Familiarization of Observation Instruments use in Astronomy

Familiarization of Observation Instruments use in Astronomy

Familiarization of Astronomical Observation

Instruments

TelescopeTelescope is an optical instrument designed to make distant objects appear nearer and bigger. Telescope magnifies the image of distant objects through an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused to a point.

In modern definition, a telescope is an instrument that aids in the observation of

remote / distant objects by collecting electromagnetic radiation. The word telescope now refers to a wide range of instruments detecting different regions of the electromagnetic spectrum. Depending upon detecting capability a telescope can be radio, infrared, x-ray or optical telescope.

The word "telescope" comes from the Greek word τῆλε, tele "far" and σκοπεῖν, skopein "to look or see"; τηλεσκόπος, teleskopos "far-seeing" was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei's instruments presented at a banquet at the Accademia dei Lincei. In the Starry Messenger, Galileo had used the term "perspicillum".

History of Optical Telescope:

he earliest known working telescopes appeared in 1608 and are credited to Hans Lippershey. The design of these early refracting telescopes consisted of a convex objective lens and a

concave eyepiece. Galileo used this design the following year. In 1611, Johannes Kepler described how a telescope could be made with a convex objective lens and a convex eyepiece lens and by 1655 astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces. Hans Lippershey is the earliest person documented to have applied for a patent for the device.

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Isaac Newton is credited with building the first practical reflector in 1668 with a design that incorporated a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Laurent Cassegrain in 1672 described the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror.

The achromatic lens, which greatly reduced color aberrations in objective lenses and allowed for shorter and more functional telescopes, first appeared in a 1733 telescope made by Chester Moore Hall, who did not publicize it. John Dollond learned of Hall's invention and began producing telescopes using it in commercial quantities, starting in 1758.

Important developments in reflecting telescopes were John Hadley's production of larger paraboloidal mirrors in 1721; the process of silvering glass mirrors introduced by Léon Foucault in 1857, and the adoption of long lasting aluminized coatings on reflector mirrors in 1932. Almost all of the large optical research telescopes used today are reflectors .

Brief History of Radio Telescope:

he era of radio telescopes (along with radio astronomy) was born with Karl Guthe Jansky's unanticipated discovery of an astronomical radio source in 1931. Karl Jansky was an engineer with Bell Telephone Laboratories, in

1932. Jansky was assigned the job of identifying sources of static that might interfere with radio telephone service. Jansky's antenna was an array of dipoles and reflectors designed to receive short wave radio signals (20.5 MHz & λ = 14.6 mtr). It was mounted on a turntable that allowed it to rotate in any direction. By rotating the antenna on a set of four Ford Model-T tires, the direction of the received interfering radio source (static) could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system. After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. Jansky finally determined that the "faint hiss" repeated on a cycle of 23 hours and 56 minutes. This period is the length of an astronomical sidereal day, the time it takes any "fixed" object located on the celestial sphere to come back to the same location in the sky. Thus Jansky suspected that the hiss originated well beyond the Earth's atmosphere, and by comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way Galaxy and was strongest in the direction of the center of the galaxy, in the constellation of Sagittarius.

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Many types of radio telescopes were developed in the 20th century for a wide range of wavelengths from radio to gamma-rays.

Classification of Optical TelescopeOptical telescopes can be classified mainly in three primary optical designs, known as refractor, reflector, or Compound.

Refractors (Dioptrics) have a lens at the front of the tube — it's the type you're probably most familiar with. While generally low maintenance, they quickly get expensive as the aperture increases. 

Reflectors (Catoptrics) gather light using a mirror at the rear of the main tube. For a given aperture, these are generally the least expensive type, but you'll need to adjust the optical alignment periodically — especially if you bump it around a lot.

Compound (or catadioptric) telescopes, use a combination of lenses and mirrors, offer compact tubes and relatively light weight; two popular designs are called Schmidt-Cassegrains and Maksutov-Cassegrains. 

Telescopes can also be classified according to the task it performed as:

a)

Astrograph h) Robotic telescope

b)

Astronomical optical interferometry i) Solar telescope

c)

Comet seeker j) Space telescope

d)

GoTo telescopes k) Spotting scope

e)

Graphic telescope l) Sun Gun Telescope

f) Infrared telescope m) Zenith telescope

g)

Meridian circle

Magnification (Power) of a Telescope: he objective's focal length (F or FL) is the key to determining the telescope's magnification ("power"). This is simply the objective's focal length divided by that of the eyepiece, which you'll find on its barrel. For

example, if a telescope has a focal length of 500 mm and a 25-mm eyepiece, the magnification is 500/25, or 20x. Most types of telescopes come supplied with one or two eyepieces; observer can change the magnification by switching eyepieces with different focal lengths.

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Telescope MountsThe best telescope in the world is useless unless it's on a solid, stable, smoothly-working mount, one that permits it to be directed to the desired part of the sky and to follow a celestial object smoothly and precisely as the Earth turns beneath it.

In realistic terms, a "stable" mount is one that, when using a moderate to high power, will not vibrate for more than a second or so after rap the tube. In particular, the view can't wiggle so much when we hold the focus knob that we can't see when we've found the sharpest focus. And when we let go, the aim must not jump to one side. This completely eliminates the typical "department-store" semi-toy telescope from consideration.

While there are variations on a theme, we will encounter two types of mount: altitude-azimuth (or "alt-az") and equatorial.

An alt-az mount operates like a tripod's pan-and-tilt head, moving the scope up-down (in altitude) and left-right (in azimuth). Equatorial mounts also possess two axes, but they're tilted so that one can be aligned with the rotational axis of the Earth.

If we are intending to use a small telescope for casual sky viewing or daytime use (say, bird watching), the alt-az mount preferable. Well engineered mounts of this type will have finely threaded slow-motion controls that enable the scope to be moved smoothly by tiny amounts, especially important when using high powers. The value of such refinements will be all too apparent when tracking a star or planet at high magnification.

