Module 5 Single Dish Radio Telescopes Swinburne Online

Module 5: Single Dish Radio Telescopes Swinburne Online Education Introductory Radio Astronomy & SETI Activity 1 : © Swinburne University of Technology Radio Ears

Summary In this Activity, we will examine: 1. radio telescopes - basic principles; 2. parabolic reflectors - focus types; 3. steering the telescope; 4. size limitations for fully-steerable radio dishes; 5. safety considerations; and 6. transit telescopes.

We have already seen a comparison between the basics of optical and radio astronomy in the Activity on “The Radio Spectrum”. For example, we compared the • types of telescopes used • the precision with which reflecting surfaces have to be made • diffraction effects & resolution • types of detectors • observing conditions required, and • limitations on image quality In this and the next Module we will investigate these and other aspects of single-dish radio astronomy in some detail.

Radio Telescopes - Basic Principles A few essential elements are needed to make a telescope which will detect radio signals from space. We start with: a) An antenna and receiver - in principle, the antenna can be as simple as a long wire acting as an aerial, and b) An amplifier - to amplify the radio signal to a convenient size. So why do radio telescopes tend to look like: rather than ?

Well, for one thing, the kid with the crystal set will • have trouble picking up anything but strong local signals, • will pick up those local signals from all sorts of stray sources, zzzt! and • have no idea about from where zzzt! in the sky any signals she does zzzt! manage to pick up are coming. zzzt!

She needs ways to • concentrate the weak incoming signals to a focus point where the antenna is located, • shield the antenna from unwanted local signals, and • make sure that the antenna picks up signals from one particular, known part of the sky. antenna local signal from starter motor of car A major step in achieving these aims is to enclose the antenna within a reflector — usually, a parabolic reflector.

The Parkes radiotelescope is a classic example of a parabolic reflector telescope: For longer radio wavelengths, as we have seen, a mesh metal surface is sufficient (64 m diameter at Parkes), but the inner part of the dish has metal cladding in order to act as a reflector for shorter radio wavelengths (45 m diameter at Parkes). Whether the surface is painted or not does not affect its reflective properties. The reflecting surface must • remain as close to parabolic as possible under all observing conditions, • reflect almost all of the radio waves which fall on it, • be as light as possible & offer as little wind resistance as possible, and • be as big as possible!

Once our young radio astronomer uses her reflector telescope to determine the location of the source and its signal strength, this information can in turn be used to form an image, usually by scanning across the source. But first, once signals are collected by the reflector, our radio astronomer still needs to work out how to • convert the incredibly weak radio signals from very weak celestial sources into electrical with a suitable antenna and receiver, • amplify them, • eliminate any stray interference still picked up by (or generated within) the telescope, and • turn the signals into radio images or spectra.

In this Module we will concentrate on the design and properties of reflectors and how they affect the signal and resulting radio image. We’ll discuss the detection and amplification of radio signals in the next Module, and the handling of interference and formation of images and spectra later in the Unit.

Parabolic Reflectors - Focus Types The Parkes radiotelescope uses ‘prime focus design’. That is, the feed antenna is located at the base of the receiver cabin, 26 m above the dish, at the prime focus of the incoming beam.

Prime Focus n’s Su This is a simple, robust and successful design, but does have the disadvantages that • access to the feed antenna and receiver can be complicated in a large telescope, and • the feed antenna can pick up thermal emissions and reflections (‘noise pickup’) from the ground. s ray warm ground In general, telescope designers have to trade off their desire to maximise the amount of the collecting surface which is actually used, against the desire to limit the amount by which the effective collecting surface is exposed to noise pickup from the ground.

Telescopes like these with a relatively short focal length F compared to their diameter D* form clear images of objects near the axis of the parabolic dish, but suffer from ‘coma’ distortion for objects relatively far off axis — that is, the image is distorted. * For example, for the Parkes dish, F/D = 0. 41

Multiple Reflector Systems ‘Multiple Reflector Systems’ are focus designs which use one or more secondary reflectors to deflect the radio beam to a feed antenna and receiver location which is more accessible and less vulnerable to noise pickup from the ground. Stray noise pickup from thermal emission by the sky still occurs, but is less of a problem as the sky is roughly 100 times ‘colder’ than the ground. Secondary reflectors are usually less weighty than receiver cabins and therefore easier to support above the primary reflector surface.

