Archive for the ‘Instruments’ Category

Achromatic Lens

Achromatic Lens – getting the colours under control

An achromatic lens is is corrected for chromatic aberration. That reduces the false colour around a star. This type of lens comprises either two or three different elements which together, correct the aberration.

An achromatic lens is made from a crown glass and a flint glass element. The crown glass is a glass type that has a positive low refractive index. The flint glass has a negative high refractive index. In that way, the two refractive indices cancel out and the ultimate result is that there is little or no scattering of the colours.

In comparison to a singlet lens (consists of only one piece of glass) the additional design freedom provided by using a doublet (consisting of two pieces of glass) design allows for further optimisation, for example, spherical aberration can also be better controlled. An achromatic lens will have noticeable advantages over a comparable diameter and focal length singlet lens.

The different elements of an achromatic lens are either cemented together or held in place by a frame that allows an air space between the two.

However, it isn’t possible to construct a perfect achromatic lens as the two glass combination will only bring the red and blue ends of the spectrum to the exact same focal point. But well made achromatic lenses make a very good job of things. When looking through a good quality doublet or triplet, to all but the most discerning and well trained eye, you will not see the coloured rings of chromatic aberration around stars.

Achromatic lenses can be used as either the objective lens ot as part of the eyepiece. The first achromatic eyepiece was devised in 1849 by Kellner. He simply replaced the single lens element closest to the eye with a doublet. The Kellner eyepiece is a good type to have for low to medium power work.

Building your own telescope is described here –

Airy Disc

Airy Disc – limiting what you can see

Larger telescopes can see smaller objects. They can also resolve closer binary stars and finer details. Why? Magnification? No, it is all to do with the Airy disc …

The apparent size of a star’s disc produced by diffraction effects in an optical telescope. No matter how well made a telescope is, it will never be perfect. The light passing through a telescope will be diffracted. The diffraction creates a sequence of rings of decreasing brightness. The central ring is the brightest and it is called the Airy disc.

The Airy disc has an inverse relationship to the aperture of the telescope. The larger the telescope, the smaller the Airy disc. The size of the Airy disc limits the resolution of a telescope.

In a refracting telescope, about 84% of the starlight reaching the telescope goes into the airy disc, the other 16% forms diffraction rings around the disc, degrading the image and limiting the resolution. Less light makes it into the Airy disc in a reflecting telescope because of the presence of the secondary mirror. It is for that reason that a reflecting telescope of a given aperture will always out perform a reflector of the same quality.

George Airy was the 7th Astronomer Royal of the UK and it is named for him. Why? Because he was the chap who worked all this out of course! But he wasn’t the first to observe the phenomenon, that honour falls to John Herschel. Or at least it was Herschel who first described it.

But how does the Airy disc arise. See the Wikipedia article for a thorough mathematical explanation.

For a simple non-mathematical treatment this will have to do. Most readers of this article will appreciate that diffraction occurs when light passes through holes of a comparable size to its wavelength. It’s not just holes that create diffraction effects. Edges can do that too. So because a lens or a mirror has a finite size, it also has an edge.  It is because of the presence of the edge that diffraction takes place.


An alidade is a sighting device that is used to measure angles.

The earliest alidades were simply long sticks with sighting slots. They enabled early astronomers to measure the relative angles between stars and thus to produce the earliest star maps.

Over the years they became more and more sophisticated and incorporated small telescopes, for example, as used in the modern theodolite by terrestrial surveyors.

Altazimuth Mount

A type of telescope mount, which allows the instrument to be moved freely in declination (altitude) and right ascension (azimuth) hence the name altazimuth mount.

The altazimuth mount is the simplest of all of the telescope mounts. It has two axes that are set at right angles to each another. These two axes permit movement vertically and horizontally, allowing a telescope to be directed at any point in the sky. The big disadvantage is that, unlike an equatorial mount, it requires continual adjustment to counteract the Earth’s rotation in order to keep an object in view. It is for that reason that the altazimuth mount has long been associated with cheap telescopes and beginners telescopes. It is simple and cheap to produce. It was the first effective  type of telescope mount and dates back to the 16th century, when telescopes were first invented.

With the advent of computers, that all changed. When drive motors controlled by computers are added to the two axes, it then becomes straightforward to follow an astronomical object.

The 6-meter Bol’shoi Teleskop Azimultal’nyi (BTA), Nizhnii Arkhyz, Russia was commissioned in 1976. It was the first large optical telescope to use an altazimuth mount. It is located on the slopes of Mount Pastukhov on the northern side of the Caucasus Mountains. The telescope uses an f/4 primary mirror giving a 26-meter tube housed in a dome 58m in height.

The computer for the Bol’shoi telescope was a complex mainframe initially but the same digital tracking  technology is now available to amateurs on their own computers or built into the telescope control system.  Examples of these telescopes can be found from the main manufacturers such as Meade, Celestron and so on. Very little setting up is required, other than the mount is perfectly level to begin with. ‘Teach’ the computer where it is by showing it two known objects and the software will do the rest.

