TELESCOPES WITH FIXED POSITION EYEPIECES Roberto Bartali

ABSTRACT A telescope is a very complex instrument and must be designed taking into account its three fundamental components:

• 1 – Optical (lenses, mirrors, cameras and filters) • 2 – Mechanical (mounting and basement) • 3 – Control system (motors, actuators, electronics, computers and software)

All three are equally importants, so the order is not an issue. A perfect optical system is not useful if the mounting is not stable and if the control system is unable to follow the apparent movement of the heavens. This document is a description of different mounting designs adopted to sustain and tracking the optical system only. After a general description of what a mounting is and its main characteristics, follows a detailed analysis of most used configurations. Today telescopes are very large and expensive, to get the best cost to performance ratio, astronomers need to couple many instruments, most of them very obstructing and heavy, they have to be placed off axis and on stable platforms sometimes far from the telescope. This implies the deviation of the light rays by the usage of auxiliary optics like in the Coudé or Nasmyth configuration, described in the second part of this work. Finally there is a description of the main difference between professional and amateur mountings. INTRODUCTION Astronomy is a joint venture of many sciences and technologies, a telescope is like a dedicated robot, order of magnitudes better than any other industrial counterpart. Technology involved in the design and construction of a modern telescope is pushed to the limit, sometimes there is not an available technology capable to construct a telescope with the performance that astronomers needs. During the first century after the invention of the telescope, both, refractor and reflector in the XVII century, the main attention was placed on the optical system, the mounting was a mere support for it, we can see fine works of art, but with questionable performance (Figure 1).

When more large mirrors and lenses were made in the XVIII and the first half of the XIX centuries, the necessity of heavy and “colossal” mounts was imperative (Figure 2).

Figure 3 One of the largest modern telescopes, the Gemini South.

Figure 2 Big mounting for a refractor at Lick observatory.

Figure 1 Antique telescope on wood pier.

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The real importance of the mounting of a telescope was considered only after the

invention of photography. When astronomers applied it to retract celestial objects, which are very faint and so needs long exposure times, the importance of a very stable mount capable to track and maintain them, during hours, in the centre of the field of view, mechanical engineers designed heavy, large and sophisticated systems (Figure 3). MOUNTING CHARACTERISTICS Now let’s see why a mounting is so important and must be designed and manufactured whit such a degree of perfection. The mounting of a telescope has three main functions which are briefly explained here:

1 – Sustain and maintain the optical system. 2 – Pointing and Tracking. 3 – Sustain the ancillary instrumentation.

Optical system The optical system (mirrors and lenses) came in a wide range of diameters, from small 6 cm beginner’s telescopes objective lens up to 1 meter Yerkes refractor (figure 4) and to 10 meters primary mirrors, but it is expected, in a near future, that this figure grows to 50 or more meters. Such giant mirrors, even when they take advantages of new technologies, are heavy, in the order of several tons. They have to be maintained in their place with a high degree of precision, otherwise the light path could change and the resulting image came out of focus or could be distorted. The optical system comprise not only the primary mirror, but also the secondary one and many times some correcting group of lenses (figure 5), which are smaller than primary mirrors, but equally huge and heavy; sometimes the astronomer seat inside the secondary mirror support (figure 6). It is the duty of the mounting to maintain optical axis well aligned. Modern adaptive and active optic systems uses deformable mirrors (figure 7) so the mechanical structure must be more complicated, to do thing even more complex we have also liquid mirror telescopes, where, instead of a polished glass, the primary mirror is a rotating bowl filled with mercury (figure 8).

Figure 5 Large lenses to correct the field

of view mounted on the secondary support mirror of the Anglo Australian Telescope

Figure 4

Largest refracting telescope (1 meter objective diameter) at Yerkes observatory.

Figure 6 Astronomer inside the telescope secondary mirror structure of the Keck observatory.

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Figure 7 Active optics system on the back of the Subaru telescope.

Figure 8 Liquid mirror telescope primary reflective surface of the LIDAR project.

Pointing and Tracking The Earth rotate and due to this, celestial objects are moving, so the telescope, which is fixed on the planet surface, must follows the object observed all night long. Hundred tons of glass, steel and aluminium must be moved smoothly and precisely with a high degree of repeatability. Earlier systems used complex train gears and chains (figure 9), something like a giant clock. The gears were moved by gravity. Later, the implementation of motors simplified the mechanics, but it needed a complex electrical and electronic system (figure 10). Modern telescopes uses powerful direct drive motors, coupled directly to both axis, the mechanical system has a reduced complexity, but the control electronics needs a set of powerful computers running sophisticated software routines.

Figure 10 Gears and belts coupled to motors are transmitting the movement to telescope axes. Example from the ASAS project telescope.

Figure 9 Gravity gears drive for telescope movements.

Sustain instrumentation The first instrument used by astronomers was the eye, observing through the telescope eyepiece group of lenses. Then they coupled photographic cameras which came in a variety of size depending on the plate used (figure 11).

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After the invention of the spectroscope, astronomers apply it to study the physics and chemical properties of stars, so they apply it to telescopes, at first in the same place of the camera, then on the focal plane. The combination of a spectroscope and a camera resulted in a huge and heavy instrument, so mechanical constrains were more demanding regarding the whole mechanical structure (figure 12). Large telescopes today are very expensive, to give the most of them many instruments must be attached, this way there is the possibility to do different kind of work simultaneously. The mechanical structure must be modified, there is no more room nor lifting capacity to sustain and move so many instruments. They must be placed, sometimes, away from the telescope in another room or in the basement. This factors leads to the design and construction of completely new configurations (figure 13).

Figure 12

Spectrograph attached to the 4 meter Kilt Peak telescope.

