Telescope Equipment Basics

Maksutov Designs

Above:  Optical layout of a typical Maksutov-Cassegrain telescope This section details the optical design and inherent aberrations of Maksutov-type catadioptric telescopes.  For a more basic overview of these designs please see the Maksutov-Cassegrain page and the Maksutov-Newtonian page.  For a review of the optical design terms, see the Optical Aberrations and Optical Design sections. Maksutov… More »

Schmidt-Cassegrain Design

Above:  Optical layout of a typical Schmidt-Cassegrain telescope This section details the optical design and inherent aberrations of Schmidt-type catadioptric telescopes.  For a more basic overview of these designs please see the Schmidt-Cassegrain page and Schmidt-Newtonian page.  For a review of the optical design terms, see the Optical Aberrations and Optical Design sections. Schmidt Camera… More »

Classical Cassegrain Design

Above:  Optical layout of a typical Cassegrain reflecting telescope This section details the optical design and inherent aberrations of Cassegrain-type reflecting telescopes.  For a more basic overview of these designs please see the Ritchey-Chrétien page and the Cassegrain page.  For a review of the optical design terms, see the Optical Aberrationsand Optical Design sections. Classical… More »

Newtonian Reflectors

Above:  Optical layout of a typical Newtonian reflecting telescope This section details the optical design and inherent aberrations of Newtonian reflecting telescopes.  For a more basic overview of this design please see the Newtonian page.  For a review of the optical design terms, see the Optical Aberrations and Optical Design sections. Newtonian Design The Newtonian… More »

Achromatic and Apochromatic Refractor Designs

Above:  Optical layout of a typical refracting telescope This section details the optical design and inherent aberrations of refracting telescopes.  For a more basic overview of this design please see theRefractors page.  For a review of the optical design terms, see the Optical Aberrations and Optical Design sections. Achromatic Refractor Design A simple lens focuses… More »

Lateral Chromatic Aberration

Lateral color is an off-axis aberration resulting from a difference of image scale at each wavelength. Above:  Lateral color arises from a difference in image scale depending on wavelength Lateral color causes colored fringes at the edge of the field.  The stars will appear to split into red and blue halves, pointed toward the center… More »

Longitudinal Chromatic Aberration

Longitudinal Chromatic Aberration Often called longitudinal color, or axial color, this is the primary aberration affecting refracting telescopes.  It can also arise in any system which uses lenses, although it tends to be most problematic in pure refracting instruments. How Longitudinal Color Arises When light rays pass from one medium to another–say, from air into… More »

Distortion

Distortion is one of the least problematic aberrations for telescopes.  Distortion is typically only large over very wide fields such as with wide-angle camera lenses.  Over the small fields of view associated with telescopes, distortion is rarely an issue.  Very wide field binoculars may show some noticeable distortion, especially less expensive pairs. Distortion arises from… More »

Field Curvature

The focal planes of most telescopes are not flat.  Off-axis light usually focuses closer to the objective than does on-axis light. Above:  A curved focal plane For visual observing, a slightly curved field is no big deal.  The eyepiece will compensate somewhat, and if the stars at the edge of the field are slightly out… More »

Astigmatism

Above:  Spot diagram of a star at the edge of the field affected by astigmatism Like coma, astigmatism is an off-axis aberration.  Unlike coma, it is a symmetrical aberration.  For professional astronomers this can be an important distinction, since asymmetrical aberrations do not allow accurate astrometric (positional) measurements to be made.  For amateur astronomers viewing… More »

Spherical Aberration

Spherical Aberration Spherical aberration is an axial aberration, affecting the entire field equally, including stars at the center.  All telescope designs strive to eliminate or minimize spherical aberration.  Normally, spherical aberration should not be visible in an optical system.  But it is important to understand how it arises to see how it is eliminated in… More »

Coma

Above:  Spot diagram of a star at the edge of the field affected by coma Coma is an off-axis aberration.  Stars in the center of the field are not affected by coma, but the effect grows stronger toward the edge of the field.  Stars affected by pure coma are shaped like little comets (hence the… More »

Optical Designs

The following section describes in detail how various optical components work to form an image in a telescope, and how they modify or correct the aberrations present.  This is intended to give more information to those who want to know exactly how all the components in their telescopes–from mirrors to focal reducers–function.  It will also… More »

Optical Aberrations

This section describes optical aberrations, why they occur, and how they can be eliminated or minimized.  More specific information on how certain optical designs deal with aberrations is described in the Optical Designs section. Spot Diagrams Optical aberrations are normally displayed by showing a highly magnified image of a theoretically perfect star as imaged by… More »