The Dobsonian is a form of alt-az mount. Inexpensive materials such as particleboard and Teflon figure in its construction, resulting in a low-cost, low-center-of-gravity mount that (ideally) glides smoothly about both axes with fingertip control. A Newtonian reflector mounted in this fashion is not only extremely easy to set up and intuitive to use, but very good value, too.

An equatorial mount makes the tracking of celestial objects easier as the Earth turns. Once correctly set up, the scope need only be turned about one axis to follow an object across the sky — and a drive motor can do this automatically. An equatorial is mandatory for most astrophotography.

For a telescope intended for astronomy, and for which photography is a future prospect, consideration should be given to some form of equatorial mount that automatically counteracts Earth's rotation. It's far easier to track a celestial object with a scope mounted this way, since observer need only to turning the scope about one axis — not two simultaneously, as in the alt-az. When an equatorial mount is properly set up, turning the slow-motion control of its polar axis is all that's required to keep an object in view.

More sophisticated mounts, including modern high-tech alt-az mounts, have built-in electric motor drives to do this, freeing observer to concentrate on observing.

So is one type of mount better than the other? Not really, since each has its strengths. For the casual observer who wants a highly portable scope that can be quickly set up in a variety of locations, an alt-az is preferable — especially a Dobsonian. An equatorial, while virtually mandatory for most forms of astrophotography and critical observations of the Moon and planets at high power, needs to have its polar axis aligned with the rotational axis of the Earth. While polar alignment is not particularly difficult and becomes routine with practice, it can take a little time at the start of your observing session if you want to do it really precisely (necessary for photography but not for just looking).

Internet Reference: http://www.skyandtelescope.com/astronomy-equipment/types-of-telescopes/

Astronomical Binoculars

BinocularsBinoculars (also known as field glasses or binocular telescopes)  are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects.

Binoculars have long held a prominent place in amateur astronomy, with many observers having them as part of their observing kit. Binoculars are simply two small telescopes mounted side by side. The best glasses for general astronomy are 7x50s or 10x50s. However, any size can work as an astronomical instrument. What size one chooses depends largely on their budget and needs.

Two basic terms are magnification and aperture. You often see them as numbers, such as 7x50 or 8x30. The first number is the magnification, or "power," telling how many times an object is magnified or how much larger it will appear. The second number is the aperture, the diameter of each of the front (or "objective") lenses in millimeters. Therefore, 8x30 binoculars (spoken as "8 by 30") will make objects appear eight times larger than with the unaided eye and have objective lenses 30 mm in diameter. The larger the objective lenses, the greater the light grasp, and the brighter the object will appear at a given magnification.

In astronomy, most targets glow only dimly in the darkest depths of night, so a large aperture becomes much more important than it is for daytime use.

Therefore 7x50 binoculars will show many more stars and deep-sky objects than 7x35s because, though both units magnify everything to the same apparent size, the 7x50s collect about twice as much light, thereby making stars appear twice as bright.

Field of view is the area of land or sky the glasses present to your eyes. Makers of binoculars often state this as width in feet at 1,000 yards. For example, engraved on 7x50 binoculars might be the phrase "372 feet at 1,000 yards."

This is the binocular's linear field of view. But astronomers are used to fields of view being expressed in degrees. The conversion is easy — divide the linear field by 52.4. So the 7x50s mentioned above have an angular field of view of slightly more than 7°. Remember that the higher the power of your binoculars, the smaller the field of view will be.

The exit pupil The exit pupil is the disk of light you see apparently floating in the eyepiece when the instrument is held a foot or so from your eyes. This little disk must be equal to or smaller than the pupil of your eye if you want the full light-collecting benefit of the instrument's aperture. You can measure the exit pupil directly with a millimeter ruler or just divide the aperture by the magnification. So both 7x35 and 10x50 binoculars have exit pupils 5 mm wide.

Young people's eyes have pupils that open to about 7 mm in the dark. So a 7-mm exit pupil is generally taken to be the largest you can use if you want all the light collected by the binoculars to enter your eye. But it's important to remember that our eyes dilate less as we age, so a 5-mm exit pupil is a better guideline for middle-aged and older people.

Eye relief indicates how far the binoculars can be held from your eyes and still allow you to see the full field of view. Long eye relief is especially useful for people who must wear glasses while observing in order to correct for astigmatism. If you're merely nearsighted or farsighted, you can take off your glasses and refocus the binoculars to compensate.

Mechanical Arrangement:  Most binocular bodies come in one of three styles : (1) The roof-prism, or "H" style, (2) The Zeiss, or "German" style, and (3) The Bausch & Lomb, or "American," style. The latter two are Porro-prism designs and often look similar, but they differ quite a bit in concept and structural integrity.

The Zeiss-style binocular is characterized on the outside by its two-piece body. The housing for the objective screws into the main body, or prism housing, which is divided in two, with a Porro prism in each half held in place by a steel spring clip.

This construction leaves the binoculars more susceptible to being knocked out of alignment than the American-style, one-piece body in which the prisms are mounted together on a "shelf."

Roof prisms vs. Porro prisms. 

We often hear people say roof-prism binoculars are better than the more familiar Porro-prism design. For astronomy, this is not true. Roof prisms allow binoculars to be made small and light — a great advantage for hikers and other active users. But in roof-prism binoculars the light beam is split into two parts, then recombined. The tolerances for doing this successfully are extremely tight, so roof-prism binoculars are expensive to manufacture and test. Moreover, due to the wave nature of light, the beams are "phase shifted" when they recombine, leading to a loss of contrast. Some manufacturers are now coating their roof prisms with "phase coatings" to increase contrast and close the quality gap with Porro-prism binoculars. Still, there's no reason for astronomers to depart from binoculars with Porro prisms.

Porro prisms Roof prisms

Interior of a roof binocular

Center focus vs. individual focus. 

Center focusMost people opt for a center-focus model, in which turning a small wheel between the eyepieces focuses both of them at once — but only after correcting for any difference in nearsightedness or farsightedness between your eyes. This is done by first focusing for the left eye (using the center wheel) and then bringing the right-eye view into focus by adjusting the right eyepiece focuser. This isn't the best choice for astronomers. After all, celestial objects don't change their distance, and so frequent refocusing isn't required.

