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Telescope Terminology
** [ Draft pending completion ] **
TBD:
1. Expand the Eyepiece section
2. Write up concluding remarks
This is a concise introduction to telescope terminology.
It is neither comprehensive nor complete. Coverage and depth of detail have been sacrificed in favour of brevity. Links to sources providing more detail have been provided as far as possible.
The intent of this article is to provide a quick run-through of commonly used telescope terms for beginning amateur astronomers. If you have the time, the author recommends Phil Harrington's Star Ware.
A telescope collects light from a distant source, and focusses it into an eyepiece, so as to form an image that can be viewed by the human eye. This is done by using a system of mirrors, lenses, or both.
There are broadly three kinds of telescopes - reflectors, refractors, and catadioptrics.
Refractors use a system of lenses to focus an image onto the eyepiece. Inexpensive achromatic refractors suffer from chromatic aberration (CA). There are also apochromatic refractors that do not have CA. Needless to say, the lack of chromatic aberration comes at a price.
(Newtonian) Reflectors use two mirrors to focus light onto the eyepiece. The bigger mirror, called the primary can either be parabolic or spherical in shape. A Dob, short for Dobson(ian) Telescope, is a popular kind of Newtonian Reflector.
Catadioptrics use a system of both mirrors and lenses. They have a spherical mirror and a corrector for the spherical mirror. They are usually smaller than comparable Reflectors or Refractors. Maksutov-Cassegrains (Maks or MCTs), and Schmidt-Cassegrains (SCTs) are two popular kinds of catadioptrics.
Telescopes are primarily specified by their aperture and focal length.
Aperture is the diameter of the telescope's objective (or primary) mirror or lens. It is specified in mm or inches. Typically, the bigger the aperture, the greater the telescope's light-gathering ability, as also its ability to resolve fine details. This is usually a good thing. With bigger aperture, however, the telescope becomes heavier and bulkier.
The Focal Length of a telescope is the distance between the objective and the point where the light is brought to focus.
Focal Ratio or f-number is another way of expressing the focal length. It is simply the ratio of the focal length to the aperture.
A telescope could probably be described as a 127mm f/12.1 telescope, or as a telescope with 127mm aperture, and 1540mm focal length. Both descriptions are exactly the same.
You might also come across the terms slow and fast telescope. A "slow" telescope has a bigger f/number than a "fast" scope.
Magnification is the ratio of the telescope's focal length to the focal length of the eyepiece being used to view through the 'scope. So if you use a 25mm eyepiece on a telescope with a focal length of 1540mm, you get a magnification of 1540/25, or 61.6x.
As magnification increases, the brightness of the image decreases. High Magnification is not necessarily a good thing. Also, there are limits to the magnification one can extract from a telescope of a given aperture.
When choosing a telescope, ignore magnification. Aperture is more important.
Achromatic Refractors are (relatively) inexpensive. They also suffer from chromatic aberration. At least one BAS member has put a 70mm achromat to excellent use, and recommends it as a first scope.
Apochromatic Refractors are expensive, and are not really beginner telescopes. That said, there's nothing that should prevent you from buying one if you can afford it, and really want one. The Vendor Information page should be of some help in this regard.
Refractors grow expensive with aperture; it is difficult to make big lenses, it is far easier to make big mirrors.
Newtonian Reflectors probably offer the best bang-for-buck with respect to aperture. They also gain bulk with increasing aperture, and lose a little on portability. This is subjective, however - the best way to understand this is to actually look at a scope in person. If you can carry it comfortably, ignore the previous discussion.
Maksutov- (and Schmidt-) Cassegrains are compact, and approach refractors in image quality. The images might be a little less bright than those from a Reflector of similar size. They are very portable, and also more expensive than Reflectors.
