Introduction

Tonight In the Sky

• Planisphere Late Evening
• Planisphere Midnight
• Planisphere Early Morning
• Lunar/Solar Calendar

Telescopes

• Optical Terms and Characteristics of Telescopes
• Optical Aberrations
• Light Grasp Table
• Refracting Telescopes
• Newtonian Reflecting Telescopes
• Catadioptric Telescopes

Mounts

Applications

• Night Time Observing
• Solar Observing
• Daytime use of Astronomical Telescopes
• Photography
• Digital Imaging

Eyepieces

• Eyepiece Calculator
• Telescope Calculator
• Light Grasp Table

Filters

• Planetary Neutral Density and Polarizing Filters
• Nebular Filters
• Solar Filters

Collimation

Astronomy Product    Websites

There are many terms used to describe various characteristics of telescope optics. Here are a few of the most important ones:

Aperture and Performance Magnification Angular Resolution Definition Light Grasp and Limiting Visual Magnitude Field of View Closest Focus

Focal Ratio (Photographic Speed or F/Stop) Focal Length Airy Disk Exit Pupil Eye Relief Wave-Front Accuracy (Rayleigh Limit)

Aperture and Performance

The aperture of a telescope objective determines its light grasp and ability to show fine detail. To get much of a start in astronomy, 3" is the minimum acceptable aperture for refracting telescopes, while a slightly larger aperture is required for reflectors and catadioptrics. When acquiring a telescope, always try to obtain as much aperture as you can, consistent with your budget and portability requirements.

Click here for a table that compares scope apertures, listed both in inches and millimeters, with the associated Light Grasp, Magnification limits and Limiting Visual Star Magnitudes.

Given your Telescope's focal length and aperture calculate the suggest powers of the eyepieces suited for your telescope with Scope City's Telescope Calculator.

Magnification

A telescope can be used at a variety of magnifications for different applications using different eyepieces. The size of an image produced by a telescope is determined by the telescope's focal length and that of the eyepiece used. To determine the total magnification of a telescope when an eyepiece is used to magnify the image, divide the telescope's focal length by the focal length of the eyepiece. For example, if a telescope's focal length is 50" and the focal length of the eyepiece is ½", then the final magnification will be 100X. Since eyepieces are interchangeable, a telescope can be used at a variety of powers for different applications.

Telescopes have upper and lower limits of useful magnification. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum useful magnification is 60 times the aperture of the telescope (in inches). Magnifications higher than this usually result in an image that is too faint. Thus, the maximum useful magnification of a 6" telescope is 360X.

Do not believe manufacturers who advertise a 600X power telescope when its aperture is only 60mm. Such advertising is misleading.

Most of your observing will be done at magnifications of 100X and less. This is because there is a lot to see conveniently at these magnifications, and because the Earth's atmosphere often sets an upper limit to the amount of magnification that you can use.

Lowest useful magnification is 3½ X per inch of telescope aperture. At this limit, the diameter of the telescope's exit pupil has become as large as that of the human pupil. Lower magnifications would not be useful visually and might actually cause a dark spot to appear at the center of the field in Catadioptric or Newtonian telescopes due to the obstruction of the secondary mirror.

Given your Telescope's focal length and the focal length and apparent field of view of a eyepiece calculate the power of the eyepiece with your telescope and the actual field of view with Scope City's Eyepiece Calculator.

Given your Telescope's focal length and aperture calculate the suggest powers of the eyepieces suited for your telescope with Scope City's Telescope Calculator.

Angular Resolution

Angular resolution is the ability of a telescope to show fine detail. The greater the aperture of a telescope, the more detail it will reveal. The theoretical angular resolution of a telescope is equal to 4.56 divided by the telescope aperture in inches. For example, a 6" telescope is theoretically capable of resolving two stars separated by 0.8 seconds of arc. This is referred to as "Dawes Limit." Part of the challenge of high-magnification viewing is waiting for wave fronts of high enough quality to arrive at the telescope after passing through the Earth's atmosphere.

Definition

Definition is a telescope's ability to reveal the contrast between two areas having nearly the same brightness in an extended object image. This is especially important in planetary observing. Dust, bad coatings, thermals, poor seeing or optical aberrations can upset a telescope's contrast factor, thus diminishing its ability to reveal low-contrast detail. Of these, poor seeing is by far the most frequently encountered.

Light Grasp and Limiting Visual Magnitude

Field tests have shown that a telescope's limiting visual magnitude for stars depends strongly on both aperture and magnification. The basic limiting magnitude formula is 8.8 + 5LOG D where the constant 8.8 is for telescopes operating at a magnification of 3.5X per inch of aperture. D is expressed in inches. As magnification increases, the constant, known of as Equivalent Magnitude, increases for stars, finally leveling off at 11.6 for telescopes operating at a magnification of 40X per inch of aperture. Thus, the stellar limiting visual magnitude of an 8" telescope is 13.3 at 28X. But at 320X the limiting magnitude is 16.1. To realize your telescope's limit, its optics must be of high quality, the star must be located at the zenith and the air must be transparent and free of light pollution.