Individual focus

Individually focused eyepieces are simpler, more rugged, and less prone to moisture infiltration. Best of all, they won't tilt back and forth when you turn the focus in and out or rock side to side when you press unequally against the eyepieces.

Anti-reflection Coatings. Few features are touted more in binocular ads than the quality of antireflection optical coatings. In the best binoculars all air-to-glass surfaces are multicoated, which not only improves light transmission but also minimizes internal reflections and ghosting. Modern multi-coatings are better than single-layer magnesium fluoride (MgF) coatings, but a lot depends on how well any coating is applied.

On many inexpensive instruments, internal surfaces are left uncoated. "Fully coated" ought to mean all air-to-glass surfaces are coated, but in practice it can mean almost anything. As for comparing multi-coatings by two reputable manufacturers, you might as well try to split hairs with an ax — there's just not much difference.

One crude in-store test of antireflective coatings is to place your hand over the eyepiece and look down at the objective lens. In glasses with simple magnesium fluoride on the objectives, you'll probably see your face quite well. In a multicoated lens you'll see a noticeably dimmer reflection. And contrary to urban legend, you can't reliably judge the quality of coatings by their color. However, a totally white reflection may indicate an optical surface that is not coated at all.

For the last few years, some companies have sold binoculars with so-called "ruby" coatings that increase the contrast between brown and green objects. This feature is fine for nature lovers trying to distinguish a deer or other brown-toned animal against the green background of foliage, but sky-gazers don't find many brown bears free-floating in space! In addition, if you look at the objectives you'll see your face very well — much better than with multicoated or MgF-coated lenses. This reflection means you're seeing light that should be going the other way — through the instrument to your eyes. With the coating reflecting so much light, the binocular's light grasp is significantly reduced. Thus, users of ruby-coated binoculars are trading image brightness for a slight improvement in contrast — a tradeoff that amateur astronomers should avoid.

Baffling. The presence of shielding against stray light and internal reflections mustn't be taken for granted. Aim the binoculars at a bright surface or scene. The field of view should be surrounded by a black or very dark background and not be affected by shiny reflections from internal parts or the improper placement of field stops. Anything less will result in a loss of contrast.

Glass Types. Another binocular feature often oversold is the type of glass used in the prisms. For most designs, prisms made of barium light crown glass (Bak4) are preferred over the industry-standard borosilicate crown glass (Bk7). Bak4 has a higher refractive index, which allows the exit pupil to be fully illuminated. The less-expensive Bk7 prisms put squarish, gray edges on the exit pupils. In practice, most people will not notice the slight loss of image brightness from Bk7 prisms, particularly if the binoculars have a large exit pupil.

Collimation. Since binoculars may be thought of as a pair of telescopes mounted side by side, an error in collimation (alignment of the two telescopes) can significantly reduce the performance of even the best binoculars. In "true" collimation, the two barrels remain aligned with each other no matter what the interpupillary distance — the gap between the pupils of observer’s eyes. When binoculars are

aligned for only one particular interpupillary distance, they're not really collimated. Inexperienced technicians often use this "conditional alignment" when aligning or realigning binoculars. As binoculars age and get bumped around, their collimation will gradually drift. Insisting on good-quality mechanics when buying a binoculars ensures that they will last longer before collimation errors become a problem.

Waterproofing. The most common reason binoculars need repair, aside from getting knocked out of collimation, is moisture stains or fungus on the prisms and lenses. Repeated changes in temperature and humidity allow condensation to damage internal parts and cloud optical surfaces. (Dunking binoculars in the lake doesn't help, either.) To "just clean" binoculars actually means an expensive disassembly, reassembly, and complete recollimation. By comparison, a premium, waterproof, dry-nitrogen-filled instrument begins to look economical.

Holding Binoculars SteadyHolding 7-power binoculars steady is not a chore for most people. But use a higher power, or point binoculars skyward, and body tremors cause the image to bounce and shake. This motion compromises observer’s view of the night sky and robs the image of fine detail. The traditional (and inexpensive) solution to the jitters is to attach the binoculars to a tripod. Most binoculars come with a threaded mounting hole. An L-shaped adapter screws into this hole and onto a camera tripod. Also available are binocular mounts specifically designed for astronomical use.

A different approach to the problem came in 1980 when Fujinon introduced the Stabiscope — a gyro-stabilized binocular. So good is this unit that a 14-power image can be stabilized enough for use from a helicopter or fast-moving boat or automobile. But its high price tag kept the unit from catching on with casual observers. Fortunately, image stabilization (IS) for astronomers has arrived. Canon is offering a whole family of lightweight, handheld IS binoculars, and other companies (Fujinon included) are entering the fray.

Pressing a button on the top of the binocular housing activates the image-stabilization system. But this new technology comes with a price — most IS binoculars are several times more expensive than their non-stabilized equivalents. While most people don't really need help holding low-power binoculars steady, the higher-power units are real astronomical workhorses. Stabilization allows you to view objects fainter than you might think possible, and it helps reveal far more detail than would be visible in equivalent non-IS binoculars. Binoculars offer novice stargazers an intuitive way to explore the sky. Veterans often regard binocular observing as an activity distinct from telescopic viewing. Regardless of your experience, quality binoculars belong in every observer's kit.

Advanced OpticalAdvanced Optical TelescopesTelescopes

Advanced Optical Telescopes

Cassegrain reflector telescopes 

The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas.

In a symmetrical Cassegrain both mirrors are aligned about the optical axis, and the primary mirror usually contains a hole in the centre thus permitting the light to reach an eyepiece, a camera, or a light detector. Alternatively, as in many radio telescopes, the final focus may be in front of the primary. In an asymmetrical Cassegrain, the mirror(s) may be tilted to avoid obscuration of the primary or the need for a hole in the primary mirror (or both).

