com/watch?v=gxp6aMhoT9U