Hyperbolic mirror The Cassegrain design shown here is a common design for optical telescopes too. The combination of parabolic primary and hyperbolic secondary mirrors produces a much longer (often 5 times longer) effective focal length than for the prime focus arrangement, and in consequence reduces coma distortion of off-axis images. Parabolic mirror with aperture

Cassegrain design dish of the Australia Telescope Compact Array. www. narrabri. atnf. csiro. au/

However, as for all multiple reflector arrangements, in order for the focus arrangement to produce clear images, the primary and secondary mirror surfaces have to be made with much greater surface accuracy than does a simple prime focus dish. This has cost implications when designing large dishes.

As we will see in the Activity “Feeds and Backends”, radio observations at significantly different frequencies require different receivers. The location and ease with which receivers can be changed is important, so that precious telescope observing time is not unduly wasted when swapping frequencies.

The off-axis Cassegrain, shown here, is a convenient arrangement for telescopes designed to work over ranges of frequencies, because several feed antennae & receivers can be located in a circle around the main reflector axis. Hyperbolic reflector set for frequency 2 1 Changing frequencies Parabolic reflector just requires a rotation of with a number of apertures the subreflector around the axis, to ‘illuminate’ To detector for the appropriate feed frequency 1 antenna. To detector frequency 2

The dishes of the VLA (Very Large Array) in New Mexico, USA are of off-axis Cassegrain design. http: //www. aoc. nrao. edu/vla/html/VLAhome. shtml

Hyperbolic reflector Other multiple reflector systems include: • The Nasmyth design, which has the advantages of locating the receivers in a cabin outside the dish, Parabolic reflector with aperture Receivers Plane reflector

The Owens Valley submillimetre array, shown here, uses the Nasmyth design. http: //www. ovro. caltech. edu/mm/main. html

Hyperbolic reflector Other multiple reflector systems include: • The beam waveguide feed design, which conveniently locates the feed antennae and receivers at ground level, and Parabolic reflector with aperture Plane reflector Feed antennae and receivers

The Nobeyama 45 m Telescope uses beam waveguide feed design. http: //www. nro. nao. ac. jp/~nro 45 mrt/index-e. html

Hyperbolic reflector Other multiple reflector systems include: Parabolic reflector • The offset Cassegrain design, where the secondary mirror does not block the incoming beam at all (an attractive but expensive design). Feed antennae and receivers

This offset Cassegrain design was proposed for the Atacoma Large Millimetre Array (ALMA) mm array (Chile), but rejected in favour of more conventional dishes. http: //www. alma. nrao. edu/index. html

Steering the Telescope Declination axis of telescope Equatorial Mounts Equatorial mounts are routinely used with good optical telescopes. Any mounting must allow the direction of the telescope to be adjusted in two directions, about two axes. In an equatorial mount, one of the axes (the declination axis) is always parallel to the plane of the equator and the other (the polar axis) is always perpendicular to it. Polar axis of telescope To South (or North) celestial pole

RA and dec Plane parallel to plane of Equator The polar axis is always set up parallel to the Earth's rotation axis, so it points to the north (or south) celestial pole. Motion about the polar axis changes the right ascension (RA). This angle is the RA Declination axis The other axis (the declination axis) is perpendicular to the Earth's axis, and outward from it, in a plane parallel to the plane of the Equator. This angle is the dec Polar axis Motion about this axis changes the declination (dec).

RA and dec in practice This slide shows how RA and dec would appear, and could be measured, for a particular (fictitious) star not far from the constellation Leo, seen from a point in the southern hemisphere. Crux dec is measured in degrees north or south of the intersection: e. g. 20 degrees RA is measured in hours and minutes East of the intersection: e. g. 1 hr east south celestial pole ecliptic Leo Virgo celestial equator Find where the ecliptic and celestial equator cross, and draw a line to the nearest pole from there. Divide the celestial equator into 24 hours and the line to the pole into 90 degrees.

However with any but the smallest radio telescopes, equatorial mounts are almost never used. The size of an equatorial mount needed to allow the telescope to observe the whole sky can be prohibitively expensive. . . Pivoting is managed here to alter declination Polar axis: aligned to rotation axis of Earth Rotation about polar axis changes right ascension

… although lighter and cheaper equatorial mounts can be designed as long as the telescope is only intended to observe a relatively small section of the sky — for example, for solar observations. Only small adjustments to RA and dec can be made Polar axis: aligned to rotation axis of Earth

The Howard E. Tatel Radio Telescope, built in 1958, was the first major radio telescope at the National Radio Astronomy Observatory (NRAO) in Green Bank, WV, USA. The Tatel telescope has an equatorial mount. http: //www. gb. nrao. edu/~fghigo/fgdocs/ tatel/tatel. html

North Azimuth Altitude-Azimuth (alt-az) Mounts Mechanically, alt-az mounts are much simpler to design and build for bulky radio telescopes than are equatorial mounts. The telescope is rotated around vertical and horizontal axes, changing the altitude and azimuth. Altitude vertical

North Azimuth Altitude and Azimuth Altitude is the angle between a celestial object and the horizon, measured vertically. Azimuth is the bearing of an object measured as an angle round the horizon eastwards from north. Altitude vertical If the telescope points in the direction shown, alt might be about 60 degrees and az might be about 120 degrees. Alt and az depend on the observer’s latitude and longitude, and the time of day.