The classic Cassegrain configuration uses a parabolic reflector as the primary while the secondary mirror is hyperbolic.[1]Modern variants often have a hyperbolic primary for increased performance( for example, the Ritchey–Chrétien design), or the primary and/or secondary are spherical or elliptical for ease of manufacturing.The Cassegrain reflector is named after a published reflecting telescope design that appeared in the April 25, 1672 Journal des sçavans which has been attributed to Laurent Cassegrain. Similar designs using convex secondaries have been found in the Bonaventura Cavalieri's 1632 writings describing burning mirrors and Marin Mersenne's 1636 writings describing telescope designs. James

Gregory's 1662 attempts to create a reflecting telescope included a Cassegrain configuration, judging by a convex secondary mirror found among his experiments.

The Cassegrain design is also used in catadioptric systems

The Schmidt–Cassegrain is a catadioptric telescope that combines a Cassegrain reflector's optical path with a Schmidt corrector plate to make a compact astronomical instrument that uses simple spherical surfaces.

The first optical element is a Schmidt corrector plate. The plate is figured by placing a vacuum on one side, and grinding the exact correction required to correct the spherical aberration caused by the primary mirror. Schmidt-Cassegrains are popular with amateur astronomers. An early Schmidt-Cassegrain camera was patented in 1946 by artist/architect/physicist Roger Hayward, with the film holder placed outside the telescope.

The Maksutov-Cassegrain is a variation of the Maksutov telescope named after the Soviet / Russian optician and astronomer Dmitri Dmitrievich Maksutov. It starts with an optically transparent corrector lens that is a section of a hollow sphere. It has a spherical primary mirror, and a spherical secondary that in this application is usually a mirrored section of the corrector lens.

In the Argunov-Cassegrain telescope all optics are spherical, and the classical Cassegrain secondary mirror is replaced by a sub-aperture corrector consisting of three air spaced lens elements. The element farthest from the primary mirror is a Mangin mirror, in which the element acts as a second surface mirror, having a reflective coating applied to the surface facing the sky.

The Klevtsov-Cassegrain, like the Argunov-Cassegrain, uses a sub-aperture corrector. It consisting of a small meniscus lens and Mangin mirror as its "secondary mirror".

Applications

The Schmidt–Cassegrain design is very popular with consumer telescope manufacturers because it combines easy to manufacture spherical optical surfaces to create an instrument with the long focal length of a refracting telescope with the lower cost per aperture of a reflecting telescope. The compact design makes it very portable for its given aperture, which adds to its marketability. Their high f-ratio means they are not a wide field telescope like their Schmidt camera predecessor but they are good for more narrow field deep sky and planetary viewing.

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Main Parts of a Telescope

Primary Mirror :

Secondary Mirror:

Mirror Support:

Clamps:

TriPod:

Mirror Adjusting Mechanism:

EyepieceAn eyepiece (Ocular Lens) is a type of lens that is attached to a telescope.  It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.

An eyepiece consists of several "lens elements" in a housing, with a "barrel" on one end. The barrel is shaped to fit in a special opening of the instrument to which it is attached. The image can be focused by moving the eyepiece nearer and further from the objective. Most instruments have a focusing mechanism to allow movement of the shaft in which the eyepiece is mounted, without needing to manipulate the eyepiece directly.

In telescopes eyepieces are usually interchangeable. By switching the eyepiece, the user can adjust what is viewed. For instance, eyepieces will often be interchanged to increase or decrease the magnification of a telescope. Eyepieces also offer varying fields of view, and differing degrees of eye relief for the person who looks through them.

Eyepieces determine the magnification and field of view of a telescope.  Different eyepieces are used to view different objects.  Some objects, such as nebulae and star clusters, appear quite large and are best viewed at low magnifications (which give a wider field of view), whereas planets appear very small and are normally viewed with high-magnification eyepieces.  One of the most common misconceptions in amateur astronomy is that magnification is the most important aspect of a telescope.  In reality, the diameter (aperture) of a telescope determines its power and different eyepieces are used to get the best view of a given object. Often the best view is at a low magnification.  

Why are eyepieces even necessary?  A telescope is an optical system that creates an image, just like a camera lens creates an image on film.  In fact, placing a camera at the focus of a telescope will also capture an image, since the telescope becomes the camera lens.  But, placing your eye at the focus point of a telescope does not produce an image.  Why not?  Because your eye is also an optical system.  Your eye focuses light just like a telescope does, and it cannot focus on a real image such as that created by a telescope.  It requires a virtual image, which is what an eyepiece creates.

Take a look at the diagram.  It shows that both a telescope and your eye focus light to a point.  Placing an eyepiece at the focal point of a telescope then creates a light beam which is neither converging nor diverging.  Your eye can then focus the light beam exiting the eyepiece.

Aspects relevant to choice of Eyepiece

MagnificationThe most important eyepiece characteristic is focal length.  This is the number, in millimeters, written on the side of every eyepiece.  It allows you to determine the magnification an eyepiece gives in combination with a given telescope.

Magnification is determined simply by dividing the focal length of the telescope by the focal length of the eyepiece.

This means that a smaller number on an eyepiece gives a higher magnification.  A 10 mm eyepiece would provide twice as much magnification as a 20mm eyepiece.  It also means that the same eyepiece gives different magnifications on different scopes.  A 10 mm eyepiece would be low power on a short-focal-length scope but high power on a long-focal-length scope.  For example, on an 80 mm short-focal-length refractor, a 10mm might only provide 40x magnification, but the same eyepiece on a 10" Schmidt-Cassegrain telescope would give 300x.

A typical eyepiece collection would include 3 eyepieces:  one low power, one medium power, and one high power.  The usual magnification range depends on the telescope, but for most scopes the normal range might be from 50x to 250x.

Increasing the magnification makes the image larger, but the image gets dimmer and the field of view gets smaller.

Eye ReliefThis is an important aspect of many eyepieces.  Eye relief is the distance from the eyepiece to the observer's eye.  The shorter this distance, the more difficult it can be to observe.  Also, if the observer must wear eyeglasses, short-eye-relief eyepieces can be very difficult or impossible to use.  Long-focal-length eyepieces (usually low power) inherently have long eye relief, so they do not need to be specially designed to increase eye relief.  Short-focal-length eyepieces (usually high power), on the other hand, do not inherently have long eye relief and must be specially designed to make them easier to use.