Typically, a "telescope" is just the optical tube, also referred to as the Optical Tube Assembly (OTA). In addition, one also needs an eyepiece to see through the telescope, and a mount to support the optical tube assembly. In most cases, one will also need a finderscope. This is the minimum setup needed to make astronomical observations. Depending upon the telescope, one might also need a Star Diagonal. The telescope is pretty much useless without these "accessories".
Primary (and associated) optics apart, the part of the telescope that makes the most difference to image quality is the Eyepiece. Some terms that we need to be familiar with in connection with eyepieces are their Focal Length, Apparent Field Of View (AFOV), Eye Relief and Exit Pupil.
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The barrel size of an eyepiece is typically 1.25" or 2". Older telescopes used to accept 0.965" eyepieces; these are not as common. That said, as recently as a few years ago, some small refractors - notably the Celestron Powerseeker 50mm - still came with 0.965" eyepieces.
Steve Coe's article has more information on eyepieces.
The telescope mount needs to support the telescope, and also needs to be able to aim the telescope at any point in the sky.
Alt-Az mounts allow the telescope to be moved along two axes of rotation. These are the horizontal axis or the azimuth, and the vertical axis or the altitude axis.
A common kind of Alt-Az mount uses an L-shaped mount head sitting on a tripod. The telescope is attached to the mount head by a metal dovetail and screws. The dovetail can either be an integral part of the scope's body (typically the case with small catadioptrics), or is holds the telescope via tube rings.
Some Alt-Az mounts come with slow-motion controls which allow you to lock the main altitude and azimuth controls, and make small changes in the altitude and azimuth axes.
A Dobsonian Telescope is an Alt-Az mounted Newtonian Reflector. Instead of mounting the telescope on a separate mount-head and tripod, the OTA and mount are part of the same construction. In the simplest design, the altitude motion is provided by a pair of circular bearings located on either side of the OTA. These altitude bearings sit in a rocker box, which itself sits atop a bigger azimuth bearing. More complicated designs are possible, but they all derive from this basic design.
The Earth's rotation (about a tilted axis) manifests itself in the apparent motion of heavenly bodies in the sky. At any location on the Earth, objects in the sky describe a circular path from East to West. This circle is identical to the latitude parallel of the location, and has the North Celestial Pole (NCP) as its centre. Objects in the sky move along this circular path at the rate of 15 degrees an hour. (The Earth completes one rotation - 360 degrees - in approximately 24 hours.)
Owing to this apparent motion, if you aim a telescope at any point in the sky, and lock its position, the object being viewed at will eventually move out of the telescope's field of view. This can happen in as little as a few seconds.
While an Alt-Az mount provides for circular motion along two axes, neither axis lies in the same plane as the latitude parallel. In order to track a heavenly object with an Alt-Az mount, one must make small (stepwise) motions in both the altitude and azimuth axes to approximate the motion of the object. An effect of this approximation is that during the process of tracking, the OTA rotates about its axis. This causes the view in the eyepiece to rotate as well. This phenomenon is called field rotation.
Equatorial mounts also allow motion about two (orthogonal) axes of rotation. One axis is oriented at right angles to the latitude parallel, pointing at the NCP. This axis is called the Right Ascension (RA) axis. The other axis is called the Declination (Dec) axis. Rotational motion about the RA axis tracks the latitude parallel at the telescope's location.
The telescope is aimed by moving it along the RA and Dec axes. Once an object of interest is located, it can be tracked by rotating the telescope about the RA axis at the rate of 15 degrees an hour, from East to West. This motion compensates for the Earth's rotation.
Equatorial mounts do not suffer from field rotation.
The process of aligning the RA axis to point at the NCP is called polar alignment.
The most common kinds of equatorial mounts are German Equatorial Mounts (GEMs) and Fork Mounts.
A GEM has the RA and Dec axes in a T-shape. The stem of the T is positioned at right-angles to the local latitude, and the telescope is mounted at one end of the "top bar" of the T. The other end is typically counterweighted to balance the weight of the telescope, and hence reduce stress on the RA bearing.
