Tools of the Astronomer

Part 1 - Ground-based Optical Telescopes

 

Telescopes are time machines

-         Carl Sagan

 

Window to the World

 

    Long before the telescope was invented, it was well known that a piece of glass with a convex surface (later called a lens) could be used as a magnifying device through its ability to concentrate or disperse light.  It was not until the beginning of the seventeenth century that a pair of lenses of differing surface curvatures were used to make a distant object appear closer and larger.  This marked the birth of the optical telescope, whose invention is generally attributed to the Dutch lens maker Hans Lipperhey in 1608.  It would have remained largely a curiosity were it not for the genius of the great Galileo Galilei (1564 - 1642) who exploited its potential fully, both for terrestrial and celestial observations, thus opening up a window to the outside world.  It was he who not only fabricated the first astronomical telescope but also used it to make most of the path breaking discoveries in modern astronomy.  They eventually led to the acceptance of the Copernican heliocentric system of planetary motions, and ushered in a far reaching revolution in scientific thought.

 

    As with anything new in the world of Science and Technology, Galileo’s invention was received initially with a lot of indifference, scepticism, disbelief and even scorn as it challenged long-held beliefs and prejudices.  Today it is looked upon as one of the greatest inventions of all time and Galileo himself as the father of modern science.

 

The Galilean Telescope

  

    Galileo’s early telescopes consisted of a biconvex objective lens and a biconcave eye piece as in the diagram below and formed a non-inverted image of the distant object. Though compact, they were very crude by later standards, had a narrow field of view, with limited magnifications, and the images were quite fuzzy because of the poor quality of the lens surfaces.  

 

     Despite these serious limitations Galileo made some epoch making discoveries that included lunar surface features, four satellites of Jupiter and the phases of Venus.  He observed that the four tiny satellites of Jupiter orbited the planet much the same way that the planets themselves went around the Sun.  This discovery gave a knockout blow to the prevailing geocentric world view which had placed the earth at the centre of everything observable.

 

    Shown in the picture below are a painting of Galileo and two of his earliest telescopes, displayed in a Science museum in Florence, Italy.

 

Refractor Telescopes

 

    The Galilean telescope is one of several types of what are called Refractor Telescopes since they use lenses. Image formation is by refraction and convergence of light through the objective lens of a large focal length.  Eyepieces of much shorter focal lengths and small apertures are used to magnify the image formed by the objective for comfortable viewing or photography.

 

        In a refractor telescope, light from a distant object is brought to focus through a biconvex objective lens.  The intermediate image so formed is magnified when viewed through the eyepiece to give a much larger (inverted) image as depicted in the diagram here.

    In most refractor telescopes the eyepiece is placed in an adjustable housing at right angles to the viewing end to facilitate comfortable viewing.  A reflecting mirror or prism is used to direct the image to the eyepiece.  An adjustable laser pointer can be mounted near the eyepiece holder to locate the desired object precisely, especially in the night sky.

 Optical Aberrations

 

    Images formed by refractors suffer from a variety of defects (called aberrations) during the process of refraction and dispersion through the glass material.  A few of them are mentioned here.

 

    A blurring of the image due to refraction, often noticeable at the edges, is called spherical aberration.  Light rays near the edges of the lens are focussed slightly closer to the lens than the axial rays as shown in the diagram. This causes the smeared out appearance of the image.

 

    A lens refracts different colours present in white light to different extents depending on the wavelength of the component and this results in what is called chromatic aberration. The inability of the lens to bring all of the colours into a common focal plane results in a slightly different image size and focal point for each of the component wavelengths.  The result is a coloured halo surrounding the image, with the colour changing as the focal point of the objective shifts.

 

    If light rays refracting through a lens in two perpendicular planes come to a focus at different points, the resulting aberration is called astigmatism. This is generally due to imperfections in the lens surfaces.  The human eye is particularly vulnerable to astigmatism, which can be corrected through properly shaped spectacle lenses.

 

    These and other optical aberrations result in a significant degradation of the image produced by lenses. It is possible to overcome them and achieve a high quality image through the use of combination of two or more lenses of differing shapes, curvatures and densities.  Such complex imaging systems are invariably employed in modern cameras and other optical instruments where image fidelity is an important consideration. 

 

    Optical aberrations in small refractor telescopes are minimised with combination lenses only in the case of small telescopes.  For large telescopes this is not attempted at all since reflector telescopes offer a vastly superior alternative to refractors as discussed later.

