Posts Tagged ‘telescope’

Exit Pupil

The exit pupil is the image formed by the eyepiece of an optical instrument.

At low magnifications, the exit pupil is wider than at higher magnifications.


An eyepiece is the combination of lenses that magnifies the image formed by the objective (main mirror or lens) in an optical instrument.

There are different types of eyepiece, each has its merits. The most common type seems to be the Plössl, which is a good general eyepiece. Huygens and Kellner are the names of two other types of eyepiece.


A lens that comprises two elements glued together in order to reduce chromatic aberration. One of the elements is made from crown glass and the other from flint glass. They work well if the light is travelling parallel to the lens’s axis but performance is worse if it is not, in which case multi-element lenses should be employed.


The angle north or south of the celestial equator to a star. It is one  of the two co-ordinates used to describe the position of an astronomical object on the so-called celestial sphere. Together with Right Ascension, declination describes where in the sky an object can be found.

GoTo telescopes are programmed with thousands of celestial co-ordinates and make it extremely easy to find an object. But where is the satisfaction in that? For an amateur astronomer, finding a faint object for yourself is extremely satisfying, users of telescopes with the RA and declination pre-programmed are missing out on half of the fun and can only really be called stargazers, not astronomy enthusiasts. Sadly, the marketing machine of telescope manufacturers suggests otherwise – they just want to get as much of your cash as they can. Cynical, aren’t I?

Dark Adapted

This is what every astronomer’s eyes should become before they begin observing. On leaving a brightly lit area and entering a dark area, you will notice a rapid increase in visibility over a short period of time. However, it takes about 20 – 30 minutes to become fully dark adapted. In low light levels, the chemicals in the eye increase the ability to see faint objects. See also the Purkinje effect

Chromatic Aberration

A defect of a lens that creates a fringe of colour round an object. It is caused by the fact that different wavelengths of light are refracted by different amounts.

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.


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.

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.


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.

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.


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.

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.

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.

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