Two short focal length eyepieces, one with normal eye relief and one

specially designed with long eye relief.  Note the difference in the size of the

eye lens.The eye relief of an eyepiece is the distance from the top lens in the eyepiece to the observer's eye

 Field of ViewThe amount of sky seen through an eyepiece (called the true field of view) is determined by both the magnification and the eyepiece's apparent field of view. Apparent field of view is a design characteristic of an eyepiece design.  Some eyepieces have narrow apparent fields and some have wide apparent fields.  If the magnification is kept the same (i.e., the eyepieces have the same focal length), an eyepiece with a wider apparent field will have a wider true field.

Changing the apparent field but not the magnification changes the field of view but not the object size.

Observer can also change field of view by simply changing magnification.  If the apparent field is kept the same, a lower power eyepiece will give a wider field of view.  To view very large objects such as the Andromeda Galaxy or Pleiades star cluster, observer need a very large field of view and hence a very low magnification.  Field of view is very important for getting the best view.

Increasing the magnification may not always result in a better view, especially if the object being viewed is very

large.

Eyepiece Sizes

There are two standard sizes of telescope eyepieces.  The sizes are determined by the diameter of the eyepiece barrel that fits into the telescope.  The two standard sizes are 1.25" and 2".  A third size, 0.965", is a smaller standard that is usually best to avoid (see below).

1.25" EyepiecesAlmost all telescopes are designed to be used with 1.25" diameter eyepieces.  Most telescopes will include at least one 1.25" eyepiece.  Accessories such as Barlow lenses and filters are designed to thread into the barrel of these eyepieces, so such accessories are also distinguished by size.  Good 1.25" eyepieces typically cost $40-200, although there are more and less expensive models.

2" EyepiecesThe second standard size is the larger 2" diameter.  Many telescopes will accept these eyepieces, though some telescopes will require an optional adapter.  Not all telescopes work with 2" eyepieces.  2" eyepieces are wide-field, low-power eyepieces. Above a certain magnification (which depends on the design), 2" diameter barrels are not required, so not all wide-field eyepieces are 2"--some will still be 1.25" and this is not a disadvantage, just a function of the design.  This is a common misconception.  Accessories such as filters and Barlow lenses are designed for 2" eyepieces as well.  2" eyepieces typically cost $200-400, with some of the largest and highest quality eyepieces costing around $600.  Some inexpensive models are also available for around $100, though these will obviously not have the features or quality of the more expensive eyepieces.

Above:  A 2" wide-field eyepiece compared to a standard 1.25" eyepiece.  Both are 26mm eyepieces.

0.965" EyepiecesThe final eyepiece size is the one to avoid.  0.965" eyepieces are the standard size for "department store" telescopes.  These inexpensive telescopes often frustrate new stargazers, and one of the primary reasons is that viewing through 0.965" eyepieces is all but impossible.  Also, standard accessories such as Barlow lenses and filters are not normally available for these eyepieces.  And you are usually stuck with the eyepieces that come with the scope since 0.965" eyepieces are rarely sold separately.  The difference between a scope with 1.25" eyepieces and one with 0.965" eyepieces is usually the difference between a scope that ends up in the yard showing you the wonders of the universe and one that ends up in the closet collecting dust.

Common Telescope Eyepiece Designs(Including the Brandon, Erfle, Huyghens, Kellner, Konig, Modified Achromat, Monocentric, Orthoscopic, Plossl, Ramsden, RKE, Tele Vue/Nagler and TMB Planetary designs)By Chuck Hawks

Brandon eyepieces. Illustration courtesy of Vernonscope & Company.

This article deals only with fixed focal length eyepieces. The top zoom eyepieces are described in detail in the article, Comparison: 8-24mm Zoom Eyepieces from Celestron, Meade, Tele Vue and Vixen.

There are many different designs, or optical formulas, for oculars. Many are named for their original designer, famous opticians with last names such as Huyghens, Ramsden, Kellner, Plossl, Konig, Erfle, Branden and Nagler.

Like camera lenses, inside of every ocular you will find a group or groups of individual lens elements. Oculars usually consist of two or more elements. Simple two element eyepiece designs are plagued by lateral color error (color fringing). Probably the best known two element optical formulas are the Ramsden and Huyghens, which date back to the 17th and 18th Centuries respectively. Two element oculars should be avoided.

Achromatic oculars, such as the original three element Kellner design that uses a cemented, achromatic doublet eye lens, focus the long and short wavelengths of visible light, red and blue, to a common plane, but green light is not properly focused and typically creates a noticeable fringe of color around bright objects. Nevertheless, the Kellner design is much better corrected than the Ramsden or Huyghens. Similar in function to the Kellner design are the Modified Achromat (MA) and Super Modified Achromat (SMA) eyepieces from Meade.

In practice, a fourth lens element is usually required to "bring it all together" and produce sharp views without intrusive lateral color and other aberrations. (All ocular designs have some residual aberrations, though.) The best eyepieces today incorporate a minimum of four elements in at least two groups in their design. Four element oculars are common and generally perform well. Orthoscopic and Plossl oculars are the most popular four element designs, while wide view Erfle type oculars use five elements.

Some modern oculars incorporate more elements (up to eight) than the traditional designs for improved correction of aberrations, increased eye relief and a wider apparent field of view (AFOV). Often these eyepieces incorporate elements made of extra low dispersion (ED) glasses. The upscale Burgess/TMB Planetary (60-degree AFOV), Celestron Axiom LX (81-degree AFOV), Celestron Ultima LX (70-degree AFOV), Celestron X-Cel (55-degree AFOV), Meade Ultra Wide (84-degree AFOV), Orion Stratus (68-degree AFOV), Orion MegaView (82-degree AFOV), Tele Vue Ethos (100-degree AFOV), Tele Vue Nagler (82-degree AFOV), Tele Vue Panoptic (68-degree AFOV), Tele Vue Radian (60-degree AFOV) and Vixen Lanthanum-LVW (65-degree AFOV) ocular lines, to name a few of the better known brands, are examples of such eyepieces. These are priced higher than standard designs and they are available in a variety of focal lengths in both 1.25" and 2" size.However, more elements are not necessarily better in terms of brightness, sharpness and contrast. The more glass photons of light have to travel through on their way to your eye, the more light is absorbed, scattered and lost, decreasing brightness and contrast. Optical design is always a balancing act, an attempt to minimize flaws (aberrations and distortion) and maximize certain desirable characteristics, such as sharpness from center to edge, brightness, contrast, eye relief and a flat field.