Figure 13 Large spectrograph attached to the Nasmyth focus of the Galileo Telescope.

Figure 11 Large format photographic plate camera attached to a telescope at Lowell observatory.

Mounting specifications A well designed mount must comply with some basically characteristics all at the same time, otherwise it will not perform as expected and data collected by the optical system will be meaningless. It must be stable, rigid, light weight, precise and balanced [Buckman 2002][Trueblood 1985]. Stability is important because it must maintain the optical system in its place no matter the position in the sky of the object observed, no flexures are allowed. The telescope is continuously moving so the centre of gravity of the instrument is ever changing, but the optical axis must be every time perfectly aligned with the object and the relative position of each lens or mirror must no change, otherwise a trailed or blurred image can be obtained. A rigid structure means that no vibration must be induced or transmitted if we want to have a point like image of a star. There are many sources of vibrations. Some are external to the instrument like vehicles moving near the observatory building, settlement of the building, wind and air currents and small earthquakes. These vibrations are easier to avoid with separated structures and basements for the telescope and for the buildings.

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Internal vibration sources are motors, gears, movement transmission systems misalignment and backlash, flexures due to unbalanced masses and optics supports, jog due to pointing

control errors, wearied or aged bearings, non perpendicularity of the axes, active and adaptive optics actuators, air currents and temperature changes inside the dome and not well fixed joint in the structure or in the instrumentation attachment. To avoid vibrations, massive and colossal basements made of concrete (figure 2) and large diameter steel tube structures were made in the past (figure 14), but with so large aperture telescopes, like

modern ones, it showed to be not feasible because flexure problems are nearly impossible to remove. Figure 14

100 inches Mt. Wilson Telescope mounted on massive structure.

Modern telescope mounting designs uses light weight material like aluminium and, in some cases, also sturdy plastics. Rigidity is accomplished by the usage of pure triangle structures

(figure 15). It is clear that the mounting and the optical tube are the same things. Al mechanical parts, as optics, must be machined with the highest possible quality, minimal imperfections can carry misalignments and produce vibrations. Many times the

structure is implemented using customized metal alloys, not commercially available, so each piece must be consistent with all others. A difference in the alloy formula may induce to a different strength of some part respect to all others damaging the stability and rigidity of the mounting. Another effect consists on differences in thermal conductivity. This is very important because metal deformations, under temperature changes, can induce structure modifications like misalignments and flexures and finally they means instability and vibrations, in the worst case, also the rupture of a joint. The whole telescope structure must be well balanced to reduce to the minimum the power of the motors and the size of the transmission components system. Moving an unbalanced load imply that the torque developed by motors is not constant, so more power is needed, but this means that motors dimensions and weight are much more than the necessary. A variable load also can induce variations in the movements and at last, vibrations;

another side effect could be some flexure due to torsion effects, without speaking about the more complex electronics and software.

Figure 15 Modern giant telescope structure are made using small diameter tubes forming triangles.

Equatorial mounting are the most difficult to balance, as we will see ahead, because the centre of gravity is not on the telescope axis. This problem is well solved by using Alt-Azimuth mountings.

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The above discussion is regarding professional telescopes, but it apply well also for small telescopes, like those used by amateurs. The reduced available budget forces amateurs to buy low quality mountings, resulting in a very instable instrument. For amateur and portable instruments, is best to use wood instead of aluminium for the tripod, because wood absorb and not transmit vibrations as any metal do. To achieve maximum stability a triangle as near the floor as possible joining legs is the better solution.(figure 16).

Figure 16 A wooden tripod wit tensors near the floor to achieve stability.

MOUNTING CONFIGURATIONS There are two main configurations for a telescope mount: Alt-Azimuth and Equatorial [Savard 2001][Malacara 1995]. In the following paragraphs I will describe both and their different and most used configurations. ALT-AZIMUTH

This kind of mounting, very useful also for terrestrial observation, was used before the “photography era” and from the third half of the XX century until now and it is the preferred for modern giant telescopes. Most telescopes with more than 1 or 2 meters aperture are placed on an Alt-Azimuth mounting. The first professional instrument that takes the full advantages of an Alt-Azimuth mounting is the 6 meter Russian Special Astrophysics Observatory made in 1976.

Figure 17 Alt-Azimuth mounting movements are one horizontal and one vertical.

There are two perpendicular axes, a horizontal one parallel to the horizon, called Azimuth axis and a vertical one, perpendicular to it, called Altitude or elevation axis (figure 17). The observed object is tracked moving simultaneously both axes. There are many advantages using this mounting, the main one is the stability because the centre of mass falls always inside the area occupied by the telescope. There is no need of balancing counterweights, reducing the total mass of the instrument. The total area occupied by the telescope is reduced; observatory dome and building are less than half if compared to a conventional equatorial mounted telescope of the same aperture, due to this, the overall cost

of the observatory is also cheaper. Huge and heavy instruments can also be placed easier because they can be mounted on the elevation axis and they are never lifted. It is easy to convert an Alt-Azimuth mounting to a fixed eyepiece position telescope. The main reason to not be adopted before 1970 is because the complexity of the pointing and tracking tasks. As stated above, there is the need of two motors, moving at variable speed depending on the position of the observed object. Only a dedicated computer may be programmed to perform according to this. When the object is near the horizon the

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elevation axis speed is very low, but as the object is moving toward the Zenith, the speed is changing reaching the maximum at an elevation of 90°. The horizontal axis speed, instead, is the minimum at the highest elevation and it increase to the maximum when the object is near the horizon. For this reason modern telescopes, adopting this configuration, are not able to be pointed to the Zenith (with the exception of Zenithal telescopes described later); normally they can be pointed between 15 to 85° of elevation. Even when the telescope can be, in theory, pointed near the horizon, it is not useful to observe there due to the high value of the airmass which increase atmospheric distortions and absorption.