Alt and az in practice This slide shows how alt and az would appear, and could be measured, for the same star we viewed when discussing RA and dec. alt is measured in degrees from the horizon: e. g. 50 degrees Leo east south celestial pole Crux az is measured in degrees east of due North: e. g. 220 degrees Virgo ecliptic angle from due North Draw a line parallel to the horizon, and through the star perpendicular to the horizon. Divide the horizontal line into 360 degrees from North and the vertical line into 90 degrees from horizon to zenith.

The dishes of the Australia Telescope Compact Array have alt-az mounts. www. narrabri. atnf. csiro. au/

Apart from the size and complexity of equatorial mounts for radio telescopes, when compared to alt-az mounts they have two other particular disadvantages. As a radio telescope dish is raised in elevation, the gravitational torques on its surface change, flexing it and thereby tending to change its shape & focus properties. Top of rim is pull towards centre, sides are twisted around centre and bottom is pulled away from centre All parts of the rim are pulled in the same direction

Signals pass through a lot of atmosphere at an angle and are refracted a lot Also, as the elevation changes, the amount by which received signals are refracted as they pass through the Earth’s atmosphere changes too. Signals pass straight down through less atmosphere and are hardly refracted at all

With an equatorial mount, flexure and refraction are dependent on both RA and dec settings. With an alt-az mount, both these effects are much easier to correct for, because the azimuth variable is not involved and corrections depend on altitude only. polar axis Flexure and refraction effects depend only on altitude

Alt-az mounts have limitations too: • In order to keep the telescope accurately pointed towards one particular source as the position of that source shifts in the sky during the Earth’s daily rotation, both the alt and az settings have to be changed at variable rates. (With an equatorial mount, the RA setting would change at a constant rate, and the dec setting would not change at all. ) Alt-az telescopes require computerised drives to control the varying rate of rotation. • If the source happens to move through the zenith, the rate at which azimuth setting needs to change to keep up with the source approaches infinity! Research radiotelescopes often sound an alarm if the source approaches the zenith, though modern computer controls can usually sort out the azimuth settings fairly quickly. In general, radio telescopes can take quite a while to slew from one widely-separated source to another — for example, up to 15 mins to slew 180 o!

Alt-az mounts have limitations too: • As the position of the source moves across the sky due to the Earth’s daily rotation, its orientation changes with respect to the orientation of the dish, when an alt-az mount is used. This can be a problem if, for example, the observers are trying to measure the plane of polarisation of the incoming signal. To overcome this, the feed antenna for an alt-az telescope could be rotated at the same rate, though in practice, the effect of the changing orientation is often corrected for in the data analysis software.

As a source moves across the sky, a computer control can calculate the appropriate alt and az settings for the radio telescope at any time and control the servo motors which drive the radio dish. In some older radiotelescopes which predate full computer control, other servo methods were devised. For example in the case of the Parkes radiotelescope, a small equatorial optical telescope located in the radiotelescope’s tower automatically tracks a laser point projected at the desired RA and dec onto a closed dome. A feedback mechanism links the orientation of the small equatorial optical telescope to the orientation of the large alt-az radio telescope, forcing the radio telescope to point to the same RA and dec!

Size limitations for fully-steerable radio This path length. . . dishes. . . suits this The main engineering challenge in designing and constructing a large radio telescope involves keeping the surface of the dish parabolic. If the surface of the dish is accurately parabolic, waves that arrive in step at the aperture (even at very different parts of the aperture) also arrive in step at the focus. There is “constructive interference” and the resulting signal is strong. path length

Size limitations for fully-steerable radio This path length. . . dishes. . . no longer suits If the surface departs from parabolic shape by more than a small fraction of the observing wavelength , then the incoming radio waves will be out of step when they reach the feed antenna, and the signal will be significantly degraded: Waves that arrive in step at the aperture arrive out of step at the focus. There is “destructive interference” and the resulting signal is weak. this path length

For example, for observations at 7 cm, the surface must be parabolic to within 7/16 4 mm at all elevations, even if the dish is 30 m in diameter! Distortions in the shape of the dish can be due to • the dish sagging under gravity by differing amounts as it tracks at different elevations, • wind loading on the dish, and • thermal variations causing differential expansion across the dish (especially between sections in sunlight and sections in shadow).