Astronomers use a magnitude scale to indicate the brightness of celestial objects. On this scale, which is based on the fifth root of 100, each successive magnitude is 2.512 times fainter than the one below it. For example, a third magnitude star is 2.512 times fainter than a second magnitude star, and 6.310 times fainter than a first magnitude star. The sun, which has a magnitude of -27, is a million times brighter than the full moon, which is of magnitude -12. The faintest star that you can see with your unaided eye is about magnitude 6.

Click here for a table that compares scope apertures, listed both in inches and millimeters, with the associated Light Grasp, Magnification limits and Limiting Visual Star Magnitudes.

Field of View

Field of view comes in two varieties: Apparent Field and True Field. Apparent Field is the field of an eyepiece and is usually 30 to 50 degrees wide. True Field is the actual amount of sky that you see through a telescope using the same eyepiece. It is quite small and may sometimes be only a fraction of a degree wide. The way to calculate True Field is dividing the Apparent Field by the telescope's magnification. For example, if your eyepiece has a 50-degree Apparent Field and the magnification is 100x, then the True Field is 0.5 degree.

Given your Telescope's focal length and the focal length and apparent field of view of a eyepiece calculate the power of the eyepiece with your telescope and the actual field of view with Scope City's Eyepiece Calculator.

Closest Focus

Closest focus is the distance to the nearest object upon which a telescope may be focused. At close enough ranges, a telescope behaves as a long-distance microscope, revealing detail smaller than what the unaided human eye can see at its own closest focus.

Focal Ratio (Photographic Speed or F/Stop)

Focal ratio is the ratio of the focal length of the telescope to its aperture. To calculate it, divide the telescope's focal length by the diameter of the objective. For example, a 6" telescope with a 48" focal length has a focal ratio of 8. This is normally expressed as f/8.

Generally, the slower and simpler an optical system is, the gentler the curvatures of its optics and the freer from optical aberrations it will be. For example, slow Newtonians and Refractors have very gentle curvatures and complete freedom from coma and field curvature. A telescope is considered slow if its focal ratio is f/10 or greater. Medium speeds occur at focal ratios of f/7 to f/9. Focal ratios f/6 or less are considered fast.

Beginners are often under the false impression that the surface brightness of an extended object image is determined by a telescope's focal ratio. In reality, surface brightness is determined by the telescope's total light grasp and the magnification. Stars differ from extended objects in this respect by actually becoming easier to see at higher magnifications, again, completely regardless of focal ratio.

Focal Length

Focal length is the distance from the telescope objective to the focal plane where the image is formed. The longer the focal length of the telescope, the larger the prime focus image will be and the more magnification will be produced with a given eyepiece. A telescope with a focal length of 2000mm will produce twice the magnification of a 1000mm telescope.

Given your Telescope's focal length and aperture calculate the suggest powers of the eyepieces suited for your telescope with Scope City's Telescope Calculator.

Airy Disk

The Airy Disc is the result of the diffraction of light, which causes stellar images to appear as diffraction patterns under high magnification. It is the part of a stellar image that is seen and appreciated by the human eye. You will be able to detect the diffraction pattern when you focus on a bright star at a magnification of 30X per inch of telescope aperture. The bright dot at the center of the pattern is the Airy disk. Look closely and you will see the first bright interference ring surrounding it. Since images formed by telescopes are made of diffraction patterns, a telescope's angular resolution will be determined by the angular size of the Airy disk. And since increasing telescope aperture results in less diffraction, large telescopes produce small diffraction patterns. The result is high angular resolution.

Exit Pupil

The exit pupil of a telescope is the circular beam of light that leaves the eyepiece. Its diameter is a function of telescope aperture and magnification. To determine exit pupil diameter, divide the aperture of the telescope by the magnification of the eyepiece in use. For example, an 8" aperture telescope at 100X will produce an exit pupil 0.080" in diameter. When exit pupils are small, observers who have astigmatism in their eyes can remove their glasses, since small exit pupils effectively stop down the human pupil, eliminating astigmatism.

Eye Relief

Eye relief is the distance from the final lens surface of an eyepiece to the pupil of the eye that allows access to the optimal field-of-view. Some eyepieces have eye relief so short that your eyelashes will actually brush against the lens. With these eyepieces, eyeglass wearers will see only a small portion of the field. If you are an eyeglass wearer, then you will need to use eyepieces that feature long eye relief.

Wave-Front Accuracy (Rayleigh Limit)

Good telescope optics are characterized by converging wave-front path differences that are smooth and do not exceed the Rayleigh Limit of 1/4 wavelength of light. As wave-front quality approaches 1/10th wavelength, definition becomes essentially perfect.