Figure 19 Carlsberg Meridian Transit Telescope.

Figure 18 Field de-rotator.

During the apparent motion of the object from East to West, the field must be rotated, if an object is on the left of the centre of the field when it rises, on the East, will be on the opposite side when it sets on the West. A so called Field Derotator (figure 18) must be placed in front of the camera or any other ancillary instrument. It consists basically in a mirror moved by precise stepping or servo motor. The speed is proportional to the sidereal rate and must be calibrated depending on the pixel size of the detector [Teare 2000]. The same effect can be achieved rotating the whole ancillary instrument, depending on its size and weight. A third motor, smaller than those used for tracking, with its relative transmission system, is used. Now it is clear that only a computer can move simultaneously all

three motors, it has to calculate, many times per second, the correct speed of each one. For visual observations, there is no need for a field derotator, the brain can do that job and, also, because the time spent viewing the object is small compared with a photographic or CCD

exposure. There are some variations of an Alt-Azimuth mounting, but they are based on the same principle. Transit Telescopes and Zenithal Telescopes mountings are two special cases. Amateurs telescopes came also with an Alt-Azimuth mounting with four different versions: a German equatorial (figure 49), a fork (figure 50), a single arm (figure 51) and a Dobsonian (figure 20, 52). Mounting for Transit telescopes

A transit, also called meridian, telescope is a small aperture refractor mounted onto an Alt-Azimuth mounting with only one axis movable: the elevation one (figure 19). The Azimuth axis is fixed because the telescope must point always to the Meridian (the line that goes from North to South passing through the Zenith). This telescope takes the advantage of the rotation of the Earth because stars always pass the Meridian line, so they are observed during their passage. Due to the particular application of this telescope to measure precisely the time when the star pass the Meridian

(collecting data for catalogues and measuring Earth rotation rate), the mounting must be

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perfectly aligned with the East-West line, if there is some error on axes perpendicularity, it must be precisely known, the basement on which rest the mounting must be very sturdy and independent from the dome building. Mounting for DobsonianTelescopes

This is a type of Alt-Azimuth mounting mostly used by amateurs because it is very easy to construct and uses cheap materials like wood or plastics (figure 20) for almost any telescope aperture. Due to lightweight materials and the capability to be dismounted easily, it is very transportable. It is used for short focal length Newtonian optical configurations. It consist mainly on a box containing the primary mirror moved in altitude by a semicircle rotating over another box which rotate horizontally over a table. Secondary mirror are supported by a box. To maintain reduced the weights it is normally an open structure with one to four tubes (figure 20 and 52).

Figure 20 Typical Dobsonian telescope made by amateurs.

A well manufactured Dobsonian mounting can perform very well for short exposure astrophotography. The rigidity depends on the stiffness of the support for the secondary mirror and on the precision of the semicircle that move the telescope around the vertical axis. Normally the movement is transmitted by friction in the case of a motorized mounting.

This represent an advantage respect to the generation of vibration, because if the friction rod is held with the correct pressure to the wheel, there is no vibrations. The same principle is applied also to some giant telescopes [Eaton 1997]. EQUATORIAL This was the main configuration used in the second half of the XIX and most of the XX centuries. It consists also on two perpendicular axes, but all the system is inclined at an

angle, over the horizon, equal to the latitude of the observing place, so the horizontal axis is parallel to the celestial equator, called declination axis and the other is perpendicular to it and it is pointing to the Celestial North Pole, called, for this reason, polar axis (figure 21). Equatorial mountings have some great advantages [Cecchini 1969][Malacara 1995][Oliver 2004]:

1) just one axis movement is needed for tracking (polar axis) while the declination axis, when the object is pointed and centred on the field of view, it remain

fixed; Figure 21 Schematic diagram of an equatorial mounting.

2) there is no need of a field de-rotator, because the object remains always in the same position;

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3) tracking the object at a constant speed, so less expensive control system to drive the telescope is needed, even a completely mechanical one like the gravity load drive (figure 9) performs well.

The real big problem with this kind of mounting is that the telescope is always unbalanced, so (such as in German type configurations) large and heavy weights must be placed on the declination axis which extent away from the optical axis. Many times the counterweight used to balance the telescope weight more than the optical system (this is particularly true for amateur telescopes). The structure of the mounting is large and occupies much more space than the used for an Alt-Azimuth one. The needed dome diameter is, then, at least 50% larger, but depending on the configuration, even twice (figure 22). The overall cost of the observatory, when an equatorial mounting is used, is greater than the cost for a similar telescope with an Alt-Azimuth mounting. As we seen before, an Alt-Azimuth mounting may be constructed with pure triangle structure (figure 15) lowering the weight, but this is not possible for an equatorial mounting

(just the optical tube can be made with triangle structure) because the mounting and the optical tube needs two separated structures. The largest reflector telescope mounted on an equatorial mounting is the 508 cm Hale at Palomar (made

in 1948) and the largest refractor is the 100 cm at Yerkes (made in1897), today, for sure, both telescopes would be on Alt-Azimuth mounting. This is because balancing the instrument and the dome size are too much budget demanding and mechanical problems raised with such giant structures very difficult to avoid.

Figure 22 The Keck dome on the right, hosting a 10 meter telescope is smaller than the Hale dome hosting a 5 meters telescope.

German Equatorial Mounting

Figure 23 Schematic diagram of a German Equatorial mounting.

