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Telescopes – Beginners Guide – Parts common to all types of telescopes December 17, 2009

Posted by aquillam in Astronomy, telescope beginners guide.
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There are several things the all amateur optical telescopes have in common. Below are some definitions and brief explanations.

Primary or Objective

In a refractor, the big lens at the front that you point at the object is called the objective lens. In a reflector, the big mirror that gathers all the light is called the primary mirror or the primary.  Since most telescopes are reflectors, the term “primary” is sometimes used to refer to both of these, even though it is more correct to say “primary mirror or objective lens”.  If you are specifically talking refractors, you should definitely say “objective”.

The primary mirror or objective lens is responsible for most of the telescope’s properties, and especially for the light gathering power and (for smaller ‘scopes) resolution.

Optical Tube Assembly

The Meade Lightbridge OTA is actually an open truss rather than a tube.

The Optical Tube assembly (OTA) is the body of the telescope.  It consists of the primary or objective and the structure that supports it and makes it possible to look through.

When the telescope is a refractor, the OTA really should actually be a tube with the objective at one end and the eyepiece at the other.  Reflector telescopes may have an open truss assembly  instead, and the location of the eyepiece can vary significantly.


The eyepiece is the lens you actually look through.  Any telescope you expect to look through yourself (as opposed to attaching a camera) needs an eyepiece.  There is a nice page with lots of info and images at http://www.rocketroberts.com/astro/eyepiece_basics.htm.

barrel diameter

a 2" and 1.25" eyepiec

Eyepieces with a 2" and 1.25" barrel diameter. They are labeled with their focal lengths, 27mm and 24 mm. (on a side note, these are very nice eyepieces, but they also cost more than my telescope!)

Eyepieces can fit directly into the OTA, into a focuser, or into a diagonal or prism. Whatever the eyepiece fits into must match the diameter of the eyepiece. In the last 30 years or so, 1.25″ has become the standard for amateur telescopes, although large telescopes may take 2″ eyepieces.  Cheap scopes may come with 0.97″ eyepieces, usually made out of plastic and apparently designed to cause eye strain and scare people away from the hobby. Earlier telescopes may have lenses made just for that telescope, so you may find eyepieces of all different sizes, especially with telescopes built before the mid-20th century.

focal length

The focal length determines the primary properties of the eyepiece (at least, when compared to eyepieces of similar configuration). Eyepieces are frequently refered to by their focal length. It determines the magnification of the telescope.  It also determines what the field of view will be.  The shorter the focal length, the bigger the magnification and the smaller the field of view.

In addition, shorter focal length eyepieces tend to have a smaller exit pupil. For the user, this means you have to have your eye really well aligned with the eyepiece to see anything.  Additionally, less light will pass through, so the image may be dimmer.


There are many many different configurations, ranging from a single bi-convex lens to large collections of 5 (or more) lenses of various shapes with different materials in between. Many of the more complex ones are designed to make the field of view wider (usually called “wide field” eyepieces) or make the image brighter. They may also be designed to improve the eye relief.  Good eye relief means you can be relatively far away and still focus on the image, very important for people who wear glasses.

One special lens is the Barlow.  It is intended to be used with another eyepiece or a camera to improve magnification.  A good Barlow can effectively double the usefulness of your other eyepieces.

Diagonals and prisms

Most telescopes (but especially refractors) can be hard to look through when pointing at things high overhead.  A diagonal or prism redirects the light to make it easier to look through.

Diagonals work either by using a plane mirror to reflect the light at a right angle, or a prism.  A prism can either reflect the light 90º or 45º.

Mirrors are generally prefered for astronomical use since they will reduce the light less (you lose a little bit of light every time the light has to pass through or reflect off  a surface.) However for small telescopes a “correct image” prism may be prefered, because you can use it for terrestrial viewing as well.


The focuser is the part that lets you bring an image into focus.  There are several different types of focusers, ranging from simply moving the eyepiece  by hand to motorized systems that move one of the mirrors.


You may be able to hold a small, low power telescope  steady enough by hand, but chances are you will need some sort of mount for your telescope.

Small telescopes may sit very nicely on a table (especially if the alternative is a tripod bigger than the ‘scope). However, most users prefer a tripod.

There are two basic types of mounts, called equatorial and alt-az. Equatorial mounts are aligned with Earth’s axis of rotation, which makes it easy to “drive” the telescope.  Alt-az mounts move in altitude (horizon to zenith) and azimuth (north, east, south…, usually measured in degrees away from due north.)  This is a fairly complex subject, so it gets its own entry.

Drive or Motor

The Earth is constantly turning.  If your telescope doesn’t move, the thing you are trying to look at will pass out of your ‘scopes field of view.  To counter that, many scopes have a drive.

Drives work best with equatorial mounts.  The Earth is rotating eastward along it’s axis.  With an equatorial mount, a motor can counteract that motion by moving the telescope westward (same direction as the sky) on it’s axis at the same rate.  You can also drive an alt-az mounted ‘scope, but in that case each axis has to move at a different rate and direction depending on where the ‘scope is pointed.  This is only really possible if the mount is computer controlled.


Telescopes severely restrict how much you can see.  With nothing in the way (like glasses) most people can see about 160º side to side. It’s pretty easy to spot the star you want that way. Most telescopes have a field of view that’s less than a degree.  And if you boost the magnification, it’s even smaller.  If you want to find anything with your telescope, you’ll need a finder.

Using the site on the galileoscope

Using the site on the Galileoscope: Align the rear post with the front notch and the target.

There are several different types of finders. The simplest finders are just a couple of raised bumps for a sight (see image), or a hollow tube aligned with the OTA.

Most telescopes use a very small refractor, typically about  40 mm in diameter with a 5º field of view. The telescopic finders usually have mount with 3 screws to adjust the alignment with the OTA.

View through a telrad

Simulated view through the window of a telrad

There are also more unusual finders, like red dot or telrad finders.  These use a red light, usually from an LED, projected onto a screen or mirror to show where the telescope is pointed.

A more recent option is a green laser pointer mount.  Green laser pointers are usually bright enough to use as a pointer at night. We can take advantage of this by putting a mount on the telescope to align the laser pointer with the OTA. Turn the laser pointer on, and you can see exactly where the telescope is pointing.



1. laser pointer - December 19, 2009


i think?

2. Valuable Internet Information » Telescopes – Beginners Guide – Parts common to all types of … - December 17, 2009

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