Different eyepiece designs do better in some areas than others and no design is perfect in all respects, so you have to choose the design that is most suitable for your intended purpose. The quality, care and precision of manufacture is also extremely important. Online specialty retailers, such as Optics Planet (www.opticsplanet.com), offer extensive eyepiece selections at a wide range of prices. Following, in alphabetical order, is a brief description of the most common eyepiece designs encountered today.

BrandonBrandon eyepieces are manufactured by Vernonscope and sold directly, as well as by Questar. Questar supplies house brand Brandon oculars with its telescopes. Vernonscope offers 1.25" Brandon eyepieces in 8mm, 12mm, 16mm, 24mm and 32mm focal lengths. (These are pictured in the color photo at the top of this page.)The Brandon uses four elements in two groups. It is a symmetrical design similar to the Plossl with a 50-degree AFOV, but its optical properties are intended to mimic those of an Orthoscopic ocular. Brandons are sharp, flat field, highly corrected eyepieces with very low distortion and exceptionally high contrast. Like Plossls and Orthos, however, short focal length Brandons are lacking in eye relief.Interestingly, unlike other premium eyepieces, Vernonscope Brandon oculars are not multi-coated, although they are fully coated. The stated reason for this is to reduce narrow angle light scatter; multi-coatings increase narrow angle light scatter, which interferes with low contrast detail. Vernonscope has tried seven layer multi-coatings and their single layer anti-reflection coating works as well or better.

ErfleHeinrich Erfle invented this design during the First World War. The basic Erfle uses five elements in three groups and delivers a wide apparent field of view of about 60-degrees. Erfle variations are also made with six elements in three groups, as in the diagram below. Erfles have a nice, flat field from edge to edge, although sharpness and contrast is slightly inferior to the Orthoscopic and Plossl designs. They also feature long eye relief, especially important for eyeglass wearers.

Erfle eyepiece diagram. Illustation courtesy of Wickipedia Commons.

This combination of features makes these oculars excellent for wide sky viewing of open clusters, star fields, etc. They are usually found in focal lengths in the 18mm to 32mm range. I have used 1.25" Erfles in 24mm and 32mm focal lengths, where they give a greater field of view than Plossl type oculars.

Erfle type oculars make excellent medium to low magnification eyepieces and today are often sold under proprietary names that do not credit Heinrich Erfle. Erfles are available in 1.25" and 2" size. Meade QX 1.25 and 2" oculars are probably the best known Erfles. 1.25" QX eyepieces are available in 15mm and 20mm focal lengths. Offering a similar apparent field in both sizes are the premium, 6-element designs called Super Wide Angle by the Meade Instrument Company, which fulfill the same role as traditional five element Erfles. In 1.25" diameter, the Meade Super Wide Angle oculars are available in focal lengths from 13.8mm to 24.5mm.

HuyghensIn the 1660's, Dutch astronomer Christian Huyghens designed a simple, two element eyepiece using a large, planoconvex field lens placed before the focus point of the telescope and a small planoconvex eye lens, separated by about half the sum of their focal lengths. The plain side of both elements faces the observer's eye and the image is formed between the two elements, making this a negative ocular.

Huyghens eyepiece diagram. Illustation courtesy of Wickipedia Commons.

Huyghenian eyepieces have a restricted AFOV and there isn't a lot of eye relief. The small eye lens makes peering through high magnification Huyghenian oculars rather like looking through a pin hole. This design served its purpose in the 17th and 18th Centuries, but is obsolete today. Because they are inexpensive to make, Huyghens eyepieces are still sometimes provided with cheap, department store telescopes.

Kellner (Achromat)Carl Kellner designed the first achromatic ocular in 1850. (Some sources report the date as 1863.) Along with the similar Meade Modified Achromat (MA), these are the least expensive practical oculars and are offered in focal lengths from about 6mm to 40mm. Kellners use three elements in two groups to minimize color fringing. However, some lateral color error is apparent when looking at bright subjects, such as Jupiter or Venus, against a dark sky.

Kellner eyepiece diagram. Illustation courtesy of Wickipedia Commons.The design uses a planoconvex, two cemented-element eye lens and a large convex field lens. Kellner type oculars generally have an apparent field of view of about 40-degrees, with moderate eye relief. They have good center sharpness, but exhibit some field curvature and astigmatism. Edge sharpness is only so-so, as is the apparent field of view. They are most useful in medium and longer focal lengths for terrestrial, planetary and lunar viewing.Kellners are better than two element achromats, but clearly inferior to four element Ortho and Plossl oculars. They can, however, provide decent views with small telescopes. I have found the moderate 16mm to 25mm focal lengths to be the most useful among the Kellner oculars I have owned. Kellner and other three-element ocular designs are available in .96", 1.25" and 2" diameter oculars. Celestron used to supply Kellner oculars with their least expensive telescopes and still may.KonigDesigned in 1915 by Albert Konig, the Konig eyepiece is similar to an Orthoscopic with a doublet field lens in place of the triplet. The original design allows for high magnification with excellent eye relief, the most eye relief proportional to focal length of any design prior to the introduction of Al Nagler's high magnification eyepieces. The Konig's apparent field of view is about 55-degrees.

Konig eyepiece diagram. Illustation courtesy of Wickipedia Commons.Modern versions of Konigs usually use lower dispersion glasses than were available to Albert during WW I and sometimes add more elements. The most common change is to add a positive, concave-convex element before the cemented doublet field lens, with the convex surface facing the doublet. These modernized Konigs offer 60 to 70-degree AFOV's and are usually sold under proprietary names that do not credit Konig as the original designer.