This is a mounting invented by Fraunhofer in 1812, very common for small telescopes, but used also for mid range and large telescopes in the XIX and the first half of the XX centuries. The 1 meter Yerkes telescope (figure 2) is one example. It is an asymmetric structure because the telescope is not supported on both sides. The declination axis is supported by the polar axis, not by the basement. The telescope weight must be balanced by a counterweight, doubling the mass of the system. Due to this, the declination axis tends to induce large flexures, so a large diameter tube

must be employed [Beish 1995]. The polar axis is supported by the basement, but the inertial load on its end is huge. The union point of the declination axis to the polar axis must be very strong. Overall weight of the structure is very high.

Polar axis is pointing to the celestial North Pole, this way the telescope can point any where (figure 23).

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The height of the pillar must be at least as the half of the length of the optical tube, this imply that the probability of flexures is very high, even if the basement is large and heavy. For this reason it is better for short focal length telescopes. To avoid large counterweight it is possible to place smaller ones, but at the expense of the lengthening of the declination axis, creating dangerous vibrations and flexures.

The most used optical configurations for telescopes supported by this mounting are refractors, astrographs, Cassegrain and Newtonian if they have short focal length. It is best suitable for photographic purposes, because, if it is used visually, the observer has to be lifted depending on the declination of the object pointed by the telescope. It is possible to convert a German Equatorial mounted telescope into a fixed position eyepiece telescope by deviate the light path at least two times, the first parallel to the declination axis and the second parallel to the polar axis. Both deviating mirrors must be rotating, incrementing the difficult of the mechanical and control system. Amateur telescopes came with this kind of mounting,

but modern professional telescopes do not, precisely because of the huge and sturdied structure that they need for supporting ancillary instruments.

Figure 24 50 cm. Newtonian on German Equatorial mount at the University of Thailand.

All professional refractors are supported on this type of mounting, also some Newtonian (figure 24) and Cassegrain.

Fork Mounting This mounting replaced German Equatorial for medium to large telescopes. The polar axis ends into a fork (figure 25). Two bearings hold the polar axis to the basement, one has to be very large in diameter because it must support all the weight of the telescope, the second is smaller because it acts as an ending support to avoid too much stress and flexure. The basement must be of heavy construction. The optical tube is held in the middle of the fork, so the declination axis length is nearly zero, this is an

advantage because there is no stress and the weight of the optical tube is shared by both sides of the fork. Figure25

Schematic diagram of a fork mounting. This configuration is best suited for short tube

telescopes like Schmidt (figure 26), Schmidt-Cassegrain, Cassegrain, Newtonian, Astrograph and short focal length refractors; it was used mainly in the XIX century for telescopes with primary mirror diameter up to 2.5 meters. It was designed by Lassell in 1861. Fork mounted telescopes do not be able to be pointed near the Zenith because the polar axis is maintained short to avoid flexures and mechanical stress. There is no need, almost in theory, to counterweight the telescope because this task is performed by the polar axis that is firmly supported by the basement. In practice, the

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declination axis is not positioned in the middle of the optical tube because the weight of the primary mirror is much higher than the weight of the secondary, if there is some instrument attached on the Cassegrain focus, the telescope must be balanced placing counterweights on

the top end of the tube. As in the German equatorial it is the polar axis that supports all the weight of the telescope. Medium to large size amateur’s telescopes came with this kind of mounting. To convert a telescope supported by this mounting on a fixed position eyepiece telescope, are valid the same considerations made for other equatorial mountings.

Figure 26

Schmidt telescope at Mt. Palomar.

English Yoke Mounting

This mounting consist of a set of two piers, one higher than the other, the polar axis forms with the basement and the piers a rectangle triangle (figure 27). The angle formed by the shortest pier and the basement is equal to the latitude of the observatory.

Figure 27 English Yoke scheme.

The telescope is held in the middle of a closed fork attached at the top of both piers, giving a symmetric

distribution of loads. The declination axis, like in fork mount, is nearly zero.

Figure 28 Crossley reflector on EnglishYoke mounting.

This mounting is best suitable for short focal length telescopes (figure 28) and it has the disadvantage to not be pointed close to the pole. If large diameter telescope are supported by this mounting, the mechanical stress on the declination axis is an issue. As for fork mounting, the telescope

must be counterweight or balanced only if there is some instrument attached to the primary mirror end. If a telescope on such

mounting would be converted to a fixed eyepiece instrument, a greater number of mirrors are needed, compared with those for

fork and German Equatorial mounts. Horseshoe Mounting This mounting is similar to the Yoke, but solve the problem for supporting large telescopes. It was used for all telescopes larger than 2.5 meters and it was introduced with the 5 meters Hale at Mt. Palomar. This is the last equatorial mounting scheme adopted

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before the advent of computer controlled Alt-Azimuth mounting for large telescopes in the decade of 1970.

Instead of a simple support at the long pier top end, as for the Yoke, there is a fork, shaped as a

hodepoba

Figure 31 Inverted fork scheme.

TEquatorsky. Fle

telescopes are the largest apertur Inverted fork Mounting This configuration is sim

Tmounting to a fixed position eye Cross-axis Mounting

betwethere there iEquato also su

Figure 29 Horseshoe mounting scheme.

rseshoe. So the closed two arms fork support the clination axis on both sides and the horseshoe support the lar axis (figure 29). To avoid vibrations, the mounting sement is independent from the dome building. This configuration solves the problem to point the telescope at the Pole because it is an open structure system.

Figure 30 An horseshoe mounting support the Anglo Australian Telescope.

his mounting share all the properties of a German ial, because the telescop where in the xures problems of the p

Like a Yoke mounted telescope, there is the need for a great number of deviating mirrors to convert the telescope into a fixed eyepiece one (at

least 4). The Anglo Australian Telescope (figure 30) and the Hale e examples.

ilar to the Fork, but now the telescope is not supported by the fork ends, it is attached to the fork base. At the fork ends there are two balancing counterweights. Polar axis must be strong enough to support the full load of the telescope, most of the mass of the mounting and the counterweight (figure 31).

e can be pointed anyo . lar axis are an issue

he conversion of a telescope supported by this kind of piece can be achieved adding two rotating mirrors.