Fork mounts have the telescope mounted in a U-shaped "stirrup". This provides motion in the Dec axis. The bottom end of the U is mounted on the RA bearing.
It is possible to couple motors to either the RA bearing, or both the RA and Dec bearings, to provide motorized slewing and/or tracking. For tracking purposes, a single motor coupled to the RA bearing is sufficient. Power for the motors can either come from a mains socket and adapter, or from a battery pack.
Usually, a motor kit is sold as an upgrade option for most equatorial mounts.
Motorized mounts that come with a database of heavenly objects are called GoTo mounts. Selecting the object of interest through the mount's interface (typically a keypad integrated with the hand controller) will result in the mount automatically slewing to the target.
Some GoTo mounts (but not all) also provide tracking.
Most low-end GoTo mounts are motorized Alt-Az mounts - they suffer from field rotation.
All GoTo mounts need power. Some do not work at all without power, and do not permit manual slewing.
In contrast to a GoTo mount, a Push-To device (usually an add-on to an existing mount) does not automatically slew to the target; instead, it provides the user feedback as to where the telescope is currently pointing. On being provided with a target, a Push-To device provides realtime directions to arrive at the target, and also confirms the same.
In other words, a Push-To device acts as a Digital Setting Circle (DSC), along with a database of heavenly objects.
The Orion IntelliScope series of telescopes (for example) come with built-in encoders to keep track of the telescope's position, and a push-to hand controller.
In addition, there are apps for smartphones like SkEye, which transform the phone into a (virtual) DSC. The author of SkEye is none other than BAS's Harshad RJ. The advantage of SkEye is that you don't need to buy a separate set of encoders and a hand controller - you can instead repurpose your smartphone to do much the same job. This happens at the cost of some accuracy; but (IMHO) this is outweighed by the benefits.
There are also off-the-shelf Digital Setting Circles like the Argo Navis, Nexus DSC, or the Sky Commander. These kits (usually) come with optical or magnetic encoders, and are pretty accurate. They come at a price too.
Equatorial Platforms were invented as a means of providing Dobs with Equatorial Tracking. The page linked to has further details.
The precise distance from the primary optics at which the eyepiece attains focus depends upon a few things - among which is the focal length of the eyepiece itself, (as well as the eye of the observer). Since one typically has eyepieces of varying focal lengths, telescope OTAs usually come with a mechanism called a focuser that helps one vary the position of the eyepiece slowly, smoothly, and in small increments (or decrements) of distance.
Small telescopes tend to come with 1.25" focusers. Some refractors, and big Dobs come with 2" focusers. It is possible to use 1.25" eyepieces in 2" focusers with the help of an adaptor.
SCTs and Maks usually have an integrated focuser, which is controlled by a knob on the back.
The most common kind of focuser is a rack-and-pinion focuser - the same kind that used to focus common school-laboratory microscopes.
If finer control is desired, there are Crayford Focusers, which work off a different mechanism than a rack-and-pinion.
Newtonian reflectors have their eyepieces at right angles to the main OTA. Reflectors and Catadioptrics do not, by design. Having the eyepiece at the rear end of the OTA is not a very desirable thing, since it makes looking through it (literally) a pain in the neck.
The solution is to use a star diagonal - a prism or a mirror arrangement which attaches to the eyepiece end of the OTA, and lets one mount the eyepiece at right-angles to the OTA. This makes for a more comfortable viewing arrangement.
The diagonal also has the effect of increasing the effective focal length of the OTA by a little (since it extends the light path).
Diagonals usually result in images flipped L-R. Some diagonals correct for this, (and also for image inversion), and deliver L-R and up-down correct images.
In order to view an object through a telescope, the telescope must first be aimed at the object. The narrow field of view of a telescope makes this a difficult proposition - except perhaps in a few wide-field scopes (but let's ignore them for now).