Modified AchromatA three element ocular design used by Meade for the oculars they supply with their least expensive telescopes. Functionally similar to a Kellner ocular. Some chromatic aberration is apparent when looking at very bright subjects, such as Jupiter or Venus. They have a narrow apparent field of view of about 40 degrees and limited eye relief in the shorter focal lengths, which makes them difficult to look through. MA's are adequate starter oculars in medium and longer focal lengths, but it is best to replace MA's with Ortho, Plossl or other more sophisticated ocular designs when the opportunity arises.

MonocentricDesigned by Adolf Steinheil in 1883, this unusual eyepiece design uses three thick elements that are segments of concentric circles. The two outer elements are crown glass cemented to a flint glass center element. The result is a solid eyepiece with only two air to glass surfaces (the front and back). The

Monocentric exhibits excellent brightness, contrast and freedom from ghost images. Unfortunately, the apparent field of view is very limited, about 25-degrees. However, this is not too much of a drawback when the Monocentric ocular is used for its intended purpose, which is viewing planetary and lunar details at high magnification.

Monocentric eyepiece diagram. Illustation courtesy of Wickipedia Commons.The modern optician Thomas M. Back did considerable work improving Monocentric eyepieces by tweaking the design, using ED glass and low scatter anti-reflection coatings, ultimately creating the TMB Super Monocentric oculars for telescopes with focal ratios slower than f/6. This was intended to squeeze the absolute maximum contrast and sharpness possible from an eyepiece and Back was willing to sacrifice eye relief, AFOV and correction for off-axis astigmatism to achieve his goal. The result was a line of eyepieces beloved by serious planetary observers, but ignored by nearly everyone else. TMB Super Monocentric focal lengths included 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 12mm, 14mm and 16mm. The AFOV was 32-degrees. The Super Monocentric eyepiece line was subsequently replaced by the current Burgess/TMB Planetary line, based on a totaly different design.

Orthoscopic (Ortho)An apochromatic ocular design by Ernst Abbe' that uses four elements asymmetrically spaced in two groups with (unlike Plossl, Branden and Erfle oculars) crown glass elements outside of the flint glass elements. There is a single eye lens and the field lens is a three element cemented group.

Orthoscopic eyepiece diagram. Illustation courtesy of Wickipedia Commons.Well designed and manufactured Orthos are sharp, well corrected, flat field eyepieces. The Ortho was designed for very low pincushion and barrel distortion and it is a good design for viewing bright objects with a lot of detail. It is excellent for planetary and lunar viewing. Orthos typically have very good center sharpness with only a small amount of field curvature and astigmatism. Orthos with 1.25" diameter mounting barrels are usually available in focal lengths from about 4mm to 25mm.

The drawbacks to Orthoscopic eyepieces are an AFOV of around 40 to 45-degrees and limited eye relief. Due to their restricted eye relief, Orthos are

difficult for eyeglass wearers to use in focal lengths shorter than about 25mm and difficult for everyone in focal lengths shorter than about 10mm, unless you don't mind your eyelashes brushing the ocular lens. Despite their limited eye relief, I have enjoyed the tack sharp views provided by the Orthos I have owned.Perhaps because of its asymmetrical design, an Ortho is less susceptible to internal reflections (ghost images) than Plossl oculars. On the other hand, the 50-degree AFOV of Plossl oculars makes them a better choice for deep sky and all-around viewing.

PlosslDesigned by Georg Simon Plossl in 1860, the Plossl design has become the mainstay of the modern ocular business and probably represents the best value in terms of performance and price, especially in medium and long focal lengths. Celestron (Omni), Meade (Series 4000), Orion (Highlight), Tele Vue, Vixen (NPL) and others offer high quality Plossl oculars. Good Plossls are expensive to manufacture, because they require good optical glass and precisely matched concave and convex doublet surfaces to prevent internal reflections. All Plossl oculars are not created equal.

Plossl eyepiece diagram. Illustation courtesy of Wickipedia Commons.The Plossl optical formula uses four elements in two symmetrical, cemented pairs. Unfortunately, as alluded to in the paragraph above, this symmetry makes Plossl oculars susceptible to internal reflections when viewing bright objects at high magnification, so an internally blackened lens barrel with thread baffles is important, as are blackened lens edges and sophisticated anti-reflection multi-coatings. Inexpensive Plossls lacking these features deliver noticeably inferior views and are not a good value.

If well made, Plossls are bright, contrasty, with a flat field, and excellent sharpness. Plossls are usually well corrected and offer about a 50-degree apparent field of view. This apparent field is wider than an Ortho or a Kellner, but not as wide as an Erfle or Radian. Their eye relief is limited to about 70-80% of their focal length. Plossls are available from many suppliers in a wide range of focal lengths in 1.25" and 2" sizes.

The focal length range for 1.25" Plossls is about 6mm to 45mm. Plossl oculars are usually parfocal within a given manufacturer's line, which means that when one is in focus, they are all (at least approximately) in focus. Plossls provide adequate eye relief for eyeglass wearers in focal lengths of about 25mm and longer. Non-eyeglass wearers can usually tolerate Plossls as short as about 10-12mm before their eyelashes start brushing the ocular lens.

Plossls are useful for most purposes, from short focal lengths designed for planetary views and splitting double stars, to long focal lengths designed for

spectacular deep sky views. A 50-56mm Plossl in 2" diameter is spectacular for wide field, deep sky viewing. Telescope companies such as Celestron, Meade, Orion, Sky View, Stellarvue, Tele Vue and Vixen generally supply a 1.25" Plossl eyepiece with their better telescope packages. There are full length articles about Celestron Omni, Meade Series 4000 and Tele Vue Plossl eyepieces on the Astronomy and Photography Online home page.

RamsdenCreated by and named for the English mathematician Jesse Ramsden in the 1700's, this is a symmetrical two element design using two planoconvex elements (eye lens and field lens). The plain side of both elements faces outward and the image is formed between the eye element and the observer's eye, making the Ramsden design a positive ocular.