This mounting is similar to the Yoke, it is an hybrid en this and a German Equatorial. Instead of a closed fork, is just an axis, the telescope is attached on one side and s the need of a counterweight the same way as in German rial (figure 32). The telescope can be pointed anywhere on the sky. It is itable for short focal length telescopes because piers must

be higher if a long optical tube is used to observe objects near the Celestial Equator. Mechanical stress over the polar

Figure 32 Cross Axis mounting scheme.

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axis is twice the measured with a Yoke structure, because there is just one axis instead of two. The load to be supported is also greater due to the presence of the counterweight, so to avoid flexures and torsions, large diameter axis must be used. Declination axis also suffered flexures and torsion problems.

As for yoke mounted telescopes, there is the need of at least 4 mirrors to convert it to a fixed position eyepiece instrument.

Figure 33 A telescope mounted on an cross axis mounting.

Many telescopes in the range from 1 to 2 meters diameter uses this kind of mounting (figure 33).

FIXED POSITION EYEPIECE CONFIGURATIONS

Figure 35 Using additional mirrors (blue), the light may be deviated to a fixed position.

Figure 34 Platforms on both sides of the telescope are where instruments can be placed (large boxes).

As seen above, huge instruments can not be fitted to the telescope focal plane nor to the optical tube assembly because of their weight and dimensions. Light path must then

modified to carry the image of the observed object away from the telescope assembly or to platforms on both sides of the telescope (figure 34), this is done by the interposition of some mirrors in the light path (figure 35). It is superfluous to say that the optical quality of such mirrors must be as good as the primary and secondary ones. These mirrors have to be supported firmly, any vibration or deformation of the

structure can distort the image, the goal is to deviate the light rays with the minimum number of additional mirrors because

each one reflects less than 100% of the incoming light. Obviously is not useful to have a very large collecting area (the primary mirror) if there is too much loss of light in the optical system [Warner 2002]. In some cases, these additional mirrors are not fixed, they

rotate depending on the position of the telescope; this fact represent a complication of the mechanical structure of the telescope and, obviously, to the control system. Another drawback is the change of the polarization of the incoming light due to this rotation.

If we disregard the number of additional mirrors, every mounting configuration can be converted to a fixed position eyepiece telescope, it

depends only on the kind and quantity of instruments they have to carry on. For this reason only giant telescopes are designed this way. Light rays can be deviated to many parts of the instrument (both sides, behind primary mirror, etc) using partially reflective mirrors or changing the position of the first deviating mirror to reflect light to different angles as showed in figure 36.

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There are two main optical configurations useful for this purpose called Coudé and Nasmyth.

A special case of Alt-Azimuth mountings with the capability for fixed position eyepieces are the Solar and Zenithal telescopes, all are described in the following paragraphs. Conventional optical configurations for telescopes are very different, depending on the mirror, lenses or both used to achieve the best possible image without aberrations. They have in common that the instrument is attached to the optical tube. The difference, with fixed position eyepiece optical designs, is that the image is obtained out of the optical tube.

Figure 36 Using rotating mirrors the light can be directed to many different instruments.

Mounting for Coudé Telescopes

Light gathered by the primary mirror of a reflecting telescope is reflected to the secondary mirror and, from this, to a series of auxiliary mirrors which carry the light through the polar axis of the mounting until the basement or a room below the telescope (figure 37).

Figure 37 Diagram of a telescope with Coudé focus.

Figure 38 Light from the telescope is redirected to the instrument on a room below the telescope.

This configuration is used when long focal length are needed as for planetary observations or for high resolution spectroscopy (figure 38). The number of auxiliary mirrors and if they

are rotating, depends on the mounting design, but the best suited for this are the English Yoke, the Horseshoe and the Cross

Axis. Coudé telescopes are most of the time Schmidt, Cassegrain and also refractors but not so common. The first auxiliary mirror can be a partially reflective or a prism, this is because, depending on the observation, it

may be necessary to have the Cassegrain and/or the Schmidt foci available at the same time than the Coudé (two instruments

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working simultaneously). To interchange the Coudé focus with the Cassegrain or the Schmidt, a tilting mirror is used; in one position is able to direct light to other mirrors creating the Coudé focus but, if it is tilted to a perpendicular position respect to the primary mirror, the light forms an image at the Cassegrain or Schmidt foci. Mounting for Nasmyth Telescopes

Figure 39 Diagram of a telescope with Nasmyth focus.

The light collect by the primary mirror is reflected to the secondary and then to an auxiliary mirror. The image is not created on a room below the telescope as in the Coudé focus, but on a side of the telescope (figure 39). So Nasmyth optical configuration is used

when short focal length are needed or just to deviate outside the optical tube the primary focus. Many times there are both possibilities: the long Cassegrain focus and the

short primary focus. The focus interchange is made by the same way as described above for the Coudé system.

Figure 40 Structure of the Gemini telescope showing platforms for instrumentation at Nasmyth focus.

Every giant telescope has almost one Nasmyth focus, but normally they have two, one on each side of the optical tube. Due to the short path that light have to travel, the image if formed just outside the optical tube, normally passing through the elevation axis of an Alt-Azimuth mounting (figure 40). The ancillary instrumentation is placed on

platforms where there are also the elevation motor.

Mounting for Solar Telescopes

Solar telescopes are not conventional telescopes in the sense that they have not a mounting structure and an optical tube. They have a set of mirrors collecting light and sending it to a laboratory where the instrumentation (camera and spectroscopes), are placed (figure 42).

Figure 41 Solar telescope.