 

Some Characteristics of Telescopes 

 

    Why are big telescopes preferable to smaller ones given the same image quality?  Here are some characteristics of optical telescopes which show why: 

 

Light Gathering Power

 

    Obviously, the larger the aperture (diameter) of the objective the more the light gathering power of the telescope.  As an example, if two similar open trays of different sizes are placed under steady rain, the tray with the larger exposed area will collect proportionately more water in a given time than the smaller tray.  For analysing the light gathered from any faint distant object this is a major consideration.  Since the exposed area is proportional to the aperture, a 20 cm aperture telescope is four times as good as a 10 cm one … and a 2 m aperture one 400 times as good!

 

Resolving Power

 

    Resolving power is another feature of a telescope, often more important than its light gathering power. It is the ability of the instrument to distinguish clearly between two points whose angular separation is less than the smallest angle that the normal human eye can resolve.  It is directly proportional to the aperture of the telescope and can be calculated from the formula RP = (D/1.22 λ), where D is the aperture and λ the wavelength of light involved (600 nm can be used as a representative value for white light). The factor 1.22 arises from the application of what is known as Rayleigh’s criterion for two closely spaced objects to be treated as resolved and takes into account the diffraction of light by the objects.

 

Magnification

 

    The magnification that a telescope can provide is another key feature to consider. It is given by m = (f_o/f_e), where f_o is the focal length of the objective and f_e the focal length of the eyepiece.  To obtain different magnifications with a given telescope (of fixed objective) eyepieces of different focal lengths can be used. The magnification is also equal to D_o/D_e, where D_o is the aperture (diameter) of the objective and D_e is the aperture of the eyepiece.

 

    Beyond a certain magnification the telescope may not be of much use since the field of view and the image brightness both decrease in inverse proportion. On the other hand, reducing the magnification increases the image brightness while maintaining the same resolution, which is a major advantage especially for photography and photometry.

 

Field of View (FOV)

 

    This is the open observable area visible through the eyepiece of the telescope and is generally specified in angular measure.  For any given instrument the FOV depends on the focal length of the eyepiece used, decreasing as this increases.  Obviously, higher the magnification lower the FOV.  To observe the whole of the full Moon, the FOV required is just over half a degree. Incidentally, binoculars are designed to provide much larger FOVs than telescopes.

 

f-number

 

    In line with the common usage associated with camera lenses, the f-number of the objective of a telescope is defined as f_o/D.  Thus, the f-number of a telescope whose objective lens has an aperture of 15 cm and focal length of 1.5m is denoted f/10.

World’s Largest Refractor

 

    Because of numerous optical aberrations which are very difficult to eliminate and huge fabrication challenges and costs, large refractor telescopes are very rare and only of historic interest today.  The largest such telescope ever built was the 40” (1.02 m) aperture instrument built at the Yerkes Observatory in Wisconsin, USA.  Sadly, it is no longer in use. Here is a picture taken against the background of this telescope when the celebrated physicist Albert Einstein visited the facility in 1921. The large proportion of women in the picture is appropriately indicative of the appeal Astronomy has always carried for women in the USA and elsewhere!

    Another great refractor telescope of historic importance is also worth mentioning here. It is the 26-inch "Great Equatorial" located on the grounds of the US Naval Observatory at Washington, DC and still in limited use, with a rich history behind it.  Completed in 1873, it was the largest refracting telescope in the world for a decade. It was from here, in August 1877, that astronomer Asaph Hall discovered the two tiny moons of Mars, Phobos and Deimos, drawing the attention of the world to the contribution of US Navy to astronomical research. This author cherishes a fond memory of his visit to this observatory in December 1966.

Binoculars

 

    Though the binoculars is a familiar and widely used piece of optical equipment, somewhat like the camera, its use for viewing the night sky objects and star fields is not widely known.  Effectively, it is two ‘folded’ telescopes placed side by side on a common framework with a focussing thumbwheel for hand-held viewing.  The compact size and re-inverted image arising from the ‘folding’ of the light path is achieved through the use of a pair of prisms as shown in the cut-away diagram. Binoculars carry numbered designations like 8x40, 7x50, 12x80, etc.  The first number is the magnification and the second one the aperture.  For astronomical use, 7x50 binoculars is probably best suited because of the right balance between the two parameters.   A seriously interested sky watcher will do well to begin with such an instrument before going for any telescope.

 

Reflector Telescopes

 

    The highly popular reflecting telescope was invented by the great Isaac Newton in 1668. The big breakthrough he made was in replacing the objective lens of a refractor with a curved reflecting mirror. Since the primary image is formed by reflection from a polished or coated surface, most of the optical aberrations either do not exist or can be more easily minimised.

