Basic optical principles Refractor Reflector Advantages and disadvantages of each Discussion of F-ratios/focal lengths/speed.

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1 PH507 Astrophysics Dr Dirk Froebrich 1 Lecture 1: Introduction to telescopes Lecture Outline. Lecture #1: Introduction to telescopes Basic optical principles Refractor Reflector Advantages and disadvantages of each Discussion of F-ratios/focal lengths/speed. Lectures #2, #3: Introduction to the E-M spectrum General discussion of E-M spectrum from radio -> gamma rays Heterodyne receivers Bolometers & Golay cells Photoconductors Hand out assignment #7. Lecture #4 and #5: Visible and beyond detectors. CCDs Grazing incident focussing (X-rays etc.) Exotic detectors (neutrino, gravity wave etc.). Lecture #5: Space and ground based observatories Descriptions of ground based observatories and instruments (JCMT, SCUBA2, ALMA etc.). Descriptions of space based observatories and instruments (SIRTF, Spitzer, Kepler etc.). List of on-line resources.

2 PH507 Astrophysics Dr Dirk Froebrich 2 Telescopes. There are two basic types of telescopes, refractors and reflectors. The part of the telescope that gathers the light, called the objective, determines the type of telescope. A refractor telescope uses a glass lens as its objective. The glass lens is at the front of the telescope and light is bent (refracted) as it passes through the lens. A reflector telescope uses a mirror as its objective. The mirror is close to the rear of the telescope and light is bounced off (reflected) as it strikes the mirror. Refractor Telescopes The refractor telescope uses a lens to gather and focus light. The first telescopes built were refractors and the invention of the refracting telescope is generally ascribed to Galileo in 1609, although it is recorded that a Dutch lens maker called Hans Lippershey built one in The small, cheap toy telescopes sold in shops are generally of the refractor type. Refracting Telescopes focus light using lenses and the Principle of Refraction. Note: n is a function of the wavelength of the incident radiation. Generally, as wavelength increase, n decreases. So shorter wavelength light (e.g. blue) is bent more than longer wavelength light (e.g. red).

3 PH507 Astrophysics Dr Dirk Froebrich 3 The Refractive Index of Air is very nearly 1 (n air = , depending on Temperature & Pressure, but astronomers need to take this into account when calculating wavelengths in the earth's atmosphere). The refractive index of water is n H2O = 1.33 and refractive indices for various kinds of glass vary from about n = A diamond's luster is partially due to its high refractive index, n = 2.4. In order to look through a telescope you need two lenses, the objective, which is the principal lens of the telescope, and an eyepiece. The image scale in the focal place is determined by the focal length of the objective; if you look through the telescope, the magnification will be determined by the ratio of the focal lengths of the objective and the eyepiece.

4 PH507 Astrophysics Dr Dirk Froebrich 4 The sensitivity of the telescope is determined by the collecting area of the objective lens (or primary mirror) which is proportional to the square of the diameter of the primary lens or mirror: Area = π d 2 2 Though excellent refractors are still made, the disadvantages of the refractor telescope have blocked the construction of very large refractors for use in astronomical research. Advantages 1. The glass surface inside the tube is sealed from the atmosphere so it rarely needs cleaning. 2. Since the tube is closed off from the outside, air currents and effects due to changing temperatures are eliminated. This means that the images are steadier and sharper than those from a reflector telescope of the same size. 3. Refractor telescopes are rugged. After the initial alignment, their optical system is more resistant to misalignment than the reflector telescopes. 4. Generally produce a sharper image than reflectors due to the lack of secondary mirror support structure. Disadvantages 1. All refractors suffer from an effect called chromatic aberration (``color deviation or distortion'') that produces a rainbow of colors around the image. Because of the wave nature of light, the longer wavelength light (redder colors) is bent less than the shorter wavelength light (bluer colors) as it passes through the lens. This is used in prisms to produce rainbows, but can it ruin an astronomical image!