Ramsden eyepiece diagram. Illustation courtesy of Wickipedia Commons.Both elements are made of crown glass and the Ramsden design was once popular with those who made their own eyepieces. Subject to lateral color error and substantial loss of sharpness at the edge of the field of view. Restricted AFOV, but longer eye relief than the Huyghens and Kellner designs.

Reversed Kellner (RKE)Designed for Edmund Scientific in the late 1960's by David Rand, the RKE is a three element eyepiece that provides a somewhat greater AFOV than the traditional Kellner. It is known as a Reversed Kellner, because the configuration of the elements is reversed from that of the Kellner.

RKE eyepiece diagram. Illustation courtesy of Wickipedia Commons.The RKE uses a single convex eye lens and a concave/convex cemented doublet field lens with the concave element facing the light source. A diagram of the RKE looks like a Konig with the two groups spaced farther apart.

Tele Vue Nagler, Radian, Panoptic and EthosTele Vue's resident head optician is Al Nagler, one of the best known optical designers in the world today. These popular premium oculars incorporate various types of ED glass elements in their multi-element designs. The Tele Vue

proprietary eyepieces include the Nagler, Radian, Panoptic and Ethos lines, all of which use more than four elements.

Al Nagler formulated a short focal length eyepiece design that incorporates a pair of elements that essentially serve as a permanent, built-in Barlow lens (note the two elements that constitute the field group at the far left in the diagram below) and achieved an incredible 82-degree apparent field of view coupled with comfortable eye relief (12mm-19mm, depending on focal length, in current Nagler eyepieces). The Nagler eyepiece design has evolved over the years with the result that Nagler Type 4, 5 and 6 are the current versions. The Nagler Types 1, 2 and 3 have been discontinued.

Nagler 2 eyepiece diagram. Illustation courtesy of Wickipedia Commons.Nagler oculars are generally big and heavy and therefore often are not appropriate for small or compact telescopes. They contain six to eight elements in four or five groups. Introduced by Tele Vue in 1982, Naglers have become the "gold standard" for short oculars, especially in the 2" diameter mounting barrel size.Nagler ultra-wide designs are primarily used for short focal length (high magnification) applications, because short focal length necessitates short eye relief and tiny eye lenses in most conventional ocular designs. The Nagler design eliminates both of these drawbacks. Nagler Types 4, 5 and 6 are more compact than the original (hand grenade size!) Naglers and retain the 82-degree apparent field of the earlier types. Focal lengths run from 4.8mm to 16mm in 1.25" mounting barrels. There are also a couple of very short focal length Nagler zoom oculars, a 2-4mm and a 3-6mm.

The Radian design incorporates six elements in four groups in focal lengths above 8mm and seven elements in five groups in the 6mm and shorter focal lengths. Radians have a uniform 60-degree apparent field of view and 20mm eye relief, regardless of focal length. They range in focal length from 3mm to 18mm, in 1.25" size only, and are primarily intended for medium to high magnification applications. Radian oculars offer full field sharpness with true orthoscopic linearity, high contrast and relatively compact size. They are a good choice for eyeglass wearers and are physically smaller than Tele Vue Nagler oculars. There is a full length article about Tele Vue Radian oculars on the Astronomy and Photography Online home page.

Tele Vue's premium Panoptic oculars have a 68-degree apparent field, generous eye relief and are available in both 2" and 1.25" barrel diameters. Their optical design uses six elements in four groups. The Panoptic 41mm delivers the widest 2" AFOV possible. There are three focal lengths in the 1.25" size: 15mm, 19mm, 22mm and 24mm. The latter has the widest 1.25" actual

FOV possible, equal to that of a 32mm Plossl with, of course, considerably greater magnification (but less brightness). Eye relief is approximately 70% of focal length.

The Ethos line is the work of lead designer Paul Dellechiaie and has the widest apparent field of view among Tele Vue eyepieces, 100-degrees. It is also the most expensive. Ethos oculars feature low distortion, high contrast and very good center sharpness. They are intended for high magnification planetary observing. There are 21mm and 17mm Ethos eyepieces in 2" mounting barrels and 6mm, 8mm, 10mm and 13mm focal lengths in dual purpose 2"/1.25" mounting barrels. Eye relief is a constant 15mm for all Ethos oculars. These are very large oculars, weighing from about one pound to 2.25 pounds for the massive 2", 21mm Ethos.TMB Planetary II (Burgess/TMB Planetary)Designed by the late Thomas M. Back and marketed by both TMB Optical and Burgess Optical, these are apochromatic, flat field, low distortion, short focal length eyepieces with many of the advantages of an Orthoscopic ocular, but without the disadvantages. This was achieved by using modern design techniques and optical glasses.

Contrast and sharpness are superb. The eye lens is sizeable and easy to look through and the eye relief is a comfortable 16mm for all focal lengths. The AFOV is a generous 60-degrees. The 9mm Planetary uses six elements, all other focal lengths use five elements.

Here is an explanation of the reason for the Planetary eyepiece from Burgess Optical: "A planetary eyepiece is optimized to view bright objects while showing maximum detail and definition. It is an eyepiece with very high light transmission, very high contrast, minimal lateral color and minimal light scatter. For Dobsonian users and others without tracking drives, lateral color and other aberrations are minimized to allow the planet to drift across the field of view while still being sharp and having high contrast. Also, because lunar and planetary observers seek to see as much detail as possible, planetary eyepieces typically have a short focal length for high magnification."I replaced all of my short focal length Plossl and Ortho oculars with Burgess/TMB Planetary eyepieces and have never looked back. The available focal lengths are 2.5mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm and 9mm. All are supplied with twist-up eyecups.

Finder Scope

A finderscope is an aiming device used in astronomy, typically a small auxiliary telescope mounted on the main astronomical telescope along the same line of sight. The finderscope usually has a smaller magnification than the main telescope, providing a much larger field of view, useful for manually aiming (also called "slewing") a telescope and locating a desired astronomical object. Some finderscopes have crosshairs to aid in accurately pointing the telescope system at a target.