     A replica of Newton’s earliest reflecting telescope is shown above together with a modern version of a handy commertial portable reflector on an equatorial mount. In Newton's instrument, the mirror was made of a copper-tin alloy.  Near the other end of this polished primary mirror Newton placed a small flat secondary mirror at a 45-degree angle, to reflect the light into an eyepiece mounted on the side of the telescope tube. The quality of the image was highly disappointing initially.  It improved greatly when the primary mirror was made of glass, with its curved surface coated with a reflective layer. Later, vacuum coating of the surface with silver or aluminium deposition marked a great advancement in telescope making.

 

    Since image formation is by reflection and not by refraction, there was no question of chromatic aberration.  Spherical aberration could also be avoided by replacing the spherical mirror surface with a parabolic one, though this was not easy to achieve.

 

Imaging in Reflector Telescopes

 

    Depending on how and where the final image is formed, reflector telescopes are classified as indicated in the following ray diagrams:

 

    The Cassegrainian telescopes offered some significant operational advantages over the other types, especially in large instruments.

 

Schmidt Corrector Plate

 

    The difference in image formation between a spherical reflecting surface and a paraboloidal reflecting surface can be seen in the following diagram:

 

    The paraboloidal surface produces an image without any spherical aberration, but it is a great deal more difficult to fabricate than a spherical surface with its attendant aberration.

 

    In 1931, Bernhard Schmidt came up with an ingenious work-around to get the best out of a spherical mirror. He designed a thin highly transparent corrector plate placed in front of the spherical surface with one of its surfaces shaped in such a manner as to effectively cancel out the spherical aberration and form a sharply focused image.  The following diagram illustrates how this is achieved:

    Both the thickness and the curvature of the corrector plate are shown highly exaggerated here.  In actual fact it is very thin, with the curvature hardly noticeable.  In addition to its primary role, the corrector plate also served to shield the mirror from the atmosphere by acting as a protective barrier and allow only light to get through. 

 

Catadioptric Telescopes

 

    The combination of a thin Schmidt type corrector lens and a reflecting spherical mirror gives a class of efficient (and relatively inexpensive) telescopes termed Catadioptric. A great additional advantage of such systems is the substantial reduction in the length and mass of the instrument. Use of a small secondary reflecting mirror at the centre of the corrector plate will result in a folded (triple) path for the light rays inside the telescope tube. A sophisticated portable computerised Schmidt-Cassegrainian telescope of this type with folded optics (owned by this author) is shown below with its corresponding ray diagram:

 

Advantages of Reflector Telescopes

 

    Reflector telescopes present numerous advantages compared to refractor types, especially where large telescopes are concerned.  Here are some of the major ones:

• While optical aberrations degrading the image quality are a major issue with refractors, these are either non-existent or greatly reduced in reflectors.  In particular, the virtual elimination of chromatic aberration is a huge advantage.
• Considering that the resolving power and light gathering power both increase in proportion to the aperture of the telescope, size for size it is easier and less expensive to build large and medium sized reflectors as compared to refractors.  This is true of portable telescopes also.
• Large refractors would also require large objective lenses and, apart from the engineering and cost factors, the significant absorption of light through the thick lenses involved is another major deterrent.
• While quite a number of reflectors are now operational with apertures of the order of 3-5 metres or larger, refractors with apertures even of the order of 1-2 metres are now virtually defunct.
• Reflector telescopes can be used effectively not only in the visible region of the electromagnetic spectrum but also in the ultraviolet and near infrared regions. This greatly enhances the utilitarian aspect of large reflectors.

Some of the largest reflector telescopes in the world

 

    Hundreds of reflectors with apertures in the range 2 – 10 metres are in use for astronomical and astrophysical research and studies all over the world.  Just a few of the more important ones are outlined below:

 

    Perhaps, of the greatest importance of all is the 100” (2.5 m) Hooker Telescope at the Mount Wilson Observatory in California, USA. Completed in 1917, it was the world’s largest until 1949 and ushered in some of the most path breaking discoveries in Astronomy.  Much of the work leading to the epoch making and now well understood theory of the origin and expansion of the universe was done here by the celebrated astronomer Edwin Hubble.  Earlier, the fact that the Andromeda Nebula was indeed a full-fledged galaxy like our own Milky Way, and external to it, had been established with its use.  Later, the first indication of the existence and nature of dark matter in the universe came from here.