5 PH507 Astrophysics Dr Dirk Froebrich 5 There are a couple of ways to reduce chromatic aberration. One way uses multiple compensating lenses to counteract chromatic aberration. The other way uses a very long objective focal length (distance between the focus and the objective) to minimize the effect. This is why the early refracting telescopes were made very long. 2. How well the light passes through the lens varies with the wavelength of the light. Ultraviolet light does not pass through the lens at all. 3. How well the light passes through decreases as the thickness of the lens increases. 4. It is difficult to make a glass lens with no imperfections inside the lens and with a perfect curvature on both sides of the lens. 5. The objective lens can be supported only at the ends. The glass lens will sag under its own weight. Note: A 40 (~1 meter) diameter refractor lens weighs approximately tons! Because of these disadvantages, the largest refractor telescope ever built is one at the Yerkes Observatory, which was built more than 100 years ago! It has an objective 1.02 meters (40 inches) across at one end of a 19.2-meter (63 feet) tube. The two largest refractors are shown below. The first picture is the 40-inch refractor at Yerkes Observatory. The second picture shows an astronomer (Kyle Cudworth) next to the objective to give you an idea of the size of the telescope. Notice the size of the people in the first picture! The third picture is the 0.91-meter (36-inch) refractor at Lick Observatory. Notice the astronomer at the lower left. The last picture is E.E. Barnard at the eyepiece of the Lick 36-inch.

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7 PH507 Astrophysics Dr Dirk Froebrich 7 Reflector Telescopes The reflector telescope uses a mirror to gather and focus light. All celestial objects (including those in our solar system) are so distant that all of the light rays coming from them reach the Earth as parallel rays. Because the light rays are parallel to each other, the reflector telescope's mirror has a parabolic shape. The parabolic-shaped mirror focuses the parallel lights rays to a single point. All modern research telescopes and large amateur ones are of the reflector type because of its advantages over the refractor telescope. Advantages 1. Reflector telescopes do not suffer from chromatic aberration because all wavelengths will reflect off the mirror in the same way. 2. Support for the objective mirror is all along the back side so they can be made very BIG! The larger reflecting telescopes have primary mirrors >8 meters in diameter; compare that to 1 meter for refractors. This gives a factor of 64 improvement in light gathering power. 3. Reflector telescopes are cheaper to make than refractors of the same size. 4. Because light is reflecting off the objective, rather than passing through it, only one side of the reflector telescope's objective needs to be perfect.. Modern reflecting telescopes use a parabolically shaped primary mirror coated with a thin film of aluminum.

8 PH507 Astrophysics Dr Dirk Froebrich 8 Disadvantages 1. It is easy to get the optics out of alignment. 2. A reflector telescope's tube is open to the outside thus thermal differences in the tubes can effect the seeing. Also the optics need frequent cleaning. 3. Often a secondary mirror is used to redirect the light into a more convenient viewing spot. The secondary mirror and its supports can produce diffraction effects: bright objects have spikes (the ``Christmas star effect''). Two famous reflector telescopes are shown below. The first picture is of the 5-meter (200-inch) Hale Telescope at Palomar Observatory. The number refers to the diameter of the objective (almost 17 feet across!). The telescope is the vertical piece in the middle with the mirror close to the floor. The huge diagonal piece is used to balance the telescope. Until recently it was the world's largest optical/infrared telescope.

9 PH507 Astrophysics Dr Dirk Froebrich 9 All modern optical/infrared telescopes are reflecting telescopes, because: 1. Reflecting telescopes do not suffer from chromatic aberration. 2. Figuring a mirror requires polishing only one precise surface rather than two (or four for a compound lens). 3. Mirrors are easier to support because they can be supported on the sides and the back; large lenses tend to sag because they can only be supported on the perimeter. Reflectors are also much more versatile than refractors because they can be used at several different foci. The world's largest optical/infrared telescopes are the twin 10-meter Keck Telescopes operated by the University of California and Caltech on the 13,700ft dormant volcano, Mauna Kea, Hawaii. These are amongst the principal research instruments of optical and infrared astronomers.

10 PH507 Astrophysics Dr Dirk Froebrich 10 The Keck Telescopes employ hexagonal segmented primary mirrors, each made out of 36 hexagonal segments, 1.8m in diameter. In order to maintain a precise optical surface the positions of the segments are monitored by sensors which relay signals to a computer which drives precision actuators, keeping each segment in proper alignment.