To make it easier to aim a telescope, we use a smaller, wide-field refractor called a finder scope. The finder is usually mounted on the main OTA, and aligned with it so that both share the same view. The idea is to see through the finder, and use this view to aim the telescope.
Most finders have a fixed magnification. A few allow for the use of eyepieces and have a small (usually helical) focuser.
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Achromatic Finders
These are the most common kind of finder scope. It is more or less a small refractor with a fixed magnification. Most small telescopes come with a 6x26 (6x magnification, 26mm objective lens) or similar finder.
Most amateurs seem to gravitate to finders with a bigger aperture, like 9x50 finders. One reason for this preference is that these finders offer similar fields of view to binoculars, and hence offer similar (and familiar) views.
Most achromatic finders also display inverted images, similar to refractors.
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[RA]CI Finders
Some finders offer a right-angle eyepiece, so that it is not very difficult to move from the finder to the eyepiece, and vice-versa. These are called Right-Angle (RA) finders.
Other finders correct for the L-R image flip, as well as the inverted image. These are Correct-Image (CI) finders. Again, some like CI finders for the similarity of their views with binoculars.
Finders that have a right-angle eyepiece and deliver a correct image are called RACI finders.
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Unity Finders (Telrads and the like)
When using a finder in conjunction with a star chart, it can be useful to not have any magnification, but instead have a feel for the angular distance between the objects in the field of view.
The Telrad and other "Unity Finders" do exactly this. They do not magnify the view, but instead superimpose a pattern - usually three concentric red circles, each 4 degrees, 2 degrees, and 0.5 degrees in diameter on the unmagnified field. This can be very useful under dark skies.
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Red-Dot Finders
Red-dot finders superimpose a red dot on the unmagnified field.
Magnifying finders and unity finders each have their own uses; some observers use both.
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Barlow Lenses
A Barlow Lens multiplies the magnification of an eyepiece. It is inserted between the eyepiece and the focuser, and has a receptacle for the eyepiece. Barlows typically provide 2x or 3x times the eyepiece's magnification. This doesn't come for free, there is usually some loss in image brightness.
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Dew Shields & Dew Heaters
In cold weather, exposed surfaces have moisture condense on them. Glass surfaces, especially Refractor objectives and Mak corrector plates, are no exception. In some cases, even Newtonian secondaries get dewed over. Solutions to avoid dew include Dew Shields - basically a tube that extends beyond the objective. Devices that electrically warm the air around the glass surface to keep it free of dew are called Dew Heaters.
For more on dew shields and dew heaters, please consult this article. It also links to plans for a DIY dew heater.
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Polar Alignment Scopes
A Polar Alignment Scope is an aid for the polar alignment of equatorial mounts. It is essentially a small refractor that comes either with cross-hairs, or a small circle, within which one should centre Polaris (or Sigma Octantis, if you're in the Southern Hemisphere) by adjusting the mount.
Some polar scopes also superimpose images of well-known constellations like Cassiopeia and Ursa Major to make it easier to locate Polaris in them.
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Collimation Tools
Collimation is the process of aligning the optics of the telescope. Collimation tools in increasing order of complexity and expense are Collimation Caps, Cheshire Collimators, Laser Collimators, and Barlowed Lasers.
References 13 and 14 have more to say about collimation tools.
- Philip Harrington, "Star Ware"
- Starizona on Refractors
- Wikipedia on Reflectors
- Wikipedia on Catadioptrics
- Wikipedia on Dobsonian Telescopes
- Post from BAS discussion on buying telescopes
- Chuck Hawks, Simple Formulas for the Telescope Owner
- Orion IntelliScope Dobsonians
- Harshad RJ: SkEye - Advanced Planetarium for Android
- Sawdust Factory: Equatorial Platforms
- Steve Coe: A Guide to Eyepieces
- BAS article on Dew Shields and Dew Heaters
- BAS Group conversation on Collimation
- Gary Seronik on Collimation Tools