 

    Commissioned in 1949, the Hale telescope, named after astronomer George Ellery Hale, and located on Mount Palomar in California, USA, was the world’s largest optical telescope following the Hooker Telescope.  With an aperture of 200 inches (5.2 m) it was twice as large as its predecessor, incorporated some path breaking technologies in its construction and contributed equally to the advancement of observational astronomy, including the direct imaging of a few exoplanets in 2010.  The mirror itself took about eleven years to fabricate starting from the molten glass that was poured into the mould.  Overall, the project took two decades to complete.  Pictured here are both the telescope and its housing dome.

 

    Appropriately named after the inventor of the reflecting telescope, the Isaac Newton Telescope or INT is a 2.54 m (100 in) optical telescope managed by the Isaac Newton Group of Telescopes at Roque de los Muchachos Observatory on La Palma in the Canary Islands since 1984. Originally the INT was situated at Herstmonceux Castle in Sussex, England, which was the site of the Royal Greenwich Observatory after it moved away from Greenwich due to light pollution. In 1979, the INT was shipped to La Palma, where it has remained ever since.

 

    India also figures among the giant optical telescopes of the world through its 3.6 m Devasthal Optical Telescope (DOT) built by the Aryabhatta Research Institute of Observational Sciences (ARIES) and located at the Devasthal Observatory site near Nainital, Uttarakhand. The telescope was activated remotely on 31 March 2016 by Indian Prime Minister Narendra Modi and Belgian Prime Minister Charles Michel from Brussels. The telescope optics has been built in collaboration with the Belgian firm Advanced Mechanical and Optical System (AMOS). The 3.6m DOT is currently the largest reflecting telescope in Asia.  The telescope is also the first of its kind in India that features an active-optics system (being discussed later).

 Segmented Mirror Telescopes

    There is a technological limit to the size and surface area of primary mirrors made of a single piece of glass substrate. They cannot be fabricated larger than about eight meters in diameter. Using a monolithic mirror much larger than about 5 meters becomes prohibitively expensive. The best way to overcome this limitation is to use multiple segmented mirrors to effectively function as a single device, an idea pioneered in the 1980s by Jerry Nelson at the Lawrence Berkeley National Laboratory of the University of California.  Many of the recently built giant sized telescopes have adopted this idea.  The mirrors need to be controlled individually with exceptional precision so that they collectively function perfectly as a single mirror of the combined aperture.

Keck Twin Telescopes

    The strategy of building a telescope from segmented mirrors was soon realised in the historic 10 m aperture Keck twin Telescopes, each made of a honeycomb of 36 hexagonal segments (as can be seen clearly in the picture), each of 1.8 m wide and only 7.5 cm thick. These telescope are part of the W M Keck Observatory setup on the summit of the 4145 m high Mauna Kea Mountain in Hawaii. The key technological development that made possible the construction of such huge telescopes was the use of advanced real time computer feedback and control systems, like active optics and adaptive optics, to operate smaller mirror segments together as a single contiguous mirror.

 

Active and Adaptive Optics

    Large telescope mirrors are very heavy and suffer from structural and thermal instabilities which can distort their surface. Even very small distortions can degrade the image significantly. This places a practical limit on the size of any type of telescope. Another complication is that the atmospheric temperature and density both fluctuate continually, causing light waves to be distorted from their original form.  Locating the telescopes at high altitudes is a partial solution to this problem. Light pollution near urban areas is another serious and continually worsening problem.  The techniques of active and adaptive optics now allow astronomers to overcome these limitations far more effectively.

    Active optics provides a way of deforming a mirror to compensate for its lack of structural rigidity. In adaptive optics, the optical elements of the telescope are instantaneously and continually adjusted to compensate for, and effectively neutralize, the blurring effect of the Earth's atmosphere. Both active optics and adaptive optics use actuators (tiny pistons) and computers in a constantly monitored electronic feedback system to make minute adjustments to the shape of the primary and secondary reflective surfaces.

    The main difference between the two techniques is that any changes made with active optics are relatively slow and intended to correct for the sagging or wind induced vibrations of the telescope as it tracks an object across the sky. In contrast, adaptive optics is intended to remove the effect of turbulence in the Earth's atmosphere and therefore makes much faster, continual, real time adjustments.

 

Some giant Optical Telescopes of today

 

    Top of the list in terms of equivalent aperture is the Large Binocular Telescope (LBT) located on top of Mount Graham in Arizona, USA.  With an effective aperture of 11.8 m, each of these two 8.4 m aperture mirrors is the single largest non-segmented mirror in the world.  Functional since 2005, it is a joint project of a number of universities and incorporates both active and adaptive optics. The primary use of this facility is in imaging through specialized techniques of aperture synthesis and long baseline interferometry.