Only small aperture telescopes for solar observation are similar to a conventional telescope. The light is collected by a mirror on the top of a tower

and a series of secondary rotating mirrors, following the Sun, deviate the light inside the tower until they are focussed (figure 41). These are very long focal length instruments to achieve the largest image as possible and to spread the spectral lines as much as possible.

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Figure 42

Left: structure of a solar telescope. Right: Mirrors collecting the sunlight and following the Sun.

Mounting for Off Axis Telescopes

Figure 44 Hobby Eberly primary mirror tilted to reflect the light to the tower behind the dome.

Figure 43 One example of an off-axis configuration.

Small off axis telescopes are normally long focal length instruments, but large aperture uses mainly their primary short focus. The main concept behind

this design is to have the possibility to collect as much light as possible without having

the secondary mirror obstruction which can reduce the incoming light and image contrast. In this optical configuration the primary mirror is not perpendicular to the incoming light, but it is tilted, so the light is deviated to a side of the

telescope. Light is reflected by flat and curved mirrors, depending on the required final focal length. Almost every type of mounting design are suited for small off axis telescopes, but for larger, the Alt-Azimuth is better.

This design is more used for radiotelescopes than for optical ones, but in recent years two observatory adopted it: the Hobby Eberly and the South African telescopes.

These large aperture telescopes are mounted on a modified Alt-Azimuthal mounting. There are no movement

of the telescope because they are pointing to some angle on the sky and reflect the light collected by the primary mirror to a secondary one or directly to the sensors (cameras or spectroscopes) placed on the top of a tower near the dome

(figure 44). The advantage of this configuration is that the mechanical structure is enormously simplified, there are no movement nor support for secondary and auxiliary mirrors, so the cost is much less than for a conventional telescope. The sensor is moved over the primary (at an offset angle) so there is not obstruction for the incoming light. As Earth rotate, stars moves over the primary and reflected to the sensor.

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A disadvantage is that the telescope is not able to be pointed to all part of the sky, but it has a coverage of some decades of degree. Mounting for ZenithTelescopes

A Zenith Telescope is pointed directly to the Zenith, they were used for timing purposes in the beginning of the XIX century. They are not supported by a conventional mounting, the are just placed with the optical axis pointing straight up. They are a special case of the transit telescopes (which has just one possible movement) because for these there are no moving axes.

Figure 45 39 cm Zenith telescope.

Mounting for Liquid Mirror Telescopes

Figure 46 Liquid mirror telescope structure.

Strictly speaking this type of telescope do not have a mounting. Liquid mirror telescopes uses a mercury bath as the reflecting surface instead of a glass primary mirror. The bath is rotating attached to an air bearing, the great density and specific weight of the metal forms, naturally, a parabolic concave reflector. This metal is highly reflective, but dangerous, so many precautions

must be taken to avoid environment and human contamination. These are short focal

length telescopes working on primary focus. At the top of a simple structure there are is a corrector and the image sensor.

Figure 47 6 m diameter liquid mirror telescope.

This is a special case of the zenith telescope, because the rotating bath must be placed in an exactly horizontal position to

avoid spilling or overflowing. The advantage is the reduced cost of the mechanical structure, because the only thing that is moving

is the bath and the mercury do the job of the glass mirror. As Zenithal telescopes the sky is scanned using the Earth rotation, so this is only a partial disadvantage.

The real great problem with this telescopes is the oxidation of the mercury so the maintenance cost of the bath is high. More advantages of a fixed eyepiece telescope As we see in previous sections having a fixed position eyepiece telescope give the opportunity to place many instruments improving the efficiency of the instrument. But there is also another very important issue that is the opportunity to show to, common people the marvels of the Universe. Placing the eye at a telescope ocular it may be very uncomfortable for many people because of the position of the telescope. Most of the time people do not see anything because light do not reach their pupil, so they only see a blank or black circle [Duran, Malacara 2004]. My own experience, during a public observation

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session, is that more than half assistants suffered this problem, it is difficult to properly align the eye with the telescope, especially when a medium to short focal length ocular is used or the telescope is pointing at more than 45° elevation (figure 48). People on wheel chairs, with broken legs or arms, whit some mobility disability, or simply

aged people unable to do “contortionism”, are automatically excluded to direct observation through a telescope. Having a fixed position ocular, bringing the focussed object far from the telescope mounting, gives to these people a chance to see stars and planets.

Figure 48 Uncomfortable position for observing.

Normally people tendency, when observing through the ocular, is to lean against the telescope or the ocular itself, most of

the time producing vibrations or, in the worst case, moving the telescope. During the observing session, the telescope is continuously moving

tracking the sky, so the position of the ocular is also changing, than observers are forced to take different positions depending on that. With a fixed position eyepiece telescope all the above are no more a problem because people may be comfortably seated without the need to touch the instrument. PROFESSIONAL AND AMATEUR MOUNTINGS DIFFERENCES

Even when there are no differences in theory, in practice the telescope mounting used by amateurs are completely different in performance and construction (figure 49, 50, 51, 52). The main difference is portability because most of the time they are forced to move to a dark sky location or simply to the roof of their home or in the backyard. But

portability does not necessary means instability because the instrument may be fitted to a dismountable or sectioned pier.

impl

FigFor

the vwith

lightweight of the moun

Figure 49 German Equatorial mount on tripod.

Some mechanical tricks may also be

emented like adding triangle structures or enlarging the axes.

ure 50 k mount on tripod.

Figure 51 Single arm mount on tripod.

Figure 52 Dobsonian mount.

The Achilles heel of a typical mounting used by amateurs consists on ery small, compared to the rest of the structure, joint of the mounting

the tripod. If an equatorial mounting is used, the telescope tilt plate is supported only by a small screw or small diameter tube, giving a lot of instability. Amateurs telescopes carries only small and

weight cameras, only a few are devoted to spectroscopy, so the size and ting is many order of magnitude less than the used by professionals.