Because of it’s cheap and robust construction, altazimuth mounts are increasingly used for the worlds largest professional telescopes. That keeps costs down and to divert money to more crucial systems.

For amateurs, the Dobsonian mount brought portability to owners of larger Newtonian reflecting telescopes. The Dobsonian mount is a version of the altazimuth mount. It is essentially a box shaped fork with two semicircular recesses ar the top of each fork. The telescope sits in the recesses in the top which permit vertical movement. The base of the fork rests on short vertical axis and a teflon coated flat plate. That permits horizontal movement. It is not uncommon now to have fairly easily portable 30cm or greater Newtonian reflectors.


Aperture is the diameter of the main lens or mirror of a telescope in inches or cm.

The larger the aperture, the greater the light grasp and resolving power. Optical instruments with a larger aperture can therfore see fainter objects and separate more closely spaced objects.

The effective aperture of a reflecting telescope is reduced by the secondary mirror. Size for size therefore, a refracting telescope is better.

Aperture Synthesis

A technique used in radio astronomy where by an array of radio telescope dishes are used together, effectively giving the observer a much larger dish size e.g. the VLA in New Mexico. More recently, as technology has improved, it has been possible to link smaller optical telescopes on the same site to act as if they have a much larger resolving power, thus extending the range and usefulness of Earth based telescopes.


The branch of astronomy dealing with the movements and positions of celestial bodies.

Astrometry dates back to the earliest days of astronomy when the first star catalogues were being produced, e.g. that of Hipparchus in 190BC. Early measurements were probably made using cross-staffs to measure the relative positions of stars from one-another and from features on the horizon. As time progressed, more sophisticated instruments were used, such as the astrolabe.

The same principles are still used in modern astronomy but have become very precise and can measure the wobbles in the movements of stars that could indicate the presence of extrasolar planets or to find astrometric binary star systems.

Astrometry also includes the measurement of parallax. If you observe an object from two widely spaced locations, you will measure a slight difference in position. From the annular differences in position and knowing the distance between the two locations of observation, the distance of stars can be determined. That was taken to a new level with the Hipparcos satellite launched by the ESA. Early parallax measurements were limited to relatively close stars, however, the distances of several cepheid variable stars was measured. Thes can then be used as ‘standard candles’ when seen in distant galaxies to give a reasonable estimate of their distances. Further refinement to astrometric determination of distant objects came with the advent of interferometry.

As with all measurements of the extremely large and extremely small, astrometric measurements require careful error correction. As knowledge and instrumentation improves, distances and speeds of star movements are constantly being refined and it is believed that it is now possible to see the peturbations in stellar motion caused by planets not much more massive than the Earth.

Astronomical Unit

The mean (average) distance of the Earth to the Sun is termed 1 Astronomical Unit (1 AU). It is a convenient way of describing distances within our Solar System.

Nominally, 93 million miles or 150 million km. It is therefore a lot more convenient to use but even this mind-bogglingly enormous distance is inadequate to express the distance to galaxies so that is when parsecs are used.


The angle measured from the south point of the horizon toward the west to a point at the foot of a star’s vertical circle. When used as an indication of the position of a star on the imaginary celestial sphere it is referred to as Right Ascension.

Barlow Lens

Put a Barlow lens between the main mirror or lens of your telescope and its eyepiece and increase the magnification. Effectively it increases the focal length of the main mirror or lens. The most usual magnification for a Barlow lens is X2. In practice, they are rarely used since they cause a large light loss in the telescope. It is a cheap way of getting a short focal length eyepiece. Better to spend more on an eyepiece, you get better quality.

Cassegrain Reflector

A type of reflecting telescope where the main mirror has a central hole. Light from an object is reflected off the primary mirror up to the secondary mirror and back through the hole in the primary to be focused in the eyepiece tube. The primary mirror is a spherical mirror and is therefore easier and cheaper to make than a parabolic (slightly elliptical) mirror. Correction for spherical aberration is made by having a parabolic secondary mirror. The path of the light is also folded on itself which makes its tube much shorter, lighter and more portable than a refracting or a Newtonian reflector of the equivalent aperture. Popularised by Meade and Celestron telescope manufacturing companies in a modified form.

Catadioptic Telescope

A type of telescope, which uses both refraction and reflection to form an image at the prime focus. This type of telescope is generally based on the Cassegrain design but uses a corrector plate to prevent spherical aberration. This means that both the primary and secondary mirror can be spherical rather than parabolic. See Cassegrain Telescope for the advantages.


Celestial Latitude

The angle north or south of the ecliptic to an object. Used to help to describe the location of a star on the imaginary celestial sphere.

Celestial Longitude

The angle that is measured eastward along the ecliptic, from the vernal equinox, to the foot of a circle that is perpendicular to the ecliptic and passing through the object.

Celestial Sphere

This does not actually exist. Early humans believed that stars were fixed to a crystal sphere in the sky, at a great distance from Earth. This is because there is no sense of distance in the night sky and the stars always seemed to be fixed in position relative to each other. The idea is a handy one when dealing with positioning and angles between objects in the sky. See also celestial latitude, longitude, azimuth.

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