 

    One of the largest multi segmented mirror telescopes in the world, the Hobby-Eberly Telescope (HET) located at the McDonald Observatory in Texas, USA, combines a number of features that differentiate it from most telescope designs, resulting in significantly reduced construction costs.  The primary mirror is a honeycomb of 91 hexagonal segments adding up to an equivalent aperture of just over 10 m.  It is fixed at a 55° angle and can only rotate around its base. A target is tracked by moving the instruments at the focus of the telescope. This provides access to about 70–80% of the sky at its location and allows a single target to be tracked for up to two hours. The telescope is used primarily for spectroscopic studies and does not need very sophisticated and expensive control systems like other telescopes of the same class. It is actively engaged in studies related to the properties and distribution of dark matter in the universe.

 

    The author of this article fondly recalls a whole day spent in visiting the numerous facilities at the sprawling McDonald Observatory and interacting with some of its staff on a rainy and gloomy day in August 2017 after viewing a total solar eclipse near faraway Nashville, Tennessee a week earlier. He also had a long interaction with a resident astronomer at the campus trying to understand the operational details of the telescope and its efficacy in the ongoing cosmological studies. The composite picture below commemorates his visit to the Hobby-Eberly telescope. 

    The Gemini Observatories consist of two identical 8.1 m aperture single mirror telescopes, one in the northern hemisphere and the other in the southern hemisphere, at two of the best locations for observational astronomy on the planet. Gemini North is part of the Mauna Kea Observatories in Hawaii and Gemini South is at Cerro Pachon in Chile, both belonging to a global consortium of seven countries.  Gemini South is close to the extensive Inter-American Observatory at Cerro Tololo.  Together they can cover all parts of the night sky visible from the Earth.

Giant Telescopes of the future

    A number of giant telescopes are either already under various stages of construction or in advanced stages of planning.  They include the following:

    The Vera C Rubin Observatory, named after a pioneering woman astronomer and formerly known as the Large Synoptic Survey Telescope (LSST), is under construction at Cerro Pachon in Chile, close to the existing Gemini South telescope.  It has a wide field 8.4 m primary mirror designed to photograph the entire available night sky once every few days for building up maps in unprecedented detail. The telescope uses a novel three-mirror design, which allows a compact telescope to deliver sharp images over a very wide 3.5-degree diameter field of view. Images are recorded by a 3.2-gigapixel CCD imaging camera, the largest digital camera produced till recently. The facility is being operated under the management of the Association of Universities for Research in Astronomy (AURA). The picture below shows the top-end assembly being lowered by a 500-ton crane.  Work is expected to be completed in another two years.

    The Thirty Meter Telescope (TMT) is one of Astronomy's next generation telescopes, coming up on Mount Mauna Kea in Hawai, home to possibly the world's largest collection of giant telescopes. Being built by an international consortium that includes India, Japan and China, Its 30m diameter prime-mirror will enable observations from ultraviolet to mid-infrared wavelengths, with state-of-the-art adaptive optics systems that will compensate for the effects of Earth's atmosphere, and produce images rivaling those of the Hubble Space Teelscope and the James Webb Space Telescope as well.

    Described as the biggest eye on the sky and simply called the Extremely Large Telescope (ELT) for want of a better name yet, the ELT has a revolutionary design and mind boggling performance specifications.  Located at Cerro Armazones in the bone-dry Atacama desert of Chile, it will have as many as five huge mirrors, all having different shapes, sizes and roles, but working together seamlessly for unprecedented imaging clarity. The main mirror alone, with a diameter of 39 m, will be composed of 798 segments and their alignment is expected to be better than ten thousandth of the thickness of the human hair.  It will employ ever more sophisticated adaptive optics to compensate for atmospheric turbulence and produce images sharper than those of any other ground-based telescope. Originally slated for completion by 2027 at a cost of over 1.3 billion Euros, it is a highly ambitious project of the European Southern Observatory (ESO) which is managing several other gigantic observatories in the world.  Here is a graphic of the  projected housing and the multi-mirror imaging system within:

An Overview

    Ground-based optical telescopes continue to provide a huge impetus to the advancement of observational astronomy and compliment the enormous contributions of space based instruments such as the great Hubble Space Telescope and its successor, the James Web Space Telescope. The following graphic presents a comparative picture of some of the world’s best known optical telescopes that are in operation or under various stages of completion.  In concert with telescopes operating in other regions of the electromagnetic spectrum, optical telescopes are expanding the frontiers of astronomy, astrophysics and cosmology at a breath taking pace and contributing to our continuing quest for understanding our universe.