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CONCLUSIONS The mechanical structure of a telescope is as important as the optical and the control system because imperfections, deformations or vibrations on the mounting structure can distort the image. For this reason many mounting designs were made, each one having some advantages over the other, but at the cost of some drawbacks too, it is not possible to design a perfect and universal mounting. The enormous light collecting capacity of modern telescopes implies the construction of mechanical structures capable to sustain loads of several to hundred of ton, but there are two difficult problems to solve: cost and size. A large instrument cost more than a small one, but the cost is not only due to the optical part, a great percentage is for the mechanical structure and the dome which has to host the instrument. Stability and rigidity are the main features involved in the design and construction of modern telescope mountings, so mid size and large telescopes are supported by Alt-Azimuth mountings instead of Equatorials because the latter demands more space, are much more heavy and do not have the capacity to sustain huge ancillary instruments. An equatorial mount is much easier to point and move to any part of the sky than an Alt-Azimuth one, but, today, computers are so powerful that controlling many motors simultaneously is not an issue. The construction of telescopes with equatorial mounting is then limited to the smallest ones. Light collected by the primary mirror of large telescopes is carried to many instruments like CCD cameras and spectrographs, to achieve the best performance, the largest field of view and to study very fine details, these instruments are huge and heavy, most of the time they must be placed far from the telescope, in the mounting basement or in an adjacent room. Mounting structures and optical systems must be modified to carry the light path to a fixed position where the cameras or the spectrographs are firmly placed, no matter where the telescope is pointing to. I am sure that amateur telescopes mountings could be designed and manufactured in a much better way, clearly at higher cost, but results obtained with a more stable and precise tracking functionality could be more interesting and they can contribute to science in a much better way. Amateurs are many orders of magnitude more than professionals and they are widespread around the world. REFERENCES Andrenelli P., L´Astronomo dilettante, Sansoni, 1968. Barney Smith E., et al, Rapid Response Telescope (RRT) for Gamma Ray Bursa (GRB) Acquisition, http://coen.boisestate.edu/EBarneySmith/GRB/mount_design.htm Barbieri C., Large optical telescopes, World Scientific, 2002, http://www.pd.astro.it Buckman A., Telescope pointing errors and corrections, AWR Technologies, 2002, http://www.awr.tech.dial.pipex.com

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Beish J., A german equatorial mount for the planetary telescope, Cecchini G., Il Cielo, Utet, 1969. Covington M., Astrophotography for the amateurs, Cambridge, 2002. Duran-Ramirez V.M., Malacara Doblado D., Some considerations regarding the exit pupil location in some visual systems, Revista Mexicana de Fisica 50 (4) 397-400, 2004. Eaton J., Structural analysys of the telescope mount, 1997 Kollmorgen, Direct drive motors: http://www.kollmorgen.com Lucchesi D., Tecnologia meccanica, Sansoni, 1974. Malacara D., Malacara J.M., Telescopios y estrellas, Fondo de Cultura Economica, 1995. Mobberley M., Telescopes, mounts and control systems, Encyclopedia of Astronomy and Astrophysics, 2005. Mt. Palomar Telescopes: http://www.astro.caltech.edu/palomar/hale.html Oliver J, Techniques of observational Astronomy (AST3722), University of Florida, 2004 http://www.astro.ufl.edu/~oliver/ast3722/ast3722.htm Orange County Astronomers, Telescope Mounts, http:// Osservatorio Astronomico di Torino, Attivita scientifica dal 1987 al 1998, OATO, 1999. Russia Astrophysical Observatory: http://www.sao.ru/Doc-en/Telescopes/bta/descrip.html Savard J., Telescope mountings, 2001, http://members.shaw.ca/quadibloc/science/opt03.htm Teare S., UNISIS Field de-rotator, 2000: http://www.ee.nmt.edu/~teare/fielddr.htm Telescope Mounting, http://www.daviddarling.info/encyclopedia/T/telescope_mounting.html Trueblood M., Genet R., Microcomputer control of telescopes, Wilmann-Bell inc., 1985. University of Texas, Astronomy 301-Introduction to Astronomy, Classnotes 7, http://www.as.utexas.edu/astronomy/education/spring01/lambert/classnotes7.html Yerkes Telescope: http://astro.uchicago.edu/yerkes/virtualmuseum/40inch.html

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Warner M., Alt-Azimuth vs. Equatorial: number of reflections to Coudé, ATST, 2002, http://atst.nso.edu Wikipedia, Telescope_mount, http://en.wikipedia.org/wiki/Telescope_mount http://www.starstuff.com/index.htm http://home.earthlink.net/~indig/DesignMusings/text/Design_Mountings.html http://www.astrosurf.com/re/history_telescope.html http://amazing-space.stsci.edu/resources/explorations/groundup/teacher/grabbag.html http://www.quadibloc.com/science/opt03.htm IMAGE CREDITS Figure 1 Antique telescopes: http://www.antiquetelescopes.org/short_reflector.jpg Figure 2 Lick refracting telescope mount: http://cassfos02.ucsd.edu/public/tutorial/images/telescopes/lick36in.jpg Figure 3 Gemini South telescope: http://www.gemini.edu/index.php?set_albumName=album03&option=com_gallery&Itemid=39&include=slideshow.php Figure 4 Yerkes telescope: http://www.lib.uchicago.edu/e/spcl/centcat/fac/fac_img11.html Figure 5 Correcting lens group for the AAT: http://crystalnebulae.co.uk/2dfscience.html Figure 6 Secondary mirror structure of the Keck telescope: http://www.astro.virginia.edu/class/oconnell/astr121/im/keck-mirror-ex.jpg Figure 7 Active optics on back of the primary mirror of the Subaru telescope: http://www.instanthawaii.com/Images/Astronomy/sub9.jpg Figure 8 Liquid mirror telescope primary reflective surface: http://www.hipas.alaska.edu/hipasweb/lidar.htm Figure 9 Gravity drive motor: http://www.palomar.edu/astronomy/Astronomy/UnitronDrive.jpg Figure 10 Transmission of movement by gears: http://archive.princeton.edu/~asas/gif/asas3_large.gif Figure 11 Astrograph at Lowell Observatory: http://www.cerritos.edu/ladkins/lowell/pluto_camera.htm