Binoculars for Astronomy

Binoculars offer a number of advantages for stargazing.  They are portable, easy to use, and provide wide-field views.  There are objects which can be seen in binoculars that are difficult to view with a telescope.  The Pleiades star cluster or the Andromeda Galaxy, for example, are too large to fit in the field of view… More »

Others Accessories

Below are some of the handiest accessories you can get to make your observing easier and more enjoyable.  Most are considered must-haves by experienced observers.  So read on to see what you might need to enhance your stargazing! Reflex Finderscope Whether you are hunting down objects manually or have a computerized system, this device might… More »

Focusers

Focusing a telescope seems like such a simple thing that, at first, it’s hard to believe there is such a multitude of accessories to make focusing easier, faster, and more precise.  There are entire companies that exist solely to build focusers for telescopes!  Additional focusing accessories can be as simple as an improved focus knob… More »

Light Pollution and Nebula Filters

Light pollution filters and nebula filters are used to enhance the view of deep sky objects.  They both work in a similar manner, but are intended for slightly different purposes.  In fact, nebula filters really are just a type of light pollution filter.  These filters are normally designed to thread into the bottom of an… More »

Solar Filters

Might as well get the obligatory warning out of the way.  The sun generates more energy in one second than all the power plants on Earth could create in a million years.  It’s really, really, really bright and without a proper solar filter it will do unpleasant things like melt your eyeballs, so be careful.… More »

Lunar and Planetary Filters

Two types of filters are often used to enhance planetary detail.  The most popular method is to use a neutral density or polarizing filter to cut down the glare from bright objects like the moon and Venus.  There are also color filters which are used to enhance specific details of certain planets.  Both types of… More »

Understanding Magnification

Magnification is the most misunderstood aspect of telescopes, but not only by beginners.  New telescope users often assume that more magnification gives a better view.  They quickly learn that this is rarely true, and that on the contrary, lower power almost always yields a better image.  Check out the Magnification Calculator to determine the power… More »

Binocular Viewers

Until recently, many new observers inquired about binocular viewers (binoviewers), but few actually bought them, primarily due to the expense.  This is starting to change, with less-expensive binocular viewers becoming available.  As with anything, you get what you pay for, so the best binoviewers will still be pricey.  Also, you have to buy two of… More »

Zoom Eyepieces

Zoom eyepieces offer a convenient method of changing magnification without having to use multiple eyepieces.  Unfortunately–there being no free lunch and all–there is a trade-off in optical quality when using a zoom eyepiece. Above:  A typical zoom eyepiece, TeleVue’s 8-24mm Click Stop Zoom   Advantages of Zoom Eyepieces The main advantage of a zoom eyepiece… More »

Long Eye Relief Eyepiece

One of the drawbacks to standard eyepiece designs is that at short focal lengths (high power on most telescopes), the eye relief is very short.  Eye relief is the distance the observer’s eye must be from the eyepiece lens (see diagram below).  Short eye relief makes viewing quite difficult, especially for eyeglass-wearers. Above:  Eye relief… More »

Wide Field Eyepieces

Wide field eyepieces (with apparent fields greater than 50°) were originally invented for military applications during World War I.  The Erfle design consists of 5 or 6 lens elements and give an apparent field of 60-70°.  They are relatively inexpensive but suffer from distortion and astigmatism at the edge of the field of view.  They… More »

Standard Eyepieces

Standard eyepieces are relatively inexpensive designs which function very well.  These include the types of eyepieces normally included with telescopes.  The apparent field of view of a standard eyepiece is usually in the range of 40-50°. Early Eyepiece Designs The first telescope eyepieces, such as those used by Galileo, consisted of a single lens.  (See… More »

Understanding Eyepieces

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… More »

Other Mounts

The most common telescope mounts are the German equatorial mount, the fork mount, and the Dobsonian mount.  Almost all telescopes sold have one of these types of mount, but there are a few other designs which are less common but still used with a variety of instruments. Altitude-Azimuth Above:  A typical alt-az mount, TeleVue’s Gibraltar… More »

Dobsonians

A Dobsonian telescope incorporates a Newtonian optical design on a unique alt-azimuth type of mount.  These telescopes are very popular for both beginners and advanced observers. Above:  A typical Dobsonian telescope There are two basic categories of telescope mounts: alt-azimuth and equatorial.  Alt-azimuth–short for altitude-azimuth–mounts allow a telescope to move up-down and left-right.  This movement… More »

Fork Mounts

Fork mounts have become very common for their versatility and ease of use.  German equatorial mounts (GEMs) used to be more common, but the advent of computerized telescopes has made the fork mount usable for a variety of applications and it is quickly becoming the most popular mount for most observers. Above:  A typical fork… More »