11 PH507 Astrophysics Dr Dirk Froebrich 11 Mauna Kea is probably the world's best observatory site because of its stable atmosphere, maintained by the island's marine layer, and its altitude. Over 20 astronomical telescopes from the US, Britain, Canada, France and Japan are in operation or under construction. The second picture shows the path light travels in the 10-meter Keck Telescope at the W.M. Keck Observatory. The objective is composed of 36 hexagonal mirrors put together to act as one large mirror 10 meters across. The small image next to it shows the 10-meter objective. The person in the red clothing at the center gives you a sense of scale. In both the reflector and refractor telescopes, the focus is before the eyepiece, so the image in astronomical telescopes is upside down. Telescopes used to look at things on the Earth's surface use another lens to re-invert the image right-side up. Most reflector telescopes will use a smaller secondary mirror in front of the large primary mirror to reflect the

12 PH507 Astrophysics Dr Dirk Froebrich 12 light to a more convenient viewing spot. Isaac Newton used a flat secondary mirror at a 45 angle to reflect the light to an eyepiece at the side of the telescope tube near the top. Such an arrangement, called a newtonian design is used by many amateur telescopes. Many reflector telescope use another light path design called the cassegrain design to reflect the light back through a hole in the primary mirror, so that detectors or the eyepiece can be conveniently placed behind the telescope. Most of the large telescopes used for research, including the Hubble Space Telescope, are of this design. Some of the largest telescopes like the Hale Telescope and the Keck Telescope have places to put detectors at the prime focus, where the light from the primary mirror first comes to a focus. The images in reflector telescopes do not have holes or shadows in them because the light rays from the unblocked parts of the primary mirror are all added together when they are focused together. Even though part of the primary mirror is blocked or missing, there is still plenty of usable primary mirror space to gather the light. Both types of telescope can suffer from a defect called spherical aberration so that not all of the light is focused to the same point. This can happen if the mirror is not curved enough (shaped like part of a sphere instead of a paraboloid) or the glass lens is not shaped correctly. Hubble Space Telescope: a classic case of spherical aberration. The Hubble Space Telescope objective suffered from this (it was too flat by 2 microns, about 1/50 the width of a human hair) so it used corrective optics to compensate. The corrective optics intercepts the light beams from the secondary mirror before they reach the cameras and spectrographs. Fortunately, the Hubble Space Telescope's spherical aberration was so perfect, that it was easy to correct for!

13 PH507 Astrophysics Dr Dirk Froebrich 13 Even before the servicing mission that installed the corrective optics 2.5 years after the Hubble Space Telescope was put in orbit, astronomers were able to get significant results from the telescope. The images were computer-enhanced to correct for the spherical aberration to produce sharper images than from any ground-based telescope. Also, astronomers were able to observe ultraviolet light from celestial objects and fainter objects than could be seen from the ground. However, the computer processing took a long time and the aberration prevented the focusing of most of the light. This meant that astronomers could not see the very faint (and distant) objects they were looking for. M100 a few days before (left) and after (right) the corrective optics (COSTAR) were installed in December 1993.

14 PH507 Astrophysics Dr Dirk Froebrich 14 Summary of optical telescopes. The principle part of the telescope is called the collecting aperture, acting as a means to focus light and to produce a primary image. Note that the arriving light comes into the front of the telescope as parallel rays. This is generally true for all astronomical objects as they can be considered to be at infinity in comparison to the dimensions of any telescope. As all the objects to be investigated are effectively at infinity, the distance of the primary images from the collecting aperture is defined as the focal length, F, of the telescope. A plane through this point and at right angles to the optic axis is defined as the focal plane. It is often convenient to describe a telescope in terms of its focal ratio (f-ratio), f, which is the ratio of the focal length of the collector to its diameter, D: f = F D This definition is synonymous with the speed of an optical system when applied to general photography. The quantity of energy which is collected per unit time by the telescope is proportional to the collecting area.

15 PH507 Astrophysics Dr Dirk Froebrich 15 When a telescope is directed at the sky, an image of part of the celestial sphere is produced in the focal plane. The relationship between the size of this image and the angular field which is represented by it is governed by the focal length of the telescope. In the lower of the pictures above, rays are drawn for two stars separated by an angle θ. For convenience, one star has been placed on the optic axis of the telescope. Rays which pass through the centre of the system are not deviated, thus the separation, s, of the two images in the focal plane is given by: s = F tan θ As θ is normally very small, this can be re-written as: s = F θ where θ is normally expressed in radians. The correspondence between an angle and its representation in the focal plane is known as the plate scale of the telescope, which can be seen to be given by: dθ 1 = ds F Normally this is expressed in arc seconds per mm, and in this case the plate scale is given by: dθ = ds F where F and s are expressed in mm and in seconds of arc respectively, with the numerical term (206265) referring to the number of arc seconds in a radian. The telescope and the collected energy. The apparent brightness of an astronomical object is increased as the square of the diameter of the collector. Stellar brightness is normally termed Flux, F, which is the energy received per unit area per unit wavelength interval per unit time, and often expressed in units of W m -2 Å -1. The number of photons arriving at the telescope aperture can be written as:

16 PH507 Astrophysics Dr Dirk Froebrich 16 N T π = D 4 2 Δt λ 1 λ Fλ λ dλ hc 2 Where λ 1 and λ 2 are the cut-on and cut-off wavelengths defined by the filter that is used, and Δt is the integration time. The arrival of photons is a statistical process. When the arriving flux is low, fluctuations are clearly seen on any recorded signal, and the measurements are said to suffer from photon shot noise. If no other sources of error are present, the uncertainty in any measurement is given by N. Hence any record and its error may be represented by N T ± NT. In these circumstances, the signal-to-noise ratio (S/N) of the observation is given by: T N T N T = N T 2 D Δt D Δt Example: At 6300Å (630nm) the flux from a source is W m -2 Å -1. Determine the photon rate passing through a telescope with a diameter, D=2.2 meters over a wavelength interval of 100 Å. Calculate the best S/N ratio of a measurement with an integration time of 30 seconds. N T π = = and if the noise is purely limited by shot noise, the S/N ratio is simply = 782. Therefore the accuracy is about a part in 782, or about 0.1% N T

17 PH507 Astrophysics Dr Dirk Froebrich 17 Chromatic aberration. Chromatic aberration corrected using a doublet of 2 differing refractive index glasses.

18 PH507 Astrophysics Dr Dirk Froebrich 18 Spherical aberration.

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20 PH507 Astrophysics Dr Dirk Froebrich 20 The Newtonian and Schmitt-Cassegrain optical system. 1) Celestron 8-inchNewtonian reflector 2) Meade 10-inch Schmidt-Cassegrain

21 PH507 Astrophysics Dr Dirk Froebrich 21 3) Skywatcher 6-inch refractor

22 PH507 Astrophysics Dr Dirk Froebrich 22 Radio Telescopes Radio astronomy has its roots back in the 1930's when Karl Jansky accidentally detected radio emission from the center of the Milky Way as part of his research on the interference on transatlantic phone lines. The British advanced radio antenna technology in their development of radar technology to fight the German warplanes in World War II. After the war, astronomers adapted the technology to detect radio waves coming from space.

23 PH507 Astrophysics Dr Dirk Froebrich 23 A radio telescope uses a large metal dish or wire mesh, usually parabolic-shaped, to reflect the radio waves to an antenna above the dish. An example of a mesh is shown at left. This was the mesh of the parabolic dish for the former 100-meter radio telescope at Green Bank, West Virginia (photos courtesy of National Radio Astronomy Observatory). Looking from underneath the radio telescope, a person could see the clouds in the sky overhead but to the much longer wavelength radio waves, the metal mesh was an excellent reflector. The signal from the antenna is sent to an amplifier to magnify the very faint signals. At the last step, the amplified signal is processed by a computer to turn the radio signals into an image that follows the shape of the radio emission. False colors are used to indicate the intensity of the radio emission at different locations. An example is shown below for Jupiter. Charged particles in its magnetic field produce a large amount of radio energy in doughnut-shaped regions around its center. A visible band image of Jupiter is shown below the radio image.

24 PH507 Astrophysics Dr Dirk Froebrich 24 Radio telescopes may be made much larger than optical/infrared telescopes because the wavelengths of radio waves are much longer than wavelengths of optical light. A rule of thumb is that the reflecting surface must not have irregularities larger than about 1/5 the wavelength of light that is being focused. By that criterion a radio telescope is several hundred thousand times easier to figure than an optical telescope of the same size. The Arecibo Observatory radio telescope (below) is a 305 meter reflecting surface in a natural limestone sinkhole in NW Puerto Rico. Because the telescope cannot be pointed independently the telescope is "steered" by tilting the instrument housing supported over the telescope. Interferometry and aperture synthesis. (Aperture synthesis: making a very large telescope out of lots of smaller ones). Because radio signals are detected as waves, signals from different telescopes can be added to simulate or synthesize the resolving or pinpointing capability of a much larger telescope. The National Radio Astronomy Observatory's Very Large Array (VLA) near Socorro, NM, consists of 27 radio telescopes, each 25 meters in diameter which are deployed on a Y-shaped track which may be extended up to 36km. The VLA has the resolving power of a 36km telescope (but only the collectingarea and sensitivity of a 130m telescope).

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