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Figure 12 Spectrograph on 4 meter Kitt Peak Observatory: http://www.noao.edu/kpno/manuals/rcspec/rcsp.gif Figure 13 Spectrograph on the Nasmyth focus of the Galileo telescope on La Palma Observatory: http://www.merate.mi.astro.it/docM/reports/ann2000/ren00/img178.png Figure 14 Mt. Wilson 100 inches telescope: http://www.mtwilson.edu/vir/100/ Figure 15 Schematic of a modern giant telescope mounting: http://imglib.lbl.gov/ImgLib/COLLECTIONS/BERKELEY-LAB/RESEARCH-1991-PRESENT/ASTROPHYSICS/images/96703309.lowres.jpeg Figure 16 Wooden tripod: Covington M, Astrophotography for the amateurs, Cambridge, 2002. Figure 17: Alt-Azimuth mounting movements: http://digilander.libero.it/andromedda/I%20telescopi%20-%20Montature%20per%20telescopi.htm Figure 18 Field derotator: http://www.ee.nmt.edu/~teare/fielddr.htm Figure 19 Carlsberg Meridian Transit Telescope: http://www.astro.ku.dk/~michael/telescope/trans.html Figure 20 Dobsonian telescope: http://www.eecs.berkeley.edu/~mihal/telescope/edited/telescope.jpg Figure 21 Equatorial mount schematics: http://www.starstuff.com/index.htm Figure 22 Keck versus Hale telescope domes: http://spacecraftkits.com/KFacts.html Figure 23 Equatorial mounting scheme: http://www.starstuff.com/index.htm Figure 24 Fork mounting scheme: http://www.starstuff.com/index.htm Figure 25 University of Thailand telescope: http://www.astrofox.nl/page.cfm?id=31&popup=1 Figure 26 Mt. Palomar Schmidt telescope: http://www.astro.caltech.edu/palomar/images/oschin_telescope_2_med.jpg Figure 27 English Yoke scheme: http://www.starstuff.com/index.htm Figure 28 Crossley reflector at Lick Observatory: http://www.ucolick.org/graphics/crossleynew_lg.jpg Figure 29 Horseshoe mounting scheme: http://members.shaw.ca/quadibloc/science/opt03.htm Figure 30 Anglo Australian Telescope:

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http://www.physics.usyd.edu.au/~bedding/images/telescopes/aat.jpg Figure 31 Inverted fork scheme: http://www.quadibloc.com/science/opt03.htm Figure 32 Cross Axis scheme: http://home.earthlink.net/~indig/DesignMusings/text/Design_Mountings.html Figure 33 122 cm telescope at Asiago Observatory: http://dipastro.pd.astro.it/progettoeducativo/telescopi.htm Figure 34 William Herschel mounting design: http://www.ing.iac.es/PR/archive/wht/wht3d.jpg Figure 35 Light path deviation example: http://members.shaw.ca/quadibloc/science/opt03.htm Figure 36 William Herschel telescope optical design: http://www.astro.ufl.edu/icons/gtc/optics_layout.gif Figure 37 Coude focus located below the telescope: http://www.astro.uwo.ca/~dfgray/observat.html Figure 38 Ondrejov observatory 2m coude telescope: http://www.asu.cas.cz/~had/coude.jpg Figure 39 Optical diagram of Nasmyth focus: http://upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Nasmyth-Telescope.svg/232px-Nasmyth-Telescope.svg.png Figure 40 Gemini telescope structure: http://www.us-gemini.noao.edu/public/images/telescope.gif Figure 41 Solar telescope at Kitt Peak: http://nsokp.nso.edu/mp/images/02170a.jpg Figure 42 Solar telescope at Koeln university: http://www.ph1.uni-koeln.de/workgroups/astro_instrumentation/this/images/sonnenturm.gif Figure 43 Off axis optical diagram: http://www.quadibloc.com/science/images/scheif.gif Figure 44 Hobby Eberly Telescope: http://txtell.lib.utexas.edu/stories/media/m0005-10.html Figure 45 Zenith telescope at Tuorla Observatory: http://www.astro.utu.fi/telescopes/zenit.htm Figure 46 Liquid mirror telescope structure: http://www.phys.psu.edu/~cowen/popular-articles/sciam/1299musser11.gif Figure 47 Liquid Mirror Telescope: http://www.orbitaldebris.jsc.nasa.gov/measure/images/lmt.jpg Figure 48 Observing stars at not comfortable position: http://www.astrofiliaurunca.com/ricerca/images/star%20party%2020021.jpg

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Figure 49 German Equatorial mount: http://www.skywatchertelescope.com/EQ1.html Figure 50 Fork mount: http://scientificsonline.com/product.asp?pn=3126807&cr=1169&bhcd2=1131880398 Figure 51 Single arm mount: http://www.celestron.com/c2/images/files/product/11022-XLT-SE-SA_nexstar8ixltspe_large.gif Figure 52 Dobsonian mount: http://www.celestron.com/c2/images/files/product/10110_starhopper10_large.gif