For example, the rim of the 100 m Effelsberg telescope deforms by about 6 cm when the telescope is tilted from zenith to horizon*. Telescope reflector frames and mounts can be designed to minimize distortions, but this adds to the cost. However if the dish and its supports can be designed so that the deformations are homologous — that is, under deformation due to gravity, the dish remains a paraboloid (with changed focal point and axis) — then computer control of the position of the feed antenna can attempt to correct for such deformations. In that case, thermal and wind deformations become the limiting factors in radio telescope design. * K. Rohlfs & T. L. Wilson, Tools of Modern Astronomy, 2 nd edition, Springer 1996

The new Green Bank Telescope will be of offset design, 100 m by 110 m and fully steerable. It replaces a 90 m telescope which collapsed in 1988. http: //www. gb. nrao. edu/

We cannot specify the maximum practicable size of a fully-steerable radio telescope precisely, because as we will see in the next Activity, that depends not only on cost and mechanical considerations. The maximum practicable size also depends on the operating frequency and the telescope’s pointing accuracy, i. e. the accuracy with which the observer can determine the direction in which the axis of the telescope is pointing.

Safety Considerations Wind loading on a fully-steerable radio telescope dish not only causes problems with deformations: it can also put the dish under threat. Wind loading on a dish which is tilted towards the horizon can damage its bearings, and can make it difficult to drive the dish pack to its ‘stowed’ position, facing zenith. wind bearings are strained wind difficult to lift dish against the wind

Strong wind is detected Building for safety Research radio telescopes have wind monitoring equipment which warns observers when wind gusts exceed previously determined limits and may automatically stow the telescope. wind The other big threat to a radio telescope is lightning: radio telescopes can observe through rain (at most wavelengths), but lightning strikes can damage or wreck the electronic equipment. fzzzt Stowed

Transit Telescopes Radio astronomers are always seeking higher resolution images, and, at a given frequency, that means bigger dishes. One solution to circumvent the problems of building huge fully-steerable radio dishes is to restrict steering to elevation only, using the Earth’s daily rotation to scan across the sky — a meridian transit telescope. Variations on this principle exist, for example the telescope that collapsed at US NRAO (National Radio Astronomy Observatory) at Greenbank, West Virginia* was a meridian transit telescope. * J. S. Hey, The Radio Universe, Pergamon Press 1971

The Green Bank 300 ft telescope before collapse.

The Green Bank 300 ft telescope after collapse, on November 15, 1988, due to failure of a key structural element--a large gusset plate in the box girder assembly that formed the main support for the antenna.

Arecibo 305 m telescope The Arecibo 305 m telescope in Puerto Rico is an extreme example: the dish is completely fixed, a spherical reflector made by shaping and cladding a natural depression in the ground.

A 25 m secondary reflector and an 8 m tertiary reflector refocus the signal from the main reflector to the receiver ‘carriage house’ which is suspended over the dish. The combination of secondary and tertiary reflectors also help to compensate for distortion to the signal due to the spherical shape of the primary reflector.

In the next Activity we will investigate the basic properties of the beam of a radio telescope, which defines the area of the sky which the telescope is observing.

Image Credits Parkes radiotelescope, from visitors' centre, © ATNF, used with permission http: //www. parkes. atnf. csiro. au/visitors_centre/pretty. htm Parkes dish backlit by sun, © ATNF, used with permission http: //wwwatnf. csiro. au/images/telescopes/parkes 1. gif 64 m Parkes radiotelescope , © ATNF, used with permission http: //wwwatnf. csiro. au/images/telescopes/parkes_dish. jpg ATCA dish, showing Cassegrain design, , © ATNF, used with permission http: //wwwatnf. csiro. au/images/telescopes/atca_scale. jpg NRAO: The Centre of the VLA http: //www. aoc. nrao. edu/intro/vlapix/vlaviews. index. html

Image Credits Oven's submillimetre telescope array http: //www. ovro. caltech. edu/mm/main. html Nobeyama 45 m Telescope http: //www. nro. nao. ac. jp/~nro 45 mrt/index-e. html NRAO: The Howard E Tatel Telescope http: //www. gb. nrao. edu/~fghigo/fgdocs/tatel. html NRAO: The Green Bank telescope http: //www. gb. nrao. edu/ Arecibo http: //www. naic. edu/about/photos/aoviews/tescop. jpg

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